Читать книгу Heart - Johannes Hinrich von Borstel - Страница 7
ОглавлениеThe Loop in the Heart
How our heart develops, how it is structured, and how its transport routes work
The Longest Theatre Play in the World
Ba-boom, ba-boom, ba-boom, ba-boom, ba-boom. The sound of a beating heart, powerfully performing its life-preserving service day after day. It beats without a break, no matter whether we’re asleep or awake. It’s already beating on the first day of our lives and continues until we draw our last breath. But what happens to our faithful ticker in the time in between, that is, during our lifetime? The answer is actually not very complicated.
I’m a passionate theatre-goer, and it occurs to me that the experience of a heart over its average 80-year existence is like a classical drama with five acts. The first act is the introduction. From the beginning of the second act, the action begins to rise. It reaches its climax in the middle of the drama, in act three. From that point on, all begins to go tragically downhill. After the fourth act, when everything moves from bad to worse, the fifth act ends with the inevitable tragedy, the curtain comes down, and the play is over.
But enough of this talk: the scene is now set for a real drama of the heart.
Act One: the unborn heart
In the theatre, plays usually begin by presenting the characters in the first act. So, allow me to introduce you. Very soon after an egg cell is fertilised, which is the point that marks the start of the rather complicated process of embryo development, the foundations are laid for the construction of a functioning heart. A rather unprepossessing collection of cells assembles, called the cardiogenic plate.* It forms two strands, which then develop into tubes.
At the same time, the pericardium, or heart sac, forms, and the heart continues its development inside this. The pericardium will later continue to envelop the adult heart. Inside the pericardium, the two parallel tubes now merge to create a larger one, called the tubular heart. It begins to move and eventually to curve in shape. Although it bears little resemblance to a rollercoaster or a display of aerobatic prowess, this process is called cardiac looping.
This isn’t the end of the heart’s development by far. Next, our heart grows ears — although not ones it can hear with. Like those fluffy bunny ears that are so popular at hen’s nights, they only look similar to the real thing. Scientists are still unsure about the precise function of these heart-ears, which are in fact nothing more than appendages to the heart’s atria. What doctors do know, though, is that they are responsible for the release of a hormone that will later stimulate urinary excretion. Our heart not only pumps blood around our bodies, it also helps us to pee.
By this stage, almost a month has gone by since the egg cell was fertilised, and the embryonic heart can now be divided into recognisable sections that will become the chambers known as the atria (where blood enters the heart) and the ventricles (where blood is expelled). Precursors to the cardiac valves form, as do the early stages of the septum, or dividing wall between the right and left side of the heart. However, that wall does not form a complete partition in the embryonic heart, and will not fully close until a few days after birth.
In fact, there is an oval hole between the right and left atria, called the foramen ovale. Blood flows through this aperture from the right atrium into the left, and then on around the embryo’s body. Why is that? The reason is simple: embryos are not yet able to breathe independently, so it would make little sense to invest in the laborious process of pumping blood through the embryo’s lungs. This short-cut is all it takes to avoid that.
What eventually results from all this development is muscly on the outside and hollow on the inside (and thus could be said to bear a resemblance to a certain former governor of California).
Act Two: the newborn heart
The heart of a newborn baby is quite different from that of an adult. About the size of a walnut, it works much more quickly. It beats up to 150 times a minute — even at rest: baby doesn’t have to have been doing any sport. That’s about twice as fast as the normal adult heart rate. The reason for this is simply that a newborn’s heart is still very small and it pumps only a small amount of blood with each contraction. However, now that the heart is working entirely on its own, the foramen ovale closes during the first few days of life. With that connection blocked, the right side of the heart now pumps blood into the pulmonary circulation system of the lungs,* and the left side pumps blood round the rest of the newborn baby’s body.
In the theatre, this is the stage when the first signs of conflict usually appear. The same is true of the heart. If something has gone seriously wrong with the development process of the heart, this is when it will become known, if it hasn’t already. Although prenatal diagnostic techniques are now very advanced in the developed world, they are still not perfect, unfortunately. When doctors listen to an abnormal infant heart, they will often be able to diagnose a heart defect based on the sounds they hear.
The most common of these is what doctors call a ventricular septal defect, when the wall dividing the heart’s two ventricles has a hole in it.† In the most serious cases, a young life must begin with major heart surgery. It depends on the size of the opening. Minor defects can heal up by themselves without any medical intervention, and as long as the newborn child appears to be vigorous and thriving, there is no immediate danger to the baby’s life. The decisive factor is whether the infant’s organs are receiving enough oxygen. If this is the case, then doctors, parents, and, most importantly, junior can breathe easy.
