Homeostasis and Kidney Function

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Learning Objectives

  • Explain principles of homeostasis in terms of feedback loop stages.
  • Describe structure of kidney.
  • Describe and explain fnction of kidney including, ultrafiltration and selective readsorption.
  • Explain loop of Henle and the counter-current multiplier mechanism in teh reabsorption of water.
  • Explain how osmoreceptors monitor the water potential of the blood.
  • Explain role of ADH.
  • Why do fish, birds, insects and mammals have different excretory products?
  • How do desert-living creatures preserve water?
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Homeostasis describes the mechanisms by which a constant internal environment is maintained.

The control of any self-regulating system requires:

  • A detector e.g. This detects any deviation from the set point.
  • A coordinator: communicates with one or more effectors.
  • A effector:carry out corrective procedures.

Negative Feedback:

occurs when the feedback causes the corrective measures to be turned off returning a system to its normal level.

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Functions of the Kidney + urea

Two main functiosn:

  • Removes nitrogenous metabolic waste from the body.
  • Osmoregulation.


Urea is a poisonous chemical made by the liver. When there is excess protein, the amino acids are deanimated, to make ammonia, which quickly converts to urea. Urea is released into the blood and travels to the kidneys.


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Structure of the Kidney

Ureter leads to the pelvis, which is surrounded by medulla. These medullas hold around a million nephrons. The medullas are encapsulated by a cortex, and then the capsule membrane.

Each kidney:

  • Receives blood from the renal artery
  • Blood leaves via renal vein
  • Ureter carries urine from the kidney to the bladder
  • Nephrons/urinferous tubules are in close association with blood vessels.
  • Bowmans capsule recieves blood first from the afferent arteriole serving the glomerulus.
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Ultrafiltration- structure

Filtration: under pressure which separates small soluble molecules from the plasma. The structure of the glomerulus and capsule allows ultrafiltration.

The basement membrane 

  • forms a selective barrier between blood and the nephron
  • acts as a molecular sieve
  • First layer is the capillary wall.
  • Second layer is the basement layer and is the filter.
  • The third layer is the wall of the Bowman's capsule.
  • Epithelium of the Bowmans capsule is made up by podocyte cells-- wrap around increasing surface area.


  • inbetween endothelial cells of capillary
  • in basement membrane
  • Podocyte feet/filtration slits
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Ultrafiltration: HOW???

  • A high filtration pressure is created by the hydrostratic pressure from the renal artery (by the heart pump),
  • but also the sudden change in diameter of the afferent arteriole and the efferent arteriole.
  • Hydrostatic pressure must overcome the high water potential of the renal capsule.

NET EFFECT: Fluid moves out the capillary and into the lumen of the renal capsule.

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  • UREA

SUMGAV?? wat is dis??


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selective readsorption

The proximal (near) convoluted (folded) tubule runs straight down from Bowman's capsule. Here there is selective readsorption of glucose, Na+ (active transport) and osmosis of water back into the blood.

The proximal convoluted tubule cells are adapted to their function of readsorption:

  • Microvilli and basal channels for large surface area
  • Lots of mitochondria for active transport
  • Close to blood cappillaries for short diffusion pathways.

Majority of water is reasborbed in the collecting duct, but some is absorbed (along with salts) in the distal tubule. ALL glucose and some salts are absorbed in the proximal tubule.

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The loop of Henle

The loop of Henle allows

  • counter- current multiplier system to operate.
  • This allows much more water to be readsorbed.
  • This is done by building up a high concentration of salts in the tissue fluid of the medulla of the kidney.
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Loop of Henle structure

  • The descending limb leads on from the renal capsule inside Bowmans capsule.
  • The ascending limb of the loop is impermeable to water.
  • Na+ ions are actively pumped out of ascending limb into tissue fluid, and diffuse into the descending limb.
  • This causes water to leave the permeable descending limb (leaves filtrate osmotically into the capillary) and the descending limb filtrate becomes more concentrated.
  • The ascending limb therefore gets a filtrate rich in Na+.

The maximum concentration occurs at the tip of the loop of Henle, both inside the loop and in the extracellular fluid.

The more water that needs to be reabsorbed, the longer the loop of Henle e.g. desert rats have long loops.

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The collecting duct and Osmoregulation

Osmoregulation- the homeostatic control of body water.

  • Most water is lost through urine. The rest is lost through sweat, moist surfaces and faeces.
  • Urine has a concentration close to that of the bottom of the loop- hypertonic to bodily fluids.

Filtrate reaching the distal convoluted tubule and the collecting duct has a high water potential, as salt ions has been actively pumped out. The collecting duct runs down parallel to the loop of Henle, and so back into the region of low water potential. Water moves out the duct via osmosis.

The distal convoluted tubule and collecting duct have restricted permeability which is controlled by the release of ADH.

Osmoregulatory system:

  • Osmoreceptors in the hypothalamus
  • Coordinators is the posterior lobe of pituitary gland and the release of ADH
  • Effectors-the collecting ducts of the kidney and change in pores.
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Negative feedback of osmoregulation

Osmoreceptors recieve that blood water content is lower/higher than normal, and restores it to normal levels.

If the water potential is low (lots of sweating/urinating) the osmoreceptors send a nervous impulse to the posterior pituitary gland causing it to release ADH

ADH increases permeability iof the collecting duct and distal tubule, by opening aquapores. More water is then readsorbed into the medulla, and then the blood. Less water reaches the bladder.

If the water potential is high, less ADH is released. The permeability of distal convolute tube decreases. Less water is readsorbed, and more reaches the bladder.


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ADAPTIONS to environment

The environment an animal inhabits is a key role in the nitrogenous waste produced and different animals deal with disposal in different ways.

  • FISH: produce AMMONIA, which is highly toxic, but very soluble. It diffuses out the gills and quickly returned to non-toxic levels.
  • BIRDS AND INSECTS: release URIC acid which is almost insoluble and is non-toxic. Large energy cost for its excretion, but little water needed. Allows these animals to live in high shortages of water.
  • MAMMALS: extrete UREA, which requres a lot of energy, but is less toxic than ammonia. Tissues can tolerate it for longer. Via the the ornithine cycle

Desert animals

  • Longer loop of Henle- allows more water readsorption: LEADS TO HIGHLY CONCENTRATED URINE.
  • Metabolic water- produced from oxidation of food reserves.
  • Living underground which is cooler and humid.
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