heart

heart Throughout human history the rhythmic beat of the heart has quintessentially represented life. Until the advent of the heart–lung machine, the lack of a heart beat, unless reversed within a few minutes, invariably signalled death. The beat of our own heart can be apparent to us in the pulse felt, or seen, at various parts of the body, occasionally heard or — because of an unusual rhythm or ‘skipped’ beat — noticeable in the chest.

The heart is a hollow muscular organ. It acts as the ‘prime mover’ for the circulation of the blood and the maintenance of the blood pressure. A certain volume of blood is delivered with each beat, and a further key aspect is the pressure at which this flow is delivered. Vital functions such as those of lungs and kidneys, or the exchange of components of the blood and tissue fluid at the capillaries, are critically dependent on the pressure achieved within the circulatory system.

Anatomy

The heart comprises a series of blood-filled chambers; the walls are composed virtually entirely of muscle cells of a type unique to the heart (cardiac myocytes). The heart is actually two double pumps acting in series; there are four chambers in all. The right side receives blood returning from the entire body (in the great veins) and pumps it into the pulmonary artery, which supplies only the alveoli (gas exchange sites) in the lungs. The left side receives blood from the lungs and pumps it into the aorta, the largest artery. (The heart is generally illustrated as seen from the front, so ‘left’ and ‘right’ appear mirrored.) The aorta branches to form the arterial tree that supplies blood to the whole body. The heart, appropriately, is itself the first organ supplied with blood from the aorta. The coronary arteries open from the beginning of the aorta and take blood to all parts of the heart tissue. Each side of the heart has an upper chamber, the atrium (plural: ‘atria’), into which the veins drain. They serve as antechambers to the respective ventricles, the thicker-walled chambers that lie below them.

Atria and valves

The arrangement of one-way valves and the prevailing pressures mainly determine blood flow from vein–to–atrium–to– ventricle during the cyclic activity of the heart beat, but some pumping of blood by the atria into the ventricles also occurs. The valves preventing back-flow from ventricle to atrium are tough, parachute-like structures partly anchored in the connective tissue plate which forms the physical union of the ventricular and atrial portions of the heart. Their free edges are restrained by several papillary muscles. These are slim extensions from the inner wall of the ventricles, each with a tendinous end fused with the valve; acting like parachute cords, they prevent the valve being pushed through into the atrium as its flaps become filled when the ventricle contracts and puts pressure on its contents. The mitral (or bicuspid) valve on the left side has two flaps, and the tricuspid valve on the right has three. The ‘parachutes’ press together forming a complete closure preventing regress of blood into the respective atrium whence it came. Instead, when the pressure has risen sufficiently, blood is directed into the pulmonary artery and the aorta through one-way valves which separate them from, and prevent back-flow into, their respective ventricles (see Figure).

The heart beat

The heart beats between 60 and 220 times per minute in a typical young adult; 40 to 50 million beats per year. The rate alters, often rather obviously, according to one's state of physical and mental activity. This results in pumping over 3 million litres of blood (per year) through the body and an equal volume through the lungs. The pump work done by the heart is equivalent to lifting a 1 kg weight to about twice the height of Mount Everest each day. This level of persistent, rhythmic, and decidedly dynamic activity may provoke a sense of awe, although it is hardly more remarkable than the prosaic activity of every other organ — except in its absolute necessity to keep at it! We will first consider the electrical processes of the heart since, like many muscles, it is triggered into activity (contraction, the heart beat) by an electrical wave. This section is followed by consideration of contraction itself.

Electrical aspects

The left and right atria beat virtually simultaneously and then, after a fraction of a second's delay, both ventricles contract. Electrical activity, as in most other muscles, triggers the contraction. This activity arises not from excitatory nerve fibres, but spontaneously within the heart itself from a small clump of pacemaker cells near the point where the vena cava joins the right atrium: the sino-atrial (SA) node. The electrical wave, or action potential, spreads across the heart from cell to cell. This spread is made possible because each heart cell is connected to its immediate neighbours at several contact regions which offer a relatively low resistance to the flow of electrical current. All the muscle cells of the heart are thus electrically linked together. This means that the activity spreads as a wave, its direction determined by the cell-to-cell couplings available. It also means that, as far as we know, every cardiac myocyte is active at some stage during every heart beat. The muscle cells of the atria and ventricles only make electrical contact in one small region, the atrio-ventricular (AV) node at the centre of the heart. Thus, activity follows a predictable, regular path — across the right and left atria, through the AV node, along specialized faster-conducting heart cells (Purkinje fibres) on the internal face of the muscular wall between the two ventricles (interventricular septum), and thence through the substance of both ventricles. Heart cells, like other electrically excitable cells, become inexcitable (refractory) for a brief period after each action potential. Consequently, once the wave has passed right through the ventricles it ceases, since there are no non-refractory cells available to excite. A new wave is spontaneously initiated at the pacemaker region.

