Blood consists of cells and a liquid component called plasma. The plasma contains proteins and other chemicals with many functions. Some of the chemicals are hormones; others are charged minerals known as electrolytes. There are also substances for energy such as glucose, as well as waste products. Proteins have many functions, for example regulating fluid balance, helping the immune system, carrying other substances and helping with blood coagulation. Renin and its associated proteins help regulate fluid, blood pressure and electrolytes.
Renin Release
Renin is a protein hormone released by special cells situated in the kidneys near the blood vessels. The structure these cells are found in is called the juxtaglomerular apparatus, or JGA. The JGA releases renin if the arterial blood pressure drops or if sodium levels decrease -- as well as when a part of the nervous system, called the sympathetic nervous system, becomes active, which is often also a response to decreased blood pressure.
Protein Cascade
After renin released, it sets off a number of reactions with other proteins. The protein angiotensinogen is released by the liver. When renin encounters angiogensinogen, it converts it into angiotensin I. Angiotensin I travels through the blood, and when it enters the lungs it is converted into angiotensin II by the enzyme agiotensin-converting enzyme, or ACE. Angiotensin II causes the adrenal glands above the kidneys to release the protein aldosterone.
Target Effects
Since renin is released in response to low blood pressure, the effects of renin proteins promote increased blood pressure. Angiotensin II is a potent vasoconstrictor. This means it causes the muscles surrounding the arteries to contract, narrowing the width of the blood vessel through which the blood is flowing. This causes the blood pressure to rise. In addition, the aldosterone increases salt and fluid re-absorption from the kidneys, increasing blood volume and blood pressure. These proteins can also increase thirst.
Disorders
Disorders of renin, angiotensin and aldosterone can lead to problems with fluid, salt and blood pressure. For example, problems with the adrenal gland that diminish aldosterone release cause low sodium, high potassium and low blood pressure. Some people with high blood pressure are given drugs that stop the activity of ACE in the lungs so angiotensin II does not form and increase the blood pressure or stimulate aldosterone release.
References
- "Physiology," 4th edition; Linda S. Costanzo; 2010
- "Harrison's Principles of Internal Medicine," 17th edition; Anthony S. Fauci et al.; 17th Ed 2008
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