Blood vessels

The human body requires a mechanism to distribute oxygen and nutrients to the tissues and to collect carbon dioxide and other waste products from them. The blood is the vehicle for this exchange, and it circulates via the blood vascular system. This consists of a pump, the heart, and two continuous systems of blood vessels.

The pulmonary circulation carries blood to and from the lungs where it is oxygenated, and the systemic circulation distributes blood to all other tissues and organs of the body. In both circulations the blood, pumped from the heart, passes through ever smaller arteries to the capillary network where exchange takes place and then through ever larger veins that take the blood back to the heart.

Arteries

Arteries are the vessels that carry oxygenated blood from the heart to the tissues. They are thick-walled muscular tubes designed to take the high pressures of blood propelled by the powerful ventricles of the heart. They are capable of widening or narrowing in order to control pressure and flow in various parts of the body.

Arteries can be felt at certain pulse points, but they are less often seen. They pulsate with every heartbeat and cannot be squeezed shut using ordinary pressure. Artery walls are made up of three basic layers. The inner layer (intima) contains elastic and fibrous elements and is lined by a smooth layer of cells called endothelium.

The middle layer (media) contains muscle cells and gives the artery the ability to expand and contract. The outer layer (adventitia) contains elastic fibres and connective tissue to form a tough protective coat. The amount of each element varies according to the size and function of the artery. The large aorta (25 mm in diameter) contains many elastic fibres, allowing it to stretch in order to take the large volume of blood forced out of the left ventricle. However it can barely constrict and actively narrow its diameter. The smaller arteries contain more muscle fibres, enabling them to change diameter and so actively influence the pressure and blood flow. In this way blood can be distributed selectively to different tissues, according to need. The smallest arteries (arterioles) are less than 0.1 mm in diameter. The pulmonary artery is unusual in that it carries venous (deoxygenated) blood from the heart’s right ventricule to the lungs. It is short and wide (5 cm long by 3 cm in diameter) and branches under the arch of the aorta to form the left and right pulmonary arteries, which lead to the lungs.

Cross-connections between arteries

Larger arteries are sometimes connected so that tissues can be supplied with blood by means of more than one artery. These mutual connections are called arterial anastomoses or collaterals. They are important when one of the arteries is closed off – for example, because of a blood clot (thrombosis). It is then possible for the other artery to carry the blood supply to the tissue suffering from the closure. Such a system of anastomoses is founded at the base of the brain. The blood supply to the brain is regulated by two pairs of arteries; the internal carotid and the vertebral arteries. Each of these arteries branches in the skull and supplies parts of the brain with blood. The so-called circle of Willis connects these two pairs of arteries, and thus connects the blood supply between the left and right sides of te brain. Should one of the four arteries be closed by a blood clot, blood may still reach the area fed by the artery concerned, via this artery circle. In this way, degeneration of brain tissue, which is very sensitive to the lack of oxygen, may be prevented, because the brain will still be supplied with oxygen-rich blood.

Capillaries

The capillary network is the place where actual exchange takes place. Oxygen, hormones, fluids and nutrients such as the blood sugar glucose reach the tissues along arteries of smaller and smaller diameter and waste products from the tissues (such as carbon dioxide) return to the blood along veins that get wider and wider. The capillary walls are only one cell layer thick, and these substances easily diffuse or are transported through the capillary walls. Most cells however, are too large and cannot pass through.

Veins

The role of the veins is to collect ‘used’ blood and to return it to the heart. There are far more veins than arteries and their diameter is usually greater, so the blood in veins is under only a tenth of the pressure of that in arteries. Therefore they have thinner walls. Their walls contain some elastic and fibrous elements and a few muscle fibres, but these are much less abundant than in arteries. They have a smooth endothelial lining and the inside walls have small valves to prevent blood flowing in the wrong direction, away from the heart. The valves look like a pair of pockets projecting into the vein, the hollow of the pocket opening in the same direction as the direction of the flow. They are necessary, especially in the lower limbs, because without them blood would flow backwards under the force of gravity. The blood on the venous side of the circulation does not rely on the pumping action of the heart. Therefore veins cannot be felt to pulsate, and they can easily be squeezed shut. The venous blood is merely collected from the tissues and propelled in the direction of the heart by three mechanisms.

First there are the valves that prevent the blood from flowing back. Then it is helped along by the action of the leg muscles for instance, as they contract with movements and exercise. This is called the muscle pump. The bigger veins often run alongside the arteries; as the arteries pulsate and dilate they help squeeze the blood through the veins. Not all the blood passes through all the capillaries at the same time; rather it is distributed according to need. The tiny arteries and arterioles may on occasions by-pass less-used tissues, sending the blood directly to the venous side of the circulation. This is called arteriovenous anastomosis. Unlike other veins, the pulmonary veins carry arterial (oxygenated) blood from the lungs to the heart. There are four pulmonary veins, two from each lung, and they connect with the heart’s left atrium. The pulmonary veins are also unusual because they have no valves. Most other veins – the portal vein is a notable exception – in the body link with either the superior vena cava (carrying blood from the head, neck and arms) or the inferior vena cava (carrying blood from the lower part of the body). These two great veins pass the blood into the right atrium of the heart. The portal vein carries venous blood containing the products of digestion from the intestines to the liver. After passing through a capillary network in the liver, the blood is ducted along the hepatic vein, into the inferior vena cava and hence to the heart.