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The excretory system consists of paired
kidneys
and associated blood supply.
Ureters
transport urine from the kidneys to the bladder and the
urethra
with associated sphincter muscles controls the release of urine.
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The kidneys have an important role in maintaining
homeostasis
in the body. They excrete the waste product urea, control the concentrations of water and salt in the body fluids, and regulate the acidity of the blood.
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A kidney consists of an outer region or
cortex,
inner
medulla
and a cavity called the
pelvis
that collects the urine and carries it to the ureter.
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The tissue of a kidney is composed of masses of tiny tubes called
kidney tubules
or
nephrons
. These are the structures that make the urine.
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High-pressure blood is supplied to the nephron via a tuft of capillaries called the
glomerulus
. Most of the contents of the blood except the cells and large protein molecules filter from the glomerulus into the (
Bowmans) capsule.
This fluid flows down a coiled part of the tubule (
proximal convoluted tubule
) where useful substances like glucose, amino acids and various ions are reabsorbed. The fluid flows to a looped portion of the tubule called the
Loop of Henle
where water is reabsorbed and then to another coiled part of the tubule (
distal convoluted tubule
) where more reabsorbtion and secretion takes place. Finally the fluid passes down the
collecting duct
where water is reabsorbed to form concentrated urine.