Ross J. Simpson, Jr., MD, PhD

• Professor of Medicine
• Director, Lipid and Prevention Clinics
• Division of Cardiology
• University of North Carolina School of Medicine
• Chapel Hill, North Carolina

The subjective feeling of air hunger may occur even when alveolar ventilation and the blood gases are normal prostate cancer 50 year old male flomax 0.4mg without a prescription. Some people experience dyspnea when they perceive that they are short of air androgen hormone video 0.4mg flomax, even though this is not actually the case prostate cancer 38 years old generic flomax 0.2mg overnight delivery, such as in a crowded elevator prostate cancer screening guidelines cheap 0.2mg flomax overnight delivery. Consequently prostate ultrasound cpt code flomax 0.2 mg with amex, they inhale more air than they exhale and man health 125 cheap generic flomax uk, over time, they increase their end-expiratory lung volume, a process referred to as dynamic hyperinflation (p. This limits how much they can inspire because the inspiratory reserve volume (the difference between total lung capacity and endinspired lung volume) decreases. A fall in the inspiratory reserve volume below a certain value, typically about 0. Chapter in Perspective: Focus on Homeostasis the respiratory system contributes to homeostasis by obtaining oxygen from, and eliminating carbon dioxide to , the external environment. Adult brain cells that depend on a continual supply of oxygen die if deprived of oxygen for more than four minutes. Even cells that can resort to anaerobic metabolism for energy production, such as strenuously exercising muscles, can do so only transiently by incurring an oxygen deficit that ultimately must be made up during the period of excess post-exercise oxygen consumption. As a result of these energy-yielding metabolic reactions, the body produces large quantities of carbon dioxide that must be eliminated. Breathing is accomplished by alternate contraction and relaxation of muscles within the lung tissue. Alveolar ventilation does not always increase when pulmonary ventilation increases. Rhythmicity of breathing is brought about by pacemaker activity displayed by the respiratory muscles. The expiratory neurons send impulses to the motor neurons controlling the expiratory muscles during normal quiet breathing. The three forces that tend to keep the alveoli open are, and. Indicate the oxygen and carbon dioxide partial pressure relationships important in gas exchange by circling > (greater than), < (less than), or = (equal to) as appropriate in each of the following statements: a. Using the answer code on the right, indicate which chemoreceptors are being described: 1. Define the following: hypoxic hypoxia, anemic hypoxia, circulatory hypoxia, histotoxic hypoxia, hypercapnia, hypocapnia, hyperventilation, hypoventilation, hyperpnea, apnea, and dyspnea. Describe the functional significance of the discharge properties of motor neurons of inspiratory pump muscles and of the skeletal muscles of the upper airway. Why does a forced maximal expiration take much longer than a forced maximal inspiration Why does airway resistance become an important determinant of flow in chronic obstructive pulmonary disease How does haemoglobin promote the net transfer of oxygen from the alveoli to the blood John is training for a marathon tomorrow and just ate a meal of pasta (assume this is pure carbohydrate, which. Why is it important that airplane interiors are maintained at, for example, a pressure equivalent to that at an altitude of 2100 m when the plane is at an altitude of 12 000 m Explain the physiological value of using oxygen masks when the pressure in the airplane interior cannot be maintained. Would hypercapnia accompany the hypoxia produced in each of the following situations Describe how his respiratory muscle activity and intra-alveolar pressure changes compare with normal to accomplish a normal tidal volume. More specifically, the supraspinal centres believed to be involved in central command are the primary motor cortex, posterior hypothalamus, subthalamus, and mesencephalon. However, it is generally agreed that these two systems respond as one (the cardiorespiratory system), and are under a common control-central command. The Central and Peripheral Neural Elements During exercise, there is a major neural outflow to the skeletal, cardiovascular, and respiratory systems. As the muscle contractions associated with exercise begin, descending signals from supraspinal centres-in conjunction with afferent neural feedback from activated proprioceptors. Thus, the initial increase in heart rate occurs via the withdrawal of parasympathetic (vagal) tone. The activation (irradiation) hypothesis of central command is considered a feedforward mechanism, which accelerates the cardiorespiratory variables until they are turned off or modified by feedback. This verifies that exercise has begun and skeletal muscle metabolism has increased, which changes the chemical milieu of muscle, interstitial fluid, and blood. The central (throughout the medulla) and peripheral (aortic and carotid bodies) chemoreceptors, along with metaboreceptors in skeletal muscles respond to changes in the levels of carbon dioxide and H1, by-products of metabolism, thereby providing feedback for modulation of central command; this feedback is likely one of the most important modifiers of cardiorespiratory function. The central chemoreceptors Evidence of Cardiorespiratory Integration during Exercise There is a parallel between an increase in ventilation and cardiac output and an increase in oxygen uptake, which suggests a common control system and integrated response. The integrators consist of motor and sensory areas of the brain, with the brain stem linking respiratory and cardiovascular controls. Neural outflow from the motor cortex initiates exercise as well as changes in respiration and cardiovascular function; this ties together all three systems: skeletal muscle, respiratory, and cardiovascular. Central command is demonstrated by the fact that the ventilatory and cardiac responses are similar over a range of submaximal exercise intensities. Both ventilation and cardiac output increase abruptly at the onset of exercise, followed by a more gradual increase over the next one to three minutes, until a steady state is achieved between minutes three and five. How quickly a steady state is attained in a healthy individual depends on exercise intensity, movement economy, and fitness level. Economy is considered the amount of energy required to exercise at a specific intensity. If one person uses less energy while exercising at the same intensity as another person, the person using less energy is said to be more economical (their economy is greater), or efficient. At the termination of exercise, ventilation and cardiac output both rapidly decline in the first minute, followed by a more gradual decline over the next several minutes. The cardiorespiratory adjustments to exercise result from the integration of both neural and humoural factors. Signals from the brain to the active muscles-the motor outflow called central command-pass through the reticular activating system of the medulla, and this command signal from the motor cortex activates (irradiates) the respiratory and cardiovascular control centres in the medulla. During submaximal steady-state exercise, respiration is adjusted in parallel to skeletal muscle production of carbon dioxide. In contrast, this is not the case during transitions from rest to exercise and during intense (but still submaximal) exercise when the peripheral chemoreceptors and muscle metaboreceptors are stimulated. The peripheral chemoreceptors provide feedback to the respiratory centres to increase respiration. Activated central and peripheral chemoreceptors and/or muscle metaboreceptors communicate with central command to modulate respiration. In essence, chemoreceptors provide feedback for control of breathing (rate and depth of respiration) by central command, as they detect mismatches between respiration and muscular work. Recall that baroreceptors are mechanoreceptors that sense change in pressure within the walls of the vessels. Likewise, the cardiopulmonary baroreflex senses changes in blood volume and filling pressure in the pulmonary arteries and veins. Note that systolic and arterial pulse pressures increase during rhythmical dynamic exercise. This is accomplished by resetting the baroreflex to regulate arterial blood pressure at a higher operating point (new set point) during exercise. The resetting of the arterial baroreflex is believed to be accomplished by central command. It can be difficult to definitively distinguish between the central and peripheral neural contributions to the cardiorespiratory response to exercise. However, many animal and human studies of these responses to exercise support the theory of central command initiating and coordinating the cardiorespiratory responses to exercise, with the peripheral elements providing information to central command to modify its control. Peripheral Humoural Elements the humoural factors that influence skeletal muscle blood flow, cardiac output, and ventilation are metabolic