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BIOLOGY A GUIDE TO THE NATURAL WORLD FOURTH EDITION DAVID KROGH Transport and Exchange 2: Digestion, Nutrition, and Elimination Copyright © 2009 Pearson Education, Inc., publishing as Pearson Benjamin Cummings. 30.1 The Digestive System Copyright © 2009 Pearson Education, Inc., publishing as Benjamin Cummings. • There are four central functions of the digestive system. Copyright © 2009 Pearson Education, Inc., publishing as Benjamin Cummings. Four Functions of the Digestive System 1. To get the foods the body ingests into a form the body can use. 2. To move food in this form out of the digestive tract and into blood circulation. Copyright © 2009 Pearson Education, Inc., publishing as Benjamin Cummings. Four Functions of the Digestive System 3. To retain the waste that remains after nutrients from the food have been transferred. 4. To eliminate this waste from the body. Copyright © 2009 Pearson Education, Inc., publishing as Benjamin Cummings. The Digestive System • The central structure of the digestive system is a muscular tube called the digestive tract that passes through the body from the mouth to the anus. • It receives input along its length from accessory digestive organs such as the gallbladder, liver, and pancreas. Copyright © 2009 Pearson Education, Inc., publishing as Benjamin Cummings. The Digestive System salivary gland mouth (oral cavity) salivary glands tongue pharynx esophagus liver stomach gallbladder pancreas duodenum jejunum colon small intestine ileum cecum large intestine appendix rectum anus Copyright © 2009 Pearson Education, Inc., publishing as Benjamin Cummings. Figure 30.1 The Digestive System • Digestion is a process of breaking down foods—first into small pieces and then into small molecules. • These small molecules leave the digestive system for transport through the rest of the body. Copyright © 2009 Pearson Education, Inc., publishing as Benjamin Cummings. 30.2 Structure of the Digestive System Copyright © 2009 Pearson Education, Inc., publishing as Benjamin Cummings. Structure of the Digestive System • The digestive tract begins with the mouth (or oral cavity). • It continues through the pharynx, esophagus, stomach, small intestine, and large intestine. • It ends at the rectum and anus. Copyright © 2009 Pearson Education, Inc., publishing as Benjamin Cummings. Structure of the Digestive System • Most of the digestive tract is a tube composed of four layers: – The serosa, or outermost layer. – The muscularis externa, whose two sets of muscles produce the waves of contraction, called peristalsis, that move food along. – The submucosa, which contains blood vessels and nerve tissue that control digestion. – The mucosa, which absorbs digested food. Copyright © 2009 Pearson Education, Inc., publishing as Benjamin Cummings. Structure of the Digestive System absorption of nutrients microvilli epithelium lymphatic vessel villus capillary network small intestine mucosa: highly folded lining of intestine where absorption occurs submucosa: connective tissue containing blood vessels and nerves muscularis externa: circular and longitudinal muscle layers used for peristalsis serosa: connective tissue Copyright © 2009 Pearson Education, Inc., publishing as Benjamin Cummings. Figure 30.2 Structure of the Digestive System • Absorbed food is taken up by adjacent capillaries (in the case of carbohydrates and proteins) or lymphatic vessels (in the case of fats). Copyright © 2009 Pearson Education, Inc., publishing as Benjamin Cummings. 30.3 Steps in Digestion Copyright © 2009 Pearson Education, Inc., publishing as Benjamin Cummings. Steps in Digestion Digestion begins in the mouth through both mechanical means (chewing) and chemical means (enzymes breaking down carbohydrates). Copyright © 2009 Pearson Education, Inc., publishing as Benjamin Cummings. The Pharynx and Esophagus • Food is pushed by the tongue into the pharynx (or throat) and moves by muscle contraction from there to the esophagus. • The esophagus’ own muscle contractions work with gravity to push food into the stomach. Copyright © 2009 Pearson Education, Inc., publishing as Benjamin Cummings. The Stomach • The stomach digests food partly by the mechanical means of churning it and partly by chemical