What Is Filtration in the Excretory System?

Blood is a complex soup of cells, proteins, small chemical molecules and, of course, water. The excretory system has the job of filtering out chemicals that the body doesn't need, as well as excess water. Filtration occurs in microscopic tubules in the kidney, where the chemicals are squeezed out of the blood and then certain chemicals are allowed to diffuse back in.

Where Filtration Occurs

• The excretory system begins with the kidneys, where urine is made by filtering the blood. Urine flows down the ureters into the bladder, and eventually leaves the body via the urethra.

  Filtration occurs in a microscopic part of the kidney called the nephron. The blood vessels are clustered in a part of the nephron called the glomerulus. The blood vessel leaving the glomerulus is smaller than the blood vessel entering it, so the blood in this area is under very high pressure.

What is Filtered Out

• In humans, one of the most important jobs of the excretory system is to filter out nitrogenous wastes, made from the breakdown of proteins and nucleic acids. The most common nitrogenous waste in humans is urea.

  Aquatic animals usually excrete nitrogen in the form of ammonia, while animals such as birds and lizards excrete uric acid instead.

How Filtration Works

• As the beating of the heart pushes blood through the glomerulus, water and small chemicals called solutes (including urea) are squeezed through the capillary walls into a structure called Bowman's capsule. Blood cells and proteins are too large to fit through, so they stay in the blood.

What Happens After Filtration

• The rest of the nephron is dedicated to bringing some of the water and solutes back into the blood, to keep the body's blood chemistry properly balanced. These other components of the nephron include the proximal tubule, the distal tubule and the loop of Henle. Animals that live in the desert conserve water by having an extra long loop of Henle, which is the part that allows water to be absorbed back into the blood.

Disorders

• If the excretory system cannot filter out enough uric acid, uric acid crystals can accumulate in the blood. This is called gout. The crystals may get lodged in joints, such as the toe joints, or they may cause kidney stones. There are drugs, such as colchicine, which can treat gout.

  Infections or autoimmune disorders can cause nephritis, an inflammation of the glomerulus.

  Nephrosis occurs when the glomerulus leaks protein (which should normally not be excreted in large quantities).

  There are many types of kidney stones, with a variety of causes. These are caused by crystals of calcium, uric acid or other substances, and they may be so large that they get stuck in the kidney or ureter. Kidney stones may be broken up with ultrasound. Sometimes surgery is necessary.