45.1 Osmotic Regulation (p. 804) (Fig. 45.1) [transp. 243]
45.2. Nitrogenous Waste Products (p. 806)
A. Eliminating Nitrogenous Wastes
1. Breakdown of nucleic acids and amino acids results in nitrogenous wastes. (Fig. 45.3)
2. Cells use amino acids derived from protein for synthesis of body protein or nitrogen-containing molecules.
3. Unused amino acids are oxidized to generate energy or are converted to fats or carbohydrates for storage.
4. In both cases, amino groups (
NH2) must be removed; animals excrete these nitrogenous wastes as ammonia, urea, or uric acid, depending on the species. (Table 45.1)
5. Removal of amino groups requires a fairly constant amount of energy that differs for each conversion.
B. Excreting Ammonia
1. Amino groups removed from amino acids immediately form ammonia (NH3) by adding a third hydrogen ion (H+).
2. This requires little or no energy. (Fig. 45.3)
3. Ammonia is quite toxic but water soluble; it requires the most water to wash it from the body.
4. Therefore, aquatic animals (e.g., bony fishes, aquatic invertebrates, and amphibians) excrete ammonia through gills and skin surfaces.
C. Excreting Urea
1. Terrestrial amphibians and mammals usually excrete urea.
2. Urea is much less toxic than ammonia and can be excreted in a moderately concentrated solution, which conserves body water in limited terrestrial environments. (Fig. 45.3)
3. Production of urea requires energy; it is produced in the liver as product of energy-requiring reactions (urea cycle).
D. Excreting Uric Acid
1. Insects, reptiles, birds, and some dogs excrete uric acid as their main nitrogenous waste.
2. Uric acid is not very toxic and is poorly soluble in water; it can be concentrated for water conservation.
3. In reptiles and birds, a dilute solution of uric acid passes from the kidneys to the cloaca, a common reservoir for the products of the digestive, urinary, and reproductive systems.
4. After water is absorbed by the cloaca, the uric acid passes out with the feces.
5. Embryos of reptiles and birds are enclosed in egg shells; production of uric acids provides nontoxic storage.
6. Uric acid is synthesized by enzymatic reactions using even more ATP than urea synthesis. (Fig. 45.3)
7. There is a trade-off between water conservation and energy expenditure.
45.3. Animals Have Organs of Excretion (p. 807)
A. Tubular organs function in Excretion and Osmotic Regulation
B. Planaria Have Flame Cells (Fig. 45.4) [transp. 245]
1. Planaria have two strands of branching excretory tubules that open to the outside through excretory pores.
2. Located along tubules are flame cells containing tufts of cilia that appear to flicker.
3. Cilia beat back and forth, propelling hypotonic fluid through the canals that empty through pores at the body surface.
4. The system functions in water excretion, osmotic regulation, and excretion of wastes.
C. Earthworms Have Nephridia
1. The earthworm's body is divided into segments; nearly every body segment has a pair of nephridia.
2. A nephridium is a coiled tubule with a ciliated opening, nephridiostome, and excretory nephridiopore.
3. Fluid from the body cavity is propelled through the tubule by the cilia.
4. Certain substances are reabsorbed and carried away by the network of capillaries surrounding the tubule.
5. The nephridia form a urine that contains only metabolic wastes, salts, and water.
6. Each day, an earthworm produces urine equal to 60% of its body weight; it can safely excrete ammonia.
D. Insects Have Malpighian Tubules
1. Insects have a unique excretory system consisting of long, thin malpighian tubules attached to the gut.
2. The Malpighian tubules take up metabolic wastes and water from the hemolymph.
3. In the gut, water and other useful substances are reabsorbed.
4. The uric acid eventually passes out of the gut.
5. Malpighian tubules of insects that live in water or that eat large quantities of moist food reabsorb little water.
6. Insects in dry climates reabsorb most water and excrete a dry, semisolid mass of uric acid.
45.4. Humans Have a Urinary System (p. 808)
A. The human urinary system is an organ system with four parts. (Fig. 45.5) [transp. 247]
1. Human kidneys are two bean-shaped, reddish brown organs, each about the size of a fist.
a. Located on sides of vertebral column just below diaphragm, they are partially protected by lower rib cage.
b. Kidneys are the sites of urine formation.
