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The body produces renin in order to keep the level of salts in the blood stream balanced with the volume of fluid in a person’s veins and arteries. Renin can be released when there is a drop in the amount of sodium, potassium, or blood fluid in the body. The kidneys are responsible for detecting these drops and releasing this enzyme into the body. Once it enters the bloodstream, renin causes a cascade of other responses, eventually leading to the release of hormones by the adrenal gland that instruct the body to reabsorb salts rather than passing them through the urine.
Renin allows the body to compensate for a decrease in sodium or potassium. Though both of these are plentiful in most people’s diets now, throughout most of human evolution, salt, especially sodium, was hard to find. The human body needed a way to hold onto salts in times of shortage and developed renin as one component in a complex system that balances the level of salt with the amount of liquid in the blood stream.
Blood needs to have a certain thickness in order to effectively transport nutrients throughout the body. Adding salt to the bloodstream is one way to thin out the blood because salt binds to water, keeping the water in the blood vessels instead of letting it seep into cells. When the blood is at the proper thickness, the blood pressure falls within a normal range and blood moves easily, without causing too much stress on the walls of the blood vessels.
The thickness of the blood is affected by renin when a set of chemical reactions in something known as the renin-angiotensin system are triggered. It is released by the kidneys when a decrease in sodium, potassium, or blood volume is detected. After it is in the blood, it travels to the liver, where it signals the release of angiotensinogen. This molecule becomes angiotensin, which is used to signal the release of aldosterone in the adrenal gland. Aldosterone instructs the kidneys to retain salt rather than letting it pass into the urine.
As the body starts to take up salt that would otherwise be lost, the salt causes the fluid level in the blood stream to rise, increasing the overall volume of blood moving through a person's body. This increase is often healthy and necessary, though it can be a cause of high blood pressure. Renin inhibitors may be administered to patients who produce too much of this enzyme as a treatment for the hypertension.