Answer to Question #305195 in Human Anatomy and Physiology for Emma

Question

How is blood pressure controlled?

Answer 1

Blood pressure is controlled by these three mechanisms:

1. Renin-angiotensin system (RAS);
2. Release of nitric oxide (NO), induced by stress;
3. Baroreflex.

The RAS is activated by low blood pressure. This blood pressure also decreases the perfusion of the juxtaglomerular apparatus in the macula densa of kidneys. The juxtaglomerular cells release the enzyme renin, which cleaves angiotensin I from angiotensinogen. Angiotensin I is converted to angiotensin II by angiotensin-converting enzyme, which is found in endothelial cells of the capillaries. Angiotensin II is binding to receptors on intraglomerular mesangial cells, and these cells, as well as blood vessels surrounding them contract. This causes the release of aldosterone from the zona glomerulosa, which is in the adrenal cortex, and sodium lingers in the bloodstream, increasing fluid volume. As a result, blood pressure rises.

Release of NO is happening under high blood pressure, when the increasing of Ca²⁺ in the cytosol of the endothelial cells causes the contraction of the smooth muscle cells of the blood vessels. Then complex Ca²⁺-calmodulin is formed, and it enhances the activity of the endothelial nitric oxide synthase, and NO is formed. NO diffuses into the adjacent smooth muscle cells of the blood vessels, and the enzyme guanylate cyclase is activated. Then cGMP are synthesized, which reduce the free Ca²⁺ in the cytosol. Thus vascular smooth muscle cells are relaxed, and blood pressure decreases.

The baroreflex is also activated during high blood pressure, when blood presses on the baroreceptors located in the aortic arch, as well as in the carotid sinuses. An action potential is formed, which is relayed to the solitary nucleus in the medulla oblongata, as a measure of blood pressure. The vasomotor center of the medulla oblongata is inhibited, and the vagal nuclei are stimulated. This leads to decreasing in heart rate, and blood pressure decreases.