vasodilators. The focus here will be on the metabolic vasodilators that influence muscle metaboreceptors. Accordingly, the focus of this section is on the response of skeletal muscle metaboreceptors to chemical factors other than carbon dioxide. For example, if oxygen delivery to the quadriceps muscle is inadequate at the onset of exercise, the concentrations of chemical factors. Nevertheless, increased vasodilation associated with stimulation of metaboreceptors is specific to the site of working muscle, making this response local. The end result ensures sufficient delivery of oxygen to active muscle tissue, while reducing delivery of oxygen to tissues not essential to exercise. The preceding discussion supports the concept that at the onset of exercise, neural outflow from central command increases, and this activates the cardiorespiratory system. Feedback from central and peripheral chemoreceptors, metaboreceptors, baroreceptors, and peripheral proprioceptors help match cardiorespiratory output and muscle perfusion with exercise intensity and the mass of muscle activated. Although the problem is respiratory-cessation of breathing for a few seconds or up to a minute as many as 500 times a night- the pathological consequences are primarily cardiovascular. Contributing factors include age and obesity, mostly because the extra weight, especially in the neck, predisposes the upper airway to collapse. This effect is exacerbated when the neural drive to the muscles in the upper airway that dilate it during inspiration decreases during sleep. As a result, the increased inspiratory efforts can suck the upper airway closed, an effect exacerbated by surface tension between the opposing walls of the collapsed airway. This is especially stressful to the heart because even at rest it consumes approximately 70 percent of the oxygen delivered to it. Therefore, unless blood flow to the heart increases, there is a risk that it will receive insufficient oxygen during the apnea to meet its increased metabolic demands-caused by the surge in sympathetic activity-and the increase in afterload-caused by peripheral vasoconstriction. Moreover, if the asphyxia is severe enough, cardiac arrhythmias can occur, possibly leading to a heart attack. Over time, the acute and repeated episodes of increases in sympathetic activity become permanent (chronic), persisting even when the individuals are awake. The resulting effects on the heart and vasculature (the walls of the blood vessels become stiffer) include hypertension, a major factor causing heart attacks and strokes. The pressure splints the airway open, preventing the recurrent episodes of airway collapse and, therefore, apnea. Unfortunately, many people have difficulty tolerating the device, and so adherence can be low. The Urinary System Body systems maintain homeostasis Homeostasis the urinary system contributes to homeostasis by helping regulate the volume, electrolyte composition, and pH of the internal environment and by eliminating metabolic waste products. Cell survival also depends on continual removal of toxic metabolic wastes that cells produce as they perform life-sustaining chemical reactions. The kidneys play a major role in maintaining homeostasis by regulating the concentration of many plasma constituents, especially electrolytes and water, and by eliminating all metabolic wastes (except carbon dioxide, which is removed by the lungs). As plasma repeatedly filters through the kidneys, they retain constituents of value for the body and eliminate undesirable or excess materials in the urine. Also crucial is their ability to help regulate pH by controlling elimination of acid and base in the urine. While mowing his lawn on yet another hot and sunny Saturday, he suddenly had an intense pain in his lower left side that radiated around his lower back. Upon arriving at the emergency room, the intensity of the pain was making him feel nauseous, and he was having problems sitting comfortably. His urine sample had a pinkish hue to it and upon analysis was found to have an osmolality of 1200 mOsm/l and contained small calcium oxalate crystals. He was given medication to help with the pain and instructed to drink lots of water to help pass the stones. This is achieved both independently and through coordination with other organs; especially those of the endocrine system (Chapter 6). The kidneys maintain water and electrolyte balance within a narrow range, despite a wide range of intake and losses of electrolytes through other avenues. In a deficit state, the kidneys cannot make up for a depleted constituent, but it can limit further urinary loss of the constituent and conserve it until the person can take in more. Consequently, the kidneys can compensate more efficiently for excesses than for deficits. The kidneys cannot completely halt the loss of a valuable substance in the urine, even though the substance may be in short supply. Because H 2 O eliminated in the urine is derived from the blood plasma, a person stranded without H 2 O eventually urinates to death. Additionally, the kidneys are the main route for eliminating potentially toxic metabolic wastes and foreign compounds from the body. These wastes cannot be eliminated as solids, and thus must be excreted in solution as waste-filled urine. They are located in the posterior aspect of the abdomen on each side of the spine and surrounded by two layers of fat. An adult kidney is about 10 cm long, 5 cm wide, 3 cm thick, and weighs about 150 g.

Less lipid-soluble molecules require the assistance of a membrane protein or vesicle to cross the membrane thyroid hormone androgen receptor order flomax paypal. Channel proteins form water-filled channels that link the intracellular and extracellular compartments prostate 180 at walgreens cheap 0.4mg flomax overnight delivery. Gated channels regulate movement of substances through them by opening and closing prostate cancer 75 year old cheap flomax 0.4 mg with visa. Gated channels may be regulated by ligands prostate foundation buy flomax 0.2mg on line, by the electrical state of the cell prostate cancer 85 years old discount 0.2mg flomax with amex, or by physical changes such as pressure prostate cancer nomograms buy cheap flomax 0.2 mg on line. Carrier proteins never form a continuous connection between the intracellular and extracellular fluid. Active transport moves molecules against their concentration gradient and requires an outside source of energy. Most secondary active transport systems are driven by the sodium concentration gradient. All carrier-mediated transport demonstrates specificity, competition, and saturation. Specificity refers to the ability of a transporter to move only one molecule or a group of closely related molecules. Saturation occurs when a group of membrane transporters are working at their maximum rate. Diffusion is the passive movement of molecules down a chemical (concentration) gradient from an area of higher concentration to an area of lower concentration. Large macromolecules and particles are brought into cells by phagocytosis or endocytosis. When vesicles that come into the cytoplasm by endocytosis are returned to the cell membrane, the process is called membrane recycling. In receptor-mediated endocytosis, ligands bind to membrane receptors that concentrate in coated pits or caveolae. In exocytosis, the vesicle membrane fuses with the cell membrane before releasing its contents into the extracellular space. Transporting epithelia have different membrane proteins on their apical and basolateral surfaces. Molecules cross epithelia by moving between the cells by the paracellular route or through the cells by the transcellular route. Larger molecules cross epithelia by transcytosis, which includes vesicular transport. The electrical gradient between the extracellular fluid and the intracellular fluid is known as the resting membrane potential difference. The movement of an ion across the cell membrane is influenced by the electrochemical gradient for that ion. The membrane potential that exactly opposes the concentration gradient of an ion is known as the equilibrium potential (Eion). The equilibrium potential for any ion can be calculated using the Nernst equation. In most living cells, K+ is the primary ion that determines the resting membrane potential. Changes in membrane permeability to ions such as K+, Na+, Ca2+, or Cl- alter membrane potential and create electrical signals. Although the total body is electrically neutral, diffusion and active transport of ions across the cell membrane create an electrical gradient, with the inside of cells negative relative to the extracellular fluid. The use of electrical signals to initiate a cellular response is a universal property of living cells. Pancreatic beta cells release insulin in response to a change in membrane potential. Which of the following processes are examples of active transport, and which are examples of passive transport Simple diffusion, phagocytosis, facilitated diffusion, exocytosis, osmosis, endocytosis. If the molecules are moved in the same direction, the transporters are called carriers; if the molecules are transported in opposite directions, the transporters