Overview
This lecture explains the renin-angiotensin-aldosterone pathway, the roles of aldosterone and antidiuretic hormone (ADH) in regulating water, sodium, and potassium balance, and how the kidneys help control blood pressure, osmolarity, and pH.
Renin-Angiotensin-Aldosterone Pathway
- Granular cells in the juxtaglomerular apparatus release renin near the afferent arteriole.
- Renin converts angiotensinogen (from the liver) to angiotensin I in the bloodstream.
- Angiotensin I is converted to angiotensin II by angiotensin-converting enzyme (ACE) in the lungs.
- Angiotensin II constricts the efferent arteriole and stimulates the adrenal glands to release aldosterone.
- ACE inhibitors block the conversion of angiotensin I to angiotensin II.
Role of Aldosterone
- Aldosterone is released from the adrenal gland in response to angiotensin II or high potassium levels in the blood.
- It acts on distal convoluted tubule and collecting duct cells to increase the number of sodium-potassium pumps.
- These pumps move sodium into the interstitial fluid and potassium into the urine.
- More sodium in the interstitial fluid increases osmolarity, causing water to follow and be reabsorbed.
- Aldosterone helps to conserve water, increases blood pressure, and allows the body to excrete excess potassium.
Role of Antidiuretic Hormone (ADH)
- ADH is made in the hypothalamus and released from the posterior pituitary when blood osmolarity is high (dehydration).
- ADH increases the number of water channels (aquaporins) in the distal tubules and collecting ducts.
- This action promotes water reabsorption, concentrating urine and preserving body water.
- ADH works independently or together with aldosterone for water conservation.
Kidney Regulation of pH and Other Functions
- Intercalated cells in the nephron secrete protons (H⁺) into the filtrate to help regulate blood pH.
- The kidneys also remove nitrogenous wastes and secrete erythropoietin to stimulate red blood cell production.
Key Terms & Definitions
- Renin — an enzyme released by kidney granular cells that initiates the renin-angiotensin-aldosterone pathway.
- Angiotensin II — a hormone that constricts vessels and stimulates aldosterone release.
- Aldosterone — a hormone from the adrenal gland that increases sodium reabsorption and potassium excretion in the kidney.
- Antidiuretic Hormone (ADH) — a hormone that increases water reabsorption by adding water channels to kidney tubule cells.
- Osmolarity — concentration of solutes in a solution; higher osmolarity means less water.
- Juxtaglomerular Apparatus — a kidney structure involved in blood pressure regulation.
- Intercalated Cells — nephron cells that assist in acid-base balance by secreting hydrogen ions.
Action Items / Next Steps
- Review the steps and hormone actions in the renin-angiotensin-aldosterone pathway.
- Understand how aldosterone and ADH individually and together regulate urine concentration.
- Learn key definitions and kidney cell types involved in electrolyte and pH balance.