Conditions That Cause Large Increases in Blood Volume and Extracellular Fluid Volume
Despite the powerful regulatory mechanisms that main-tain blood volume and extracellular fluid volume rea-sonably constant, there are abnormal conditions that can cause large increases in both of these variables. Almost all of these conditions result from circulatory abnormalities.
In congestive heart failure, blood volume may increase 15 to 20 per cent, and extracellular fluid volume some-times increases by 200 per cent or more. The reason for this can be understood by re-examination of Figure 29–12. Initially, heart failure reduces cardiac output and, consequently, decreases arterial pressure. This in turn activates multiple sodium-retaining systems, especially the renin-angiotensin, aldosterone, and sympathetic nervous systems. In addition, the low blood pressure itself causes the kidneys to retain salt and water. There-fore, the kidneys retain volume in an attempt to return the arterial pressure and cardiac output toward normal. Indeed, if the heart failure is not too severe, the rise in blood volume can often return cardiac output and arte-rial pressure virtually all the way to normal, and sodium excretion will eventually increase back to normal, although there will remain excess extracellular fluid volume and blood volume to keep the weakened heart pumping adequately. However, if the heart is greatly weakened, arterial pressure will not be able to increase enough to restore urine output to normal. When this occurs, the kidneys continue to retain volume until the person develops severe circulatory congestion and eventually dies of pulmonary edema.
In myocardial failure, heart valvular disease, and congenital abnormalities of the heart, an important circulatory compensation is an increase in blood volume, which helps to return cardiac output and blood pressure to normal. This allows even the weakened heart to maintain a life-sustaining level of cardiac output.
Any condition that increases vascular capacity will also cause the blood volume to increase to fill this extra capacity. An increase in vascular capacity initially reduces mean circulatory filling pressure (see Figure 29–12), which leads to decreased cardiac output and decreased arterial pressure. The fall in pressure causes salt and water retention by the kidneys until the blood volume increases sufficiently to fill the extra capacity. For example, in pregnancy the increased vascular capac-ity of the uterus, placenta, and other enlarged organs of the woman’s body regularly increases the blood volume 15 to 25 per cent. Similarly, in patients who have large varicose veins of the legs, which in rare instances may hold up to an extra liter of blood, the blood volume simply increases to fill the extra vascular capacity. In these cases, salt and water are retained by the kidneys until the total vascular bed is filled enough to raise blood pressure to the level required to balance renal output of fluid with daily intake of fluid.