How your kidneys help control your blood pressure
BMJ Group Medical Reference
Your kidneys take excess salt and water out of your blood and turn them into urine. They do this to get rid of the salt and water your body doesn't need. It's also one of the ways your body regulates your blood pressure.
If your blood pressure is low, your kidneys make a substance called renin. Renin is an enzyme. Enzymes are substances in your body that help chemical changes to happen.
Renin begins a series of chemical reactions in your body that play a part in raising your blood pressure. Renin helps your body to make another chemical, called angiotensin I.
Angiotensin I needs to be changed to another form before it can work. This is done by an enzyme called angiotensin converting enzyme (ACE). ACE turns angiotensin I into angiotensin II. Angiotensin II is the active form that increases your blood pressure.
Angiotensin II raises your blood pressure by stopping your kidneys taking salt and water out of your blood. It also makes your blood vessels narrower. This means there is more blood struggling to get through a smaller space. This causes higher blood pressure. 
Some medicines for high blood pressure work by interfering with the chemicals in your body that cause your blood pressure to go up.
Drugs called renin inhibitors stop renin working.
Some drugs for treating high blood pressure stop the body turning angiotensin I into the active form, angiotensin II. These drugs are called ACE inhibitors.
Other drugs for treating high blood pressure block the effects of angiotensin II so it can't make blood vessels narrower. These drugs are called angiotensin II receptor blockers (or ARBs for short).
Enzymes are chemicals in your body. They have lots of different functions, including playing a part in helping to digest food and starting other chemical reactions that keep the body working.
Your kidneys are organs that filter your blood to make urine. You have two kidneys, on either side of your body. They are underneath your ribcage, near your back.
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