means. • Proteins are a primary target of the stomach’s digestive juices. Copyright © 2009 Pearson Education, Inc., publishing as Benjamin Cummings. The Stomach • The stomach’s contents are very acidic—a quality that is valuable both for breaking food down and for killing microorganisms that come in with the food. • The material that leaves the stomach is a mixture of food and digestive juices called chyme. Copyright © 2009 Pearson Education, Inc., publishing as Benjamin Cummings. The Stomach esophagus muscular wall stomach pyloric sphincter duodenum of small intestine rugae Copyright © 2009 Pearson Education, Inc., publishing as Benjamin Cummings. Figure 30.3 The Small Intestine • Eighty percent of the digestive tract’s absorption of nutrients takes place within the small intestine. • It begins at the stomach and ends at the large intestine. Copyright © 2009 Pearson Education, Inc., publishing as Benjamin Cummings. The Small Intestine small intestine duodenum: receives chyme from stomach and secretions from pancreas and liver jejunum: region of most digestion and nutrient absorption ileum: absorption continues Copyright © 2009 Pearson Education, Inc., publishing as Benjamin Cummings. large intestine Figure 30.4 The Pancreas • The pancreas is an organ that secretes, into the small intestine, three classes of digestive enzymes that help break down fats, proteins, and carbohydrates. • It also secretes “buffers” that raise the pH of the acidic chyme coming from the stomach. Copyright © 2009 Pearson Education, Inc., publishing as Benjamin Cummings. The Liver, Pancreas, and Gallbladder liver right lobe left lobe common hepatic duct hepatic ducts common bile duct pancreas cystic duct pancreatic duct gallbladder duodenum of small intestine Copyright © 2009 Pearson Education, Inc., publishing as Benjamin Cummings. Figure 30.5 The Gallbladder • The gallbladder stores and concentrates bile, a substance produced by the liver that aids in the digestion of fats. Copyright © 2009 Pearson Education, Inc., publishing as Benjamin Cummings. The Liver • The liver is a large organ that plays a central role in digestion. • All blood carrying nutrients from the digestive tract is channeled through the hepatic portal vein to the liver, making the liver a first stop for much digested material. Copyright © 2009 Pearson Education, Inc., publishing as Benjamin Cummings. The Liver • The liver controls which nutrients it will store and which nutrients it will send to the rest of the body. • The liver is also the first stop in the body for ingested toxins such as alcohol, making it vulnerable to damage. Copyright © 2009 Pearson Education, Inc., publishing as Benjamin Cummings. The Large Intestine • The large intestine holds and compacts material left over from digestion, turning it into feces. • It also returns water to general circulation and absorbs vitamins produced by resident bacteria. Copyright © 2009 Pearson Education, Inc., publishing as Benjamin Cummings. The Large Intestine • One of the large intestine’s regions, the rectum, is usually empty except when peristaltic contractions force fecal materials into it. • The resulting stretching triggers the urge to defecate. Copyright © 2009 Pearson Education, Inc., publishing as Benjamin Cummings. 30.4 Human Nutrition Copyright © 2009 Pearson Education, Inc., publishing as Benjamin Cummings. Human Nutrition • Nutrition is the study of the relationship between food and health. Copyright © 2009 Pearson Education, Inc., publishing as Benjamin Cummings. Human Nutrition • A nutrient is a substance, contained in food, that does at least one of three things: – Provides energy. – Provides a structural building block for the body. – Helps regulate a process in the body. Copyright © 2009 Pearson Education, Inc., publishing as Benjamin Cummings. Human Nutrition • There are six classes of nutrients: – – – – – – water minerals vitamins proteins carbohydrates lipids Copyright © 2009 Pearson Education, Inc., publishing as Benjamin Cummings. 