Excretion is the removal of harmful wastes like urea by kidneys, sweat from skin and CO2 by lungs.
Functions of Kidneys
The two kidneys remove urea from blood with the help of tubular structures called nephrons. Kidneys are made of nephrons. In excretion, waste substances are filtered by uriniferous tubules or nephrons. The useful substances (like water, glucose) are reabsorbed back into blood.
The renal vein and renal artery are blood vessels of the kidney. Renal artery brings blood to kidneys. Blood entering the kidneys contains lots of oxygen, glucose, H2O, salts from food and urea. Renal vein takes blood from kidneys. Blood leaving the kidneys contains less oxygen, glucose and urea (removed in urine), right amount of water and salts (their excess is removed in the urine).
Kidneys also control the amount of water in blood. When water needs to be conserved, concentrated urine is expelled out of the body. Such controlled removal of water in urine is called osmoregulation.
In case of kidney failure, dialysis is done in which blood is taken out and filtered in an artificial kidney or dialysis fluid and then pumped back into the body.
Urine is made in kidneys from waste like urea present in blood. Blood reabsorbs all the useful substances like glucose and amino acids from the blood filtered in the kidneys. Urinary bladder stores urine till it is removed from the body.