The extracellular fluid compartment is a complex entity composed not only of two major divisions, the vascular and the interstitial fluid compartments, but also of several subdivisions of these, each with differing characteristics. It is not surprising, therefore, that the control of ECF fluid volume, in marked contrast to control of the ECF osmotic concentration, is accomplished by a variety of mechanisms, many of which are poorly understood. It is customary to speak of control of extracellular fluid volume, but it is apparent that many of the various mechanisms are triggered by factors related to the volume of the vascular compartment. Changes in the volume of the interstitial compartment ordinarily take place indirectly as a result of changes in the dynamic steady state of the Starling forces governing fluid movement across capillary walls separating the vascular and interstitial compartments.

A. The volume of the high pressure arterial compartment is primarily a direct function of the inflow, that is, the cardiac output, and the outflow, which is governed by arterial pressure and peripheral resistance. The compliance or dispensability of the muscular arterial wall is small and normally is a minor factor. Thus the volume is maintained within narrow limits. Changes in volume are sensed as changes in pressure by the baroreceptors in the carotid sinus and aortic arch. They alter sympathetic neural and humoral input into the kidney and thereby alter renin secretion and salt and water excretion. They also can inhibit ADH secretion. The afferent arteriole in the kidney also serves as a baroreceptor and alters the rate of renin release by the juxtaglomerular apparatus.

Fig. 8-1. The vascular compartment, its subdivisions, its relation to the interstitial compartment and the location of receptors.

B. The low pressure venous compartment includes the pulmonary circulation and all the heart except for the left ventricle (Fig. 8-1). The volume of the venous compartment is primarily a direct function of the inflow, which is controlled by arterial pressure and total peripheral resistance, the outflow, and to a major extent, the venous compliance. The major factor controlling compliance is the tissue pressure exerted inward on the wall of the veins. It is also partially controlled by the activity of the venous smooth muscle. In muscles of the trunk and the extremities, the tissue pressure is the result of muscular contraction which compresses the veins and reduces their capacity. In the abdomen, contraction of the diaphragm and abdominal muscles compresses the abdominal veins. Most important, in the thorax, contraction of the rib muscles and diaphragm expands the thorax and reduces intra thoracic pressure, increasing the capacity of the great veins, the pulmonary circulation, and the atria. The atria and the great veins in particular are highly distensible: small increases in transmural pressure produce large changes in their volume. Stretch receptors, primarily in the atria respond to stretch induced by increased volume within the atria.

C. The volume of the interstitial compartment changes primarily as a result of changes in the balance between the capillary hydrostatic pressure (P_c) and the colloid osmotic pressure of the blood (_b)(Fig.8-1). Changes in these pressures result in shifts of isotonic fluid between the vascular and interstitial compartments. Thus, in one sense, the interstitial compartment can be viewed as a reservoir participating in the maintenance of vascular volume. There are no known receptors that respond to changes in the volume of the interstitial compartment. Rather its volume changes as a result of changes in the vascular compartment.

QUESTIONS: 
1. What is the effect on the volume of the interstitial compartment of each of the following: a rise in venous pressure, constriction of precapillary sphincters, a rise in arterial pressure resulting from an increase in blood volume, leakage of protein into the interstitial space, intravenous infusion of a large volume of isotonic saline?

2. What is the effect on the volume of the cellular and extracellular compartments of an infusion of 5% dextrose? An infusion of isotonic saline?

3. What is the effect on the volume of the intracellular and interstitial compartments of a loss of blood by hemorrhage?

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