Contractile (mechanical) aspects

All the heart muscle cells are thus electrically excited and it is this that triggers them to contract. The wave of contraction, therefore, follows the same sequence: atria first, then ventricles. The electrical activity triggers an abrupt rise in the concentration of ‘free’ calcium ions inside the cells — a common feature in signalling contraction in muscle of every type. The calcium ions required are derived in part by influx from the extracellular fluid, in part by release from intracellular stores in the sarcoplasmic reticulum. The influx is through calcium-selective channels in the surface membrane which are opened by the depolarization. The influx itself transiently promotes further influx, and also triggers the release of more calcium from the intracellular store.

In each ventricle, as the muscular walls contract (develop tension and shorten) they press upon the blood they enclose. The pressure rises and the AV valve fills out and closes. At this stage of the cycle, the exit valve into the relevant artery (pulmonary artery or aorta) is also closed because the pressure in the arteries is higher than that in the ventricles. Temporarily, each ventricle is thus a closed chamber, it can neither lose nor gain blood, so pressure rises quickly until it exceeds that in the exit artery; the exit valve is then pushed open and blood is ejected, squirted from the ventricles as their muscular walls continue to shorten. The pressure at which the valve opens is much higher on the left side than on the right side, in accordance with the higher blood pressure in the aorta and its branches than in the pulmonary artery and its branches. The resistance offered by the lungs to blood flow is much less than that by the body generally; thus the pressures required of the right ventricle can be lower, yet achieve the same flow rate. Both ventricles eject the same volume of blood (the stroke volume): in the adult heart, about 70 ml (half a teacup) which is half or less of the volume it contained. As action potential finishes, the intracellular calcium concentration has already started to reduce again: some calcium is being ‘pumped’ back into the store, and some is leaving the cell by an ion exchange process. With the raised calcium concentration signal thereby removed, the force of contraction quickly wanes in the muscle, so ventricular pressure falls. The elasticity of the arteries, which were dilated when blood was ejected into them, now ensures that a higher pressure is sustained in them than in the rapidly relaxing ventricles (the ‘garden hose’ effect, familiar to those who have turned off a hose-pipe supply tap only to see water continue squirting as the elastic pipe collapses). The respective exit valves are thus pushed closed again, preventing reflux into the ventricles. Blood pressure, therefore, falls more slowly in the arteries than in the ventricles. At this stage about 90 ml of blood remains in each ventricle. Pressure continues to fall quickly until it is below that in the atria. Thus, the AV valves are pushed open, allowing blood to flow from the atria into the ventricles ‘topping them up’ with more blood. (Despite the appearance in some published schematic diagrams and ‘cartoon’ sequences, at all stages of the heart beat the chambers are ‘full’ of blood. It is the enclosed volume which changes, depending on the tension and elasticity of the muscular walls and the status of the inlet and outlet valves.)

The return of the ventricle to its ‘resting’ shape between beats is due to its own elasticity. Like a squeezed sponge or hollow rubber ball, this significantly ‘sucks’ blood from the atria, thereby contributing to its own filling. The reduction of this factor in old age or its enhancement by athletic training have a major effect on overall cardiac function. These effects are analogous to problems associated with ‘stiff’ inelastic valves which perhaps more obviously compromise effective flow in and out of the chambers of the heart.

The state when the heart is contracting is termed systole (sis'-toe-lee); the relaxed state is termed diastole (di-a'-stoe-lee).

Control of pump function

The cardiac output is the volume of blood pumped per minute by each ventricle — some 5 litres/minute at ‘rest’ — and is simply the product of heart rate and stroke volume. Cardiac output will thus alter if either varies. The stroke volume is in turn influenced by cardiac filling and by the contractility of the cardiac muscle itself — its intrinsic ability to contract (shorten and/or produce tension).

Heart rate

The earliest human hunters will have noticed, like later horror film makers, that even when removed from the body, the heart continues to beat for a time. Other organs also continue to live, but their activity is hardly as impressive as that of the heart.

