From Figure 1 it is apparent that blood volume and extracellular fluid volume are usually controlled in parallel with each other. Ingested fluid initially goes into the blood, but it rapidly becomes distributed between the interstitial spaces and the plasma. Therefore, blood volume and extracellular fluid volume usually are controlled simultaneously.
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.
There are circumstances, however, in which the distribution of extracellular fluid between the interstitial spaces and blood can vary greatly. As discussed in Chapter 25, the principal factors that can cause accumulation of fluid in the interstitial spaces include (1) increased capillary hydrostatic pressure, (2) decreased plasma colloid osmotic pressure, (3) increased permeability of the capillaries, and (4) obstruction of lymphatic vessels. In all these conditions, an unusually high proportion of the extracellular fluid becomes distributed to the interstitial spaces.
Figure 2 shows the normal distribution of fluid between the interstitial spaces and the vascular system and the distribution that occurs in edema states. When small amounts of fluid accumulate in the blood as a result of either too much fluid intake or a decrease in renal output of fluid, about 20 to 30 percent of it stays in the blood and increases the blood volume. The remainder is distributed to the interstitial spaces. When the extracellular fluid volume rises more than 30 to 50 percent above normal, almost all the additional fluid goes into the interstitial spaces and little remains in the blood. This distribution occurs because once the interstitial fluid pressure rises from its normally negative value to become positive, the tissue interstitial spaces become compliant and large amounts of fluid then pour into the tissues without interstitial fluid pressure rising much more. In other words, the safety factor against edema, owing to a rising interstitial fluid pressure that counteracts fluid accumulation in the tissues, is lost once the tissues become highly compliant.
Fig2. Approximate relation between extracellular fluid volume and blood volume, showing a nearly linear relation in the normal range but also showing the failure of blood volume to continue rising when the extracellular fluid volume becomes excessive. When this condition occurs, the additional extracellular fluid volume resides in the interstitial spaces and edema results.
Thus, under normal conditions, the interstitial spaces act as an “overflow” reservoir for excess fluid, sometimes increasing in volume 10 to 30 liters. This situation causes edema, as explained in Chapter 25, but it also acts as an important overflow release valve for the circulation, protecting the cardiovascular system against dangerous overload that could lead to pulmonary edema and cardiac failure.
To summarize, extracellular fluid volume and blood volume are often controlled simultaneously, but the quantitative amounts of fluid distribution between the interstitium and the blood depend on the physical properties of the circulation and the interstitial spaces, as well as on the dynamics of fluid exchange through the capillary membranes.
