Despite the powerful regulatory mechanisms that maintain blood volume and extracellular fluid volume reasonably constant, there are abnormal conditions that can cause large increases in both of these variables. Almost all of these conditions result from circulatory abnormalities.

INCREASED BLOOD VOLUME AND EXTRACELLULAR FLUID VOLUME CAUSED BY HEART DISEASES

 In persons with congestive heart failure, blood volume may increase 15 to 20 percent and extracellular fluid volume sometimes increases by 200 percent or more. The reason for these increases can be under stood by re-examination of Figure 1. Initially, heart failure reduces cardiac output and, consequently, decreases arterial pressure. This effect 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. Therefore, the kidneys retain volume in an attempt to return the arterial pressure and cardiac output toward normal.

Fig1.  The basic renal–body fluid feedback mechanism for control of blood volume, extracellular fluid volume, and arterial pressure.  Solid lines indicate positive effects, and dashed lines indicate negative effects.

If the heart failure is not too severe, the rise in blood volume can often return cardiac output and arterial pressure virtually all the way to normal and sodium excretion will eventually increase back to normal, although increased extracellular fluid volume and blood volume will remain to keep the weakened heart pumping adequately. However, if the heart is greatly weakened, arterial pressure may not be able to increase enough to restore urine output to normal. When this situation occurs, the kidneys continue to retain volume until severe circulatory congestion develops and the person may eventually die of pulmonary edema unless corrective measures are taken.

In myocardial failure, heart valvular disease, and con genital abnormalities of the heart, increased blood volume serves as an important circulatory compensation that helps to return cardiac output and blood pressure toward normal. This compensation allows even the weakened heart to maintain a life-sustaining level of cardiac output.

INCREASED BLOOD VOLUME CAUSED BY INCREASED CAPACITY OF CIRCULATION

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 1), 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.

In pregnancy the increased vascular capacity of the uterus, placenta, and other enlarged organs of the woman’s body regularly increases the blood volume 15 to 25 percent. 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.

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مواضيع ذات صلة

Integrated Responses to Changes in Sodium Intake

Role of Atrial Natriuretic Peptide in Controlling Renal Excretion

Role of Aldosterone in Controlling Renal Excretion

Role of Ang II in Controlling Renal Excretion

Sympathetic Nervous System Control of Renal Excretion: Arterial Baroreceptor and Low-Pressure Stretch Receptor Reflexes

Importance of Pressure Natriuresis and Pressure Diuresis in Maintaining Body Sodium and Fluid Balance

Integration of Renal Mechanisms for Control of Extracellular Fluid

Osmoreceptor-ADH Feedback System

Urine Specific Gravity

Renal Mechanisms for Excreting Dilute Urine

Physiological Control of Glomerular Filtration and Renal Blood Flow

Renal Blood Flow