It may be due to blood loss, dehydration, or anemia Blood pressure is determined mainly by cardiac output, blood volume, and peripheral resistance Resistance Peripheral resistance is the resistance that the blood encounters in the vessels as it travels away from the heart. From capillaries to vena cava, velocity increases again as many small vessels merge to spill blood into a single larger vessel. If a tissue is inadequately perfused, its metabolites accumulate and stimulate vasodilation. As the bloodstream delivers oxygen and carries away wastes, the vessels constrict. The body is able to adjust its blood routing according to the needs of the body After a large meal, proportionately more blood is directed toward the organs of digestion and less toward skeletal muscle.
Physical exertion increases perfusion to the lungs.
Solutes more concentrated in tissue fluid diffuse into the blood, and vice versa. Lipid-soluble substances diffuse through the plasma membrane. Other substances must pass through clefts, fenestrations, or membrane channels. The COP of the blood is due to albumin. The difference between the COP of blood and that of tissue fluid is called the oncotic pressure, which tends to draw water into the capillary by osmosis, thus opposing hydrostatic pressure Net filtration pressure, then, is the difference between net hydrostatic pressure and oncotic pressure.