Physiology of excretion

Kidneys filter all of the blood that comes from the heart. The process of cleaning takes place in the nephrons present in kidneys in three stages. They are ultrafiltration, reabsorption and tubular secretion.

The nephron cleans all your body blood in 45 minutes. Everyday nephron sends about six cups of urine to the urinary bladder. The main components of urine are water and urea.

1.0Ultrafiltration

The first step in clearing the blood is filtration under pressure, the passage of a liquid through a filter to remove impurities. Filtration occurs in the glomeruli. Blood pressure helps plasma (the liquid portion of the blood) to pass through the capillary walls in the glomerulus. Glomerular capillaries are more permeable than other capillaries. The filtrate contains water, glucose, amino acids, ions and urea.

Normally in a healthy adult, the initial filtrate in the kidneys is about 180L daily. However, the volume actually excreted is only a liter or two a day because the remaining filtrate is reabsorbed in the renal tubules.

2.0Reabsorption

During reabsorption, substances considered necessary in the filtrate are reabsorbed by renal tubules which then travel back into the bloodstream. Maximum reabsorption occurs in PCT because of the presence of microvilli.

3.0Tubular secretion

The passage of unwanted substances out of the capillaries directly into the renal tubules is called as tubular secretion. This is an additional way of getting waste materials into the urine.