Microcirculation

Blood leaving the heart travels through large arteries into progressively smaller vessels. Microcirculation refers to the circulation and processes within the smallest vascular network, which consists of arterioles, capillaries, and venules (also discussed in Fundamentals of blood, lymph and edema).

• Arterioles — Act as gates that regulate the amount of blood entering tissues.
• Capillaries — Have very thin walls and serve as pathways for the exchange of oxygen, nutrients, carbon dioxide, and waste products. They are so narrow that red blood cells can barely pass through in single file.
• Venules — Collect blood after exchange has taken place and serve as the entry point for its return to the veins.

As blood passes through this network, oxygen and nutrients are delivered to tissues, carbon dioxide and metabolic waste products are collected into the bloodstream, and fluid movement between tissues and blood vessels is also regulated (Capillary exchange).

Pressure inside blood vessels, pressure in the surrounding tissues, vessel wall permeability, and signals from vascular endothelial cells work together to determine the direction and amount of fluid movement into and out of blood vessels. Starling forces describe the physical basis of this balance and provide a foundation for understanding why swelling occurs.

Blood vessels continually widen and narrow in response to temperature, activity, and the energy needs of tissues. Examples include increased blood flow to muscles during exercise and reduced blood flow to the fingertips on cold days. This regulation is a normal part of the body's everyday responses.

Because the legs are below the heart, venous blood and lymph must travel upward against gravity. Each time the calf muscles contract, they squeeze the veins within and around them, pushing blood upward. This is the skeletal muscle pump (calf muscle pump). When prolonged standing or sitting reduces this pumping action, blood pools in the lower legs, causing a feeling of heaviness and swelling. This is also why shoes may feel tight in the evening. Heaviness and dependent edema are discussed in this context.

“Peripheral circulation” broadly refers to blood flow to the body's outer tissues, such as the hands, feet, skin, and muscles, rather than the large vessels near the heart. The flow of blood to tissues through arteries and back through veins is a separate system from the lymph flow that collects tissue fluid.

Peripheral blood flow varies with temperature, posture, activity, and autonomic nervous system responses. Exposure to cold increases sympathetic nervous system activity, narrowing blood vessels near the skin (Vasoconstriction and vasodilation) to reduce heat loss. During exercise or in hot conditions, the vessels widen instead to release heat. Cold or reddish-looking hands and feet are therefore often temporary changes related to the environment and activity.

Recurrent coldness in the hands and feet with noticeable color changes is discussed separately in Cold intolerance and cold extremities and Raynaud phenomenon.

• Move frequently. At least once an hour, get up and walk or move your ankles up and down to support the calf muscle pump.
• Rest with your legs raised. Before bed, leg elevation, with your legs above heart level, can ease the feeling of pooling.
• Walk regularly. Regular exercise is associated with greater development of capillaries within muscles.
• Keep your body warm. Keep your hands and feet warm on cold days, and warm up with a bath or foot bath.
• Avoid tight clothing and shoes. Choose loose-fitting clothes so that blood and lymph flow are not compressed.

Have an assessment at a hospital if just one leg suddenly swells and becomes painful or red; if swelling does not subside with rest; or if your hands or feet repeatedly turn white or blue.