are called carriers. A transport protein that moves only one substrate is called a(n) carrier. A molecule that moves freely between the intracellular and extracellular compartments is said to be a(n) solute. Rank the following individuals in order of how much body water they contain as a percentage of their body weight, from highest to lowest: (a) a 25-year-old, 74-kg male; (b) a 25-year-old, 50-kg female; (c) a 65-year-old, 50-kg female; and (d) a 1-year-old, 11-kg male toddler. In your own words, state the four principles of electricity important in physiology. Two compartments are separated by a membrane that is permeable to glucose but not water. A 2 M NaCl solution is placed in compartment A and a 2 M glucose solution is placed in compartment B. The compartments are separated by a membrane that is permeable to water but not to NaCl or glucose. If water moves, it will move from compartment to compartment. Explain the differences between a chemical gradient, an electrical gradient, and an electrochemical gradient. A material that allows free movement of electrical charges is called a(n), whereas one that prevents this movement is called a(n). Sweat glands secrete into their lumen a fluid that is identical to interstitial fluid. Insulin is a hormone that promotes the movement of glucose into many types of cells, thereby lowering blood glucose concentration. Propose a mechanism that explains how this occurs, using your knowledge of cell membrane transport. The following terms have been applied to membrane carriers: specificity, competition, saturation. Integral membrane glycoproteins have sugars added as the proteins pass through the lumen of the endoplasmic reticulum and Golgi complex (p. Based on this information, where would you predict finding the sugar "tails" of the proteins: on the cytoplasmic side of the membrane, the extracellular side, or both Label the solutions with all the terms that apply: hypertonic, isotonic, hypotonic, hyperosmotic, hyposmotic, isosmotic. Use large letters for solutes with higher concentrations and small letters for solutes with low concentrations. Define the following terms and explain how they differ from one another: specificity, competition, saturation. The cells have an osmolarity of 300 mOsM, and the solution has an osmolarity of 250 mOsM. If you give 1 L of half-normal saline (see question 32) to the patient in question 31, what happens to each of the following at equilibrium The following graph shows the results of an experiment in which a cell was placed in a solution of glucose. The cell had no glucose in it at the beginning, and its membrane can transport glucose. In this article, we examine the basic patterns of cell-to-cell communication and see how the coordination of function resides in chemical and electrical signals. To maintain homeostasis, the body uses a combination of diffusion across small distances; widespread distribution of molecules through the circulatory system; and rapid, specific delivery of messages by the nervous system. Signal pathways that once seemed fairly simple and direct are now known to be incredibly complex networks and webs of information transfer, such as the network shown on the opening page of this chapter. In the sections that follow, we distill what is known about cell-to-cell communication into some basic patterns that you can recognize when you encounter them again in your study of physiology. As with many rapidly changing fields, these patterns reflect our current understanding and are subject to modification as scientists learn more about the incredibly complex network of chemical signals that control life processes. Those cells face a daunting task-to communicate with one another in a manner that is rapid and yet conveys a tremendous amount of information. Surprisingly, there are only two basic types of physiological signals: electrical and chemical. The cells that respond to electrical or chemical signals are called target cells, or targets for short. Protein binding of chemical signals obeys the general rules for protein interactions, including specificity, affinity, competition, and saturation [p. Long-distance communication (4) uses a combination of chemical and electrical signals carried by nerve cells and chemical signals transported in the blood. For example, a molecule can act close to the cell that released it (local communication) as well as in distant parts of the body (long-distance communication). A gap junction forms from the union of membranespanning proteins, called connexins, on two adjacent cells [p. When the channel is open, the connected cells function like a single cell that contains multiple nuclei (a syncytium). In addition, gap junctions are the only means by which electrical signals can pass directly from cell to cell. Movement of molecules and electrical signals through gap junctions can be modulated or shut off completely. Scientists have discovered more than 20 different isoforms of connexins that may mix or match to form gap junctions. The variety of connexin isoforms allows gap junction selectivity to vary from tissue to tissue. In mammals, gap junctions are found in almost every cell type, including heart muscle, some types of smooth muscle, lung, liver, and neurons of the brain. In this test, blood is drawn after an overnight fast, and the glucose concentration in the blood is measured. Endocrine System (d) Hormones are secreted by endocrine glands or cells into the blood. Blood Nervous System (e) Neurotransmitters are chemicals secreted by neurons that diffuse across a small gap to the target cell. Electrical signal Neuron Target cell Response Endocrine cell Cell without receptor Cell with receptor Target cell (f) Neurohormones are chemicals released by neurons into the blood for action at distant targets. No response Response Blood Neuron Cell without receptor Cell with receptor No response Response 166 6. The similarities between neurohormones and classic hormones secreted by the endocrine system bridge the gap between the nervous and endocrine systems, making them a functional continuum rather than two distinct systems. A paracrine signal para-, beside + krinen, to secrete is a chemical that acts on cells in the immediate vicinity of the cell that secreted the signal. A chemical signal that acts on the cell that secreted it is called an autocrine signal auto-, self. In some cases, a molecule may act as both an autocrine signal and a paracrine signal. Because distance is a limiting factor for diffusion, the effective range of paracrine signals is restricted to adjacent cells. A good example of a paracrine molecule is histamine, a chemical released from damaged cells. When you scratch yourself with a pin, the red, raised wheal that results is due in part to the local release of histamine from the injured tissue. The histamine acts as a paracrine signal, diffusing to capillaries in the immediate area of the injury and making them more permeable to white blood cells and antibodies in the plasma. Fluid also leaves the blood vessels and collects in the interstitial space, causing swelling around the area of injury. Cytokines May Act as Both Local and Long-Distance Signals Cytokines are among the most recently identified communication molecules. Initially the term cytokine referred only to peptides that modulate immune responses, but in recent years the definition has been broadened to include a variety of regulatory peptides. Most of these peptides share a similar structure of four or more -helix bundles [p. Families of cytokines include interferons, interleukins, colony-stimulating factors, growth factors, tumor necrosis factors, and chemokines. In development and differentiation, cytokines usually function as autocrine or paracrine signals. In stress and inflammation, some cytokines may act on relatively distant targets and may be transported through the circulation just as hormones are. Many research laboratories are interested in cytokines because of their importance in disease processes. First, cytokines are not produced by specialized epithelial cells the way hormones are. Second, cytokines are made on demand, in contrast to protein or peptide hormones that are made in advance and stored in the endocrine cell until needed. And finally, the intracellular signal pathways for cytokines are usually different from those for hormones. For example, erythropoietin, the molecule that controls synthesis of red blood cells, is by tradition considered a hormone but functionally fits the definition of a cytokine. Long-Distance Communication May Be Electrical or Chemical All cells in the body can release paracrine signals, but most longdistance communication between cells takes place through the nervous and endocrine systems. The endocrine system communicates by using hormones hormone, to excite, chemical signals that are secreted into the blood and distributed all over the body by the circulation. The nervous system uses a combination of chemical signals and electrical signals to communicate over long distances. An electrical signal travels along a nerve cell (neuron) until it reaches the very end of the cell, where it is translated into a chemical signal secreted by the neuron.