Because all the cells of the heart are electrically connected to their neighbours, the whole behaves as a unit. Most regions are inactive, unless artificially stimulated. The activity of the regions with the property of ‘firing’ spontaneously is conducted to all their inactive neighbours, so they act as pacemakers. The inherent pacemaker firing rate, typically about 100 per minute, is influenced by nerve actions of the autonomic nervous system: sympathetic nerves release noradrenaline which increases rate, and parasympathetic (vagus) nerve fibres release acetylcholine which slows the rate. Heart rate typically varies between 60 per minute (in deep sleep) to approaching 200 per minute (during brief bursts of maximal exercise). The normal ‘resting’ rate while sitting, relaxed, is about 70 per minute, but shows wide variation amongst entirely healthy individuals. (In one university class of 350 twenty-year-old students, the range was 48 to 90 per minute.) One common feature is a marked variation within the breathing cycle: breathing in usually increases the rate. Physical fitness, particularly that associated with endurance rather than muscle strength, is often associated with a low resting rate. Extremes such as the tennis player Bjorn Borg, or the professional cyclist Miguel Indurain, with resting values in the low 30s per minute, are well known. Young children have higher resting rates; whilst still in the womb, a baby will have a rate of 120 to 160 beats per minute; it is often reported that rates above 140 indicates a female baby, but there are more reliable tests!

Cardiac filling

‘Filling’ reflects the flow of blood back into the heart (venous return from the lungs and the body). William Harvey observed that the presence of valves requires that blood in the larger veins can only flow towards the heart, the key to recognizing that blood circulates. Amongst other factors, the extent of muscular activity, breathing movements, and body positions (standing, lying, arms or legs raised) all affect the rate of return of blood to the heart. Cardiac muscle shows the unusual property that, within limits, it contracts more powerfully when starting from stretched lengths, so that the ventricle ‘empties’ more forcibly when it is ‘filled’ more than usual. This is achieved at trivial extra metabolic cost; the efficiency of pumping thus increases as output increases; surely a paradigm for ‘productivity gains’. This property allows the heart to compensate automatically when the volume of blood within it at the start of the beat (the end diastolic volume) is greater than previously, by pumping more forcefully, thus ejecting a larger volume. This feature is termed Starling's ‘Law of the Heart’, after one of its discoverers.

Contractility

It is obvious that an intrinsically stronger heart will be able to eject blood more forcefully and more completely. Unlike our voluntary (skeletal) muscles, the ‘strength’ of heart muscle can vary quickly, even from one beat to the next. This is because it is sensitive to chemical influences (especially of adrenaline/noradrenaline) and electrical influences that can rapidly modify the intracellular processes that underlie contraction. Additionally, as with voluntary muscle, the extent of growth and development of the heart muscle will affect the overall strength of the organ; athletes generally have thicker heart walls which match the larger muscles in their thicker limbs. A normal, sudden increase in contractility is associated with the onset of physical activity or even with its anticipation; this is signalled to the heart, along with the increase in heart rate, by activity in the sympathetic nerve fibres which release noradrenaline. The combination of higher rate and stronger, more rapid contraction tends to match cardiac output to the increased ‘demands’ for blood flow to the exercising muscles.

The heart of the matter and the matter of the heart

The control systems which influence the heart rate and strength of beating are the same as those implicated in such apparently diverse processes as blushing, breathing rate, sexual arousal, mental stress, or alertness. These links seem to have been recognized by our forebears in advance of the definitive precision of the discoveries of cardiovascular physiology. Poets report that hearts leap, hearts are strong, hearts are united, hearts are hot, heart strings are plucked, hearts are ‘in the mouth’, hearts become feeble, hearts are chilled, hearts tremble, and hearts are broken. In human history, the nature of the circulation of the blood and the (quite literally) central role of the heart in this system are still recent discoveries, even though they rank with the very earliest of the truly ‘modern’ scientific method. Nevertheless, the heart (with perhaps the eye) is the organ most quoted in literature and song to define the essential qualities of life and even its very presence. The ready perception of the action of the heart, its racing rate when we are excited or surprised, aroused or shocked, the shallow, rapid beat encountered in feverish poor health, the occasional irregularity of beat that can concern us all (often, thankfully, quite unnecessarily), together form the shared ‘heart’ experiences of mankind that writers and poets have ever drawn upon. We are generally blissfully unaware of the other hives of metabolic industry that contribute to our physiology. The liver, the thyroid, the hypothalamus, the pituitary, the spleen, the pancreas, not one of these is dignified with a property recognizable to their owners. It is surely the literal vitality of the heart's rhythmic beating, the recognition of its link to the movements of blood, the necessary identity between this continual activity and life itself (outside an operating theatre) that validates the continuing truth of poetic notions of ‘heart’

David J. Miller

See cardiovascular system.See also autonomic nervous system; blood pressure; blood circulation; blood vessels; cardiac muscle; heart attack; heart block; heart failure; heart sound.