The heart is the organ that pumps the blood round the body. Very many animals have hearts, from earthworms to blue whales, and of course Man. This Page is mainly about the human heart.
A word used a lot when talking or writing about the heart is cardiac - this comes from a Latin word and just means something to do with the heart.
This Page is about a normal healthy heart. Apart from a “hole in the heart” most heart problems are the result of an unhealthy life style and are considered only on another Page of this Web Site.
Parts of this Page are still under construction.
If we have a cylinder with a piston in it, as we pull down the piston air is drawn into the cylinder, and as we push up the piston air is expelled from it.
Valves control the flow of a fluid (gas or liquid) through a system. On a high-performance motor car the valves which control the way in which the fuel-air mixture enters the cylinders and the exhaust gases are expelled can be very complicated; in a very basic mechanical pump such as this simple pressure-operated non-return valves are all that are needed.
When the pressure at A is greater that the pressure at B the valve will open and let the air through, but when the pressure at B is greater than the pressure at A the valve will shut and not let any air through. This is one sort of mechanical non-return valve; non-return valves inside our bodies may not look like this but they work the same way.
A basic pump has two valves, an inlet valve and an outlet valve. Next time you are using a balloon pump to blow up balloons for a party you might like to see if you can find the valves on it.
When the piston starts to move upwards the inlet valve shuts. The pressure in the cylinder builds up, and so there is a back pressure on the inlet valve. It must be able to withstand this pressure without leaking. The higher the pressure the pump is designed to produce the more difficult it is to stop the valves from leaking.
Pumps designed to produce high pressures often consist of two cylinders in tandem.
In this arrangement the back pressure on the inlet valve is much less than it would be in a single stage pump. The drawing shows an air pump. Because air is compressible, so that the higher the pressure the smaller the volume, the second stage is smaller than the first stage. Liquids are incompressible, so in a pump designed for liquids the the two stages will be the same size.
When we think of muscles we usually think of the muscles like those which move our arms and legs. These consist of a bundle of muscle fibres with a tendon at each end. When we use this sort of muscle it gets shorter and fatter and pulls on the joint so that it is moved.
You can see this most easily in your biceps, and you can also feel the tendon at the elbow end of the biceps quite easily. A muscle like this can only exert a pull, not a push. So each joint in our body must have two muscles, one to move the joint one way and the other to move it the other way, like the biceps and triceps. The two muscles need not be of equal strength, for example the muscles which enable a crocodile to shut its jaws are very powerful indeed, enabling it to grip its prey, whereas those that enable it to open them do not need to be nearly so strong, in fact, they are very weak. If you get near a crocodile when its jaws are open and it grabs you you will not be strong enough to stop its jaws from closing on you, and you will get eaten. But if you come up to the crocodile when its jaws are closed you can easily tie them closed, and so stop it from eating you, with something as thin as the cord (not elastic!) from your pyjamas.Caution: adult supervision recommended.
There is however another type of muscle, where the muscle fibres are arranged in a ring like a doughnut with a hole in it: the mathematical name for this shape is a torus.
Muscles like this are used a lot in many animals. One use is to seal a duct or tube. The muscle contracts and seals the tube. A muscle used in this way is called a sphincter.
Ring-shaped muscles also push the food along inside our intestines.
The simplest heart is just a swelling in a blood vessel with an inlet and an outlet valve inside a cavity in a muscle. As the muscle first squeezes and then relaxes the blood is pumped round the body.
This is a one-chambered heart. A simple one-chambered heart such as this can only produce a low blood pressure because the inlet valve can only withstand a low back pressure without leaking. Some animals have one-chambered hearts, some two-chambered and some four-chambered. Two- and four-chambered hearts can produce a much higher blood pressure than a single-chambered heart because the inlet valve does not have to withstand such a high back pressure. Man has a four-chambered heart. But all hearts are basically exactly the same: one, two or four chambers with valves inside a cavity inside a muscle.
Oxygen is needed for respiration. It is however only one of a number of substances which are carried round the body in the blood and are essential for life. But oxygen differs from all the others because the body cannot make it and cannot store it. It is important therefore that the heart circulates the blood in a way which ensures that it is kept well oxygenated.
The blood also carries the carbon dioxide produced by respiration. When we talk about “oxygenating the blood” we really mean not only adding oxygen to the blood but also removing carbon dioxide from it, which is just as important, although this is not always explicitly stated.
Oxygenated blood is carried round the body in the arteries and is red; deoxygenated blood is carried in the veins and is blue. If you are a young boy with skin of a nice pale pinky colour you should be able to see the blueness of your veins very clearly by looking at the front (palm side) of your own wrist; if you are not a young boy and your skin is a beautiful browny colour you may not be able to see the blueness in your own wrist so you might need to look at someone else's - the blueness is more clearly seen in boys than girls and children than adults so it is best to ask a young boy with skin of a nice pale pinky colour... (But do make certain you tell him why you want to look at his wrist!)