If the slow waves reach threshold at the peaks of depolarization prostate q complex flomax 0.2mg discount, a volley of action potentials is triggered at each peak prostate cancer vs breast cancer order flomax 0.2mg on-line, resulting in rhythmic cycles of contraction in the sheet of smooth muscle cells driven by the pacemaker man health report garcinia test generic 0.4 mg flomax fast delivery. Contractions within the tract occur at the rate of slow-wave frequency within a given section of tract prostate cancer age safe 0.2 mg flomax. The parasympathetic nervous system increases smooth muscle motility and secretion of digestive enzymes man health 99 generic 0.4 mg flomax. Parasympathetic stimulation induces production of a large volume of watery saliva that is rich in enzymes prostate cancer mayo clinic purchase on line flomax. Sympathetic stimulation produces a small volume of thick saliva that is rich in mucus. During swallowing, respiration is temporarily inhibited while the entrance to the trachea is closed off to prevent food from entering the respiratory airways. Contraction of the laryngeal muscles tightly aligns the vocal folds across the glottis. The pharyngoesophageal sphincter at the upper end of the esophagus separates the pharynx from the esophagus and prevents air from entering the esophagus and stomach during breathing. This sphincter remains tonically contracted except during a swallow, when it opens. The gastroesophageal sphincter at the lower end of the esophagus separates the esophagus from the stomach and prevents reflux of gastric contents into the esophagus. A strong peristaltic contraction in the stomach antrum pushes the luminal contents downward toward a slightly open pyloric sphincter. A small portion of the chyme is forced through the sphincter before the peristaltic wave reaches the sphincter and closes it tightly. When food that is moving forward hits the closed sphincter, it is tossed backward, only to be propelled forward again by the next peristaltic wave. This retropulsion (cycles of food being moved forward and backward in the antrum) shears and grinds the food into smaller pieces and thoroughly mixes it with gastric secretions, converting it into chime before emptying. The cephalic phase of gastric secretion occurs in response to foodrelated stimuli such as thinking about, tasting, smelling, seeing, chewing, and swallowing food. Finally, the entire stomach lining undergoes significant turnover, with the cells being replaced every three days. The pancreas produces proteolytic enzymes for digesting food proteins, but these enzymes could also digest the proteins of the cells that produce them if they were not stored in inactive form until they are secreted. These inactive enzymes are stored in secretory zymogen granules in the pancreatic acinar cells until appropriate stimuli induce their secretion. They are activated to protein-digesting enzymes (trypsin, chymotrypsin, and carboxypeptidase) only when they reach the duodenal lumen, where they act on food, not the pancreatic cells. Bile salts contribute to dietary fat digestion by their detergent action (fat emulsification) in the small intestine. The droplets do not recoalesce into the large globule because bile salts adsorb on the surface of the smaller droplets, creating a lipid emulsion consisting of many small fat droplets suspended in the watery chyme. This action increases the surface area of fat available for attack by pancreatic lipase. Three major structural features increase the surface area of the small intestine available for absorption: (1) Large circular folds on the small-intestine luminal surface increase the surface area threefold; (2) microscopic finger-like projections, known as villi, that sit atop the circular folds increase the surface area another tenfold; and (3) a multitude of hairlike protrusions, referred to as microvilli or the brush border on each villus surface, collectively increase the surface area another twentyfold. Together these three adaptations increase the absorptive surface area of the small intestine approximately 600 times greater than compared to a completely smooth tube of the same length and diameter. Absorption of most dietary carbohydrate and protein is accomplished by secondary active transport that involves the cotransport of N1 and the nutrient molecule into the absorptive cell. The nutrient molecules move out of the cell into the blood across the basal membrane by facilitated diffusion, completing the absorptive process. Large-intestine haustral contractions are very similar to small intestine segmentation contractions in that both are rhythmic, autonomous, oscillating ringlike contractions initiated by pacemaker cells, but haustral contractions occur much less frequently (once every 30 minutes) than segmentation contractions (9 to 12 per minute). Haustral contractions, which are nonpropulsive, shuffle the colonic content back and forth, facilitating salt and water absorption, which compact the content and form a firm fecal mass. In contrast, segmentation in the small intestine is both a slowly propulsive and mixing movement. For example, gastrin is trophic to the gastric mucosa, in addition to stimulating secretion by the parietal and chief cells. The significance is that the hormone helps maintain the gastric mucosa, despite harsh conditions, thereby maintaining the secretory capabilities of this stomach lining. The area of a sphere with that radius is terial enzymes, is responsible for the brown colour of feces, which are greyish 4 p(0. Therefore, the total volume did not change as a result of emulsification, as would be expected, because the total volume of the Check Your Understanding lipid is conserved during emulsification. Internal work constitutes all biological energy expenditure that Points to Ponder does not accomplish mechanical work outside the body; it includes (Questions on pp. Patients who have had their stomachs removed must eat small quantities of shivering and all energy-expending activities essential for sustaining food frequently instead of consuming the typical three meals a day because life, such as pumping blood or breathing. Metabolic rate is the rate at they have lost the ability to store food in the stomach and meter it into which energy is expended during both external and internal work: the small intestine at an optimal rate. If a person without a stomach conmetabolic rate 5 energy expenditure/unit of time. Appetite signals give sumed a large meal that entered the small intestine all at once, the luminal the sensation of hunger, driving us to eat. Satiety signals give the sensacontents would quickly become too hypertonic as digestion of the large tion of being full, suppressing the desire to eat. Adiposity signals are nutrient molecules into a multitude of small, osmotically active, absorbable indicative of the size of fat stores in adipose tissue and are important units outpaced the more slowly acting process of absorption of these units. Adipokines refer to hormones As a consequence of this increased luminal osmolarity, water would enter secreted by adipocytes. Visceral fat is the deep, "bad" fat that surrounds the small intestine lumen from the plasma by osmosis, resulting in circulathe abdominal organs and is likely to be chronically inflamed and tory disturbances as well as intestinal distension. Subcutaneous fat is deposited under the syndrome" from occurring, the patient must "feed" the small intestine only skin (it is the fat you can pinch) and is less harmful than visceral fat. The basal metabolic rate can be determined indirectly by measuring a products can keep pace with their rate of production. Neuropeptide Y, from the arcuate nucleus of hypothalamus, increases pathogens into the body proper. Defecation would be accomplished entirely by the defecation reflex Orexins, from the lateral hypothalamus, increase appetite. Voluntary control of the external anal sphincter would be hypothalamus, decreases appetite. Removal of the stomach leads to pernicious anaemia because of the resulof heat between objects of differing temperatures that are in direct contant lack of intrinsic factor, which is necessary for absorption of vitamin B12. Clinical Consideration Chapter 16 Energy Balance and Temperature Regulation (Question on p. As the gallbladder contracts and bile is squeezed into the blocked bile