Activated when blood volume and blood pressure drop.
Purpose: Increase blood volume and blood pressure.
Components
Renin: First component released.
Angiotensin: Second component, created from angiotensinogen by renin.
Aldosterone: Third component, released by adrenal glands.
Kidneys and Nephrons
Nephron: Filtration unit of the kidney (~1 million per kidney).
Glomerulus: Capillary network for filtration.
Kidneys need to maintain a glomerular filtration rate (GFR) of 120 mL/minute.
Release of Renin
Drop in Blood Pressure: Detected by juxtaglomerular cells (granular cells) in the afferent arteriole.
Drop in Sodium Concentration: Detected by macula densa cells in the distal convoluted tubule; signals granular cells to release renin.
Sympathetic Nervous System Activation: Direct innervation of granular cells increases renin release.
Renin's Role
Renin: An enzyme (not a hormone) released into the systemic circulation.
Angiotensinogen: Produced and stored by the liver; converted to angiotensin I by renin.
Angiotensin I: Slight vasoconstrictor, converted to angiotensin II by angiotensin-converting enzyme (ACE) in the lungs.
Angiotensin II's Effects
Vasoconstriction: Generalized vasoconstrictor, increases blood pressure by constricting arterioles.
Constriction of Efferent Arteriole: Increases glomerular filtration rate (GFR) by backing up blood into the glomerulus.
Stimulates Aldosterone Release: From the adrenal cortex; aldosterone increases sodium reabsorption in the distal convoluted tubule, which increases blood volume and pressure.
Stimulates ADH Release: From the posterior pituitary gland; ADH increases water reabsorption in the distal convoluted tubule and collecting ducts, further increasing blood volume and pressure.
Summary
Stimulus: Drop in blood volume and pressure.
Outcome: Increase in blood volume and blood pressure through vasoconstriction, sodium reabsorption, and water reabsorption.