Kidney Function

The kidney is concerned with both excretion and homeostasis. It:

• excretes urea and other waste solutes as urine
• regulates the water content of the blood (osmoregulation)
• regulates the pH of the blood by excreting hydrogen ions if  necessary.

It is not possible to filter waste material from the plasma selectively. Instead, in the process of ultrafiltration, the filtration membrane allows all molecules below a certain size to pass through. To avoid the body being depleted of useful materials, it is followed by selective reabsorption. Most of the water lost from the blood is returned to the blood and osmoregulation takes place.

Ultrafiltration

• The afferent arteriole supplying the capillaries of the glomerulus is wider then the efferent arteriole. This, and the short distance of the kidneys from the aorta, provides the necessary hydrostatic filtration pressure.
• The filtration membrane is the basement membrane of glycoproteins surrounding the endothelium of the capillaries. The inner layer of the Bowman's Capsule consists of podocytes , which provide support for the membrane with minimum interruption to filtration.
  
• Water and crystalloid solutes, including urea, are filtered from the blood into the Bowman's capsule. Cells, platelets and colloidal solutes, including the blood proteins with a relative molecular mass greater than 69000, do not pass through the undamaged filtration membrane. Hormones in the blood pass into the filtrate, so urine analysis can test for pregnancy or anabolic steroid misuse. 

Selective Reabsorption

• Glucose, amino acids, vitamins and some ions are reabsorbed by the proximal convoluted tubule (PCT)
• Sodium ions are actively transported out of the PCT cells through the plasma membranes next to the blood capillaries,…