When your body is in balance, it is said to be in homeostasis. If something becomes off, such as your blood pressure is too high or your body does not have enough fluid, your organs work to maintain balance. Because of their role in filtering fluids and wastes through the body, your kidneys are considered a major homeostatic organ of your body.
Your kidneys are paired organs in your body that rest deep within your abdominopelvic cavity, underneath the stomach and intestines, resting on either side of your spine. Your kidneys filter all the fluid in the body, removing nutrients for cellular use and also to remove wastes -- meaning the kidneys play a part in controlling urination. When fluids exit the kidneys, they move to the ureters and then the bladder where they are excreted as your urine. The kidneys are vital to your body's functioning, which is why those whose kidneys do not work properly must turn to artificial means, like dialysis, to cleanse the blood. Although the kidneys use a number of hormones to regulate homeostasis, the two major ways they maintain balance is through releasing hormones to regulate blood pressure and through altering sodium and electrolyte balances to maintain proper fluid amounts in the body.
Your blood pressure is a measure of how hard your heart works to pump blood through the body. If your blood pressure is too high, your heart is working overtime to move blood. Too low, and your heart is not working hard enough. Either is dangerous to your health. Your kidneys contain special cells called juxtoglomerular cells that monitor your blood pressure as blood flows through the kidneys for filtration. When these cells sense your blood pressure is too low, they release a hormone known as renin that sets off a chain reaction in your body to increase blood pressure.
When your kidneys release angiotensin, your blood converts this into two different forms: first, angiotensin I, then angiotensin II. Angiotensin II then signals the blood vessels to constrict. This signals the heart to work harder to pump blood through the body. Angiotensin II also the signals the release of aldosterone, a hormone triggers the kidneys to absorb more sodium and fluids. This increases your blood volume -- the amount of blood in your body -- meaning the heart must work harder to support this increasing amount of blood. These and other smaller actions help to increase your blood pressure, working toward homeostasis and allowing balance to return to the body.
The renin mechanism of the kidneys works to maintain fluid balance that affects your other organs. However, the kidneys internally work to maintain homeostasis as well. The kidneys filter blood, which contains sodium, potassium and other salts that are necessary for your cells to function. The kidneys maintain this fluid balance by speeding up or slowing the rate at which they filter. If your body needs more fluids, such as when you are dehydrated, the kidneys will take in more fluids and you will urinate less. However, when your body has too much fluid -- such as when you have had a large glass of water to drink -- your kidneys will increase their filtration rate, which causes you to urinate more. This maintains homeostasis by ensuring you have the proper amount of bodily fluids to maintain cell functioning.