Longterm Regulation Of Arterial Pressure

Although neural reflexes may buffer moment-to-moment changes in MAP, they can quickly adapt to a continued stimulus, thereby negating their effectiveness in long-term control of blood pressure. The most important long-term controller of arterial pressure is the kidney, which makes this contribution through its ability to regulate blood volume. Fluid loss by the kidney is determined directly by the pressure perfusing it. The higher the kidney's perfusion pressure, the more fluid will be filtered, which ultimately contributes to the urine. When perfusion pressure is low, less urine is formed and less salt and water will be lost. Also, the renin-angiotensin system amplifies the relationship between renal perfusion pressure and fluid loss. Angiotensin II causes the kidneys to selectively retain

_Normal fluid_ intake

_Normal fluid_ intake

50 100 150 200

Arterial pressure (mmHg)

FIGURE 7 Relationship between arterial pressure and urine output in a normal person. The intersection of the broken line with the curve (point A) represents the point at which fluid intake equals urine output—normal arterial pressure. Point B illustrates the influence of a reduced arterial pressure on urine output, and point C shows the urine output response to an increased arterial pressure.

salt and water, both through a direct effect and through its effect on the release of aldosterone, a hormone that controls salt excretion (see Chapter 32 for a detailed account of aldosterone's action). By retaining salt and water when MAP is low, blood volume and the mean circulatory filling pressure are increased, which, in turn, increases cardiac output and finally restores the MAP. This renal-blood volume mechanism for regulating blood pressure requires a few hours to manifest a significant response; however, unlike the baroreceptor reflex, it never fatigues or adapts, and it will eventually restore blood pressure all the way back to normal.

Figure 7 shows the effect of arterial pressure on urine output in a normal person and it demonstrates the sensitivity of the renal responses to changes in arterial blood pressure. In a normal individual with a MAP of 100 mm Hg, urine output over a 24-hr period will equal the amount of fluid intake (point A in Fig. 7). However, if this individual experiences significant blood loss, the arterial pressure will initially fall (with an effort by neural mechanisms to raise the pressure toward normal) and the kidneys will produce less urine (point B in Fig. 7). If fluid intake remains normal, then the fluid retention that accompanies the reduced urine output will eventually (requiring days to weeks) restore blood volume back to normal, leading to reestablishment of arterial pressure and urine output. On the other hand, if arterial pressure is elevated above normal, the renal output of water (and salt) increases profoundly above fluid intake (point C in Fig. 7); the resulting fluid loss reduces blood volume, and arterial pressure eventually returns to normal. The steepness of the relationship between urine output and arterial pressure illustrates how effectively the kidneys can respond to even small changes in arterial pressure and in doing so use adjustments in urine output as a means for precise regulation of arterial pressure.

Get Rid of Gallstones Naturally

Get Rid of Gallstones Naturally

One of the main home remedies that you need to follow to prevent gallstones is a healthy lifestyle. You need to maintain a healthy body weight to prevent gallstones. The following are the best home remedies that will help you to treat and prevent gallstones.

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