The Kidneys & Excretion

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Ultrafiltration (in the Glomerulus)

High Pressure Filtration

  • pressure created by the heart
  • lumen of the efferent arteriole is smaller than that of the afferent artieriole so the presure is amplified

What Passes Through?

  • electrolytes (Na+, K+ etc)
  • Glucose
  • Water
  • Amino Acids
  • Wastes (urea)

Plasma can pass throguh the leaky capillary wall, basement membrane and pdocytes in the Bowman's capsule

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Selective Reabsorption (in proximal convuluted tub

Pinocytic Vesicle - a from of endocytosis where the bulk transport of  liquids from th efiltrate occurs and these then diffuse back into the blood

Mitochondria - realsea ATP througgh aerobic respiration giving energy for active transporrt

Microvilli - increase the surface are for diffusion to occur across

Tight Junction - connects the cells so they are water tight - prevents filtrate leaking back into the blood

Folded Basal Membrans - increase the surface area for diffusion to occur across

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Loop of Henle - Reabsorption of water

...allows the concentration of urine in order to conserve water - The Countercurrent Multiplier Hypothesis:

  • Cl- and Na+ ions are actively pumuped out of the filtrate frrom the ascending limb and into the surrouunding tissue fluid. No water follows as this part of the loop is impermeable to water
    • 1)Water flows out of the descending limb and collecting cudt into the tissure fluid by osmosis
    • 2)Na+ and Cl- at a  very high concentration in the tissue fluis so difffuse down the concentration gradient into the descending limb
  • When the filtrate reaches the bottowm of the ascending limb, the fluid in the loop has lost a lot of water and is very concentrated. The fluid surrounding, in the bottom of the loop (medulla) is also very concentrated because of the accumulate ion Na+ and Cl- ions
  • As the fluid then goes up the ascending limb, Na+ and Cl- ions are pumpe out so it gets more dilute
  • This couunter current multiplier allows concentrated urine to be produced - it is more concentrate than plasma
  • More water is drawn out of the collecting duct by osmosis when the filtrate passes through the medulla again -> water is conserved
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The Water Balance - control of ADH

1) Blood concentration monitored by osmoreceptors in the hypothalamus

2) If the concentration is too high (more negative WP), the osmoreceptors shrink

3) This stimulates neurosecretory cells in the hypothalamus to release Anti Duiretic Hormone

4) ADH is manufactured in the cell body of these cells but flows down their axon to the posterior pituitary gland

5) When neurosecretory cells are stimulated, they send impulses to the posterior pituitary gland which then releases ADH

6) ADH causes the kidney to reabsorb water from urine

7) THe water level will be brought back to normal and the impulses stop

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Balancing water potentials

More Neagtive

  • too little water in the blood
  • osmoreceptors detect this in the hypothalamus
  • tthe posterior pituitary releases ADH
  • ADH makes the collecting duct more permeable so more water is reabsorbed
  • THe urine formed in very concentrated

Less Negative

  • Too much water in the blood
  • Detected by osmosreceptors in the hypothalamus
  • Posterior pituitary gland stops releasing ADH
  • No ADH means the collecting dusts are less permeable so less water is released
  • Urine formed is more diluted
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ADH molecules bind to the receptor proteins and triggers the vesicles containing aquaporins to bind with the plasme membrane next to the lumen

Aquaporins are large channel proteins

ADH causes vesicles ao aquaporins to be inserted into the membrane of the collecting duct wall cell

Water enters the cell througgh the aquaporins by osmosis

water moves fromt he cell to the capillary throuhg their interstitial fluid

1)ADH detected by cell surface receptors

2) Enzyme controlled reaction

3) Vesicles of aquaporins fuse to membrane

4)water is reabsorbed into the cell

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Kidney Failure

Kidney Infection - inflammation and damages cells and disrupts the normal filtering reabsorption. High Blood Pressure - damges the glomeruli so large molecules like proteins can get through

  • Waste products build up - cause weight loss and vomiting
  • Fluid accumulates -swelling in the abdomen, legs and face
  • Ions in blood become unbalanced - blood may become too acidic. An imblance of CA2+ causes brittle bones and salt build up causes fluid retention
  • Long term failure causes a lack of haemoglobin in blood and death as toxic substances accumulate


  • Transplant - a diseased kidney is replaced with a healthy one from a donor. To prevent rejection by the immune system, the donor kidney must a similar tissur type to the recipient and the recipien is trreated with immunosuppresant drugs
  • Renal dialysis - blood taken from vein and pumped througgh machine. CLeaning is done by getting waste to diffuse across a selectively permeable membrane into a plasma-like fluid and then returned to vein. It restores the concentration of dissolved substances to normal and has to be carried out regularly
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Using Urine to test...

Pregnancy tests:

  • detect the hormone Human Chorionic Ganadotropin which is only fouund in the urine of pregnant women.
  • The ***** contains artificual monoclonal complementary antibodies specific to hCG.
  • If the ***** is dipped in urin with hCG present, the hCG binds to the antibodies (that are tagged with a coloured dye)
  • A control line is used for comparison - if there are two line, the woman is pregnant

Misuse of Anabolic Steroids

  • Anabolic steriods increase protein synthesis and so help to build muscle. It is filtered in the kidneys and so is present in urine
  • It is tested for using gas chromatography - unrine is vapourised and passed througgh a column. Different components move through at different rates
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