Act Three: the strong heart
The heart of a healthy 20-year-old human contracts somewhere between 60 and 80 times a minute. If it is well trained, it can beat quite significantly more slowly when its owner is at rest. And this bundle of muscle is practically bursting with energy. The best way to gain an idea of its internal structure is to cut it open and take a look. For me, as a student of medical anatomy, this was an extremely exciting experience. But it might not be everyone’s idea of fun.
Let’s take a look at it from the point of view of a red blood cell, also known to scientists as an erythrocyte. It, and its many fellow red blood cells, gets its name from the red pigment haemoglobin, which it contains. Its main job is to transport oxygen from our lungs to the rest of our body, and, on return, to transport carbon dioxide back to our lungs.
Imagine you are an RBC (the slang term among medical types for red blood cell). You are transporting carbon dioxide — bonded to your haemoglobin — from one of the organs of the body, let’s say the brain, through a blood vessel back towards the heart. So you must be in a vein, since that’s the term for all the vessels that transport blood to the heart, while those that carry blood from the heart to the rest of the body are called arteries. After a few twists and turns, you eventually end up in the superior vena cava, a vessel that empties directly into the heart. And it is into the heart’s right atrium that you are now swept, along with your cargo of carbon dioxide. From there, you pass into the right ventricle of the heart. Hurry now, don’t dawdle, we have a mission to complete!
To get from the heart’s right atrium to the right ventricle, you pass through an atrioventricular valve known to medics as the tricuspid valve (the Latin word cuspis means ‘point’ or ‘tip’). Once you have left the right atrium via that valve, there is no going back — if you are in a healthy heart. All the heart’s valves are unidirectional: they only let blood flow one way. This is a trusty means of making sure blood does not flow in the wrong direction, from the right ventricle back into the atrium. Thus, in a healthy heart, blood always only flows in one direction, and does not, for example, slosh back and forth between the ventricle and the atrium.
Continuing your journey, you leave the right ventricle via another valve — the pulmonary valve — heading towards the lungs. Having passed through that valve, you now find yourself in the pulmonary artery, the artery of the lung. This shows, by the way, that the much-quoted rule ‘arteries transport oxygenated blood and veins transport deoxygenated blood’ is in fact nonsense. After all, you’re still carrying your cargo of carbon dioxide, making you ‘deoxygenated’, although you are currently floating through an artery, not a vein. Once more for clarity, the more accurate rule is: arteries carry blood away from the heart, veins towards it (although there are still some small exceptions to this rule, e.g. in connection with the liver*).
On arrival in the lungs, you complete the first part of your mission as an RBC by unloading your carbon dioxide and taking on a fresh cargo, this time of oxygen. With that freight on board, you now set out on a return journey through the pulmonary vein (!) back towards the heart. There, you and your many fellow erythrocytes flow into the left atrium and on, through a third valve, into the left ventricle, the last ventricle on your voyage. The valve between the left ventricle and the left atrium is known as the bicuspid† or mitral valve, so called because its shape reminded anatomists of the kind of bishop’s hat known as a mitre.
The left ventricle is the bodybuilder among the chambers of the heart. It has by far the thickest muscle wall. This isn’t surprising, since it needs to build up a great deal of pressure to keep our blood constantly flowing and to pump it to even the furthest reaches of our body. Now, on we travel, through a final valve, the aortic valve, and into the aorta, the body’s main artery. This vessel describes a graceful curve around the heart, from which vessels branch off towards the head and the arms. It then continues into the abdomen, where it splits into ever-smaller branches to provide fresh blood to all our organs and tissues, right down to the tips of our toes.
We are now approaching the climax of our drama of the heart. Everything is working fine, the heart and vascular system seem to be indestructible. But things are about to take a tragic turn.
Act Four: the ailing heart
After just 25 years, the first ‘deposits’ begin to appear on the walls of the coronary arteries (arteries that supply the heart muscles themselves with blood). At this stage, it’s not a big problem, but it lays the foundation for a very serious condition: arteriosclerosis, sometimes called ‘hardening of the arteries’. It is the number-one cause of the world’s most-common killers: heart attacks and strokes. Deposits of fatty plaques on the walls of the blood vessels will continue to build up, getting thicker and thicker and restricting the flow of blood until, in a worst-case scenario, a vessel eventually becomes completely blocked (like a water pipe with limescale).
