2.68a Excretion & 2.68b Osmoregulation

2.68 Understand how the kidney carries out its roles of excretion and of osmoregulation.

Part A

To illustrate this we will think about excretion of the molecule known as urea.
It is important to remember that urea contains nitrogen which is toxic to our body so it cannot be stored. The original form of nitrogen circulating in our blood stream which is potentially toxic are in the amino acids.

The amino acids are used for growth in our body, but we do have extra amino acids which must be removed because they are toxic. Doing this is the role of both the liver and the kidney.

The first stage of this is when blood circulates into the liver and the amino acids are broken down and converted into urea. This re-enters the blood stream and circulates to the kidneys (both) so it can be removed from the body.

The kidneys will filter the urea from the blood, and it will be added to water to form urine. This is then collected in the bladder, and has been removed from the body.

The filtered blood is then returned to the body from the kidney.

Part B

In the diagram there are some cells in the body surrounded by tissue fluid, which must be isotonic with the cytoplasm of the cells. This means that the amount of water going into and out of these cells is equal so the cells can maintain the same size, shape and function.

However the danger to the tissue is that blood circulating into the tissue will be concentrated causing either a hypertonic or hypotonic tissue fluid. Both of these would be bad because they would either remove, or add too much water to the cell. Maintaining the isotonic relationship between the cells and the tissue fluid is achieved by controlling the composition of blood. It is the role of the kidney to do this. The kidney removes excess water and salts and excretes them. This controls the content of water and salt in the blood, so the kidney can keep the blood isotonic with the cytoplasm of the cells.