A drop in overall blood pressure in the body also usually leads to a corresponding drop in filtration pressure in the kidneys. The function of renin is to restore normal blood pressure, thereby increasing filtration rates of water and solutes in the kidney tubules, so that filtration proceeds in proper balance. Renin achieves this purpose along with angiotensinogen, a substrate from the liver, combining to form what is called angiotensin I. Studies have revealed that, when normal renin functioning is absent, it leads to significantly lower blood pressure levels, or hypotension. When blood volume and pressure increase, the release of renin stops until triggered once again by low pressure levels.
Tests for the proper function of renin are done when high blood pressure — hypertension — or hypotension are suspected. Plasma renin activity (PRA) test measurements are compared against levels of natural aldosterone hormone from the adrenal glands. Aldosterone and parathyroid hormone regulate the absorption of electrolytes such as sodium and potassium in the kidneys and are involved in water retention. Studies of the development of end-stage renal disease in those suffering from diabetes mellitus suggest that, too often, triggering of the renin-angiotensinogen system in the early stages of diabetes leads to hypertension.
In the renin-angiotensin system, the angiotensin I is changed into angiotensin II by an enzyme from the lungs known as angiotensin-converting enzyme (ACE). This leads to blood vessel constrictions, which cause aldosterone and other hormones to be stimulated and create the thirst reflex. When a person drinks more water because of the thirst reflex, blood pressure can then normalize, due to the water retention capacity from aldosterone.
The function of renin not only affects kidney filtration balances, but also plays a role in healthy balances for the heart. An overactive renin-angiotensin-aldosterone system can lead to hypertension and sometimes cardiac dysfunction. Therefore, medical professionals use several drugs to inhibit the activity of this system. Sometimes drugs that inhibit the ACE from initiating angiotensin II are administered, and sometimes blockers of angiotensin II are used. These two blocking mechanisms are established treatments after heart failures.
There are several types of cancer that affect the function of renin. Kidney cancers can sometimes form in the juxtaglomerular cells that produce renin; this is known as reninoma. Additionally, carcinomas can develop in the renal cells themselves, due to a malfunction of gene expression. One blastoma cancer that produces renin, usually only found in children, is called Wilms' tumor. This cancer is said to be caused by a chromosome mutation; however, early detection and treatment leads to a very high survival rate.