Extracellular fluid distribution between interstitial space and blood vessels

2021-05-06 09:17 AM

When the volume of extracellular fluid increases by more than 30 to 50 percent from normal, almost all of the extra fluid will enter the interstitial spaces, and less remains in the blood. 

The control of extracellular fluid volume and blood volume

It is clear that the blood volume and extracellular fluid volume are often controlled in parallel. Oral and dietary fluid initially enters the bloodstream but is rapidly distributed between the interstitial spaces and the plasma. Therefore, the blood volume and the extracellular fluid volume are often controlled simultaneously.

Basic renal-body feedback mechanism to control blood volume, extracellular fluid volume and  arterial pressure.

Figure. Basic renal-body feedback mechanism to control blood volume.

Extracellular fluid volume and arterial pressure.

Solid lines indicate positive effects, and dashed lines indicate negative effects.

There are cases, however, in which the distribution of extracellular fluid between the interstitial spaces and blood can differ greatly. Major factors that can cause fluid accumulation in the interstitial spaces to include:

(1) increased capillary hydrostatic pressure.

(2) decreased plasma colloidal osmolality.

(3) increased permeability of the capillaries.

(4) lymphatic vascular obstruction. 

In all these problems, abnormally high rates of extracellular fluid will be distributed into the interstitial spaces.

The relationship between extracellular fluid volume and blood volume

Figure. The relationship between extracellular fluid volume and blood volume.

Indicating a nearly linear relationship within the normal range but also shows an inability to continue to increase blood volume as extracellular fluid volume becomes exceeded. When this occurs, an additional volume of extracellular fluid builds up in the interstitial spaces and results in oedema.

The figure shows the normal distribution of fluid between the interstitial spaces and the vascular system and the distribution occurs in a state of oedema. When a small amount of fluid builds up in the blood due to too much fluid absorption or a decrease in the amount of fluid in the kidneys, about 20 to 30 percent of the fluid stays in the blood and increases the blood volume. The remainder is distributed to alternating spaces. When the volume of extracellular fluid increases by more than 30 to 50 percent from normal, almost all of the extra fluid will enter the interstitial spaces and less remains in the blood. This distribution occurs because when the interstitial fluid pressure increases from the normal negative value to the positive, the tissue interstitial spaces become compliant and a large amount of fluid then flows into the tissues without fluid pressure. interstitial increases even more. In other words, the factor is safe against oedema, due to increased interstitial fluid pressure against fluid accumulation in tissues.

Therefore, under normal conditions, the interstitial space acts as an "overflow" tank for excess fluid, sometimes increasing volume by 10 to 30 litres. This condition causes oedema, but it also acts as a vital drain valve for circulation, protecting the cardiovascular system against dangerous overload that can lead to pulmonary oedema and heart failure.

In summary, the extracellular fluid volume and the blood volume are usually controlled simultaneously, but the amount of fluid distribution between the interstitial and the blood depends on the physical characteristics of the circulation and the interstitial space, as well as the dynamics of the process of fluid exchange through capillary membranes.

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