6 Hormonal Regulation
Hormones also regulate GFR. One such mechanism is activated when juxtaglomerular (JG) (or granular) cells are stimulated to secrete the enzyme renin. This enzyme begins a process that results in the production of angiotensin II, a powerful systemic vasoconstrictor that also constricts the afferent and efferent arterioles. Angiotensin II also stimulated the contraction of mesangial cells resulting in a decrease in the surface area of the glomerulus. Because renin is released when blood pressure is low, the resulting constriction of the efferent arteriole helps maintain net filtration pressure in the glomerular capillary and consequently, GFR. Another hormone, atrial natriuretic peptide (ANP) increases GFR. ANP is released when the atria of the heart is stretched, for example, in heart failure when blood volume increases due to sodium and water retention. ANP relaxes the mesangial cells of the glomerulus, making more surface area available for filtration. This increases GFR.
Glomerular Filtration Regulation
| Regulation | Primary Stimulus | Mechanism/Activity Site | Effect |
|---|---|---|---|
| Renal autoregulation | Systemic rising or falling of arterial blood pressure | Adjusts resistance to the flow of blood, when systemic arterial blood pressure rises or falls | Prevents significant fluctuations in GFR |
| Myogenic mechanism | Smooth muscle fibers in walls of afferent arteriole are stretched when blood pressure increases | Contraction of smooth muscle fibers narrows lumen of afferent arterioles | GFR decrease |
| Tubuloglomerular feedback | Increased delivery of sodium ions and chloride ions to the macula densa when blood pressure increases | Constriction of afferent arterioles due to the release of adenosine by macula densa cells | GFR decrease |
| Vasoconstrictor adenosine | Decreased delivery of sodium ions and to the macula densa when blood pressure drops | Dilation of afferent arterioles due to inhibition of adenosine release by macula densa cells | GFR increase |
| Neural regulation | Release of norepinephrine due to increased activity of renal sympathetic nerves | Constriction of afferent arterioles due to activation of alpha-adrenergic receptors and renin release | GFR decrease |
| Hormone regulation | Stimuli cause juxtaglomerular cells to secrete renin. | Once juxtaglomerular cells secrete renin, angiotensin II is produced. | Maintains GFR |
| Angiotensin II | Production of angiotensin II due to decreased blood volume or blood pressure | Constriction of afferent arteriole and contraction of mesangial cells | GFR decrease |
| ANP | Secretion of ANP due to stretching of atria of heart | Capillary surface area available for filtration increased due to relaxation of mesangial cells in glomerulus | GFR increase |
Hormonal Regulation of Reabsorption and Secretion
Five hormones control the absorption of water, and sodium, chloride, and calcium ions: angiotensin II, aldosterone, antidiuretic hormone, atrial natriuretic p