We say that human beings have a four-chambered heart, but it is really two separate two-chambered pumps contained inside a single muscle. The right-hand pump takes deoxygenated blood from the body and pumps it to the lungs for oxygenation; the left-hand pump takes oxygenated blood from the lungs and pumps it round the body. Remember that the terms left-hand and right-hand refer to their position inside the chest: because we almost invariably draw the heart as it is seen from the front, in a diagram of the heart the right-hand side is drawn on the left - to make this clearer, the boy in the picture is raising his right hand.
Our heart is about the size of our clenched fist. It is actually in the middle of our chest, but because it is the left side which pumps blood round the body we are more conscious of the left hand side of the heart beating, and so when we put our “hand on our heart” we usually put our right hand on the left side of our chest.
At first sight a drawing of the heart can be a little difficult to understand: here is a schematic diagram.
At the beginning of the heart cycle the inlet valves (sphincters) are open, and the deoxgenated blood from the body enters the first chamber, the right atrium, from the two main veins from the body, the superior vena cava and the inferior vena cava - superior and inferior just mean upper and lower, not more important and less important. At this stage the tricuspid (three leafed) valve is also open, so some blood also flows into the right ventricle. Now the sphincters close the vena cavae and the heart muscle exerts a gentle squeeze, pushing the blood in the right atrium through the tricuspid valve and into the right ventricle. Next the tricuspid valve closes and the heart muscle exerts a harder squeeze, pushing the blood through the watchpocket valves and into the two pulmonary arteries. These carry the deoxygenated blood to the left and right lungs. Here the blood is oxygenated and the carbon dioxide removed.
Now the oxygenated blood from the lungs passes through the pulmomary veins and two more sphincters into the third chamber, the left atrium. The bicuspid (two leafed) valve is also open so some blood flows into the fourth chamber, the left ventricle. Now the sphincters close and the heart muscle exerts a gentle squeeze, forcing the blood in the left atrium through the bicuspid valve into the left ventricle. Now the bicuspid valve shuts and the heart muscle exerts a much harder squeeze. The watchpocket valve opens and blood is pushed at high pressure into the body's main artery, the aorta.
In a healthy young adult each beat pumps about 70 cm3 of blood into the aorta, and of course the same amount into the pulmonary arteries; the total volume of blood in the body is about 6 dm3 (6 litres). In growing children and young people these volumes are of course less.The two atria contract at the same time: this is called the atrial systole . Similarly the two ventricles contract at the same time: this is called the ventricular systole. The period when the atria are filling and the heart muscle is relaxed is called the diastole. In a normal heart cycle atrial systole makes up about one eighth, ventricular systole three eighths and diastole about one half. Here is the normal heart rythm in graphical form.
The normal resting heart rate of a healthy young adult is about 70 beats per minute, about 90 for a fifteen year old, and higher for children.
All mammals have four-chambered hearts; in smaller mammals such as mice the heart rate is much faster than in humans (about 700 beats per minute), in very much larger mammals such as blue whales it is much slower (about 8 beats per minute), but the rythm is the same whatever the heart rate.
The heart is mainly muscle. It never stops working so it is made of a special type of muscle, called cardiac muscle, which, unlike the type of muscle from which our other muscles are made, never gets tired. At one time heart was often eaten, but today it is not so popular and is used mainly in pet food. Because heart is made of cardiac muscle rather than ordinary muscle it has a different, very strong, taste which not everyone likes.
The sphincters controlling the flow of blood into the atria can withstand only a low pressure difference, but the bicuspid and tricuspid valves must be able to withstand a much higher back pressure without leaking and so are controlled by tendons. As described earlier the heart is mainly a large muscle with a cavity inside containing valves and tendons. When a heart is being prepared for cooking the valves and tendons have to be taken out - the tendons are like pieces of rather stiff string and so are commonly known as the heart-strings. In the Harry Potter Books Mr Ollivander uses dragon heart-strings in some of his wands.
In a healthy young person the arteries (but not the veins) are very elastic, like a balloon. This means that at ventricular systole the blood is forced into them at high pressure and they stretch, then during diastole this stretchiness keeps the blood pressure in the arteries high. This is rather like the bag-pipes, where the bag contains air under pressure the whole time and not only when the piper is blowing into it.
Blood pressure in the body is measured with a sphinctomanometer. Originally these contained a mercury manometer, but now most are electronic. But blood pressure is still always measured in millimetres of mercury.
The blood pressure produced by the left side of the heart is much higher than that produced by the right side, because this is the side which pumps blood round the body, including of course up to the brain. The blood pressure on the right hand side cannot be so high because the blood from it passes to the lungs and the air in the lungs is at atmospheric pressure. Animals such as lizards with two-chambered hearts are usually long and thin so that the head is about the same level as the heart and a high blood pressure is not needed to pump the blood to the brain. A lizard cannot keep its head higher than its body for very long without its brain being starved of blood and so oxygen. Crocodiles have four-chambered hearts. What follows is about the left hand side of a human heart.