When this happens to the coronary arteries, small or even larger sections of the heart muscle are left with an insufficient supply of nutrients and oxygen, and they begin to change. This is the infamous heart attack. Undersupplied areas of muscle transform into a kind of scar tissue that no longer contributes to the beating action of the heart. And, as we all know, a team is only as strong as its weakest member. The result is that the heart loses both strength and stamina.
At this point in a play, drama theorists speak of a ‘delaying factor’ in the plot, when the pace of the story slows down as the final denouement approaches. In the case of heart-attack patients, the role of delaying factor is played by medicine. To delay, or, even better, avert the oncoming catastrophe, doctors can prescribe medication, insert catheters (thin plastic tubes) into the coronary arteries, and try to alter the patient’s life circumstances to take some pressure off the heart and thus minimise the risk of another heart attack.
Act Five: the (c)old heart
Chest pain. Irregular heartbeat. A listen to the chest with a stethoscope shows it is no longer beating with its regular ba-boom, ba-boom, ba-boom rhythm. Now, it sounds more like ba … boom, ba-ba-boom, boom, ba-boom. Difficulty breathing and weakness set in. After beating without a break for almost a century, the heart is now significantly weaker. It’s been through a lot. For some, it may be experiencing its second or third attack. It pumps ever less powerfully and, in a final act of valour, it gathers all its resources and tries to work faster. But in the end, all is in vain. The heart is no longer able to work properly; it twitches uncontrollably for a brief while and eventually becomes still. And then that’s it: curtains.
This is the inescapable end to the drama. Predictable, but nonetheless tragic. Although all of our hearts will eventually stop, the time before this happens need not be dramatic. On the contrary, a hale and hearty life is more reminiscent of a comedy than a tragedy. The heart still ends up still, but at least its owner has laughed a lot and spent the time in a fulfilled way.
The good news is that anyone can take preventive measures to make sure the time when their heart stops beating comes as late as possible. And, at best, without cardiovascular disease ruining our existence before it does.
The first step towards this goal is keeping a sense of humour. Life can sometimes be an extremely serious business, but everything is easier when you’re smiling. You could try laughter yoga. Or just search ‘quadruplet babies laughing’ on YouTube.
It’s not only hypochondriacs who interpret trivial symptoms as the harbingers of death-bringing illness. No one is completely free of this crippling habit — not you, not me, not any of us. But the great thing is that, as a rule, human beings are basically healthy creatures. And that’s true of the heart, too. When some part of our body feels strange, it’s usually not due to a rare disease that will carry us off in a matter of hours, but due to something completely harmless. True to my favourite saying: ‘If you hear hoof beats, think horses, not zebras.’ So there is nothing standing in the way of personal happiness and physical health. But, still, I sometimes like to listen carefully to my heart.
Medics and Money before Midnight
I lie in bed and listen to my own heart hard at work. It’s beating a little harder than normal because I swam a few lengths before going to bed. Looking at my alarm clock, I count 19 beats in 15 seconds. I do the math: four times 19, or 19 times two times two. Or two times 38, equalling 76 beats a minute. I look down at my chest and watch it pulsating with each beat of my heart.
As a doctor-to-be, I have my stethoscope at hand. I listen to my chest. Ba-boom, ba-boom, ba-boom, ba-boom, ba-boom. I’ve just celebrated by 25th birthday. That means my heart has already beaten around 900 million times during my lifetime. Faithfully and dependably fulfilling its duty of keeping me alive. Thank you, dear heart, for doing that monotonous job for me.
But listening a little more carefully, it becomes clear that the work of the heart is not quite so monotonous after all. It does not simply go boom, boom, boom, boom like the bass from the speakers of your stereo system or boombox. In fact, a heartbeat consists of more than just the simple contraction of the entire organ. It is a precisely timed and coordinated operation of the muscles of the atria and the ventricles, as well as the opening and closing of the heart’s valves.
First, the atria contract and press blood into the ventricles. This process can’t normally be heard through a stethoscope. A short time after, normally about 150 milliseconds, the ventricles contract, transporting the blood into the lungs and eventually the rest of the body. That contraction of the ventricle muscles is what causes the ‘ba-’. The subsequent ‘boom’ isn’t caused by the heart muscle itself, however, but by the closing of the semilunar valves of the aorta and the pulmonary artery. I place the stethoscope on another part of my chest. The sound changes. A little further up, and it changes again. I could happily spend hours just moving my stethoscope around.
