The Kidneys and Controlling Water Levels

Your body needs to be able to regulate its water levels to prevent cells from getting damaged. Due to osmosis, if your body has too much water, your body cells will swell up and burst. On the other hand, if your body doesn’t have enough water, cells will shrivel up and die. Thankfully, as a result of homeostasis, which is your body’s ability to maintain a constant internal environment in order to function properly, your body is able to control its water levels to prevent this from happening.

The Kidneys

In order to understand how the body is able to maintain water levels, we need to first start off with the kidneys. These organs work to filter out substances in the blood that are not needed or are harmful to the body. For example, urea, which is a toxic waste product that needs to be removed and excess ions. These excess substances are removed from your body through your urine. The kidneys do two very special processes that allow them to achieve this. These are Ultrafiltration and Selective Reabsorption.

During ultrafiltration, small molecules in the bloodstream are filtered out into the kidneys such as glucose, urea, ions and water molecules. Larger molecules and substances such as red blood cells and platelets cannot be filtered out as they are too big. This process allows unwanted molecules to be filtered out and put into the urine in order for the body to get rid of them.

The kidneys cannot differentiate between the molecules they have absorbed. This means that it also takes in substances that the body requires such as glucose, and some water molecules, ions, in order to live. Therefore, this process is essential in order to make sure that molecules that the body needs doesn’t get taken away. During selective reabsorption, useful substances are put back into the blood and the unwanted ones make urine.

Here is what the kidneys look like. Image by OpenClipart-Vectors from Pixabay


Osmoregulation is just the fancy term for regulating water levels. This process is a negative feedback loop meaning that when a certain factor (in this case water levels) become too high, the body works to bring them back down and vice versa.

What happens when water levels are too high?

  1. Receptors in the brain detect an increase in water levels
  2. The Pituitary Gland receives these signals and releases less ADH
  3. The lower amount of ADH causes the kidneys to reabsorb less water, bringing water levels down to normal

ADH stands for anti-diuretic hormone which prevents a person from peeing more often, by making the kidneys reabsorb more water. This makes sense because if you have too much water in your body, your body will want to get rid of it and if you have too little, your body will not want to lose that water.

What happens when water levels are too low?

  1. Receptors in the brain detect a decrease in water levels
  2. The pituitary gland receives these signals and releases more ADH
  3. The higher amount of ADH causes the kidneys to reabsorb more water, bringing water levels back up to normal
Image by rony michaud from Pixabay

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