30.5 Water, Minerals, and Vitamins Copyright © 2009 Pearson Education, Inc., publishing as Benjamin Cummings. • Water makes up 66 percent of the human body and is vital to health. Copyright © 2009 Pearson Education, Inc., publishing as Benjamin Cummings. Water • People generally do not need to be concerned about whether they are getting too much or too little water in their diet. • Recent research has shown that thirst is the best guide as to whether the body is sufficiently hydrated. Copyright © 2009 Pearson Education, Inc., publishing as Benjamin Cummings. Minerals • Minerals are chemical elements needed by the body either to help form bodily structures or to facilitate chemical reactions. Copyright © 2009 Pearson Education, Inc., publishing as Benjamin Cummings. Minerals • Dietary minerals are divided into two categories: – Major minerals, which are needed in amounts of 100 milligrams or more per day. – Trace minerals, which are needed in amounts of less than 100 milligrams per day. Copyright © 2009 Pearson Education, Inc., publishing as Benjamin Cummings. Minerals Required by the Human Body Copyright © 2009 Pearson Education, Inc., publishing as Benjamin Cummings. Table 30.1 Vitamins • Vitamins differ from minerals in that vitamins are compounds, rather than elements. • In addition, vitamins do not help form bodily structures and thus are needed strictly in small quantities—sometimes milligram quantities per day, but often microgram quantities. Copyright © 2009 Pearson Education, Inc., publishing as Benjamin Cummings. Vitamins • There are 13 vitamins that the body must have to function properly. • Of these, eight are B vitamins that facilitate chemical reactions by acting as coenzymes or parts of coenzymes. Copyright © 2009 Pearson Education, Inc., publishing as Benjamin Cummings. Vitamins as Coenzymes coenzyme substrate enzyme Copyright © 2009 Pearson Education, Inc., publishing as Benjamin Cummings. Figure 30.8 Vitamins in the Human Diet Copyright © 2009 Pearson Education, Inc., publishing as Benjamin Cummings. Table 30.2 Do We Need Supplements? • Most nutritionists have concluded that, while a well-balanced diet can supply all the vitamins and minerals the average American needs, there is little harm to be had, and possibly some gain to be realized, from taking a one-a-day vitaminand-mineral supplement. Copyright © 2009 Pearson Education, Inc., publishing as Benjamin Cummings. Do We Need Supplements? • Nutritionists do not, however, recommend taking large “megadoses” of any vitamin. • The average American diet may sometimes be deficient in vitamins A and C and often seems to be deficient in the minerals calcium and iron. Copyright © 2009 Pearson Education, Inc., publishing as Benjamin Cummings. 30.6 Calories and the Energy-Yielding Nutrients Copyright © 2009 Pearson Education, Inc., publishing as Benjamin Cummings. Calories and Energy-Yielding Nutrients • All the calories in the human diet are provided by three classes of nutrients: proteins, carbohydrates, and lipids. • Calories are units of energy, not units of weight. Copyright © 2009 Pearson Education, Inc., publishing as Benjamin Cummings. Calories and Energy-Yielding Nutrients • In nutrition, a calorie (sometimes written as Calorie) is the amount of energy it takes to raise the temperature of 1,000 grams of water by 1 degree Celsius. • Proteins, carbohydrates, and lipids sometimes are referred to as the energy-yielding nutrients in recognition of the fact that they alone provide the body with energy. Copyright © 2009 Pearson Education, Inc., publishing as Benjamin Cummings. Calories and Energy-Yielding Nutrients • Quantities of energy-yielding nutrients that we consume but do not burn up end up being stored within us, mostly as fat tissue. • Foods are caloric in accordance with how much energy they contain per unit of weight. Copyright © 2009 Pearson Education, Inc., publishing as Benjamin Cummings. Calories and Energy-Yielding Nutrients • Lipids are much more caloric than either proteins or carbohydrates. • Each gram of lipids yields 9 calories of energy. • Each gram of proteins or carbohydrates yields 4 calories of energy. Copyright © 2009 Pearson Education, Inc., publishing as Benjamin Cummings. Daily Calorie Requirements Copyright © 2009 Pearson Education, Inc., publishing as Benjamin Cummings. Table 30.3 Calories in Portions of Food Copyright © 2009 Pearson Education, Inc., publishing as Benjamin Cummings. Table 30.4 30.7 Proteins Copyright © 2009 Pearson Education, Inc., publishing as Benjamin Cummings. Proteins • Proteins are composed