What particularly fascinates me on this evening are the sounds made by the valves of my heart. They make sure that the blood travelling through our heart always moves in one direction and doesn’t suddenly go into reverse gear. As we have seen, anatomists identify four valves, two of which are atrioventricular (the mitral and tricuspid valves) and two of which are semilunar.* Always alternating, they open and close, creating specific sounds that can be attributed to each valve. Medics distinguish between four heart sounds, although only two can be heard through a stethoscope.
The first of these heart sounds is caused by the contraction of the ventricular muscles. It is sometimes called S1 by medics. The second, higher-pitched sound is shorter in duration than the first and is somewhat sharper and louder. Known as S2, this sound is caused by the closing of the two semilunar valves. During inhalation, it can change and split into two components if the aortic valve closes a little earlier than the pulmonary valve.
As any parent or teacher will confirm, children and teenagers make more noise than adults — and the same is true of their hearts. The third and fourth heart sounds can’t be heard through a stethoscope in a healthy adult, but they can sometimes be picked up in younger people. The third sound (S3) is heard when the left ventricle of the heart is filled with blood. It is normal in pre-adulthood. When it occurs in older patients, it can be a sign of problems. More precisely, it can indicate a problem with the mitral valve between the left ventricle and the left atrium,* an abnormal increase in the size of the ventricular cavity,† or cardiac insufficiency ( failure of the heart to work hard enough). And if the amount of blood remaining in the ventricle when it refills is too great, the new lot of blood will slosh against the old and this will also cause a sound.
The fourth sound (S4) results from the contraction of the atria. If it’s heard in adults, it can be an indication of high blood pressure, a thickening of the muscle of the ventricular wall, an obstruction in the left ventricular outflow tract, or — more rarely — a narrowing, or stenosis, of the aortic valve. It is generally followed immediately by the first heart sound.
Hearing all this with a stethoscope is a true art. Some doctors have such a finely tuned ear that they can hear not only the tiniest changes in the sounds of the heart, but also microtumours in the lungs. This involves placing the stethoscope on the chest and tapping in specific places. Theoretically, it’s possible to identify such tumours by listening to the resulting echoes, but I have never managed such an impressive act. I suppose this is a case of constant ‘practice makes perfect’.
Despite this, my stethoscope is always a great aid, for listening not only to the heart, but also to the rest of the body. I grew up in Germany’s Harz Mountains, an area of the country that is very popular with motorcyclists in the summer. Serious accidents are common in the biking season and those horrific crashes often leave bikers with terrible injuries. On arriving at the scene of such an accident as part of an emergency medical team, I would begin by listening to the patient’s lungs and abdomen. I did this because it’s not uncommon to hear no sound of breathing on one side of the chest, even if the patient is still breathing.
The cause of this apparent contradiction is usually a collapsed lung (pneumothorax) on the side where no breathing sound can be heard, but it can also sometimes indicate an accumulation of blood in the chest cavity (haemothorax) or, in a worst-case scenario, a combination of both (haemopneumothorax). The sound made by tapping on the chest while listening through a stethoscope (doctors call this ‘percussion’) can allow a medic to distinguish between accumulated air and accumulated blood. An accumulation of air gives a sound reminiscent of beating on a drum, while an accumulation of fluid will dampen the sound, like striking a kettledrum filled with water. If the patient were able to sing and play the guitar along with all this percussion, they would almost be ready to take to the stage for a performance — if it weren’t for the fact that they were in need of urgent medical treatment.
During a routine medical examination, the doctor will often listen to the abdomen using a stethoscope, to check the function of the intestines. After a motorbike accident, the medic will ‘percuss’ the abdomen to identify whether there is any internal bleeding or accumulation of fluids. As you can see, the stethoscope is a constant and valuable companion of medical practitioners; it is indispensable in many areas of treatment, but especially that of the heart.
However, like everything else, it has its limits. There are specialist cardiology stethoscopes with which you can almost hear the worms moving beneath the soil, but even they do not allow doctors to perceive everything — for example, the third and fourth heart sounds. For that, a special ultrasound examination of the heart is necessary (called an echocardiogram). It allows doctors to check the size of the heart, its atria and ventricles, the thickness of its walls, its overall mobility, its valves, and any defective blood flows. Often, a doctor will also be able to monitor for pathological changes to a patient’s heart, including defects in the valves or constrictions in the coronary blood vessels.
