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Download Lesson 7b - Urine Formation
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Formation of Urine Why do we need to excrete wastes from the kidneys? Although cells of the body obtain energy by converting complex organic compounds into simpler compounds, many of these simpler compounds can by harmful. To maintain life processes, the body must eliminate waste products. The lungs eliminate carbon dioxide. The intestines remove toxic wastes from the digestive system. The liver transforms ingested toxins (i.e., alcohol, medication) into soluble compounds that can be eliminated by the kidneys. The liver also transforms the hazardous products of protein metabolism into metabolites, which are then eliminated by the kidneys. Therefore, the kidneys play a crucial role in removing waste, balancing blood pH, and maintaining water balance. Proteins (unlike carbohydrates) contain nitrogen. These nitrogen molecules (NH2) are characteristic of amino acids and must be discarded by the body. The removal process, called deamination, occurs in the liver. The by-product of deamination is ammonia. Since ammonia is toxic to humans, it must be eliminated from the body. The liver combines 2 molecules of ammonia with 1 molecule of carbon dioxide to form two waste products: Urea – 100,000 times less toxic than ammonia; Uric acid – formed by the breakdown of nucleic acids. Both urea and uric acid are eliminated by the kidneys. Urine formation depends on three functions: Filtration accomplished by the movement of fluids from the blood into the Bowman’s capsule; Reabsorption the transfer of essential solutes and water from the nephron back into the blood; Secretion the movement of materials from the blood back into the nephron. A) Filtration: Each nephron has its own blood supply. Dissolved solutes pass through the walls of the glomerulus (high-pressure filter) into the Bowman’s capsule, along the concentration gradient. Water, sodium chloride, glucose, and hydrogen ions can pass from one structure to the other (because they are small). Plasma proteins, blood cells, and platelets cannot (because they are too large). B) Reabsorption: On average, about 600 mL of fluid flows through the kidneys every minute. About 20% of this fluid (120 mL) is filtered into the nephrons. That filtrate must be reabsorbed; otherwise, humans would pass 120 mL of urine every minute! One would also have to consume 1 L of fluids every 10 minutes to maintain homeostasis. However, only 1 mL of urine is formed for every 120 mL of fluids filtered into the nephron. The remaining 119 mL of fluids and solutes is reabsorbed. Selective reabsorption occurs by both passive and active transport. Mitochondria in the cells that line the nephron provide the energy needed to run active transport. However energy supply is limited. Reabsorption occurs until the threshold level of a substance is reached. The purpose of reabsorption is to return essential solutes and water to blood. Carrier molecules move Na+ ions across the cells that line the nephron. Cl- and HCO3- follow by charge attraction. Excess NaCl remains in the nephron and is excreted with the urine. Highly concentrated solutes in the intercellular space creates an osmotic force, moving water out of the nephron. Other molecules are actively transported from the proximal tubule. Glucose and amino acids attach to special carrier molecules which allow them to be shuttled out of the nephron, into the blood. The amount of solute that can be reabsorbed is limited. As water is reabsorbed, the remaining solutes become more concentrated. Urea and uric acid will also diffuse from the nephron back into the blood. See pg. 350, fig. 1. C) Secretion: The movement of wastes from the blood into the nephron. Nitrogen-containing wastes, K+, drugs, medications can all be secreted. Cells containing mitochondria line the distal tubule. Tubular secretions occur by active transport. However, molecules are transported from blood into nephron. Summary of urine formation: 1. Glomerulus & Bowman’s Capsule: Water and dissolved solutes (Na+, Cl-, glucose, amino acids, vitamins, minerals, urea, uric acid) are filtered from glomerulus into Bowman’s capsule. This is accomplished by fluid pressure in the capillaries. Passive transport. 2. Proximal Tubule: Reabsorption of nutrients (NaCl, water, K+, HCO3-, glucose, vitamins) from filtrate back into blood. Secretion of H+ and ammonia (NH3) back into nephron purpose: to restore pH Passive and active (glucose) transport. 3. Descending Limb of Loop of Henle: Permeable to water. Large amounts of water are reabsorbed into blood. NaCl becomes concentrated in filtrate as descending limb penetrates inner medulla of kidney. Passive transport. 4. Ascending Limb of Loop of Henle: Highly permeable to NaCl. Results in diffusion of salt out of ascending limb…therefore, reabsorption of NaCl. Passive and active transport. 5. Distal Tubule: Regulation of K+, HCO3- , and NaCl concentration in body fluids, by reabsorption of NaCl and water. Secretion of H+ and K+ back into nephron. Active transport. 6. Collecting Duct: Urine is formed (water, NaCl, urea, uric acid, minerals). Several collecting ducts from neighbouring nephrons connect here. Passive transport. Review Questions 1. Describe the main processes that are involved in urine formation 2. Explain why individuals who consume large amounts of sugars might do the following: excrete large amounts of glucose in the urine excrete large amounts of urine 3. Marine fish, such as herring and cod, live in a hypertonic environment. These fish lose water through their gills by osmosis. To replace the water, the fish drink seawater. Explain why these fish must actively transport salt from their bodies.
