of blood flow. The tunica media is much thicker in arteries than in veins since arteries are usually carrying blood under high pressure and their walls require extra reinforcement. The smooth muscle layers are innervated by sympathetic nerve fibres which are under the influence of the vasomotor centre within the medulla oblongata of the brain. This is the region of the brain that regulates vascular tone and therefore blood pressure by controlling the processes of vasoconstriction and vasodilation.
The tunica intima: This is the thinnest and innermost layer of the blood vessel. It is composed of a single layer of incredibly smooth squamous epithelial cells (the endothelium) and is separated from the smooth muscle cells of the tunica media by a thin layer of collagen-rich tissue termed the lamina. In arteries the smooth, silky nature of this innermost layer affords minimal resistance, ensuring that blood flows rapidly in concentric layers (laminar blood flow).
Figure 3.7 The internal structure of an artery and vein
Arteries
Arteries are muscular, pulsatile, elastic blood vessels that circulate blood under high pressure with most carrying oxygenated blood away from the heart. The aorta is the major systemic artery and has a greater stretch than other arteries because its walls have a higher elastin content. It carries blood directly away from the left ventricle of the heart into the systemic circuit. On exiting the left ventricle, the aorta curves over the superior portion of the heart (aortic arch), delivering blood into its descending portion which branches and supplies blood to the major abdominal and pelvic organs. The major arteries of the body are typically named according to the organ or region that they supply, e.g. the hepatic artery supplies blood to the liver, the splenic artery to the spleen and the renal arteries to the kidneys. The large arteries continually subdivide into smaller and smaller vessels before eventually terminating in arterioles which are the smallest arteries of the body.
Capillaries
Arterioles supply high-pressure blood directly into complex vascular structures termed capillary beds (Figure 3.8). These function as distribution vessels ensuring that all cells within a tissue or organ are adequately perfused with oxygenated blood. It is useful to visualise capillaries as completing the circuit of blood flow by forming bridges between the arteries and veins of the body. Pre-capillary sphincters are tiny rings of smooth muscle which act as valves to regulate the flow of blood into each capillary bed; these are under the control of the autonomic nervous system and a variety of locally acting chemical signals and hormones.
Figure 3.8 Capillary bed structure
Source: OpenStax (2013) Anatomy and Physiology. Rice University. Available at: https://openstax.org/books/anatomy-and-physiology/pages/1-introduction
Haemodynamics of the capillary bed: filtration and the formation of interstitial (tissue) fluid
When the pre-capillary sphincters open, blood flows into the capillary beds under high pressure (around 35 mmHg) directly from the arterioles. Each individual capillary is composed of a tube of squamous epithelial cells.
Capillaries are just wide enough to allow erythrocytes to squeeze through and travel along their length. Erythrocytes themselves are deformable because of their biconcave structure (Chapter 9); this allows the membranes of each erythrocyte to be in close proximity to the capillary wall, increasing the efficiency of oxygen diffusion into the tissues.
The adjacent cells in a capillary have regular tiny slits/gaps in their junctions which function as crude mechanical filters. When blood is forced into these porous vessels, fluid containing low-molecular-weight molecules such as oxygen, salts (sodium, potassium calcium, chloride), amino acids and sugars such as glucose is driven out through the vessel wall by a process called filtration. This fluid is termed interstitial or tissue fluid and is continually being produced to act as a medium to deliver useful molecules to the local cells. Most cells are continually bathed in a thin layer of this interstitial fluid, which also forms a medium into which waste materials such as carbon dioxide and urea can be discharged.
During the process of filtration larger molecules such as plasma proteins are too big to fit through the porous capillary walls and are therefore retained in the capillary blood. This retention increases the osmotic potential of the blood towards the venous end of the capillary bed, which serves to pull tissue fluid, now rich in dissolved waste products, back in through the capillary walls.
The role of lymphatic vessels
Resting within the interstitial spaces of most tissues are blind-ended lymphatic vessels which absorb excess interstitial fluid. This fluid is discharged into larger lymphatic vessels where it mixes with products of fat digestion to form a milky fluid termed lymph. Lymph travels through the lymphatic vessels before eventually being discharged back into the blood (at the right and left subclavian vein) to maintain the total blood volume (explored further in Chapter 9). The lymphatic system can be regarded as a second circulatory system that runs parallel to the cardiovascular system. It is often referred to as the body’s drainage system since it plays a key role in preventing over-accumulation of interstitial fluid which would otherwise lead to oedema.
Veins
Blood exiting the venous end of the capillary bed does so under very low pressure, entering the venules which are the smallest veins of the body. Venules from multiple capillary beds join up to form larger and larger veins. Most large- and medium-sized veins are equipped with semi-lunar valves to help prevent the backflow of blood under the influence of gravity. Since the pressure in veins is so low, physical movement of the body is essential to keep blood moving and avoid venous stasis, which can increase the risk of thrombosis. During bodily movement, contraction of the major muscle groups, such as those in the legs, will squeeze the thin-walled veins, ensuring blood is kept mobile, while the valves ensure the blood flows in the correct direction towards the heart.
This mechanism is termed the skeletal muscle pump and is particularly important for ensuring venous return from the lower regions of body. All veins ultimately drain into the superior and inferior vena cavae which deliver deoxygenated blood directly to the right atrium of the heart. Since veins are thin-walled vessels, they show a high degree of compliance (ability to distend) and many of the larger veins of the body act as capacitance vessels with around 60 per cent of the total blood volume found within the venous system.
Immobility and hospital bed rest
In immobile patients, e.g. those with severe disabilities or those confined to hospital beds, the skeletal muscle pump may no longer remain active, resulting in accumulation of blood in the legs and an increased risk of static blood (venous stasis) and thrombus (clot) formation. Risk of thrombosis in hospital patients confined to bed may be reduced by encouraging as much physical movement as the patient can safely undertake or via nurse-led bed exercises or regular visits from the physiotherapist. If frailty makes exercise difficult or impossible then the use of support stockings to compress the veins of the legs can also be effective in reducing the risk of thrombosis. Some patients undergoing surgery may be given subcutaneous low-molecular-weight heparin, for example enoxaparin and dalteparin, post-operatively to reduce the risk of clot formation
