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Maintaining Water Balance The Kidneys Homeostasis • Regulation of a stable internal environment no matter where we are or what we do… • We put our bodies through A LOT on a daily basis – Adding food, liquid, chemicals – Changing temperature & activity levels • BUT…. Our bodies roll with it! • How (examples)??? Excretory System • Star QB of maintaining homeostasis in our bodies – Responsible for maintaining the right amount of water and dissolved substances in your body (osmoregulation) • Includes: – – – – Kidneys Bladder Ureter Urethra Osmoregulation • How our bodies get rid of the “stuff” we don’t need such as… – Byproducts of metabolized food • Makes sure we don’t get dehydrated • Your body’s GREATEST balancing act The Excretory System • Not all animals excrete waste in the same way… • Main byproduct of metabolizing food (mainly protein) is ammonia (NH3) TOXIC – Converted into either urea or uric acid depending on the amount of water available to the animal Urea: Compound made from combining ammonia with carbon dioxide (in the liver) Low toxicity: can hang out in circulatory system for a while with no ill effects BUT…. You have to have extra water around to dissolve it and get rid of it (you pee) Uric Acid: Have to be light enough to fly and don’t have a lot of spare water Excreted as paste (not a lot of water is needed) Have you ever seen bird poop? ** The white part of it is actually the uric acid How do humans excrete urea? • Starts with the kidney – Maintains levels of water and dissolved materials – Controls blood pressure • Very Inefficient Process – Filter the water and dissolved material out of blood • 99% is reabsorbed back • 1% is sent on to make urine • Avg day: kidneys filter out 180L of fluid from your blood, BUT only 1.5L gets peed out How do your kidney’s do this? • Use nephrons – Fun Fact: If you were to unravel all of your nephrons, they would stretch ~180 km • Must follow blood flow to understand how they work… – – – – – – – – Heart Kidney (enters via renal arteries) Glomerulus Bowman’s Capsule Proximal Convoluted Tubule Loop of Henle Distal Tubule Collecting Duct Glomerulus & Bowmans Capsule • Start point for single nephron • Pressure is high enough that 20% of the fluid is squeezed out and enters the Bowmans Capsule – Cup like sac (fluid is no longer blood, but filtrate: water, urea, small ions) Promimal Convoluted Tubule • Site of osmoregulation – Reabsorb water and other materials to create whatever balance your body needs – Mainly reabsorbs organic solutes (ie: glucose); recaptures Na, K and water Loop of Henle • Site where most of the reabsorption happens – Extracts most of the water that we need – Pumps salts – Creates a concentration gradient to cause more water to be extracted ***By the time the filtrate gets to the bottom of the loop it is HIGHLY concentrated Distal Tubule • Regulates Na, K, Ca levels – Controlled by pumps and hormones that control the reabsorption process • By the time its done, we have taken everything we want OUT of the filtrate – Filtrate is now mainly excess water, urea and excess waste Collecting Duct reabsorbed H2O excretion urea passed through to bladder Descending limb Ascending limb