While I was a medical student, I learned a mnemonic that has stayed with me ever since: ‘All Physicians Take Money at 22.45’. I’m sure all physicians are willing to take money at other times of the day, but what does this have to do with the heart? Well, this mnemonic helps young medics remember where to place their stethoscope when listening to the function of the heart valves.
Having memorised this little sentence, all a doctor has to remember is that the listening points are described from top right to bottom left. The time 22.45 describes the intercostal spaces (between the ribs), and the initial letters of the words in the sentence are the same as those of the names of the valves (Aortic, Pulmonic, Tricuspid, and Mitral). Once you know this, you can listen very precisely to your own heart-valve sounds and any possible murmurs. However, interpreting these sounds is a complicated business and should be left to an experienced cardiologist, since recognising the subtle differences and changes is almost impossible without decades of practice.
There is a six-level scale for grading the loudness of heart sounds, ranging from ‘difficult to hear even by expert listeners’ via ‘readily audible but with no palpable thrill’ (medical word for tremor or vibration) to ‘loudest intensity, audible even with the stethoscope raised above the chest’. In addition, doctors distinguish between different ways the sounds change over time, using criteria like crescendo or decrescendo, i.e. getting louder or getting softer; or diamond-shaped, which means getting louder then softer again; or a constant, unchanging intensity. The heart is an instrument that can play many kinds of music. Doctors use these distinctive features as the basis for diagnosing problems with the valves of the heart, and prescribing the best treatment.
All Physicians Take Money at 22.45 — the stethoscope listening points
The way all the components of the heart work together is complex, but also absolutely fascinating. However, even the greatest, most powerful engine is useless if there are no roads for the vehicle to drive on. Our blood vessels are precisely those ‘roads’, without which our heart, as the central pump, would have no meaning. In the end, the heart’s strength and stamina, as well as its intricate valve construction and conduction system, all serve one single purpose: to send our blood rushing at full throttle along those roads.
The Body’s Highway System
Our blood vessels transport blood and nutrients to the farthest reaches of our body. In fact, there are only a very few areas that are not permeated by them. Those include the corneas of our eyes, the enamel of our teeth, our hair, our fingernails, and the outermost layer of our skin. To transport all that blood, our body needs a proper system of pipes and ducts: our blood vessels. They are also the highway system of our bodies. With the difference, however, that taken together, our arteries, veins, and capillaries (the finest branches of our blood vessels within tissues), are more than ten times longer than Germany’s famous autobahn system — totalling around 150,000 kilometres (over 93,205 miles).
Unlike the pipes that form the sewerage systems beneath our cities, blood vessels are very elastic. This is a good thing, because it allows the body to regulate the diameter of the blood vessels. It’s what enables the body to provide certain organs and structures with a greater or lesser supply of blood according to whether they require more or fewer nutrients and oxygen molecules at any given time. When it comes down to it, this is no different to the engine of a car: the more you step on the accelerator, the more fuel is injected into the motor’s cylinders.
When we are out jogging, our muscles need a better supply of blood to satisfy the increased need for oxygen. At the same time, our skin also receives more blood, so that it can release some of the increased heat into the environment via the cool, sweat-moistened surface of the skin. To make this increase in blood supply happen, our body reduces the amount of blood it provides to other parts of the body — for example, the gut. After all, digestion can always wait till later. A similar thing happens in our lungs: if a section of the lung registers a reduced oxygen supply, the vessels in that section will constrict. There is no point in sending blood to pick up oxygen where there is none to be had.
All this is possible because our arteries and veins have an elastic structure. The two types of blood vessel are similar, but there are certain differences between them. All have walls consisting of three layers, with the innermost layer made up of supporting connective tissue and what doctors call the endothelium. The endothelial cells line the inside of our blood vessels, serving as a barrier to protect the tissue of the vessel wall, and they can play an active part in the regulation of the cardiovascular system. They are the interior decoration and the Anaglypta of our blood vessels, but they are also much more than that. For instance, they can release nitric oxide, which acts as a signal to the vessels surrounding the heart and those of the skeletal muscles to relax, allowing them to dilate. This happens during physical exercise and other times of exertion to supply more oxygen-rich blood to the muscles as they work.
Structure of a blood vessel wall
The middle layer is muscular, or, more accurately, it consists 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.
