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EXCRETION Topic 12 All living organisms need to be able to excrete their waste products, because they would otherwise build up and become toxic. Plants excrete oxygen, animals excrete carbon dioxide and nitrogenous waste that come from proteins. The way an animal disposes of its nitrogenous waste depends on its evolution and habitat. Types of nitrogenous waste: Ammonia: (NH3) very toxic, must be disposed of quickly. Very soluble in water and diffuses across the cell membrane of animals that live in the water. Some can excrete through their bodies, others, like fish, excrete across their gills. Uric Acid: non-toxic, relatively insoluble, can be precipitated out into a solid, and stored for later excretion. It takes more energy to produce this molecule, but the animal does not loose water in disposing of it. Birds, reptiles, insects and some amphibians that live in the desert excrete uric acid Urea: non-toxic, soluble. Produced in the liver and transported via the blood to the kidneys for excretion. Mammals, sharks, bony fish EXCRETION IN HUMANS THE KIDNEY THE NEPHRON Efferent arteriole- Afferent going out arteriole going into glomerulus vasa recta There are four main processes in excretion: Ultrafiltration Reabsorption Secretion Excretion ULTRAFILTRATION Takes place between the glomerlus and the Bowman’s capsule 1. Blood enters the glomerular capillaries under high pressure. This pressure results from the fact that the efferent vessel (exiting the renal artery) is narrower than the afferent vessel (entering the renal artery). 2. The high hydrostatic pressure causes fluid and small molecules to be pushed out of the capillaries into the capsule. 3. Specialized structures: fenestrated capillaries (tiny holes) and podocytes in the basement membrane of the capsule facilitate this movement. ULTRAFILTRATION 4. Large molecules like proteins and cells can’t move out. 5. The concentration of salts, amino acids, glucose, vitamins and urea in the filtrate are equal to those in the blood. 6. Filtration is passive and non-selective Specialized cells of the capsule called podocytes wrap around the capillaries like fingers so that the entire surface is covered The filtrate moves from the capsule to the Proximal Tubule REABSORPTION 1. Water and solutes from the filtrate need to get reabsorbed. The volume and concentration of the filtrate will be altered in the proximal tubule 2. Some substances move back into the capillary by diffusion: HCO3, water, K+, Cl- 3. Some are actively transported: Na ( this sets up concentration gradient so water can move), glucose, amino acids SECRETION 1. Some substances are secreted into the filtrate from the capillaries 2. Ammonia, toxins, H+ ions and drugs are actively transported. 3. Urea moves by diffusion The filtrate now moves down the descending limb of the Loop of Henle 1. The epithelium here is permeable to water but not to solutes. 2. The concentration of water in the interstitial fluid is lower than the filtrate (hyperosmotic) so water continues to move out of the descending loop, making the filtrate more concentrated 3. The filtrate is at its greatest concentration (1200mosm/L) at the bottom of the Loop. The filtrate now moves up into the ascending limb. 1. The epithelium in the ascending limb is permeable to solutes but not to water. 2. The filtrate is very concentrated as it enters the ascending limb and so Na+ and CL- move out down their concentration gradients but water can’t move in, due to the selective permeability of the cells. 3. About 1/2 way up the ascending limb salts are actively transported out 4. The filtrate becomes less concentrated as it moves up, until it is even less concentrated than it was when it entered the descending limb The passive and active movement of solutes ensures that the interstitial fluid will have a high osmolarity. The filtrate now moves into the distal tubule The distal tubule’s function is to regulate the concentration of K+, Na+ and Cl-. It also regulates pH by controlling secretion and reabsorption of H+ and HCO3 - ions The filtrate now enters the collecting duct 1. Filtrate goes from the medulla to the renal pelvis then to the ureter. 2. The cells here are permeable to water but not salts, so water moves out due to high osmolarity in the interstitial fluid. 3. Na+ and Cl- can be actively reabsorbed if needed. This is dependent on hormonal control. 4. Some urea diffuses out as the filtrate becomes more concentrated. This helps to keep the interstitial fluid very concentrated. The steep concentration gradient that is maintained in the interstitial fluid around the nephron means that mammals can excrete concentrated urine and therefore conserve water. This is achieved by the : Countercurrent multiplier system A countercurrent exchange system is one in which the opposite flow of adjacent fluids maximizes transfer rates. The system is also a multiplier system because the nephron achieves a steeper concentration gradient of solute concentration in the the medulla than would be possible by just the counter current mechanism. There is also a countercurrent system in the vasa recta, the vessels that parallel the structures in the nephron. To review: •As urine passes up the ascending loop solutes are actively transported out- this increases the concentration in the interstitial fluid up to 1200mosm) and makes the urine dilute (100 mosm.) •This makes a steep gradient across the kidney: the cortex is very isotonic (300mosm) and the medulla is very hypertonic (1200msom) •The reason such steep differences can be achieved is due to the countercurrent multiplier system. The active transport of Na can only achieve a gradient of 200mosm, if you start with 300mosm, then it is only possible to get 500mosm in the interstitial fluid. How does the concentration get to 1200?? A. Loop is initially filled with filtrate that is isotonic to the interstitial fluid B. Na is pumped out of the ascending loop- this raises the concentration outside and lowers it inside to 200 mosms C.Water moves out of the descending limb by osmosis. This raises the concentration inside the tubule to 400mosm D. New filtrate enters from the glomerlulus- this pushes the concentrated fluid (400) into the ascending limb E. Now there is a second round of pumping. The Na pump produces another 200mosm gradient across the membrane. But it is starting from a more concentrated solution, so the concentration rises to 500mosm now. F. In the third round of Na pumping, the interstitial concentration rises to 700 mosm. This process continues. Notice that the concentration is higher in the medulla compared to the cortex. So, the high osmotic gradient is achieved because: 1. Na is pumped out of the ascending loop by active transport 2. Water leaves the descending loop by osmosis 3. Glomerular filtration, driven by high pressure is constantly pushing new fluid into the loop 4. The vasa recta has its own countercurrent system which prevents it from dissipating the gradient in the interstitial fluid. So the concentration in the blood is high in the medulla and low in the cortex The concentration of urine is regulated by hormones ADH: antidiuretic hormone Other hormones can influence water and salt concentrations as well.
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