duct, the duct becomes distended prior to the blockage. The thermoconforming fish would not run a fever when it has a systemic infection because it has no mechanisms for regulating internal heat production or for controlling heat exchange with its environment. The body temperature of fish varies capriciously with the external environment, no matter whether the fish has a systemic infection or not. It is not able to maintain body temperature either at some normal set point or at an elevated set point. Clinical Consideration At the higher metabolic rate during exercise, t 5 (638C)[1. The lower O2 need of cooled tissues accounts for the occasional survival of drowning victims who have been submerged in icy water considerably longer than one could normally survive without O2. Spread out consumption of the food throughout the day instead of just eating several large meals. Engaging in heavy exercise on a hot day is dangerous because of problems arising from trying to eliminate the extra heat generated by the exercising muscles. First, there are conflicting demands for distribution of the cardiac output-temperature-regulating mechanisms trigger skin vasodilation to promote heat loss from the skin surface, whereas metabolic changes within the exercising muscles induce local vasodilation in the muscles to match the increased metabolic needs with increased blood flow. Further exacerbating the problem of conflicting demands for blood flow is the loss of effective circulating plasma volume resulting from the loss of a large volume of fluid through another important cooling mechanism: sweating. Therefore, it is difficult to maintain an effective plasma volume and blood pressure and simultaneously keep the body from overheating when engaging in heavy exercise in the heat, so heat exhaustion is likely to ensue. When a person is soaking in a hot bath, loss of heat by radiation, conduction, convection, and evaporation is limited to the small surface area of the body exposed to the cooler air. Heat is being gained by conduction at the larger skin surface area exposed to the hotter water. In both sexes, these primary reproductive organs perform the dual function of producing gametes (sperm in males and ova in females) and secreting sex hormones (testosterone in males and estrogen and progesterone in females). The seminiferous tubules are the highly coiled tubular component of the testes where spermatogenesis takes place. The Leydig cells, which lie in the interstitial spaces between the seminiferous tubules, secrete testosterone. Sertoli cells are epithelial cells that lie in close association with and protect, nurse, and enhance the developing sperm cells throughout their development in the seminiferous tubules. Spermatogenesis is the sequence of steps by which relatively undifferentiated primordial germ cells proliferate and are converted into extremely specialized, motile spermatozoa. Spermiogenesis refers to the remodelling or packaging of the haploid spermatids into spermatozoa. Spermiation is the final release of a mature spermatozoon from the Sertoli cell to which it has been attached throughout development. Spermatogonia are the undifferentiated primordial germ cells that have a diploid number of singlestrand chromosomes. Two mitotic divisions of a spermatogonium yield four primary spermatocytes, each with a diploid number of double-strand chromosomes. The second meiotic division of these secondary spermatocytes yields 16 spermatids, each with a haploid number of single-strand chromosomes. The spermatids are packaged into highly specialized spermatozoa, each with a haploid number of single-strand chromosomes. Erection, hardening of the normally flaccid penis to permit its entry into the vagina, is accomplished by engorgement of the penis erectile tissue with blood as a result of marked parasympathetically induced vasodilation of the penile arterioles and mechanical compression of the veins. Ejaculation occurs in two phases: the emission phase, emptying of sperm and accessory sex gland secretions (semen) into the urethra, is accomplished by sympathetically induced contraction of smooth muscle in the walls of the reproductive ducts and accessory sex glands. The ovarian follicle secretes estrogen; the corpus luteum secretes progesterone (most) and estrogen. Estrogen stimulates growth of the endometrium and myometrium and induces synthesis of progesterone receptors in the endometrium. Progesterone acts on the estrogen-primed endometrium to convert it into a hospitable and nutritious lining (with loose, edematous connective tissue, glycogen stores, and increased blood supply) suitable for implantation. The menstrual phase of the uterine cycle takes place during the first half of the ovarian follicular phase, after the end of the last cycle when the estrogen and progesterone supply was withdrawn upon degeneration of the corpus luteum. The proliferative phase of the uterine cycle occurs during the last half of the ovarian follicular phase, as the endometrium stops sloughing and starts to repair itself and proliferate under the influence of rising levels of estrogen from the newly recruited, rapidly growing antral follicles. The secretory, or progestational, phase of the uterine cycle begins concurrent with the ovarian luteal phase, as progesterone from the corpus luteum converts the thickened, estrogenprimed endometrium to a lush environment capable of supporting an early embryo, should the released egg be fertilized and implant. The zygote is the fertilized ovum, following union of the male and female chromosomes. The blastocyst is a single-layer hollow ball of about 50 cells resulting from mitotic cell divisions of the zygote; the blastocyst is the developmental stage that implants in the endometrium. The inner cell mass is a dense mass on one side of the blastocyst that is destined to become the fetus. The trophoblast is the thin outermost layer of the blastocyst that accomplishes implantation, after which it develops into the fetal part of the placenta. The placenta is the specialized organ of exchange between the maternal and fetal blood that is derived from both trophoblastic embryonic tissue and decidual maternal tissue and that secretes peptide and steroid hormones essential for maintaining pregnancy. The embryo is the product of fertilization during the first two months of intrauterine development, when tissue differentiation is taking place. The fetus is the product of fertilization during the last seven months of gestation, after differentiation is complete and tremendous tissue growth and maturation occur. During parturition, a positive-feedback loop involving oxytocin, a powerful uterine muscle stimulant, is responsible for the progression of labour. Oxytocin promotes uterine muscle contractions that force the fetus against the cervix, dilating it and triggering a neuroendocrine reflex that results in secretion of even more oxytocin, which stimulates even stronger contractions, and so on as labour progresses until the cervix is dilated sufficiently for the baby to be pushed out. During breastfeeding, oxytocin causes milk ejection (milk letdown) by stimulating contraction of the myoepithelial cells surrounding the milk-secreting alveoli. Testosterone hypersecretion in a young boy causes premature closure of the epiphyseal plates so that he stops growing before he reaches his genetic potential for height. The child would also display signs of precocious pseudopuberty, characterized by premature development of secondary sexual characteristics, such as deep voice, beard, enlarged penis, and sex drive. Both divisions of the autonomic nervous system are required for the male sexual activity. Parasympathetic activity is essential for accomplishing erection, and sympathetic activity is important for ejaculation. Posterior pituitary extract contains an abundance of stored oxytocin, which can be administered to induce or facilitate labour by increasing uterine contractility. Exogenous oxytocin is most successful in inducing labour if the woman is near term, presumably because of the increasing concentration of myometrial oxytocin receptors at that time. Thus, treatment with these hormones would not cause estrogen and progesterone secretion. A tubal pregnancy must be surgically terminated because the oviduct cannot expand as the uterus does to accommodate the growing embryo.

