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The Kidney

  • Performs two main functions:
    -Removal of nitrogenous metabolic waste from the body.
    -Osmoregulation, the mechanism of balancing levels of water and solutes.
  • Amino acids are deanimated in the liver and excreted as urea by the kidneys.
  • Humans have two kidneys and are the organs that filter waste products from the blood.
  • Each kidney recieves blood from a renal artery and returns blood via the renal vein.
  • A narrow tube called the ureter carries urine to the bladder.
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  • Blood passes through the glomerulus (tight knot of capillaries in the Bowman's capsule) and then through capillaries serving the proximal convoluted tubule and then through capillaries serving the Loop of Henle; known as the vasa recta.
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  • Filtration under pressure that seperates small soluble molecules from the blood plasma; water, glucose, urea and salts are filtered, whereas proteins cannot be filtered.
  • The rate at which fluid passes from the blood in the glomerular capillaries into the Bowman's capsule is called the glomerular filtration rate. This is rapid and is determined by the difference in water potential between both areas.
  • The basement membrane of the capillary forms the selective barrier between the blood and filtrate, and acts as a selective molecular sieve:
    -The first cell layer is the wall of the capillary, which has many gaps.
    -The basement membrane is the middle layer that acts as the sieve.
    -The epethial cells on the next membrane are called podocytes and hold the basement membrane in place.
  • Most pressure comes from the hydrostatic pressure in the glomerulus, the pressure is amplified by the fact that efferent vessel is more narrow than the afferent; the blood plasma also affect the pressure. 
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Selective Reabsorption

  • Where useful molecules are reabsorbed back into the blood, such as glucose.
  • All the glucose and most of the water and salt are reabsorbed.
  • These proximal convoluted tubule cells are adapted by:
    -Microvilli providing a large area for absorption.
    -Numerous mitochondria for the active transport of glucose and salts.
  • Small amounts of water and absorbed by the distal tubule and most of the remaining water is reabsorbed by the collecting duct.
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Loop Of Henle and Collecting Duct

  • The loops of Henle concentrate salts in the tissue of the medulla, the high concentration of salt causes an osmotic flow of water out of the collecting duct; concentrating the urine.
  • At the ascending limb the walls are impermeable to water, the cells in the wall at first diffuse and then actively transport salts out of the tubule and into the tissue fluid; producing low water potential in the surrounding tissue.
  • The walls of the descending limb are permeable to water and also to salt ions, as the fluid passes down, water moves out and the filtrate gets more concentrated as it moves down; evenutally being its most concentrated at the bottom of the loop.
  • The concentrated fluid now flows up the tubule and this forms a counter-current multiplier; where the two loops side by side casuses the maximum concentration to be built up.
  • The fluid flowing up gets less concentrated near the top, as ions flow back out of solution; when it reaches the collecting duct it flows back down and into a region of low water potential; the water flows out of the collecting duct and back into blood.
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Loop Of Henle

(Sort of a rather vague diagram, hope it helps :'))

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  • Water is gained from food and drink, it is lost via:
    -Loss in faeces
    -Keeping exchange surfaces moist
  • Osmoregulation operates via a negative feedback:
    -The permeablity of the collecting duct walls are affected by the hormone ADH.
    -ADH makes the walls more permeable and so water is reabsorbed more.
    -A fall in water potential (may occur by sweating/less water intake) means that the hypathalmus will detect a change and release more ADH into the bloodstream.
    -So more water is reabsorbed into the tissues of the medulla.
    -More water back into the blood and the small amount of urine produced is concentrated. 
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Negative Feedback Loop


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Adaptations To Kidneys

  • Fish and other aquatic animals produce ammonia, which is highly toxic but extremely soluble in water and thus does not harm them once diluted.
  • Birds and insects produce uric acid, which is non-toxic and insoluble, it uses a lot of energy to be produced but very little water.
  • Mammals excrete urea, requires energy but isn't as toxic as ammonia.
  • Loop of Henle is also adapted:
    -Desert animals have long loops of Henle to reabsorb as much water as possible, so the urine contains very little water.
    -Terrestrial animals have medium loops, moderate reaborption.
    -Animals that live in/near water have shorter loops due to no lack of water.
  • Desert animals also live off of metabollic water, where they get water produced from the breakdown of food during respiration; they reduce water loss and energy loss and intake very little water. 
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