of chemical building blocks called amino acids. • Tens of thousands of human proteins are put together from a starting set of only 20 amino acids. Copyright © 2009 Pearson Education, Inc., publishing as Benjamin Cummings. Proteins • Proteins have many structural and regulatory functions in the body, but they provide very little energy compared to carbohydrates or lipids. • This is so in part because the body has no stored reservoir of proteins. Copyright © 2009 Pearson Education, Inc., publishing as Benjamin Cummings. Proteins • As such, the body can only derive energy from proteins by breaking apart proteins already in use. • This is an inefficient process that the body turns to only when its supplies of carbohydrates and lipids are running low. Copyright © 2009 Pearson Education, Inc., publishing as Benjamin Cummings. Proteins • Proteins consumed in food are broken down in the digestive tract into their amino acid building blocks. • It is these amino acids, rather than fully formed proteins, that move into circulation. Copyright © 2009 Pearson Education, Inc., publishing as Benjamin Cummings. Proteins • Eleven of the 20 amino acids needed for protein synthesis can be produced by the body and are called nonessential amino acids. • There are, however, nine amino acids that cannot be produced by the body and that must be obtained from food: the essential amino acids. Copyright © 2009 Pearson Education, Inc., publishing as Benjamin Cummings. Proteins • Nearly every source of animal protein supplies a complete, balanced set of essential amino acids. • Among plant proteins, however, only soy protein contains all nine essential amino acids in their proper proportions. Copyright © 2009 Pearson Education, Inc., publishing as Benjamin Cummings. Protein Complementation methionine histidine Rice contains only 8 of the 9 essential amino acids isoleucine leucine phenylalanine threonine tryptophan valine lysine Beans contain only 8 of the 9 essential amino acids Copyright © 2009 Pearson Education, Inc., publishing as Benjamin Cummings. Figure 30.10 30.8 Carbohydrates Copyright © 2009 Pearson Education, Inc., publishing as Benjamin Cummings. Carbohydrates • The primary function of dietary carbohydrates is to provide energy. • The building blocks of carbohydrates are molecules called monosaccharides. Copyright © 2009 Pearson Education, Inc., publishing as Benjamin Cummings. Carbohydrates • A single monosaccharide can be linked to a second monosaccharide to form a molecule called a disaccharide. • Monosaccharides and disaccharides form one of the three principal classes of dietary carbohydrates, the simple sugars. Copyright © 2009 Pearson Education, Inc., publishing as Benjamin Cummings. Carbohydrates • The other two classes of dietary carbohydrates, collectively known as complex carbohydrates, are: – Starches, defined as complex carbohydrates that are digestible. – Fibers, defined as complex carbohydrates that are indigestible. • Both are composed of thousands of linked units of the monosaccharide glucose. Copyright © 2009 Pearson Education, Inc., publishing as Benjamin Cummings. Carbohydrates and Nutrition • Nutritionists recommend a diet that is: – Low in simple sugars that have been added to processed foods. – High in complex carbohydrates that come from fresh fruits, vegetables, and whole-grained cereals. Copyright © 2009 Pearson Education, Inc., publishing as Benjamin Cummings. Processed and Unprocessed Copyright © 2009 Pearson Education, Inc., publishing as Benjamin Cummings. Figure 30.11 Carbohydrates and Nutrition • One of the reasons for this advice is that foods high in added simple sugars often are sources of “empty” calories, meaning calories that are not accompanied by vitamins or minerals. Copyright © 2009 Pearson Education, Inc., publishing as Benjamin Cummings. Carbohydrates and Nutrition • The consumption of large quantities of simple sugars can result in a surge of glucose into the bloodstream. • The consumption of fresh and whole-grained complex carbohydrates results in a slow, steady entry of glucose into the bloodstream. Copyright © 2009 Pearson Education, Inc., publishing as Benjamin Cummings. Carbohydrates and Nutrition • Glucose surges result in surges in blood levels of insulin, a phenomenon that has been linked to: – increased hunger – higher levels of lipids in the bloodstream – a lowering of levels of “good” cholesterol, among other effects Copyright © 2009 Pearson Education, Inc., publishing as Benjamin Cummings. Glycemic Load • One means of measuring the effect of different carbohydrates on blood levels of glucose is the glycemic load: a measure of how blood glucose levels are affected by defined portions of given carbohydrates. • Carbohydrates with low glycemic loads are in general more healthy than carbohydrates with high glycemic loads. Copyright © 2009 Pearson Education, Inc., publishing as Benjamin Cummings. Fiber • The indigestible complex carbohydrates known as fibers have several good health effects, among them a lowering of blood levels of cholesterol and a reduction in glycemic load. Copyright © 2009 Pearson Education, Inc., publishing as Benjamin Cummings. Fiber • Whole, unrefined grains tend to be high in fiber, while processed, “white” grains have less fiber, and added simple sugars have no fiber at all. Copyright © 2009 Pearson Education, Inc., publishing as Benjamin Cummings. Phytochemicals • Most carbohydrates come from plant sources. • Plants in general—and fresh fruits and vegetables in particular—are rich sources of phytochemicals: nonnutritive substances found in plants that promote health. Copyright © 2009 Pearson Education, Inc., publishing as Benjamin Cummings. Phytochemicals • Phytochemicals seem to protect against heart disease, cancer, and age-related macular degeneration. • Nutritionists recommend choosing from among a “rainbow” spectrum of brightly colored fruits and vegetables to ensure an adequate intake of phytochemicals. Copyright © 2009 Pearson Education, Inc., publishing as Benjamin Cummings. Phytochemicals Copyright © 2009 Pearson Education, Inc., publishing as Benjamin Cummings. Figure 30.13 30.9 Lipids Copyright © 2009 Pearson Education, Inc., publishing as Benjamin Cummings. Lipids • Lipids are a great source of energy, but they also have significant structural functions in the body as they are major components of all cell membranes. • In addition, lipids are used to make hormones, and they provide insulation and serve a shockabsorbing function. Copyright © 2009 Pearson Education, Inc., publishing as Benjamin Cummings. Lipids • The building blocks of most dietary lipids are molecules called triglycerides that are composed of a “head” composed of glycerol and three attached fatty acid chains. Copyright © 2009 Pearson Education, Inc., publishing as Benjamin Cummings. Triglycerides glycerol fatty acids Copyright © 2009 Pearson Education, Inc., publishing as Benjamin Cummings. Figure 30.14 Triglycerides • Triglycerides can be broken down into these component parts to yield energy or can be stored in fat cells. • When the body has sufficient energy, carbohydrates and proteins are transformed into triglycerides that are then stored away. Copyright © 2009 Pearson Education, Inc., publishing as Benjamin Cummings. Oils and Fats • In everyday speech, all lipids are referred to as fats. • Nutritionists, however, make a distinction between: – Oils, which are dietary lipids that are liquid at room temperature. – Fats, which are dietary lipids that are solid at room temperature. Copyright © 2009 Pearson Education, Inc., publishing as Benjamin Cummings. Saturated and Unsaturated Fatty Acids • Fats are saturated or unsaturated in accordance with their fatty acid makeup. • A “fat”—meaning a fat or oil—that is saturated is predominantly composed of saturated fatty acids. • A fat that is unsaturated is predominantly composed of unsaturated fatty acids. Copyright © 2009 Pearson Education, Inc., publishing as Benjamin Cummings. Saturated and Unsaturated Fatty Acids • Fatty acids are defined in a saturated-tounsaturated spectrum in accordance with the number of carbon double bonds they have in their hydrocarbon chains. • A saturated fatty acid is a fatty acid with no double bonds between the carbon atoms of its hydrocarbon chain. Copyright © 2009 Pearson Education, Inc., publishing as Benjamin Cummings. Saturated and Unsaturated Fatty Acids • A monounsaturated fatty acid is a fatty acid with one double bond between carbon atoms. • A polyunsaturated fatty acid is a fatty acid with two or more double bonds between