Our hearts are organic pumps, far more capable and efficient than anything our technology can yet create. In romantic literature and popular culture they are frequently linked to love. But the physiology of love is still unknown, despite the amazing advances in scientific knowledge over the last two centuries. However, love’s importance to people’s emotional and psychological well being has been understood since before the dawn of human civilization. Which is probably why love, in the vast majority of human cultures, has been associated with the heart, the vital organ our physical well being is most dependent on.
The purpose of the heart is quite simple, it pumps the resources our bodies need from where they enter, to where they are used and the waste byproducts of our cellular processes from where they are created, to where they can be expelled, using our blood as the carrying medium. While we often call the heart a pump, it is actually two pumps side by side, each pump having two chambers, contained within the single organ.
The upper chambers are called atria and they receive blood into the heart; the lower chambers are called ventricles and they push the blood back out. The clearest way to describe heart function is to follow the path of the blood flow, so let’s start with that and then discuss how the heart pumps and is controlled.
The right atrium receives blood from the venous system of the body and the heart; from the upper body through the superior vena cava, the lower body through the inferior vena cava and the coronary (heart) veins through the coronary sinus. The oxygen in the blood from the body is depleted, having been used for cellular respiration, so this is referred to as deoxygenated blood. The blood enters the right atrium as the heart relaxes after a contraction, the period of the cardiac cycle called diastole. The cardiac cycle is one beat sequence of the heart, for example from the start of one contraction to the start of the next contraction. The period of the cycle when the heart contracts is called systole.
Towards the end of diastole some of the blood from the right atrium will be entering the right ventricle through the connecting valve called the tricupsid atrioventricular (AV) valve. Tricupsid means the valve has three flaps, which open towards the ventricle. At the start of systole the right atrium contracts first, squeezing the blood inside it and forcing it through into the right ventricle. The valves of the heart open only one way so that backflow does not occur in a normal, healthy heart. The tricuspid valves of the vena cava veins and the coronary sinus are forced closed as the right atrium contracts, while the AV valve is forced fully open. As the right ventricle begins to contract, the AV valve is forced closed. The increasing blood pressure in the ventricle forces open the semilunar valve, also tricupsid, between it and the pulmonary artery, allowing the blood to be pumped through to the lungs. The pulmonary artery divides in two, just beyond the semilunar valve, one going to each lung.
The left atrium receives blood from the pulmonary veins which come from the lungs. The blood becomes oxygenated when it passes through the lungs, so the pulmonary veins are the only veins in the body that carry oxygenated blood. Just as with the right side of the heart, towards the end of diastole blood is flowing through the AV valve from the left atrium to the left ventricle. However this AV valve is bicuspid, it has only two flaps. This provides greater strength than the tricuspid arrangement, used in all the other heart valves, can supply.
At the start of systole the left atrium contracts first, squeezing the blood inside it and forcing it through into the left ventricle. As the left ventricle begins to contract, the AV valve is forced closed. The increasing blood pressure in the ventricle forces open the semilunar valve between it and the aorta, the primary and largest artery, allowing the blood to be pumped through to the rest of the body. The coronary artery branches off the aorta just past the semilunar valve, taking five percent of the oxygenated blood to supply the muscle cells of the heart, called the myocardium.
Because the left ventricle pumps the blood to the body it needs to be stronger than the right ventricle that pumps it to the lungs only. The myocardium of the left ventricle is approximately twice as thick as that of the right to provide the greater power. This is also why the left AV valve needs to be bicupsid, to resist the greater pressure produced in the left ventricle.
Because of the above arrangement, the human cardiovascular system is considered to be made up of two closed circuits. The pulmonary circuit goes from the right ventricle to the left atrium via the lungs and the systemic circuit goes from the left ventricle to the right atrium via the rest of the body. They are called closed circuits because they do not empty out into any of our body cavities. Most insects have open circuits, with their equivalent of blood called hemolymph partially in blood vessels and partially free flowing within their exoskeletons.
Our hearts are able to pump blood for so long and so well because of their structure. The human heart is primarily composed of the specialized muscle tissue of the myocardium. This is made up of cardiac muscle cells, found nowhere else in the human body. Surrounding the myocardium is an epithelial layer of cells similar to skin cells called the epicardium. Within the myocardium, lining the heart’s chambers is a thin layer of connective tissue called the endocardium. The endocardium is smooth and slick to allow blood to move with a minimum of friction.
The cardiac muscle cells are striated or lined in appearance, the same as skeletal muscle, due to the structure of the myofibrils within them that enable the contraction and relaxation to occur. They have fewer nuclei than skeletal muscle cells, usually just one or two, but an even higher concentration of mitochondria to supply them with energy. They are comparatively small and fat, and unlike skeletal muscle cells, they branch. This allows them to connect to multiple other cardiac muscle cells at junctions called intercalated discs. These junctions allow the passage of electrical signals that trigger contraction as well as holding the muscle cells together throughout the cardiac cycle.
The timing of the heart is kept synchronized by electrical impulses from an organic pacemaker called the sinoatrial node, located in the wall of the right atrium close to where the inferior vena cava enters. In a healthy heart, it sends out rhythmic electrical impulses that control the beat. It is this node that artificial pacemakers assist or replace when it is no longer working properly.
Each impulse first travels across the atria triggering atrial systole, contraction of both atrium, before reaching the atrioventricular node located in the wall between the right and left atria, called the interatrial septum. From there it travels through the atrioventricular bundle in the interventricular septum, the wall between the ventricles, to spread through the Purkinje fibers located in the ventricular myocardium, triggering ventricular systole. It is this travel or propagation time that results in the atria contracting before the ventricles, allowing the whole system to work.
The sinoatrial node will continue signaling at its set rate until it receives instructions to alter that rate. This is why the heart will continue to beat even when a person is brain dead, as long as the cardiovascular system itself is intact. The heart rate setting of the sinoatrial node can be modified in a number of ways, this is known as regulation:
* The cardioacceleratory center of the brain will speed up the heart rate in response to stress or increased physical exertion.
* The cardioinhibitory center of the brain will slow down the heart rate when stress is reduced or passed.
* Baroreceptors located in the aorta and the internal carotid artery measure blood pressure and will trigger a slower heart rate in response to increasing blood pressure.
* Epinephrine, a hormone known commonly as adrenaline, will increase the heart rate. It also increases the contractility of the myocardium, making each heart beat more powerful.
* Thyroxine, another hormone, increases heart rate.
The brain centers and the baroreceptors signal the sinoatrial node via the nervous system, the hormones travel to it through the cardiovascular system. It is possible for all of the above to be acting on the heart rate at the same time, if a person is particular stressed and physically active. The overall heart rate would be dependent on how they all averaged out. These regulatory controls all operate on the base heart rate, increasing or decreasing it.
The base heart rate is age dependent, varying between 60 and 80 beats per minute in healthy human adults with an appropriate body mass index, faster in those overweight or unfit. It is also faster in children, because they are growing and those “building materials” have to be moved around to where they are needed. Babies should be between 130 and 150 beats per minute, young children between 100 and 120 and early teens around 70 to 90 beats per minute. A steady progression towards a slower heart beat as they age.
