How do your kidneys remove waste from the blood?

he kidneys are the body's filtration system. The kidneys filter the blood of waste products such as urea, excess sugar, and excess water in individual functional units called nephrons. A pair of healthy kidneys contains a total of about 2 million nephrons. These nephrons filter about 500 gallons or 1900 liters of blood daily.

The way the nephrons filter blood is very complicated. In the structure called the glomerulus, almost everything except red blood cells and larger proteins is filtered into the proximal convoluted tubule. The glomerulus is the actual filter which sifts out all the wastes and the extra water and recycles the proteins and other healthy cells. The filtrate goes through a large network of tubules where most ions, water and proteins are reabsorbed into the blood, leaving only waste products and some water. The wastes and extra water are gotten rid of by being sent down the tubule in the form of urine and are stored in the bladder until urination.

More specifically, ultrafiltiration is the process by which fluid is filtered out of the blood by the glomerulus. The blood pressure in a glomerulus causes part of the blood plasma to leak through the capillary walls. The red blood cells and plasma proteins are too big to pass out of the capillary, so the fluid that does filter through is plasma without the protein. The fluid thus consists mainly of water with dissolved salts, glucose, urea and uric acid.

The kidney then uses selective reabsorption to take back into the blood the substances needed by the body. To accomplish sleective reabsorption, the filtrate from the glomerulus collects in the renal capsule and trickles down the renal tubule. As it does so, the capillaries which surround the tubule absorb the substance the body needs back into the blood. Glucose is reabsorbed with much of the water.Some of the salts are taken back to keep the correct concentration in the blood. Salts not needed by the body are left to pass down the kidney tubule with the urea and uric acid.