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Homeostasis - kidneys and water balance

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Diffusion, osmosis and active transport

Substances are moving in and out of the cells of your body all the time. To understand why the water balance is so important in homeostasis, and to make sense of how your kidneys work, you need to know about diffusionosmosis and active transport.

Diffusion is the spreading out of the particles of a gas or any substance in solution. It is caused by the random movement of the particles. The higher the temperature, the faster the particles move and the faster diffusion takes place. Diffusion results in the overall (net) movement of particles from an area of high concentration to an area of low concentration down a concentration gradient. If there is a big concentration difference diffusion will take place more rapidly than if there is only a small concentration difference. The difference in concentration is known as the concentration gradient.

Diffusion is very important in the body for the movement of substances eg the movement of oxygen from the air into the blood and carbon dioxide out of the blood into the air in the lungs, or the movement of glucose from the blood to the cells.


Osmosis takes place when two solutions are separated by a partially permeable membrane. A partially permeable membrane lets some particles through but not others. Cell membranes are partially permeable. Water can move freely through them but other particles, such as sugar molecules, cannot.

Osmosis is the movement of water through a partially permeable membrane down a concentration gradient from a dilute solution (where there is a high concentration of water) to a concentrated solution (where there is a relatively low concentration of water).

The movement of water by osmosis is the main reason why it is so important to control the water balance of the body.


Active transport uses energy to move substances against (up) a concentration gradient or across a partially permeable membrane. In active transport a special transport protein in the cell membrane picks up the useful particle on one side of the membrane. The transport protein then rotates through the membrane and releases the particle on the other side of the membrane. This uses energy from cellular respiration.

Active transport is used to move substances into cells and out of cells. Cells which carry out a lot of active transport often have lots of mitochondria to give them the energy they need. Active transport is important in the kidney for hanging on to the substances needed by the body such as glucose and some sodium ions.

Question 2