Does the volume of blood in our body have anything to do with our body’s arterial pressure? It is said that the average adult has approximately 5 liters of blood in his body. Taking into consideration its variability due to several factors (including body size), blood volume is estimated to be about 8 percent of body weight.
On the other hand, arterial pressure refers to “the pressure of the circulating blood on the arteries; (it) is the product of cardiac output and vascular resistance.”
There, indeed, is a direct relationship between blood volume and arterial pressure, and the same is of a purely physical nature. Consider, for example, the fluid pressure existing in any closed elastic system of a given capacity; the more fluid the system holds, the greater will be the pressure of that fluid. This principle applies whether or not the fluid is in motion.
Therefore, in a situation when blood volume becomes greatly diminished, the arterial pressure may drop to the point that an adequate circulation becomes difficult to maintain. In a sudden extensive loss of blood, the immediate danger to the victim is not the decrease in number of red cells but the drop in arterial pressure.
In such an emergency, there usually is a compensatory generalized vasoconstriction, elevating the arterial pressure of the blood which remains. This is probably effected in part by epinephrine, which is apparently produced in greater amounts by the adrenal glands during such emergency situations. Artificial remedial measures are intended for immediately addressing the deficiency in blood volume. Here, fluids are dispensed extensively by mouth, under the skin, or through a vein directly into the circulation, the last mode being the most effective.
As might be expected, blood transfusion would be the most efficacious measure for correcting the defect, as this would restore lost cells and fluid. In an emergency, however, when a significant amount of human blood is not obtainable for transfusion, salt solution of the correct osmotic concentration may raise the arterial pressure sufficiently to prevent death.
The significance of blood volume in relation to the maintenance of arterial pressure emphasizes the importance of three physiological processes which assist in the maintenance of blood volume. These are: the ingestion and absorption of water in the alimentary canal; kidney secretion, which decreases once the blood volume and arterial pressure go down; and the filtration-osmosis water balance between the blood and tissues.