carbon atoms. Copyright © 2009 Pearson Education, Inc., publishing as Benjamin Cummings. Saturated and Unsaturated Fatty Acids Palmitic acid saturated (no double bonds) Oleic acid monounsaturated (one double bond) Copyright © 2009 Pearson Education, Inc., publishing as Benjamin Cummings. Figure 30.15 Saturated and Unsaturated Fatty Acids • Lipids that contain a high proportion of unsaturated fatty acids tend to be oils, while those that contain a high proportion of saturated fatty acids tend to be fats. • In general, lipids are healthy to the extent that they are unsaturated rather than saturated. Copyright © 2009 Pearson Education, Inc., publishing as Benjamin Cummings. Saturated and Unsaturated Fatty Acids • Trans fats and saturated fats increase the longterm risk of heart disease. • Polyunsaturated and monounsaturated fats are at least neutral with respect to it. • Polyunsaturated fats containing omega-3 fatty acids seem to protect against it. Copyright © 2009 Pearson Education, Inc., publishing as Benjamin Cummings. Saturated and Unsaturated Fatty Acids • Omega-3 fats are found in greatest abundance in certain kinds of fatty fish. • The usual source for other polyunsaturated and monounsaturated fats is plant-based oils. • Saturated fats normally are found in animalbased fats. • Trans fats are found in a number of packaged and fast foods. Copyright © 2009 Pearson Education, Inc., publishing as Benjamin Cummings. Saturated and Unsaturated Fatty Acids • Trans fats are the product of an industrial process in which naturally occurring oils are turned into fats by partially hydrogenating them—by infusing them with hydrogen atoms. Copyright © 2009 Pearson Education, Inc., publishing as Benjamin Cummings. 30.10 Elements of a Healthy Diet Copyright © 2009 Pearson Education, Inc., publishing as Benjamin Cummings. Elements of a Healthy Diet • Diets can be judged as healthy to the extent that they contain a high proportion of healthy foods. • Food pyramids are one means of getting a sense of proportionality in food intake. Copyright © 2009 Pearson Education, Inc., publishing as Benjamin Cummings. Foods in the Proper Proportion White rice, white bread, white pasta, potatoes, soda, and sweets Red meat, butter Use sparingly Dairy or calcium supplement, 1–2 times/day Multiple vitamins for most Fish, poultry, eggs, 0–2 times/day Nuts, legumes, 1–3 times/day Alcohol in moderation (if appropriate) Vegetables (in abundance) Whole grain foods (at most meals) Fruits, 2–3 times/day Plant oils (olive, canola, soy, corn, sunflower, peanut, and other vegetable oils) Daily exercise and weight control Copyright © 2009 Pearson Education, Inc., publishing as Benjamin Cummings. Figure 30.16 30.11 The Urinary System in Overview Copyright © 2009 Pearson Education, Inc., publishing as Benjamin Cummings. The Urinary System • The urinary system: – Filters waste materials from the blood. – Regulates blood volume. – Conserves useful materials, such as water, nutrients, and ions. Copyright © 2009 Pearson Education, Inc., publishing as Benjamin Cummings. 30.12 Structure of the Urinary System Copyright © 2009 Pearson Education, Inc., publishing as Benjamin Cummings. Structure of the Urinary System • It consists of: – Two kidneys that produce urine. – The left and right ureters that the urine travels through on leaving the kidneys. – The muscular urinary bladder, which receives the urine from the ureters and temporarily stores it. – The tube called the urethra through which urine passes from the bladder out of the body. Copyright © 2009 Pearson Education, Inc., publishing as Benjamin Cummings. Structure of the Urinary System vena cava aorta renal artery renal vein kidney: produces urine ureter: transports urine toward urinary bladder urinary bladder: temporarily stores urine urethra: transports urine out of body Copyright © 2009 Pearson Education, Inc., publishing as Benjamin Cummings. Figure 30.17 Structure of the Urinary System • The working unit of the kidneys is the nephron, composed of a nephron tubule, its associated blood vessels, and the interstitial fluid in which both are immersed. Copyright © 2009 Pearson Education, Inc., publishing as Benjamin Cummings. Structure of the Urinary System • Several nephron tubules empty into a common collecting duct, which