Kidney

GCSE BIOLOGY

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Kidneys
The kidney has three functions:
­ To remove urea and other waste substances from the blood
­ To control the amount of ions in the blood
­ To control the water content of the blood
A healthy kidney produces urine by:
- first filtering the blood
- reabsorbing all the sugar
- reabsorbing the dissolved ions needed by the body MENTION ALL 5 POINTS IN EXAM
- reabsorbing as much water as the body needs
- releasing urea, excess ions and water as urine.
The blood enters the kidney from the renal artery into the glomerulus or capillary bundle
which is in the Bowman's capsule. This blood contains urea, ions, glucose and water
which are carried by the plasma of red blood cells.
This blood enters at high pressure into the glomerulus which forces urea, ions, water
(and glucose) out of the blood, through cell membranes and into the tubule. Protein
molecules and blood cells are too big to squeeze through the capillary wall and so they
remain in the blood.
Once the blood has been filtered it may need to reabsorb certain substance that were
filtered out of the blood depending on the conditions of the body. This is called selective
reabsorbtion. The tiny blood vessels which wrap around the tubule selectively reabsorb
what is needed.
All of the glucose is reabsorbed by active transport (as although the concentration
gradient of the blood is high the body can't really have too much glucose as it is needed
for respiration.) The glucose moves against the concentration gradient form an area of
low concentration in the nephron to high concentration in the blood. As this requires
energy for active transport to take place some of this glucose is used up so not all of it
gets transported as some is used up.
Water required by the blood is reabsorbed by osmosis as the concentration of water in
the nephron is higher than the concentration of water in the blood.
As the capillaries/blood vessels are only one cell thick they are ideal for diffusion. Ions
that are needed by the body can diffuse back into the blood through diffusion.
Blood flows out of the kidney through the renal vein.
Urea and waste substances pass through the tubule into the collecting duct and are
stored in the bladder as urine ready to be released out of the body through the urethra.

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If the kidneys don't work properly waste substances will build up in the blood and you will be unable
to control the levels of ions and water in the body. This will eventually lead to death.
People with kidney failure can be kept alive by having kidney dialysis or a kidney transplant.
Blood is drawn from the vein of the body and enters the dialysis machine.
Blood flows along a selectively permeable membrane surrounded by dialysis fluid.…read more

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An alternative to this is a kidney transplant ­ this is the only cure for kidney disease. Healthy kidneys are
either transplanted from dead people on the organ donation register or living people e.g. a relative as we
have 2 kidneys.
­ A donated kidney can be rejected by the body and treated as a foreign body and attacked by
antibodies. To prevent this we:
­ Select a donor with a tissue type that matches closely to the patient.…read more

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