2. Each kidney is connected to a ureter; each conducts urine from a kidney to the urinary bladder.
3. The urinary bladder stores urine from the kidneys until it is voided from the body through the urethra.
4. The single urethra conducts urine from the urinary bladder to the exterior of the body.
5. Male urethra passes through penis, which conducts semen; in females, it opens ventral to vaginal opening.
B. Kidneys Have Three Regions
1. If a kidney is sectioned longitudinally, three major regions can be distinguished.
a. The renal cortex is the thin, outer region of a kidney and appears granular. (Fig. 45.6) [transp. 248]
b. The renal medulla consists of striped, pyramid-shaped regions that lie on the inner side of the cortex.
c. The renal pelvis is the innermost hollow chamber of the kidney.
2. Each human kidney is composed of about one million tiny tubules called nephrons.
3. Some nephrons are located primarily in the cortex but others dip down into the medulla. (Fig. 45.6b)
C. Nephrons Are Numerous
1. Each nephron is comprised of several parts. (Fig. 45.7) [transp. 249]
2. End of a nephron pushes in to form cuplike structure called the glomerular capsule (Bowman's capsule).
a. Its outer layer is composed of simple squamous epithelium.
b. The inner layer is made of specialized cells that allow easy passage of molecules.
3. Nearest the glomerular capsule is the proximal convoluted tubule; it is lined by cells with many mitochondria and an inner brush border (tightly packed microvilli). (Fig. 45.8)
4. Simple squamous epithelium is loop of the nephron (loop of Henle), the middle portion of nephron tubule.
5. Distal convoluted tubule is distal portion nephron tubule; several deliver urine into collecting ducts.
6. The loop of nephron and the collecting duct give the pyramids of the medulla their striped appearance.
7. Each nephron has its own blood supply. (Fig. 45.7) [transp. 249]
a. The renal artery branches into small arteries, which branch into afferent arterioles, one for each nephron.
b. Each afferent arteriole supplies blood to a glomerulus; divides to form a capillary tuft or glomerulus.
c. Glomerular capillaries drain into efferent arteriole, which branches into peritubular capillary network.
d. The peritubular capillaries drain into a venule; venules from many nephrons drain into a small vein; small veins join to form the renal vein, a vessel that delivers blood to the inferior vena cava.
D. How Urine is Made
1. Urine production requires three distinct processes. (Fig. 45.9) [transp. 250]
2. Glomerular filtration occurs at the glomerular capsule.
3. Tubular reabsorption, occurs at the proximal convoluted tubule.
4. Tubular secretion occurs at the distal convoluted tubule.
E. Glomerular Filtration
1. When blood enters glomerulus, blood pressure moves small molecules from glomerulus across inner membrane of glomerular capsule and into lumen of glomerular capsule; this is pressure filtration.
2. The glomerular walls are 100 times more permeable than walls of most capillaries.
3. Molecules that leave blood and enter glomerular capsules are called glomerular filtrate.
4. Plasma proteins and blood cells are too large to be part of the glomerular filtrate.
5. Failure to restore fluids would soon cause death from loss of water, nutrients, and lowered blood pressure.
F. Tubular Reabsorption
1. Tubular reabsorption from nephron to blood occurs primarily through walls of proximal convoluted tubule.
2. Reabsorption recovers much of the contents of the glomerular filtrate.
a. Simple diffusion moves small nonpolar nutrient and waste molecules.
b. Nonfilterable proteins remain in blood and exert an osmotic pressure that moves water back.
c. Sodium ions are actively reabsorbed, pulling along chlorine; these increase osmotic pressure on water.
3. Cells of the proximal convoluted tubule have numerous microvilli increasing surface area for absorption, and numerous mitochondria, which supply energy needed for active transport. (Fig. 45.8)
4. Only molecules by carrier molecules are transported through the tubule into interstitial spaces.
5. Diabetes Mellitus
a. Glucose is usually reabsorbed completely; there are plentiful carriers for the glucose molecules.
b. If all carriers are in use, excess glucose can appear in the urine.
c. In diabetes mellitus, there is a large amount of glucose because the liver fails to store glucose as glycogen.