What chemical and physical characteristics do hormones prostate cancer stage 4 order flomax 0.4 mg overnight delivery, enzymes prostate cancer jewelry generic flomax 0.4mg with mastercard, transport proteins prostate 101 flomax 0.2 mg overnight delivery, and receptors have in common that makes specificity important Use the pathway to show how suppressing gonadotropins decreases sperm production and testosterone secretion prostate oncology specialists marina del rey discount flomax 0.4mg. Researchers subsequently suggested that a better treatment would be to give men extra testosterone mens health 082012 discount 0.4mg flomax. Draw another copy of the reflex pathway to show how testosterone could suppress sperm production without the side effect of impotence man health lean belly lean belly flomax 0.2mg visa. Based on what you have learned about the pathway for insulin secretion, draw and label a graph showing the effect of plasma glucose concentration on insulin secretion. Jessell, in the preface to their book, Principles of Neural Science, 2000 Neurons (blue) and glial cells (red) 8. As the camera zooms in on one tank, no fish can be seen darting through aquatic plants. The lone occupant of the tank is a gray mass with a convoluted surface like a walnut and a long tail that appears to be edged with beads. Floating off the beads are hundreds of fine fibers, waving softly as the oxygen bubbles weave through them. It is a brain and spinal cord, removed from its original owner and awaiting transplantation into another body. The brain is regarded as the seat of the soul, the mysterious source of those traits that we think of as setting humans apart from other animals. The brain and spinal cord are also integrating centers for homeostasis, movement, and many other body functions. They are the control center of the nervous system, a network of billions of nerve cells linked together in a highly organized manner to form the rapid control system of the body. Nerve cells, or neurons, carry electrical signals rapidly and, in some cases, over long distances. They are uniquely shaped cells, and most have long, thin extensions, or processes, that can extend up to a meter in length. In most pathways, neurons release chemical signals, called neurotransmitters, into the extracellular fluid to communicate with neighboring cells. Using electrical signals to release chemicals from a cell is not unique to neurons. For example, pancreatic beta cells generate an electrical signal to initiate exocytosis of insulin-containing storage vesicles [p. Single-celled protozoa and plants also employ electrical signaling mechanisms, in many cases using the same types of ion channels as vertebrates do. Scientists sequencing ion channel proteins have found that many of these channel proteins have been highly conserved during evolution, indicating their fundamental importance. Although electrical signaling is universal, sophisticated neural networks are unique to animal nervous systems. Reflex pathways in the nervous system do not necessarily follow a straight line from I one neuron to the next. One neuron may influence multiple neurons, or many neurons may affect the function of a single neuron. The intricacy of neural networks and their neuronal components underlies the emergent properties of the nervous system. Emergent properties are complex processes, such as consciousness, intelligence, and emotion that cannot be predicted from what we know about the properties of individual nerve cells and their specific connections. The search to explain emergent properties makes neuroscience one of the most active research areas in physiology today. In many instances, multiple terms describe a single structure or function, which potentially can lead to confusion. Information flow through the nervous system follows the basic pattern of a reflex [p. Sensory receptors throughout the body continuously monitor conditions in the internal and external environments. Dozens of paralyzed children-some attached to respirators to assist their breathing-filled the ward to overflowing. Efferent neurons are subdivided into the somatic motor division, which controls skeletal muscles, and the autonomic division, which controls smooth and cardiac muscles, exocrine glands, some endocrine glands, and some types of adipose tissue. However, clinically, the term motor neuron (or motoneuron) is often used to describe somatic motor neurons that control skeletal muscles. Autonomic neurons are further divided into sympathetic and parasympathetic branches, which can be distinguished by their anatomical organization and by the chemicals they use to communicate with their target cells. Many internal organs receive innervation from both types of autonomic neurons, and it is common for the two divisions to exert antagonistic control over a single target [p. In recent years, a third division of the nervous system has received considerable attention. The enteric nervous system is a network of neurons in the walls of the digestive tract. It is frequently controlled by the autonomic division of the nervous system, but it is also able to function autonomously as its own integrating center. You will learn more about the enteric nervous system when you study the digestive system. There is no cure, but usually the paralysis slowly disappears, and lost sensation slowly returns as the body repairs itself. Organize the following terms describing functional types of neurons into a map or outline: afferent, autonomic, brain, central, efferent, enteric, parasympathetic, peripheral, sensory, somatic motor, spinal, sympathetic. Neurons Carry Electrical Signals the neuron, or nerve cell, is the functional unit of the nervous system. These processes are usually classified as either dendrites, which receive incoming signals, or axons, which carry outgoing information. The shape, number, and length of axons and dendrites vary from one neuron to the next, but these structures are an essential feature that allows neurons to communicate with one another and with other cells. Structurally, neurons are classified by the number of processes that originate from the cell body. In other structural neuron types, the axons and dendrites may be missing or modified. Because physiology is concerned chiefly with function, however, we will classify neurons according to their functions: sensory (afferent) neurons, interneurons, and efferent (somatic motor and autonomic) neurons. In contrast, sensory neurons in the nose and eye are much smaller bipolar neurons. The axons may divide several times into branches called collaterals col-, with + lateral, something on the side. Bipolar (b) Bipolar neurons have two relatively equal fibers extending off the central cell body. Nerves that carry only afferent signals are called sensory nerves, and those that carry only efferent signals are called motor nerves. Many nerves are large enough to be seen with the naked eye and have been given anatomical names. For example, the phrenic nerve runs from the spinal cord to the muscles of the diaphragm. The Cell Body Is the Control Center the cell body (cell soma) of a neuron resembles a typical cell, with a nucleus and all organelles needed to direct cellular activity [p. The position of the cell body varies in different types of neurons, but in most neurons the cell body is small, generally making up one-tenth or less of the total cell volume. Dendrites increase the surface area of a neuron, allowing it to receive communication from multiple other neurons. The primary function of dendrites in the peripheral nervous system is to receive incoming information and transfer it to an integrating region within the neuron. Dendritic spines can function as independent compartments, sending signals back and forth with other neurons in the brain. Dendritic spines can change their size and shape in response to input from neighboring cells. Because of these associations, dendritic spines are a hot topic in neuroscience research. In our model neuron, each collateral ends in a bulbous axon terminal that contains mitochondria and membrane-bound vesicles filled with neurocrine molecules [p. Synaptic vesicle Rough endoplasmic reticulum 1 Golgi apparatus 2 3 5 Soma 6 6 Old membrane components digested in lysosomes 5 Retrograde fast axonal transport 4 Synaptic vesicle recycling 4 3 Vesicle contents are released by exocytosis. At the distal end of the axon, the electrical signal usually causes secretion of a chemical messenger molecule. Establishing Synapses Depends on Chemical Signals the region where an axon terminal meets its target cell is called a synapse syn-, together + hapsis, to join. Although illustrations make the synaptic cleft look like an empty gap, it is filled with extracellular matrix whose fibers hold the presynaptic and postsynaptic cells in position. The vast majority of synapses in the body are chemical synapses, where the presynaptic cell releases a chemical signal that diffuses across the synaptic cleft and binds to a membrane receptor on the postsynaptic cell. Communication at electrical synapses is bidirectional as well as faster than at chemical synapses. During embryonic development, how can billions of neurons in the brain find their correct targets and make synapses How can a somatic motor neuron in the spinal cord find the correct pathway to form a synapse with its target muscle in the big toe The answer lies with chemical signals used by the developing embryo, ranging from factors that control differentiation of stem cells into neurons and glia to those that direct an elongating axon to its target. The axon