Massage Therapy Journal Winter 2024

Winter 2024 • 45

The semipermeable membrane of the blood capillary allows this vital function to take place. Diffusion is the most important process in the nourishment of tissues and cells. Alongside diffusion, another process known as filtration occurs, which forces fluid (water) out of the blood capillary into the interstitium. Blood capillary pressure (BCP), also known as hydrostatic pressure, is higher within the blood capillaries than the pressure of the interstitial space, therefore a movement of fluid occurs from the capillary to the interstitium. The rate of filtration can change according to changes with BCP. The muscle walls of the smaller arterioles leading to the blood capillaries are regulated by the sympathetic nervous system (fight/flight) and maintain the tone of these vessels. Oxygen concentration, metabolism of tissues and thermal and hormonal changes affect the dilation and constriction of arterioles, which in turn regulates the volume of blood and rate of filtration. Increased activity resulting in the body needing more oxygen and nutrients causes the

precapillary arterioles to dilate, allowing for an increase in blood volume. This in turn increases both the amount of interstitial fluid and the rate of filtration, which leads to an increased lymphatic load. This entire process is known as active hyperemia . Many factors can create active hyperemia, including inflammation, massage and the application of heat. Passive hyperemia occurs when there is a blockage within the venous system causing a “backup” or “pooling” within the veins. In this situation, both BCP and the rate of filtration will increase, leading to an increase in the lymphatic load. When the rate of filtration from blood capillaries exceeds the rate at which the lymphatic vessels can remove the excess fluid from the interstitium, edema results. Changes in the rate of filtration, capillary permeability and a reduced colloid osmotic pressure can all influence interstitial fluid volume. Edema can result from any of the following conditions: • Congestive heart failure

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