G. Tubular Secretion
1. Tubular secretion moves substances in blood back into tubular lumen by other than glomerular filtration.
2. Secretion back into the filtrate is primarily associated with the distal convoluted tubule.
3. This helps rid the body of potentially harmful compounds that were not filtered into the glomerular capsule.
4. Urine results as the liquid waste product made by a kidney; it has the composition given in Table 45.2.
H. Product of Concentrates Urine
1. Reptiles and birds rely primarily on the gut to reabsorb water, but mammals rely on the kidneys.
2. The long loop of nephron is comprised of a descending limb, (fluid flows downward) and an ascending limb, (fluid flows upward).
3. This countercurrent flow enables mammals, including humans, to excrete a hypertonic urine.
4. Salt (NaCl) passively diffuses out of the lower portion of the ascending limb, but the upper, thick portion of the limb actively transports salt out in to the tissue of the outer renal medulla. (Fig. 45.10) [transp. 251]
5. Less and less salt is available for transport from tubule as fluid moves up thick portion of the ascending limb.
6. Urea leaks from lower portion of collecting ducts causing concentrations in lower medulla to be the highest.
7. Because of the solute concentration gradient within the renal medulla, water leaves the descending limb of the loop of the nephron along its length.
8. This occurs because decreasing water concentration in descending limb encounters an increasing solute concentration.
9. Fluid received by a collecting duct from distal convoluted tubule is isotonic to cells of cortex; however, as this fluid passes through renal medulla, water diffuses out of the collecting duct into the renal medulla.
10. The urine that is finally delivered to the renal pelvis is usually hypertonic to the blood plasma.
I. Hormones Maintain Water Balance
1. Antidiuretic hormone (ADH) is a hormone released from the posterior lobe of the pituitary.
a. It acts on collecting ducts by increasing their permeability to H2O, thereby increasing H2O retention.
b. If osmotic pressure of blood increases, ADH is released, more water is reabsorbed, and there is less urine.
c. If osmotic pressure of blood decreases, ADH is not released, more water is excreted and more urine forms.
d. Drinking alcohol causes increased urine flow; it inhibits secretion of ADH and increases fluid intake.
e. Diuretics help high blood pressure; they increase urinary excretion and reduce blood volume and pressure.
2. Aldosterone is secreted by the adrenal cortex.
a. It acts on the distal convoluted tubules to increase reabsorption of Na+ and the excretion of K+.
b. Increased Na+ in blood causes water to be reabsorbed leading to increased blood volume and pressure.
c. Aldosterone production is triggered by renin-angiotensin-aldosterone system. (Fig. 45.11) [transp. 252]
d. Blood pressure is constantly monitored within juxtaglomerular apparatus.
e. When blood pressure is insufficient to promote glomerular filtration, afferent arteriole cells secrete renin.
f. Renin catalyzes conversion of angiotensinogen (a protein produced by the liver) into angiotensin I.
g. Later, angiotensin I is converted to angiotensin II in the lungs by angiotensin-converting enzyme.
h. Angiotensin II stimulates cells in the adrenal cortex to produce aldosterone.
i. Angiotensin II increases blood pressure by stimulating secretion of aldosterone; also a vasoconstrictor.
3. Atrial natriuretic factor (ANF) is a peptide hormone produced by the heart.
a. Both ADH and aldosterone increase blood volume and blood pressure.
b. When blood pressure rises, the heart produces ANF to inhibit secretion of renin and release of ADH.
c. Therefore, this hormone serves as a check or balance to the other two.
J. How the pH is Adjusted
1. The whole nephron takes part in maintaining blood pH level.
2. The excretion of H+ ions and NH3, and reabsorption of bicarbonate ions (HCO3-
) is adjusted.
a. If blood is acidic, H+ ions are excreted with ammonia, while Na+ and HCO3- ions are reabsorbed; Na+ ions promote formation of hydroxide ions and bicarbonate takes up H+ ions when carbonic acid is formed.
b. If blood is basic, fewer hydrogen ions are excreted and fewer sodium and bicarbonate ions are reabsorbed.
3. Reabsorption or excretion of ions by kidneys is a homeostatic function; maintains pH of blood and osmolarity.
K. When Kidneys Fail
1. Hemodialysis is a therapy for persons who have suffered renal failure.
a. The blood of a patient is processed by an artificial kidney machine.
b. Blood passes through a semipermeable membranous tube in contact with balanced salt (dialysis) solution.
c. Waste molecules diffuse out of blood while others can be added to blood, such as bicarbonate ions.
d. The system is efficient enough to permit the treatment to be spaced at twice a week.