How does the kidney filter blood?

The kidney is able to filter blood as it is made up of about a million filtering units. These units are called nephrons which are assosiated with capillaries. The blood flows to the glomerulus, which is a filter. This filter allows small molecules such as water, glucose, salts and urea pass through it into the nephron, however the blood capillary walls prevent large molecules such as proteins and blood cells from entering. The Bowman's capsule collects this filtrate and it enters the tubules. Here, all glucose is reabsorbed back into the blood capillary immediately. As the rest of the filtrate passes through the tubules, water and salts are absorbed back into the blood according to how much is needed by the body, the loop of Henle helps to maintain blood water levels by filtering out salt . This leaves rest of the filtrate, the waste called urine. Urine contains all urea, excess salt and excess water. This is collected in the collecting duct. It gets transported to the bladder by the ureter. The bladder eventually expels the waste. This means that the waste material urea is able to leave the blood but useful materials such as glucose, salts and water are able to return to the bloodstream. Hence the blood has been filtered as the urea has been filtered out.