sympathetic nerves which keep the
vessels in a moderate state of contraction and keep some
capillary beds partially closed.
The total blood volume of an individual varies by only 100-200 ml. This volume is efficiently maintained by the capillary shift, kidney function and autonomic nervous control. The autonomic nervous system also controls the capillary beds and vascular tone. This action will also adjust rapidly to changes in blood pressure, hence it is not as important in the long run as it is in the short term.
Hypervolemia is an increase in blood volume while hypovolemia is a decrease in blood volume. When blood volume changes the body manages either of these events in such a way as to re-establish a normal volume of five liters.
Potentially, there is much more vascular space than the
available five liters of blood could fill. This space is
maintained by the action of the sympathetic nerves which keep the
vessels in a moderate state of contraction and keep some
capillary beds partially closed.