blood vessels

blood vessels are the system of branching and converging tubes which convey blood from the heart to all the various parts of the body and back again, and from the heart to the lungs and back (see blood circulation). The size of blood vessels varies enormously, from a diameter of about 25 mm (1 inch) in the aorta to only 8 μm in the capillaries. This is a 3000-fold range.

The thickness of blood vessel walls also varies enormously, being largest in the large arteries, much less in veins of comparable diameter, and only a single cell thick in the capillaries. Despite the range of sizes the components of the blood vessel walls have a common pattern. All vessels are lined with a single layer of flattened cells called the endothelium. Except for capillaries, all vessels also contain elastic fibres, stiff collagen fibres (similar structure to muscle tendons), and smooth muscle fibres which can constrict or dilate in response to chemical and nervous stimuli. The relative proportions of these components vary in different blood vessels in accordance with their functions.

Recently, the endothelium has been recognized to be of importance in the regulation of the state of constriction or dilatation of the vessels themselves. Of particular note in this respect is ‘endothelial derived relaxation factor’, later shown to be nitric oxide: when this is released, notably in response to the shearing force of the blood on the vessel, it causes dilatation of the vessel.

Large arteries

The aorta and its main branches are called elas-tic arteries. Although they also possess fibrous collagen tissue and smooth muscle, about half of their structure is composed of elastic fibres. These give large arteries a characteristic pale yellow colour. Their wide bore means that they offer little resistance to blood flow, so there is little pressure drop throughout the system of major arteries. The physiological significance of the elastic fibres is that they allow the vessels to expand when blood is ejected intermittently into them from the heart and to constrict again as blood flows out of them into the smaller vessels. The combination of a distensible large vessel and a downstream resistance (arterioles) transforms an intermittent cardiac ejection into a continuous capillary flow.

Small arteries and arterioles

These are the resistance vessels of the circulation and are responsible for determining blood pressure. Arterioles are the vessels at the end of the arterial tree and have a diameter of 20 to 30 μm. Their particular significance is that they have very thick walls in relation to their diameters. Furthermore, the main constituent in their walls is smooth muscle, and the degree of contraction of this muscle regulates the diameter of the vessels and consequently the amount of blood flowing through them. Arterioles are responsible for the largest pressure drop in the circulation. Blood pressure in arteries typically varies from 120 to 80 mm Hg, depending on the phase of the cardiac cycle. In capillaries, the pulsatility is lost and pressure is only about 30 mm Hg.

The muscle in the walls of arterioles possesses an inherent tone. That means that they are normally partly contracted, reducing the size of the lumen to less than the widest possible. The degree of contraction is modified by factors external to the vessels. In particular, the chemical products that are formed as tissues use up energy — the ‘metabolites’ — reach the muscle fibres in the walls of the arterioles and cause them to relax and dilate. This local vasodilatation has the effect of matching local blood flow to tissue energy requirement.

Arterioles can also be regulated by nerves and hormones. These effects tend to be widespread and are concerned mainly with the regulation of arterial blood pressure. Sympathetic nerves have an important role in the control of arterioles. As the frequency of sympathetic nerve impulses increases, more of the transmitter, noradrenaline, is released at the nerve endings, and this causes arterioles to constrict. The adrenal glands also release noradrenaline into the blood but their secretion is mainly of adrenaline. Adrenaline also constricts blood vessels — except those in skeletal muscle, where it dilates them. This diverts blood to the muscle and prepares the body for emergencies as part of the ‘fight or flight’ response.

Capillaries

These are the ‘exchange vessels’, allowing passage of substances between blood and the fluids outside them which surround the body cells. They consist of a single layer of endothelial cells, with microscopic spaces between adjacent cells which allow the solutes of the blood, including salts, glucose, and dissolved oxygen, to pass into the tissues, and products of tissue metabolism, including carbon dioxide, to pass into the blood. The number of capillaries is so vast that even though they are microscopic their overall resistance to blood flow is low and blood passes through them slowly. The high density of capillaries means the distance for diffusion by the nutrients and gases is small. The more active tissues tend to have a denser supply of capillaries.

Capillaries are formed as a complex system of branching blood vessels between arterioles and venules (microscopic veins). Those near the arteries are at a higher pressure than those near veins. The gaps between endothelial cells are small enough to be almost impermeable to the protein molecules present in the blood, causing the capillary bed to function as a semipermeable membrane. These molecules exert an osmotic force which tends to draw fluid from the tissue spaces into the capillary. This is opposed by the hydrostatic pressure forcing fluid out. A dynamic equilibrium is established, such that at the higher pressure capillaries fluid leaves the circulation, and at the lower pressure ones it is drawn back in. An additional system of vessels, the lymphatics, are fine tubes which provide an alternative route for tissue fluid, via the lymph nodes and back to the circulation.

Disturbance of the balance of the fluid exchange at capillaries can lead to oedema, which is swelling caused by excess tissue fluid. Major causes of this are: a generalized increase in tissue fluid as in heart failure; obstruction to flow through veins or lymphatic vessels such as by cancer growths; and deficiency of blood protein, as in liver or kidney disease or malnutrition, which reduces the osmotic reabsorption force.

Veins

Blood returns from the tissues to the heart along veins. Larger veins possess valves which ensure that blood travels in the correct direction and prevents the development of undue back pressure. Sometimes the valves may cease to function, causing veins to distend abnormally and permanently. This is the cause of varicose veins.

Veins have another important role in addition to being conduits. Approximately 70% of the entire blood volume is contained within the veins, and these are very distensible. This means that they can readily accommodate quite large changes in their volume, either as a result of a change in the total quantity of blood in the circulation (haemorrhage or transfusion), or because of changes in blood distribution (leg veins distend on standing up, for example). The reason that veins can change their volume with little change in pressure is partly because they collapse when empty, which applies to veins above heart level. When filled, the elastic tissue in their walls is readily distensible, although expansion is eventually limited by the relatively indistensible fibrous tissue (collagen).

There is another, active, way in which the volume of blood in veins can be controlled: some veins have the ability to constrict in response to nerve stimulation. Sympathetic nerves supply smooth muscle in the vein walls, and an increase in sympathetic activity, resulting for example from a decreased stimulus to baroreceptors (falling blood pressure), causes venous volume to decrease. The effect of this is to increase filling of the heart and to enhance its output.

Pulmonary vessels

Although the total flow of blood per minute through the lungs is the same as that through the systemic circulation, the pressures are very much lower. Pressure in the pulmonary artery is typically 25/12 mm Hg (systolic/diastolic) compared with 120/80 mm Hg in the aorta and its main branches. The pressure in the lung vessels is lower because they are shorter, wider, have less muscle in their walls, and are very numerous. In particular, there are no muscular resistance vessels like those in the systemic circulation. The pulmonary vessels form a vast low-resistance capillary network which encircles the microscopic air-sacs (alveoli). Gas exchange — of oxygen for carbon dioxide — takes place between blood in the pulmonary capillaries and air in the alveoli.

Roger Hainsworth

See cardiovascular systemSee also blood circulation; blood pressure; body fluids; lymphatic system; microscopy.