of elastic fibres and smooth muscle cells that encircle the blood vessel. Here, the fibres of the autonomic nervous system — that is, the part of our nervous system that we don’t consciously control — regulate the dilation or constriction of the vessel by tensing or relaxing the smooth muscle cells. Of course, the more dilated a vessel is, the more blood can flow through it.
The outermost layer of blood vessels is made up of connective tissue fibres, which anchor the vein or artery to the surrounding parts of the body. This layer houses those nerves that control the smooth muscles in the middle layer. But blood vessels also need oxygen themselves. This is provided by a network of tiny blood vessels, called the vasa vasorum. These ‘blood vessels of the blood vessels’ are also contained in the outer layer.
The arteries are like the sporty types within our bodies, while the veins are the couch potatoes. Their layered structures are basically the same, but the arteries are considerably more muscular. By the same token, veins have a larger internal diameter. These differences are due to the fact that the pressure inside our arteries is higher and they must be able to resist that pressure to avoid blowing up like wobbly, water-filled balloons.
Arteries can be divided into three types: elastic, muscular, and the smallest branches of the arterial system, the arterioles. The elastic arteries tend to be those closer to the heart, and the best-known artery of this type is the body’s largest: the aorta, our main artery. It’s about as thick as a garden hose. When the heart beats, the aorta dilates to accommodate a rush of extra blood, before contracting again to maintain internal pressure. Medics call this the Windkessel effect,* and it helps significantly in reducing fluctuations in the flow of blood to the peripheral areas of the body.
So, the arteries change their size by tensing or relaxing the muscles of their walls, and this regulates the amount of blood flowing to our muscles and organs. When they have almost reached their destination, the vessels branch out more and more to form arterioles. These get smaller and smaller until their walls are no longer made up of three layers, but just one layer of smooth capillary epithelial cells. At this level and smaller, scientists speak of capillaries. Every part of the body that has a blood supply contains a very extensive interwoven network of these tiny blood vessels, which can be so narrow that blood corpuscles* can only travel down them in single file, one behind the other.
The capillaries form the connection between the high-pressure arterial system and the low-pressure venous system. And since their walls are only one cell thick, oxygen can flow out of them and into the surrounding tissue much more easily than from other blood vessels. In fact, the endothelium is so porous that when tissue is infected and inflamed, white blood cells — which can be quite chubby little chappies — can leave the bloodstream through them. Eventually, the blood removes the carbon dioxide that has accrued in the body’s cells and flows with it through venules and ever-larger veins, back to the heart.
Apart from a few exceptions, there is a clear division of labour between the arteries and the veins. In general, arteries transport oxygenated blood away from the heart, while veins take deoxygenated blood back to it. The exceptions to this rule are veins that transport blood directly from one organ to another without going via the heart — for instance, the portal venous system of the liver. This is the system that transports blood from the gut straight to the liver before it continues on to the heart. And that’s practical, because some of the toxins we ingest along with our food are broken down in the liver before they can cause any damage in the rest of the body.
As we have seen, the pulmonary vein and artery are also exceptions to the general rule. The pulmonary artery, like all its fellow arteries, leads away from the heart. However, it does not transport oxygenated blood, but rather blood that is on its way to the lungs to be enriched with oxygen. This blood then flows, packed full of oxygen, from the lungs, along the pulmonary vein, to the left atrium of the heart, where it is finally thrust out into the rest of our body via the left ventricle and the aorta, our main artery. This thrust is what we feel as our pulse.
The fact that our arteries are rarely to be found close to the surface of our bodies is a clever trick on the part of evolution, since a damaged artery bleeds heavily. Imagine the mess when you cut your finger while chopping carrots — and the risk of bleeding to death if you’re unlucky. However, since our arteries are buried deep in our tissues, it takes more than a scratch to damage them.
Having survived a cut finger without bleeding to death, we need to ask how our blood gets from the tips of our fingers back to the heart. After all, it needs to return to the lungs to be recharged with oxygen. Before reaching the right atrium, it gathers in two large blood vessels, the superior and inferior vena cava. The superior vena cava receives blood from the upper body, the arms, and the head, while its ‘inferior’ counterpart gathers blood from the abdominal organs, the legs, and the torso.
But how does the blood in the veins of our lower legs manage the 130-or-so centimetre climb to the heart? This is only possible because our veins are equipped with small valves, positioned every few centimetres, that open for blood flowing towards the head but not the other way. Like the valves of the heart, they stop blood flowing back in the undesired direction. In addition, when we move, the muscles surrounding the veins do the rest of the work, pressing blood up towards the heart. This effect is appropriately known as the skeletal-muscle pump.
As we get older, more and more of our venous valves may stop working properly. When one valve gives up the ghost, the pressure is increased on the still-intact valve immediately below it, and the section of vein between them swells up. One unsightly result of this can be varicose veins, although they can also be caused by a general weakness of the connective tissue. This is often also the cause of another unpleasant vascular problem: haemorrhoids, which occur when the veins and arteries of the rectum swell and cause bleeding and itching round the back door.
However, it’s not only the venous valves and the skeletal-muscle pump that are important in transporting blood back to the heart; this process is also aided by the body’s respiratory pump. When blood has eventually made it back to the chest area, our breathing muscles help to transport it into the right atrium. This works because, during abdominal breathing, the pressure in the chest sinks as we suck air into our lungs, and that allows the inferior vena cava to take in blood more easily from the lower body. When we then exhale, the pressure on those vessels rises again, and the blood is literally squeezed into the right atrium of the heart.
As long as all these systems are working properly and all parts of the body are well supplied with blood, there’s usually nothing to worry about. Our cells get what they need and we carry on merrily with our lives. But it would be too good to be true if those systems were not prone to error. And, indeed, just like actual highway systems, the cardiovascular system is susceptible to congestion and, when push comes to shove, even gridlock.
*The word ‘cardiogenic’ comes from the Ancient Greek words kardia, meaning ‘heart’, and genesis, meaning ‘origin’ or ‘creation’.
*Pulmo is Latin for ‘lung’.
†See also, ‘The Holey Heart’, p. 270.
*See p. 34 for more on the portal venous system.
†Meaning ‘two-tipped’.
*Meaning ‘crescent-shaped’.
*Also called ‘mitral insufficiency’, i.e. a failure of the mitral valve to close properly.
†Ventricular dilatation.
*From the German for ‘air chamber’, part of a water-pumping system.
*Blood cells.
