Blood and lymph flows through circulatory system and lymphatic system respectively. The circulatory system consists of the heart and blood vessels through which the blood circulates.
The lymphatic system consists of lymph nodes, lymph organs and lymph vessels, through which colorless lymph flows. It consists of central and peripheral lymphoid system. Central lymphoid system includes thymus and bone marrow and peripheral includes lymph nodes, spleen, lymph vessels, etc.
COMPONENTS OF CIRCULATORY SYSTEM (FLOW CHART 1.1)
- Systemic circulation (Fig. 1.1): The blood pumped out from the left ventricle is carried by the branches of the aorta around the body and is returned to the right atrium of the heart by the superior and inferior vena cava.
- Pulmonary circulation: This consists of the circulation of blood from the right ventricle of the heart to the lungs and back to the left atrium. In the lungs, carbon dioxide is excreted and oxygen is absorbed.
- Coronary circulation: The blood is circulated through coronary arteries, which is a branch of ascending arteries and supplies the heart itself and returned to right atrium. Compare to other circulation heart receives its blood supply during diastole.
The heart pumps blood into vessels that vary in structure, size and function, and there are several types of blood vessels—arteries, arterioles, capillaries, venules and veins (Table 1.1).
Arteries and Arterioles
Arteries and arterioles are the blood vessels that transport blood away from the heart. They vary considerably in size and their walls consist of three layers of tissue:
- Tunica media or middle layer of smooth muscle and elastic tissue
- Tunica intima or inner lining of squamous epithelium called endothelium
Arterial walls are thicker because of more smooth muscles compared to veins. In the large arteries, elastic arteries, the tunica media consists of more elastic tissue and less smooth muscle. In the arteries, the tunica media consists almost entirely of smooth muscle (Fig. 1.2).
Anastomoses and End-arteries
Anastomoses are arteries that form a link between main arteries supplying an area, e.g. the arterial supply to the palms of the hand and soles of the feet, the brain, joints and to a limited extent, the heart muscle. When there is a block in the arteries, these arteries ensures the circulation is bypassed through a collateral circulation. End-arteries are the arteries with no anastomoses or those beyond the most distal anastomoses, e.g. the branches from the circulus arteriosus (circle of Willis) in the brain or the central artery to the retina of the eye. When an end-artery is occluded the tissues it supplies die because there is no alternative blood supply.
The smallest arterioles break up into a number of minute vessels called capillaries. Blood cells and large-molecule substances such as plasma proteins do not normally pass through capillary walls. The capillaries form a vast network of tiny vessels, which link the smallest arterioles to the smallest venules (Fig. 1.3). The capillary bed is the site of exchange of substances between the blood and the tissue fluid, which bathes the body cells.
Venules connect the capillaries (arterial system) to the venous system. Venules are very small in diameter and oval-shaped at rest. When pressures increase, they become more circular. While blood vessels typically have three distinct layers, the end venules have only an endothelial layer with a thin layer of collagen fibers (tunica intima).
As venules increase in diameter and become visible veins, they have the typical three layers mentioned earlier. Veins in the upper extremities and lower extremities are superficial and deep. The two superficial vein extremities are of upper extremities are cephalic and basilic vein. The deep veins are named corresponding to the accompanying arteries or arterial system at the similar level of the vascular system. The basilic vein originates from the dorsal venous network of the hand. It ascends the medial aspect of the upper limb. At the border of the teres major, the vein moves deep into the arm. Here, it combines with the brachial veins to form the axillary vein. The cephalic vein arises from the dorsal venous network of the hand. It ascends the anterolateral aspect of the upper limb, passing anteriorly at the elbow. At the shoulder, the cephalic vein travels in the deltopectoral groove and enters the axilla. Within the axilla, the cephalic vein terminates by joining the axillary vein. In cubital fossa, they are connected by median cubital vein, which is common site of intravenous (IV) injection. Another site for IV is dorsal venous arch of hand. Median cubital vein is commonly affected with superficial thrombophlebitis and dorsal venous arch is a common site for cellulitis.
There are three different types of lower extremity veins, which are superficial, perforating, and deep veins. The superficial veins conduct blood from the skin and subcutaneous tissue. The perforating veins connect the superficial veins to the deep veins that convey blood from the periphery to the heart. One anatomical structure the perforating, superficial and deep veins have in common is valves. The superficial veins have fewer valves than the deeper veins. Vein valves are bicuspid and avascular. They consist of thin sheets of collagen and smooth muscle with an endothelial covering. Valves appear to become less flexible as people age. The valves prevent retrograde blood flow, thus allowing veins to overcome gravity effects (Figs 1.4A and B).
The main, superficial leg veins include the greater saphenous, lesser saphenous, and the lateral (subdermis) venous system. Descriptions of saphenous veins are listed in Table 1.2 and their difference is given in Table 1.3. These veins are very thin walled and distensible. The superficial veins lie above the main fascia plane and are the primary blood collection system for the lower leg. They lack the extensive fascial restriction experienced by the deep veins. Consequently, superficial veins may undergo dramatic volume changes or distention. The lateral venous system (lateral subdermic) above/below the knee is a common area for varicosities during pregnancy and occasionally puberty.
Figs 1.4A and B: (A) Valve in a closed position which prevents retrograde flow; (B) Valve in open position which allows proximal flow
The superficial veins deliver blood to the deep veins, such as the femoral and popliteal veins. However, the superficial system also connects to perforating veins.
The perforating veins penetrate fascia and connect the superficial venous system to the deep veins (Table 1.4). Muscle contraction produce pressure that assists blood movement from perforators into the deep veins (muscle pump). Perforating veins have valves, which prevent retrograde blood flow, i.e. back to superficial. The lower leg and foot have more perforating veins than the upper leg.
