Describe the process of ultrafiltration in the Kidneys.

Ultrafiltration occurs in the nephron, the functional unit of the kidney. Blood enters a coiled network of vessels called the glomerulus from the afferent arteriole at a high hydrostatic pressure. The glomerulus is in contact with Bowman's capsule which acts a filtration barrier. The filtration barrier is composed of 3 layers: The capillary endothelium, basement membrane and then podocytes which have foot-like projections called pedicels which act to form filtration slits.This filtration barrier allows small ions like sodium, potassium, glucose, water, urea and some amino acids to be part of the filtrate whilst preventing larger products in the blood such as erythrocytes and white blood cells from being filtered. The glomerular filtrate then passes to the proximal convoluted tubule where the filtrate undergoes selective reabsorption.