cytoplasm is filled with many types of fibers and filaments but lacks ribosomes and endoplasmic reticulum. For this reason, proteins destined for the axon or the axon terminal must be synthesized on the rough endoplasmic reticulum in the cell body. The proteins are then moved in vesicles down the axon by a process known as axonal transport. Forward (or anterograde) transport moves vesicles and mitochondria from the cell body to the axon terminal. Backward (or retrograde) transport returns old cellular components from the axon terminal to the cell body for recycling. Nerve growth factors and some viruses also reach the cell body by fast retrograde transport. The current model for axonal transport proposes that the neuron uses stationary microtubules as tracks along which transported vesicles and mitochondria "walk" with the aid of attached footlike motor proteins [p. Even soluble proteins, which were once thought to move by cytoplasmic flow, appear to clump together into complexes that associate with vesicles being transported. The motor proteins kinesin-1 and dynein are the major motor proteins for axonal transport. Axonal Transport Is Classified by the Speed at Which Material Moves Fast axonal transport goes in both directions and can move material at rates of up to 400 mm (about 15. Slow axonal transport moves soluble proteins and cytoskeleton proteins from the cell body to the axon terminal at a rate of 0. Recent research suggests that slow transport may be slow because it is "stop and go," with bursts of movement followed by a pause. As an analogy: fast transport is like driving on an interstate highway while slow transport is similar to driving down a street with many stop lights. Mutations or alterations in proteins associated with axonal transport have been linked to a variety of inherited and acquired disorders. Growth cones depend on many different types of signals to find their way: growth factors, molecules in the extracellular matrix, and membrane proteins on the growth cones and on cells along the path. However, synapse formation must be followed by electrical and chemical activity, or the synapse will disappear. The survival of neuronal pathways depends on neurotrophic factors trophikos, nourishment secreted by neurons and glial cells. There is still much we have to learn about this complicated process, and it is an active area of physiological research. This "use it or lose it" scenario is most dramatically reflected by the fact that the infant brain is only about one-fourth the size of the adult brain. Further brain growth is due not to an increase in the number of cells but to an increase in size and number of axons, dendrites, and synapses. This development depends on electrical signaling between sensory pathways, interneurons, and efferent neurons. Babies who are neglected or deprived of sensory input may experience delayed development ("failure to thrive") because of the lack of nervous system stimulation. On the other hand, there is no evidence that extra stimulation in infancy enhances intellectual development, despite a popular movement to expose babies to art, music, and foreign languages before they can even walk. Variations in electrical activity can cause rearrangement of the synaptic connections, a process that continues throughout life. Maintaining synapses is one reason that older adults are urged to keep Play BioFlix Animation learning new skills @Mastering Anatomy & Physiology and infor mation. Label the presynaptic and postsynaptic ends of each neuron, the cell bodies, dendrites, axons, and axon terminals. For many years, scientists thought that the primary function of glial cells was physical support, and that glial cells had little influence on information processing.

When alveolar pressure decreases below atmospheric pressure as a result of decompression of the pleural space prostate oncology nursing discount flomax 0.2 mg otc, air flows into the lungs mens health xmas gift guide flomax 0.4mg without prescription. When alveolar pressure increases above atmospheric as a result of increased lung recoil at higher lung volumes androgen hormone metabolism buy cheap flomax, and this recoil pressure is not opposed by an equal and opposite pleural pressure (due to contraction of inspiratory muscles) androgen hormone yakiniku order cheap flomax on-line, air flows out of the lungs androgen hormone kinetics order generic flomax. Alveolar ventilation 5 (tidal volume 2 dead space volume) X respiratory dissolved prostate cancer oncology buy generic flomax 0.4mg online, (2) 30 percent is bound to haemoglobin, and (3) 60 perrate ace volume) X respiratory rate. The generated H1 the partial pressure of a gas is that fraction of the total binds to Hb. The outputs are directed to both respiratory pump between the tissues and blood and then between the blood and muscles and those that control the size of the upper airway. An increase in arterial Pco2 is uble in blood, they must be transported primarily by mechanisms the most potent chemical stimulus for increasing ventilation. This filtrate is identical in composition to plasma, except for the plasma proteins, which are held back by the glomerular membrane. They eliminate unwanted plasma constituents in the urine while conserving materials of value to the body. The urine-forming functional unit of the kidneys is the nephron, which is composed of interrelated vascular and tubular components. More than 99 percent of the filtered plasma is returned to the blood through reabsorption. The transport of Na1 out of the cells into the lateral spaces between adjacent cells by this carrier induces the net reabsorption of Na1 from the tubular lumen to the peritubular capillary plasma. The purpose of this relatively high blood flow is to supply the kidney with sufficient plasma for filtration, secretion, and reabsorption. Specific cotransport carriers located at the luminal border of the proximal tubular cell are driven by the Na1 concentration gradient to selectively transport glucose or an amino acid from the luminal fluid into the tubular cell, from which the nutrient eventually enters the plasma. Because these carriers, like the organic-nutrient cotransport carriers, can become saturated, each exhibits a maximal carrierlimited transport capacity (Tm). Once the filtered load of an actively reabsorbed substance exceeds the Tm, reabsorption proceeds at a constant maximal rate, and the additional filtered quantity of the substance is excreted in urine. Sixty-five percent of the filtered H2O is reabsorbed from the proximal tubule in unregulated fashion, driven by active Na1 reabsorption. Reabsorption of H2O increases the concentration of other substances remaining in the tubular fluid, most of which are filtered waste products. The small urea molecules are the only waste products that can passively permeate the tubular membranes. Accordingly, urea is the only waste product partially reabsorbed as a result of being concentrated. Only wastes and excess electrolytes not wanted by the body are left behind, dissolved in a given volume of H2O to be eliminated in the urine. Because the excreted material is removed, or cleared, from the plasma, the term plasma clearance refers to the volume of plasma cleared of a particular substance each minute by renal activity. This variable reabsorption is made possible by a vertical osmotic gradient in the medullary interstitial fluid, which is established by the long loops of Henle of the juxtamedullary nephrons via countercurrent multiplication and preserved by the vasa recta of these nephrons via countercurrent exchange. Vasopressin secretion is inhibited in response to a H2O excess, reducing H2O reabsorption. In this way, adjustments in vasopressin-controlled H2O reabsorption help correct any fluid imbalances. Micturition can transiently be voluntarily prevented until a more opportune time by deliberate tightening of the external sphincter and surrounding pelvic diaphragm. Inputs to the pool occur by way of ingestion or metabolic production of the substance. Outputs from the pool occur by way of excretion or metabolic consumption of the substance. This figure varies among individuals, depending on how much fat (a tissue with a low H2O content) a person has. Varying Na1 filtration and Na1 reabsorption can adjust how much Na1 is excreted in the urine to regulate plasma volume and, consequently, arterial blood pressure in the long term. The amount of vasopressin secreted determines the extent of free H2O reabsorption by distal portions of the nephrons, thereby determining the volume of urinary output. However, because the volume of fluid drunk is often not directly correlated with the intensity of thirst, control of urinary output by vasopressin is the most important regulatory mechanism for maintaining H2O balance. Furthermore, between the time of its generation and its elimination, H1 must be buffered within the body to prevent marked fluctuations in [H1]. Hydrogen ion concentration is often expressed in terms of pH, which is the logarithm of 1/ [H1]. A pH higher than normal (lower [H1] than normal) characterizes a state of alkalosis. Chemical buffer systems, the first line of defence, each consist of a pair of chemicals involved in a reversible reaction, one that can liberate H1 and the other that can bind H1. By acting according to the law of mass action, a buffer pair acts immediately to minimize any changes in pH. Normally, H1 is buffered by the urinary phosphate buffer pair, which is abundant in the tubular fluid because excess dietary phosphate spills into the urine to be excreted from the body. The