In the legs, deep veins often run parallel to superficial veins. The perforating veins connect superficial veins to deep veins like slanted rungs of a ladder. The deep veins are named corresponding to the accompanying arteries or arterial system at the similar level of the vascular system.
Functions of Vein
- To carry blood to the heart/lungs for gas, nutrient and waste exchange.
- Storage of large blood volume.
- Veins are barriers between intravascular and extravascular tissues. The proximal venules allow movement of interstitial fluids, large molecules and white blood cells through the venule wall.
- Vein walls also have cellular functions. White blood cells attach to vein endothelium in order to be available should there be an injury or disease process.
- Veins also are a factor in cardiovascular pressures. In the capillary bed, veins influence arterial output resistance. Vein walls produce nitric oxide that causes vasodilation via decreased vascular tone.
- Veins also have a role in heart filling pressure. The skeletal muscle pump, venous myogenic response and venous smooth muscle tone prevent orthostatic hypotension.
- The venules also appear to be a site of angiogenesis.
- Venous endothelium has a role in lessening platelet aggregation to prevent clot development via the formation of prostacyclin.
- Other unique or regional functions:
- Facial veins allow for blushing and temperature regulation of the head.
- Cutaneous veins help regulate body/skin temperature.
- The internal jugular vein facilitates cranial pressure regulation.
- Proximal portion of the vena cava and pulmonary veins may play a role in cardiac pacing. Their tunica media layers have cardiac myocytes that might provide cardiac pacing during some pathological conditions.
Mechanism, Which Helps in Back Flow of Blood from the Legs to the Heart
- Pressure from behind: The slight pressure pushing blood onwards from the capillaries into the venules. Although this is sufficient when we lie down, it is totally inadequate when we are standing or walking.
- Suction effect of the lungs: When we take a deep breath a negative pressure, rather similar to a vacuum, is created in the chest that helps to draw blood upwards to the heart.
- Pumping action of the leg muscles: These muscles are enclosed in a dense sheath of fibrous tissue (called the deep fascia). When you walk about, the muscles contract and the veins contained in the fibrous sheath are squeezed.
- Valves in the deep vein: At intervals along the insides of the veins are beautifully constructed valves, consisting of two flaps that meet each other exactly. Though very simple they are quite sufficient nevertheless to make sure that the blood can be squeezed in one direction only back to the heart.
- Valves in the perforating vein: The veins that drain these superficial tissues are linked up with the deep veins through a number of perforations in the fibrous sheath. Each linking or communication vein is guarded by a one-way valve. As we walk, the muscle pump squeezes the deep veins in our legs; blood is pushed upwards towards the heart; and the negative pressure within the deep veins sucks in blood through the communicating veins from the skin and the fat.
- Controlled venous diameter, high pressure on the arterial side, and the low right atrium pressure that draws blood from the great veins.
LYMPHATIC CIRCULATION (FIG. 1.5)
In the interstitial space, majority of the tissue fluid drains to their venous end whereas the remainder diffuses through permeable walls of the lymph capillaries and becomes lymph. Lymph is a clear watery fluid, similar in composition to blood plasma, with the important exception of plasma proteins and identical in composition to interstitial fluid.
The lymphatic system consists of:
- Lymph vessels
- Lymph nodes
- Lymph organs, e.g. spleen and thymus
- Diffuse lymphoid tissue, e.g. tonsils
- Bone marrow.
The lymphatic system function are to drain tissue fluid, plasma proteins and other cellular debris back into the bloodstream, and is also involved in immune defense. Fat and fat-soluble materials, e.g. the fat-soluble vitamins, are absorbed into the central lacteals (lymphatic vessels) of the villi.
Lymph capillaries originate as blind-end tubes in the interstitial spaces. They have the same structure as blood capillaries, i.e. a single layer of endothelial cells, but their walls are more permeable to all interstitial fluid constituents, including proteins and cell debris. The tiny capillaries join up to form larger lymph vessels.
Larger Lymph Vessels
The walls of lymph vessels are about the same thickness as those of small veins and have the same layers of tissue, i.e. a fibrous covering, a middle layer of smooth muscle and elastic tissue and an inner lining of endothelium. Lymph vessels have numerous cup-shaped valves, which ensure that lymph flows in one way only, i.e. towards the thorax. There is no ‘pump’, such as the heart, involved in the onward movement of lymph, but the muscle tissue in the walls of the large lymph vessels has an intrinsic ability to contract rhythmically (the lymphatic pump). In addition, any structure that periodically compresses the lymphatic vessels can assist in the movement of lymph along the vessels, commonly including the contraction of adjacent muscles and the pulsation of large arteries. Lymph vessels are divided into superficial and deep lymphatic vessels. Lymph vessels become larger as they join together, eventually forming two large ducts, the thoracic duct and right lymphatic duct, that empty lymph into the subclavian veins.
Thoracic duct begins at the cisterna chyli, which is a dilated lymph channel situated in front of the bodies of the first two lumbar vertebrae. The duct is about 40 cm long and opens into the left subclavian vein in the root of the neck. It drains lymph from legs, the pelvic and abdominal cavities, the left half of the thorax, head and neck, and the left arm.
Right Lymphatic Duct
Right lymphatic duct is a dilated lymph vessel about 1 cm long. It lies in the root of the neck and opens into the right subclavian vein. It drains lymph from the right half of the thorax, head and neck and the right arm.
Lymph nodes are oval or bean-shaped organs that lie, often in groups, along the length of lymph vessels. The lymph drains through a number of nodes, usually 8–10 nodes, before returning to the venous circulation. Function of lymph nodes includes filtering, phagocytosis and proliferation of lymphocytes.