We normally measure the blood pressure in the left arm. Two readings are always made: the systolic pressure, which is the pressure produced by the heart at ventricular systole, and the diastolic pressure, which is the pressure in the arteries during diastole. In a healthy young adult the blood pressure is about “120 over 80”, which is written as 120/80, that is, the systolic pressure is about 120 mm of mercury and the diastolic pressure is about 80 mm of mercury. For children the blood pressure is a little less. Units of pressure are discussed on another Page and to link to it please click here 
A pressure of 120 mm of mercury would produce a fountain of blood more than one and a half metres high...
The pressure in the pulmonary arteries is much lower, about 30/12. This much lower pressure, only very slightly above atmospheric pressure, allows better oxygenation of the blood.
The arteries are usually deep inside the body and surrounded by soft tissue. Pressing on an artery will just push it further into the soft tissue, in the same way that stepping on a garden hose running over muddy ground will just push it into the mud, not stop the flow of water. Sometimes however for one or two centimetres an artery passes through a narrow gap between the skin and a bone. If we put a finger on the artery at this point and push it gently against the bone we compress it slightly and we shall feel it expanding and contracting: this is a pulse. Places where an artery passes between the skin and a bone are called pulse points. There are several pulse points in the body, the easiest one to find is in our wrist, and this is usually where we “take a pulse”.
Never take a pulse with your thumb: there is a pulse point in your thumb so you will be measuring your own pulse not your patient’s!
Finding a pulse point involves pressing on an artery and if this is not done right you may restrict the blood flow. Never try to find pulse points anywhere other than in your wrist unless you have been shown how to do so and are being supervised by an adult.
The big arteries in the neck carrying the blood to the brain are called the carotid arteries, and the big veins carrying it back to the heart are called the jugular veins. There is a pulse point in the carotid artery in the neck, and in films you often see The Detective putting a finger on the body’s neck and a second later pronouncing “He’s dead.” But this only happens in films - lesser mortals such as doctors and nurses would not usually be quite so quick so pronounce someone dead, if only because the absence of a carotid artery pulse could be due to many factors other than death!
Jugular actually means something to do with the neck, so you sometimes hear the expression “going for the jugular”, but this includes both the carotid arteries and the jugular veins.
There are no valves in the arteries: the diastolic pressure due to their elasticity keeps the blood flowing along them. In the veins however the blood pressure is very much lower, and there are non-return valves every few centimetres along them.
The normal resting heart rate of a healthy young adult is about 70 beats a minute, while the volume pumped into the aorta each stroke (the stroke volume) is about 70 cm3. These give a blood flow round the body of nearly 5 dm3 (5 litres) a minute.
The heart is controlled by two cardiac pacemakers, located at the top of the heart. These control both the heart rate and stroke volume. (Sometimes if they are not doing their job properly a heart surgeon may fit an artificial electronic pacemaker.)
During exercise both the heart rate and stroke volume are increased. The heart rate of a fit and healthy but untrained young adult may increase to 180 beats a minute, and the stroke volume to 120 cm3. This gives a blood flow of more than 20 dm3 (20 litres) a minute; athletes in training may have even higher blood flows. There is much more about this on a Heart Rate Web Page
The cardiac pacemakers are also affected by other factors: adrenaline or alcohol, a big meal, or a sudden loud noise or shock, fear, happiness, excitement, and of course “being in love”. You do not have to be playing football to have a raised heart rate: just watching your favourite team score a goal on television can have the same effect.
We do not usually feel our heart beating unless it is beating much faster than normal. This leads us to associate our heart with our emotions. Valentine’s Day cards show hearts, King Richard 1st of England was called the Lion Heart because of his bravery in battle, while a coward might be called faint-hearted, the thief who mugs an 80 year old woman in a wheel-chair might be called heartless, someone making a promise might say cross my heart, when American children swear their allegiance to the United States flag they put their hand on their heart. You can thank someone with all your heart, you can leave your heart in San Francisco, your heart can go out to someone who is very sad. In Harry Potter and the Philosopher's Stone the Mirror of Erised shows not your face but your heart's desire. There are lots of other examples of the way in which we link our heart with our emotions, even our thoughts.
The Ancient Egyptians carried this idea to the extreme: they believed you actually thought with your heart, so your heart stored the memories of all your deeds, good and bad. This is why the heart had to be left inside the body when it was mummified: you needed it when you were judged after your death. For more about this go to to the mummification page of my Web Site.
Sometimes a baby is born with a hole, or a weakness which develops into a hole in later life, between the two sides of the heart. This allows a small amount of blood to leak from the right side to the left side, and so be circulated round the body before it has been oxygenated. Sometimes babies with this condition are called blue babies, because when the heart is working hard, for example when the baby is using a lot of energy by crying, the blood in the arteries is very poorly oxygenated and the baby may appear blue. The hole can usually be mended by surgery.
A hole anywhere else in the heart, that is, which allows blood to flow out of the heart into the chest, would of course almost always lead to death within a very short time.
Most other heart conditions are usually the result of a poor life style, and will be considered on another Page of my Web Site.
