Excretory System

Excretory System - Excretory systems have several major...

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Unformatted text preview: Excretory systems have several major functions: 1. Removing nitrogenous wastes from protein and nucleic acid metabolism Maintaining water and salt balance ("osmoregulation") Controlling blood levels homeostatically for a variety of metabolites Keeping a stable pH and salinity in the blood and in extracellular fluids 1. 1. 1. 1. Removal of nitrogenous wastes from protein and nucleic acid metabolism Ammonia is a very highly toxic cellular product of nitrogen metabolism. How it happens: When amino acids are used as energy sources, they are first "deaminated". As the NH2 radical is removed, the amino acids are converted to pyruvate or to other intermediates in ither the glycolytic pathway or the Krebs cycle. Ammonia is produced as a byproduct. Ammonia is also produced during nucleic acid metabolism. In some animals, ammonia is first converted to a less toxic nitrogenous waste molecule before being excreted in the urine. Image taken from http://faculty.clintoncc.suny.ed These three kinds of nitrogenous wastes differ from each other in toxicity: Of the three nitrogenous wastes, ammonia is the most toxic and requires the most water for its excretion by the kidney. Uric acid is the least toxic of the three and does not require very much water for excretion. Urea is intermediate in toxicity and requires a moderate amount of water for excretion. The bloodstream carries ammonia (NH ) to the Liver All three nitrogenous wastes )NH urea, and , uric acid( are excreted in urine. C an you name the human excretory organ ?that produces urine Kidney 2. A second important role of the excretory system is osmoregulation. Remember: Cells that are surrounded by a hypertonic environment will dehydrate. Those whose surroundings are hypotonic also cannot survive. Why is this true? Some organs involved in osmoregulation include gills, salt glands, kidneys, and nephridia. A human kidney Above: Salt glands in an albatross, a pelagic bird Nephridia in an earthworm IMPORTANT: Elimination, excretion, and secretion are not synonymous! The term "elimination" refers to indigestible materials that are removed from the body via the digestive system, as feces. "Excretion" is the term used to describe the removal from the body of cellular wastes, primarily in urine and sweat. "Secretion" is sometimes used to describe production and removal from the body of substances such as breast milk and pheromones. "Secretion" is also used to refer to the release of substances into ducts of exocrine glands (or, in the case of hormones, into the bloodstream). Functions of the mammalian kidney: Regulation of blood levels of important ions, especially Na+, K+, Cl, and Ca++ Regulating the volume of water in the blood, which in turn affects blood pressure Keeping the pH in the blood and extracellular fluid stable Returning metabolites such as glucose and amino acids to the bloodstream Secreting hormones such as erythropoietin (important in regulating production of erythrocytes) Removal from the body of nitrogenous products of protein and nucleic acid metabolism Gross and microscopic anatomy of the human urinary system: Filtration in Bowman's Capsule "Filtration" means separating components of a mixture based on their particle size. Blood pressure forces fluid from the blood in the capillaries of the glomerulus into the space within Bowman's capsule. Filtration: Water and small solutes, but not blood cells and not large molecules such as plasma proteins, normally pass through the specialized capillaries of the glomerulus. The filtration of the small solutes is selective ONLY by size. Not only do nitrogenous wastes such as urea get through, but so do important salts, glucose, amino acids, vitamins, and so on. The concentrations of these various solutes in the filtrate mirrors those of the plasma of the blood that enters the kidney. Bowman's capsule of a nephron, surrounding a specialized bed of capillaries known as a "glomerulus" Selective tubular secretion and tubular reabsorption in the nephrons: As the filtrate passes through the nephron and the collecting duct, it is acted on by selective transport epithelia. Part of this processing involves reabsorption of solutes and water. Roughly 1100 to 2000 liters of blood pass through your kidneys every day. That would fill about 50 bathtubs! Of that, about 180 liters ends up in the filtrate and is processed. Most of the sugar, vitamins, and other organic nutrients and about 99% of the water are reabsorbed into the blood, leaving only about 1.5 liters of urine produced per 24hour period. Structure of a nephron: Specialized pumps (red arrows) in the tubule's epithelium return vital components back to the blood1 and concentrate others2 in what will leave the body3 as urine, by active transport. 1 2 3 reabsorption secretion excretion Differential permeability along the length of the tubule allows for the selective return ("reabsorption") of various solutes and of water, to the bloodstream. Countercurrent mechanism of solute concentration in the Loop of Henle Many essential nutrients are actively returned to the blood, in the region of the proximal tubule. As the filtrate travels toward the inner medulla of the kidney, the surrounding environment becomes increasingly salty. This gradient is created by a sodium pump in the ascending limb of the loop and also in the collecting duct. It is further aided by differential permeability to water in the descending but not in the ascending loop and in the collecting duct. The thin segment of the ascending loop is permeable to Na+ and Cl, which diffuse out of the tubule. That saltiness later causes osmosis from the collecting tubule to the surroundings. The collecting tubule is also permeable to urea, some of which leaks back out of the tubule into the surroundings. The end result is a concentrated urine, in the collecting duct, which joins other such ducts and leaves the kidney via the ureter. Osmotic concentration of the surround ing fluid (in milliosmols) What happens to the urine made in the kidney? 1. Urine travels from the ureters to the urinary bladder, where it is stored until urination. From the bladder, urine travels through the urethra. Urine leaves the body through the urethral opening. In human males the urethra is shared with the reproductive system. That is not the case for females. 1. 1. 1. The end! ...
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This note was uploaded on 04/09/2008 for the course BIOLOGY 101 taught by Professor Beitch during the Fall '07 term at Quinnipiac.

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