Overview
This lecture explains the Renin-Angiotensin-Aldosterone System (RAAS), its cellular and hormonal components, and how it regulates blood pressure in the body.
The RAAS Pathway
- RAAS stands for Renin-Angiotensin-Aldosterone System, which controls blood pressure.
- The key cell in RAAS is the juxtaglomerular (JG) cell, a specialized smooth muscle cell in the kidney's blood vessels.
- JG cells release the hormone renin when blood pressure is low.
- Three main triggers for renin release: low blood pressure, sympathetic (nerve) stimulation, and low salt sensed by macula densa cells in the distal convoluted tubule.
- Macula densa cells detect low sodium in the distal tubule and release prostaglandins to stimulate JG cells.
Hormonal Cascade
- The liver produces angiotensinogen, an inactive circulating hormone.
- Renin converts angiotensinogen to angiotensin I.
- Endothelial cells (mainly in the lungs, but also elsewhere) convert angiotensin I to active angiotensin II.
Effects of Angiotensin II
- Angiotensin II acts on four main cell types to increase blood pressure:
- Smooth muscle cells in blood vessels, causing vasoconstriction and increased resistance.
- Kidney cells, causing increased water retention and blood volume.
- Pituitary gland, stimulating release of antidiuretic hormone (ADH), which increases water retention and vessel resistance.
- Adrenal gland, stimulating release of aldosterone, which also increases kidney water and sodium retention.
Key Terms & Definitions
- Juxtaglomerular (JG) cell — Specialized smooth muscle cell in the kidney that releases renin.
- Renin — Enzyme hormone that starts the RAAS cascade, released by JG cells.
- Macula densa — Kidney cells in the distal tubule that sense sodium concentration.
- Prostaglandins — Local messenger molecules used by macula densa cells.
- Angiotensinogen — Inactive hormone made by the liver, converted by renin.
- Angiotensin I — Inactive form produced from angiotensinogen by renin.
- Angiotensin II — Active hormone that raises blood pressure through multiple effects.
- Aldosterone — Hormone from the adrenal gland that increases sodium and water reabsorption in the kidneys.
- ADH (Antidiuretic Hormone) — Hormone from the pituitary gland that increases water retention and vessel constriction.
Action Items / Next Steps
- Review the steps of the RAAS cascade and its role in blood pressure regulation.
- Memorize the key triggers and effects of each hormone in the system.