Formation of glomerular filtrate by ultrafiltration

• Blood enters the kidney through the renal artery, which branches frequently to give one million tiny arterioles, each of which enters a renal (Bowman's) capsule of a nephron. This arteriole is known as the afferent arteriole.
• The afferent arteriole divides to give a complex of capillaries known as the glomerulus.
• The glomerular capillaries later merge to form the efferent arteriole, which then further divides again into capillaries which wind their way around the various tubules of the nephron before combining to form the renal vein.
• The walls of the glomerular capillaries are made of endothelial cells with pores between them.
• As the diameter of the afferent arteriole is greater than that of the efferent arteriole, there is a build up of hydrostatic pressure within the glomerulus. Consequently, water, glucose and mineral ions are squeezed out of the capillary to form the glomerular filtrate.
• Blood cells and large proteins cannot pass across the renal capsule as they are too large.
• The movement of the glomerular filtrate out of the glomerulus is resisted by the connective tissue of the blood capillary, the epithelial cells of the renal capsule, the hydrostatic pressure of the fluid in the renal capsule space, the low water potential of the blood in the glomerulus and the endothelial cells of the capillaries.
• Some modifications are made to reduce the barrier to the flow of filtrate out of the glomerulus. For example, the inner layer of the renal capsule is made up of highly specialised podocyte cells. These have gaps between them, allowing filtrate to pass beneath them and through gaps between their branches.
• Another mechanism that allows filtrate to pass through the glomerulus is the endothelium of the glomerular capillaries, which possesses spaces up to 100nm wide between its cells, allowing fluid to pass through.
• The hydrostatic pressure of the blood in the glomerulus is sufficient to overcome the resistance. Therefore, the filtrate passes from the blood into the renal capsule. The filtrate contains urea, however doesn't contain any cells or plasma proteins which are too large to pass across the connective tissue.