Renin is a peptide hormone secreted by the kidneys. It functions in the renin-angiotensin system (RAS) which regulates extracellular fluid volume and arteriol blood pressure. As with other peptide hormones, the production and secretion of renin is tightly regulated by physiological processes within the body.
Blood pressure is monitored by sensory cells called baroreceptors located within the lining of blood vessels. These cells signal for the secretion of renin by the kidneys in response to low blood pressure to increase blood volume.
Sodium ion concentration in the nephron is an indicator of blood volume. A drop in ion concentration results in a subsequent drop of blood volume through osmosis. This process is monitored by a group of cells in the distal tubule called the macula densa, which signals the release of renin in the kidney to increase blood volume and restore blood pressure.
Sympathetic Nervous System
The sympathetic nervous system is part of the fight-or-flight mechanism of the autonomic nervous system. In times of stress, the sympathetic nervous system prepares the body for response by innervating a variety of tissues throughout the body, including increasing heart rate, dilating the pupils and raising blood pressure. This increase in blood pressure is accomplished by signaling the release of renin in the kidneys.
Function of Renin in the RAS
After renin is released from the kidney, it breaks down angiotensinogen into angiotensin I. This peptide is then broken down further into angiotensin II, which acts as a vasoconstrictor narrowing the blood vessels. In addition, angiotensin II signals the release of the aldosterone causing an increase of sodium and water resorption thus increasing blood volume.