Ultrafiltration:

Effect of afferent arteriole being wider than efferent arteriole:

The blood vessel to the glomerulus is wider than the blood vessel leaving the glomerulus. This increases the blood pressure within the glomerulus and forces most of the fluid and dissolved substances out of the blood vessels and into the Bowman’s capsule. This process is called ultrafiltration.

Constituents of filtrate:

The filtrate contains water, salt, glucose, amino acids, urea and other waste products. Blood cells and proteins are too large to be squeezed out of the blood vessels and so stay in the blood. If kidneys are damaged protein may sometimes enter the filtrate and this is a sign of the damage.

Podocytes:

Podocytes are foot-like cells with extensions called pedicels which wrap around the capillaries. The pedicels from one podocyte interlink with the pedicels from an adjacent podocyte. The result is small gaps between the pedicel. These are like the holes in a sieve and it is through these gaps that ultrafiltration takes place. The gaps are too small for blood cells or large proteins to pass, so these remain in the blood.

Structure of glomerular capillaries:

The glomerular capillaries are similar in structure to normal capillaries. They have a single layer of endothelial cells and an outer layer of basement membrane. The basement membrane is made of collagen and other fibres. It acts like filter paper only allowing small molecules through it. What is different about the glomerular capillaries is that they have more pores between the individual endothelial cells than in normal capillaries. This is to allow ultrafiltration to occur as most substances except blood cells and large protein molecules move through the pores during ultrafiltration

Substances reabsorbed into blood in proximal convoluted tubule:

Substances such as amino acids, lipids, glucose and water are still needed by the body and so it would not be a good thing if they were to be lost in the urine, so they are reabsorbed back into the blood from the proximal convoluted tubule. There is a very good blood supply around the nephron to allow this. Bicarbonate ions are also reabsorbed here leaving the hydrogen ions to continue in the urine.

Substances reabsorbed into blood in distal convoluted tubule and collecting duct:

Most substances have already been reabsorbed from the proximal convoluted tubule. The distal convoluted tubule is for the reabsorption of water.