three classes of energy-rich nutrients are digested into absorbable units as follows: (1) Dietary carbohydrates in the form of the polysaccharides starch and glycogen are digested into their absorbable units of monosaccharides, especially glucose. The digestive system consists of the digestive tract and accessory digestive organs (salivary glands, exocrine pancreas, and biliary system). From innermost outward, they are the mucosa, submucosa, muscularis externa, and serosa. Gastric filling is facilitated by vagally mediated receptive relaxation of the stomach muscles. Gastric storage takes place in the body of the stomach, where peristaltic contractions of the thin muscle walls are too weak to mix the contents. Gastric mixing in the thick-muscled antrum results from vigorous peristaltic contractions. The specific factors in the duodenum that delay gastric emptying are fat, acid, hypertonicity, and distension. Gastric secretion is increased during the cephalic and gastric phases of gastric secretion before and during a meal by mechanisms involving excitatory vagal and intrinsic nerve responses along with the stimulatory actions of gastrin and histamine. Protein digestion is initiated by pepsin in the antrum of the stomach, where vigorous peristaltic contractions mix the food 15. More important than its minor digestive function, saliva is essential for articulating speech and plays an important role in dental health. Salivary secretion is controlled by a salivary centre in the medulla, mediated by autonomic innervation of the salivary glands. The swallowing centre in the medulla coordinates a complex group of activities that result in closure of the respiratory passages and propulsion of food through the pharynx and esophagus into the stomach. This neutralization is important to protect the duodenum from acid injury and to allow pancreatic enzymes, which are inactivated by acid, to perform their important digestive functions. Bile salts aid fat digestion through their detergent action and facilitate fat absorption by forming water-soluble micelles that can carry the water-insoluble products of fat digestion to their absorption site. Its folds bear a rich array of finger-like projections, the villi, which have a multitude of even smaller hairlike protrusions, the microvilli. Together, these surface modifications tremendously increase the area available to house the membrane-bound enzymes and to accomplish both active and passive absorption. Mass movements several times a day, usually after meals, propel the feces long distances. Movement of feces into the rectum triggers the defecation reflex, which the person can voluntarily prevent by contracting the external anal sphincter if the time is inopportune for elimination. Secretion: the alkaline mucous secretion of the large intestine is primarily protective in function. Digestion and absorption: No secretion of digestive enzymes or absorption of nutrients takes place in the colon, as all nutrient digestion and absorption was previously completed in the small intestine. Absorption of some of the remaining salt and water converts the colonic contents into feces. Digestion: the pancreatic enzymes continue carbohydrate and protein digestion in the small-intestine lumen. Gastrin is released primarily in response to the presence of protein products in the stomach, and its effects promote digestion of protein, movement of materials through the digestive tract, and maintenance of the integrity of the stomach and smallintestine mucosa. Cholecystokinin is released primarily in response to the presence of fat products in the duodenum, and its effects optimize conditions for digesting fat and other nutrients and for maintaining the integrity of the exocrine pancreas. This mechanism is primarily important in the long-term matching of energy intake with energy output, thereby maintaining body weight over the long term. Energy output or expenditure includes (1) external work, performed by skeletal muscles to move an external object or move the body through the external environment; and (2) internal work, which consists of all other energy-dependent activities that do not accomplish external work, including active transport, smooth and cardiac muscle contraction, glandular secretion, and protein synthesis. Furthermore, all the energy expended to accomplish internal work is eventually converted into heat, and 75 percent of the energy expended by working skeletal muscles is lost as heat. The metabolic rate, which is energy expenditure per unit of time, is measured in kilocalories of heat produced per hour. For a neutral energy balance, the energy in ingested food must equal energy expended in performing external work and transformed into heat. If more energy is consumed than is expended, the extra energy is stored in the body, primarily as adipose tissue, so body weight increases. By contrast, if more energy is expended than is available in the food, body energy stores are used to support energy expenditure, so body weight decreases. Usually, body weight remains fairly constant over a prolonged period of time (except during growth), because food intake is adjusted to match energy expenditure on a long-term basis. Food intake is controlled primarily by the hypothalamus by means of complex regulatory mechanisms in which hunger and satiety are important components. Feeding or appetite signals give rise to the sensation of hunger and promote eating, whereas satiety signals lead to the sensation of fullness and suppress eating. The skin exchanges heat energy with the external environment, with the direction and amount of heat transfer depending on the environmental temperature and the momentary insulating capacity of the shell. The four physical means by which heat is exchanged between the body and external environment are (1) radiation (net movement of heat energy via electromagnetic waves); (2) conduction (exchange of heat energy by direct contact); (3) convection (transfer of heat energy by means of air currents); and (4) evaporation (extraction of heat energy from the body by the heat-requiring conversion of liquid H2O to H2O vapour). Because heat energy moves from warmer to cooler objects, radiation, conduction, and convection can be channels for either heat loss or heat gain, depending on whether surrounding objects are cooler or warmer, respectively, than the body surface. Normally, they are avenues for heat loss, along with evaporation resulting from sweating. Peripheral thermoreceptors inform the hypothalamus of the skin temperature, and central thermoreceptors-the most important of which are located in the hypothalamus itself- inform the hypothalamus of the core temperature. The primary means of heat gain is heat production by metabolic activity; the biggest contributor is skeletal muscle contraction. Heat loss is adjusted by sweating and by controlling to the greatest extent possible the temperature gradient between the skin and surrounding environment. The layer of cool skin between the core and environment increases the insulating barrier between the warm core and the external air. On exposure to cool surroundings, the core temperature starts to fall as heat loss increases, because of the larger-thannormal skin-to-air temperature gradient. The hypothalamus responds to reduce the heat loss by inducing skin vasoconstriction, while simultaneously increasing heat production through heat-generating shivering. A fever occurs when endogenous pyrogen released from macrophages in response to infection raises the hypothalamic set point. An elevated core temperature develops as the hypothalamus initiates cold-response mechanisms to raise the core temperature to the new set point. Testosterone stimulates the mitotic and meiotic divisions required to transform the undifferentiated diploid germ cells, the spermatogonia, into undifferentiated haploid spermatids. The epididymis and ductus deferens store and concentrate the sperm and increase their motility and fertility prior to ejaculation. Prostaglandins are produced throughout the body, not just in the reproductive tract. These ubiquitous chemical messengers are derived from arachidonic acid, a component of the plasma membrane. By acting as paracrines, specific prostaglandins exert a variety of local effects. Union of a sperm and an ovum at fertilization results in the beginning of a new individual that has 23 complete pairs of chromosomes, half from the father and half from the mother. The externally visible portions of the reproductive system constitute the external genitalia. In the presence of masculinizing factors, a male reproductive system develops; in their absence, a female system develops. The cooler temperature in the scrotum than in the abdominal cavity is essential for spermatogenesis. Testosterone is responsible for maturation and maintenance of the entire male reproductive tract, for development of secondary sexual characteristics, and for stimulating libido. It consists of two stages: (1) emission, the emptying of semen (sperm and accessory sex gland secretions) into the urethra; and (2) expulsion of semen from the penis. The latter is accompanied by a set of characteristic systemic responses and intense pleasure referred to as orgasm. During the female sexual response, the outer portion of the vagina constricts to grip the penis, whereas the inner part expands to create space for sperm deposition. The same steps in chromosome replication and division take place in oogenesis as in spermatogenesis, but the timing and end result are markedly different.

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