will merge with other such ducts to feed into the renal pelvis, which feeds into the ureter. Copyright © 2009 Pearson Education, Inc., publishing as Benjamin Cummings. Structure of the Urinary System Proximal tubule: Water and nutrients move back into capillaries. The Nephron Glomerulus: Blood flows into glomerulus under pressure, driving some blood components out into the Bowman’s capsule. nephron tubule H2O nutrients Bowman’s capsule renal arteriole Distal tubule: Water continues to move back into capillaries; toxins ions, and acids move from capillaries into tubule. blood from renal artery blood to renal vein Kidney Collecting duct: As water continues to move back into blood circulation, waste concentrates, becoming urine that drains to the renal pelvis. renal pelvis ureter urine drained to renal pelvis Copyright © 2009 Pearson Education, Inc., publishing as Benjamin Cummings. Figure 30.18 30.13 How the Kidneys Function Copyright © 2009 Pearson Education, Inc., publishing as Benjamin Cummings. How the Kidneys Function • A knotted network of capillaries within a nephron, the glomerulus, receives arterial blood and is porous enough to allow much of the fluid portion of the blood to flow out of it along with smaller molecules such as vitamins, nutrients, and waste products. Copyright © 2009 Pearson Education, Inc., publishing as Benjamin Cummings. How the Kidneys Function • These materials enter the surrounding Bowman’s capsule, thus moving into the nephron’s tubule as a fluid called filtrate. • At the nephron’s next structure, called the proximal tubule, much of the original water and almost all the original nutrients are moved back into blood circulation. • Waste products remain in the nephron tubule, however, because of their chemical composition. Copyright © 2009 Pearson Education, Inc., publishing as Benjamin Cummings. How the Kidneys Function • This general process continues over the length of the nephron tubule: water and nutrients move back into circulation, while waste products become ever more concentrated within the tubule. Copyright © 2009 Pearson Education, Inc., publishing as Benjamin Cummings. How the Kidneys Function • By the time the filtrate has reached the collecting duct, it has become urine. • The body is able to control how much water the kidneys send to the bladder (in urine) or retain in circulation. Copyright © 2009 Pearson Education, Inc., publishing as Benjamin Cummings. How the Kidneys Function urine from kidneys ureter ligament bladder ureteral openings internal sphincter prostate gland urethra external sphincter Copyright © 2009 Pearson Education, Inc., publishing as Benjamin Cummings. Figure 30.19 How the Kidneys Function • Retention of water by the kidneys is regulated by a part of the brain, called the hypothalamus, that controls the secretion of antidiuretic hormone (ADH). • An increased secretion of ADH means that more water will move out of the kidney’s tubules and collecting ducts and back into circulation. Copyright © 2009 Pearson Education, Inc., publishing as Benjamin Cummings. 30.14 Urine Storage and Excretion Copyright © 2009 Pearson Education, Inc., publishing as Benjamin Cummings. Urine Storage and Excretion • Waves of muscle contraction squeeze urine out of the renal pelvis, moving it through the ureters and into temporary storage in the urinary bladder. Copyright © 2009 Pearson Education, Inc., publishing as Benjamin Cummings. Urine Storage and Excretion • The tube called the urethra then carries the urine from the urinary bladder to the exterior of the body. • An internal sphincter muscle provides involuntary control over the discharge of urine. Copyright © 2009 Pearson Education, Inc., publishing as Benjamin Cummings. Urine Storage and Excretion • The urethra also contains an external sphincter that is under voluntary control. • We become aware of the need to urinate when the bladder is stretched beyond a certain threshold. • We then relax the voluntary, external sphincter, which relaxes the involuntary internal sphincter, and the urine moves out of the bladder and the body. Copyright © 2009 Pearson Education, Inc., publishing as Benjamin Cummings. The Urinary System PLAY Animation 30.1: The Urinary System Copyright © 2009 Pearson Education, Inc., publishing as Benjamin Cummings.