The cardiovascular system is an organ system responsible for the circulation of blood and the transport of oxygen, carbon dioxide, nutrients, hormones, blood cells, and fluid throughout the body.
This system includes the arterial system that transports oxygenated blood via arteries and capillaries from the heart to all parts of the body and the venous system that transports deoxygenated blood via veins back to the heart.
In addition, the vascular system includes the lymphatic system, which is a network of lymphatic vessels transporting lymph fluid directionally towards the heart thereby clearing the body of waste products, toxins, and dispensable materials.
The average-size adult has about 4 to 6 liters of blood which are pumped through the body almost 1500 times a day. With approximately 7000 liters of blood that need to be returned to the heart every day, the venous system has a difficult job to do, especially because it must overcome the force of gravity to transport the blood upwards.
In the venous system, we differentiate between the superficial and the deep vein system. The superficial venous system makes up only 10% of the venous system and can be described as a network of smaller veins primarily located in the subcutaneous tissue layer between skin and muscles. The deep vein system, which is located within the muscle tissue, is responsible for transporting approximately 90% of the blood, with help of the surrounding muscles, from the extremities towards the heart.
As superficial veins are not directly surrounded by muscles, blood cannot be moved through muscle contraction, however perforator veins (or connector veins) exist. These connect the superficial vein system to the deep vein system, enabling blood to drain from the superficial veins into the deep vein system.
Due to the decreasing blood pressure in the more distal vessels, the most important mechanisms in transporting blood against gravity towards the heart in the lower extremities, are the lower leg muscle pump system (e.g. foot muscle pump, calf muscle pump, gastrocnemius pump, and thigh pump) and the venous valves.
Muscle pump promotes blood transport towards the heart
When we walk, the muscles of the leg repeatedly contract and relax. Every time these muscles contract, they compress the veins of the deep vein system in the legs, promoting blood transport towards the heart.
Vein valves close to prevent blood flowing backwards
The calf muscle pump is the strongest muscle pump of the lower leg muscle pump system. Veins are equipped with crescent-shaped vein valves dividing the vessels into smaller segments. Venous valves consist of two U-shaped elastic tissue flaps that open as soon as the muscles contract, forcing the blood upwards to the next segment. If the muscle pump is at rest and the pressure in the veins is reduced, it creates a backflow of blood that fills the U-shaped valves, closing them firmly. As the closed and intact vein valves form a physical barrier, they prevent the blood from flowing backwards.
The calf muscles compress the deep veins to pump the blood towards the heart. Backflow of blood into the U-shaped valves closes them, preventing the blood from flowing further backwards.
The lower leg muscle pump system, including the calf muscle pump, is only activated when you use your muscles. Whenever your muscles of the leg and feet are working, for instance while walking or running, the muscle pump system is active. Moving your legs keeps the muscle pumps working to ensure good blood circulation in your legs.
Constant standing or sitting impedes the flow of blood towards the heart. Under certain circumstances, this can lead to venous insufficiency, which is characterized by improperly functioning vein valves that interfere with venous return and cause the blood to pool in the veins. Venous insufficiency can be the starting point of different venous disorders, including common spider or varicose veins, but also more severe venous disorders, such as edema, skin changes, and ulcerations.
Lymph is formed from the fluid that filters out of the blood circulation into the interstitium and taken up by the blind-ended lymph-sinuses. The lymph consists of interstitial fluid, proteins (smaller than albumin), fibrinogen, and other coagulation factors, small molecules and ions of the serum and interstitium, leucocytes, immunoglobulins, fat in form of chylomicrons, cellular debris, waste products, and bacteria.
The lymphatic system is responsible for the majority of fluid uptake from the interstitial spaces. This collection of fluid is carried out by the initial lymph-sinuses, that are blind-ended epithelial lined vessels with fenestrated openings that allow fluids and particles as large as cells or proteins smaller than albumin to enter.
The fluids are then sucked and pressed into lymph pre-collectors. The lymph collectors and continually larger lymph vessels have a lymph valve system and lymphangiones (e.g. enlarged units with smooth muscle cells between two lymph vessels).
Through active contraction of the lymphangiones and restricted back flow of lymph due to the lymph vales, the lymph is transported in proximal direction through the lymph vessels and through lymph nodes.
Hundreds of lymph nodes are located within the human body (e.g. around the neck, intestinal tract or in the axilla or groin area) and are responsible for the removement of debris, regulation of the protein content of the lymph, the immune response, recirculation of lymphocytes, and re-absorption of water (approximately 5 to 8 liters per day).
Every day the lymphatic system takes up approximately 10 liters of interstitial fluid.