3 mechanisms:
1. Renal Autoregulation of GFR
A) Myogenic Mechanism: occurs when stretching triggers contraction of smooth muscle cells in the walls of afferent arterioles. As blood pressure rises, GFR also rises due to the renal blood flow increase. Elevated blood pressure also stretches the walls of the afferent arterioles. In response, smooth muscle fibres contract narrowing the arteriole’s lumen decreasing renal blood flow, and decreasing GFR to its previous level. Conversely, when blood pressure drops, smooth muscle cells relax causing dilation of the afferent arterioles and renal blood flow and GFR increase. The myogenic mechanism can normalize renal blood flow in seconds after a change in arterial blood pressure. B) Tubuloglomerular Feedback: When GFR is above normal due to elevated systemic blood pressure, filtered fluid flows more rapidly along the renal tubules. As a result, the proximal convoluted tubule and loop of Henle have less time to reabsorb Na+, Cl- and water. Cells in the Macula Densa are thought to detect increased delivery of Na+, Cl- and water and inhibit the release of nitric oxide (which causes vasodilation). This causes the afferent arterioles to constrict when the level of NO declines. As a result, there is less blood flow into the glomerular capillaries and GFR decreases. When blood pressure falls, causing GFR to be lower than normal, the opposite sequence of events occurs. 2. Neural Regulation of GFR: Blood vessels of the kidneys are supplied by sympathetic ANS fibres that release NE. NE causes vasoconstriction slowing renal blood flow and GFR. At rest, sympathetic stimulation is moderately low, and the afferent and efferent arterioles are dilated.
3. Hormonal Regulation of GFR: 2 hormones contribute to the regulation of GFR.
A) Angiotensin II reduces GFR . Angiotensin is a potent vasoconstrictor reducing renal blood flow and therefore GFR. B) Atrial Natriuretic Peptide (ANP) increases GFR . Cells in the atria of the heart secrete this hormone. Stretching of the atria occurs when blood volume increases. This stimulates the secretion of ANP. ANP relaxes the glomerular cells, which increases capillary surface area available for filtration. GFR rises as surface area increases.
Tubular Reabsorption and Tubular Secretion
Reabsorption , the return of most of the filtered water and many filtered solutes to the bloodstream is a function of the nephron and collecting duct. Normally 99% of the filtered water is reabsorbed. Epithelial cells along the renal tubule and duct carry out reabsorption with proximal convoluted tubule cells making the largest contribution. Solute reabsorbed by either active or passive processes include glucose, amino acids, urea, and ions such as Na+ and K+, Ca++, Cl-, HCO3, and HPO4.
321
Made with FlippingBook Publishing Software