Renin-angiotensin-aldosterone system blood pressure regulation

BOX 22-D THE RENIN-ANGIOTENSIN-ALDOSTERONE SYSTEM AND THE REGULATION OF BLOOD PRESSURE... 

Box22-D The Renin-Angiotensin-Aldosterone System and the Regulation of Blood Pressure... 

When the temporal sequence of adjustments of blood pressure is analysed it seems, that CNS mechanisms (e.g., baroreflexes) will provide regulation of the circulation within seconds to minutes. Other mechanisms, such as the renin-angiotensin-aldosterone system and fluid shifts, occur over minutes to hours. Only the... 

Aldosterone A hormone produced in and secreted by the zona glomerulosa of the adrenal cortex. Aldosterone acts on the kidneys to reabsorb sodium and excrete potassium. It is also a part of the renin-angiotensin-aldosterone system, which regulates blood pressure and blood volume. 

Renin-angiotensin-aldosterone system (RAAS) The hormonal system controlled mainly by the kidneys and adrenal glands that regulates blood pressure, blood volume, and electrolyte balance. 

Although the kidneys are not considered endocrine glands per se, they are involved in hormone production. Erythropoietin is a peptide hormone that stimulates red blood cell production in bone marrow. Its primary source is the kidneys. Erythropoietin is secreted in response to renal hypoxia. Chronic renal disease may impair the secretion of erythropoietin, leading to development of anemia. The kidneys also produce enzymes. The enzyme renin is part of the renin-angiotensin-aldosterone system. As will be discussed, these substances play an important role in the regulation of plasma volume and therefore blood pressure. Other renal enzymes are needed for the conversion of vitamin D into its active form, 1,25-d i hyd ro xyv itamin D3, which is involved with calcium balance. 

There is another system involved in blood pressure regulation the renin-angiotensin-aldosterone system (Fig. 2). The arterial blood pressure in the kidney influences intrarenal baroreceptors which together with the sodium load at the macula densa lead to renin liberation, angiotensin formation and aldosterone secretion, which by influencing the sodium balance changes the blood volume and influences the arterial blood pressure. 

Physiologically, in both normal and hypertensive individuals, blood pressure is maintained by moment-to-moment regulation of cardiac output and peripheral vascular resistance, exerted at three anatomic sites (Figure 11-1) arterioles, postcapillary venules (capacitance vessels), and heart. A fourth anatomic control site, the kidney, contributes to maintenance of blood pressure by regulating the volume of intravascular fluid. Baroreflexes, mediated by autonomic nerves, act in combination with humoral mechanisms, including the renin-angiotensin-aldosterone system, to coordinate function at these four control sites and to maintain normal blood pressure. Finally, local release of vasoactive substances from vascular endothelium may also be involved in the regulation of vascular resistance. For example, endothelin-1 (see Chapter 17) constricts and nitric oxide (see Chapter 19) dilates blood vessels. 

Angiotensin II is involved in the renin-angiotensin-aldosterone system, which regulates blood pressure, sodium and water homoeostasis by the kidneys, and cardiovascular function. Angiotensin II stimulates the synthesis and secretion of aldosterone and raises blood pressure via a direct vasoconstrictor effect. 

Assuming the capsular pressures opposing the movement of water out of the blood and into the top of the nephron are constant, the net filtration pressure is due largely to the blood pressure. Any fall in blood pressure can have a dramatic effect on the efficiency of filtration and therefore clearance of waste materials. So important is the pressure within the renal vasculature that the kidney is critical in regulating systemic blood pressure via the renin-angiotensin-aldosterone (RAA) axis, a physiological process which relies on transport mechanisms within the renal tubules. 

Hypertension is an increase of blood pressure to levels greater than normal that arises because of a mismatch between the volume of the vascular tree and the volume of blood. Blood volume depends on total body sodium content, which is a balance between sodium intake and output. Total body sodium is controlled by variable excretion of sodium by the kidneys. To regulate sodium balance, the primary variable that the kidney monitors is not total body sodium but rather systemic blood pressure. Renal regulation of blood pressure is via the release of the peptide hormone renin from specialized renal cells. Release of rerun ultimately leads to the production of angiotensin H. Angiotensin II increases total peripheral resistance and blood pressure and also leads to an increase in aldosterone. Aldosterone is a steroid... 

The renin-angiolensin system is a hormonal system that plays a central role in (he control of sodium excretion and body fluid volume. It interacts clasely with (he sympathetic nervous system and aldosterone secretion in (he regulation of blood pressure. Figure 19-13 shows Ihe relationship of Ihe component parts of (he renin-angiotensin system and (heir main physiological cffccis. 

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