The heart and cardiac cycle

Why large active animals need mass transport

A flatworm has no heart, no blood and no vessels. It is flat and thin, no cell in it lies more than a fraction of a millimetre from the surface, and diffusion supplies oxygen to every cell. Increasing the size of such an animal creates two separate problems.

Describe the drawing

Three cubes drawn side by side with sides of one, two and four centimetres, each labelled with its surface area to volume ratio: six, three and one point five per centimetre. The cubes get larger from left to right while the ratio falls, showing that a bigger body has proportionally less surface across which to exchange materials.

The first is the surface area to volume ratio. Volume rises with the cube of length while surface area rises only with the square, so a big animal has proportionally far less exchange surface per gram of tissue than a small one. The second problem is the diffusion distance. Doubling the distance does not double the time taken; it roughly quadruples it. Over a few micrometres diffusion is effectively instantaneous; over a few centimetres it takes days.

A third factor applies to mammals and birds. They are endothermic and have a high metabolic rate, so their demand for oxygen and glucose per gram of tissue is high and their production of carbon dioxide is correspondingly high. Large size, high activity and endothermy together make a mass transport system necessary.

Mass transport system

A system in which substances are moved in bulk, in a fluid driven by a pump, over distances too great for diffusion to serve.

Surface area to volume ratio

The exchange surface available per unit of volume; it falls as an object gets larger.

Metabolic rate

The rate at which an organism uses energy, and so the rate at which it must be supplied with oxygen and relieved of carbon dioxide.

Single and double circulation

A fish has a two-chambered heart and a single circulation: blood is pumped from the heart to the gills, and from the gills it passes directly to the body before returning. Passage through the gill capillaries causes a large drop in pressure, so blood leaves the gills at low pressure and circulates slowly through the rest of the fish. The arrangement is adequate because a fish's oxygen demand per unit mass is comparatively low; not maintaining a body temperature above the surroundings is part, but not the whole, of the explanation.

In a double circulation the blood returns to the heart between the two circuits. The right side sends it to the lungs and it returns to the left side, which pumps it out again at a pressure the gas exchange surface does not have to withstand. Each circuit therefore operates at the pressure suited to it.

Keep the two circuits straight by name. The pulmonary circuit runs right ventricle → lungs → left atrium and works at roughly 24/8 mmHg. The systemic circuit runs left ventricle → body → right atrium at roughly 120/80 mmHg. Five times the pressure, on the same volume of blood per minute.

Within the systemic circuit the organs are supplied in parallel: most receive their own branch of the aorta and drain back into the vena cava. Learn the names of the vessels serving the heart, lungs and kidneys. The renal artery carries blood from the aorta to each kidney and the renal vein returns it to the vena cava. The lungs are supplied by the pulmonary artery and drained by the pulmonary veins, and the heart muscle is supplied by the coronary arteries, the first branches of the aorta.

The structure of the mammalian heart

The heart is two pumps side by side, sharing a wall and contracting together. The right side receives blood returning from the body and pumps it to the lungs; the left side receives blood returning from the lungs and pumps it to the rest of the body. The septum between them keeps oxygenated and deoxygenated blood from mixing, which matters because mixed blood would deliver less oxygen per unit volume.

Describe the drawing

A vertical section through the heart seen from the front, so the left side of the heart is drawn on the right. Deoxygenated blood arrives from the vena cava into the right atrium, passes an atrioventricular valve into the thin-walled right ventricle and leaves by the pulmonary artery. Oxygenated blood returns by the pulmonary vein into the left atrium, passes the left atrioventricular valve into the left ventricle, whose wall is drawn three times thicker, and leaves by the aorta. Semilunar valves are drawn at the base of both arteries and a septum separates the two sides.

Trace the route of the blood. Vena cava → right atrium → atrioventricular valve → right ventricle → semilunar valve → pulmonary artery → lungs → pulmonary vein → left atrium → atrioventricular valve → left ventricle → semilunar valve → aorta. Learn that sequence in full: questions about septal defects, faulty valves and blocked coronary arteries all depend on it.

Two vessels are named against the usual pattern. The pulmonary artery carries deoxygenated blood and the pulmonary vein carries oxygenated blood. An artery is a vessel carrying blood away from the heart and a vein one carrying it back; oxygenation is not part of the definition.

The wall thicknesses relate directly to the pressures generated. Atria have thin walls: they push blood a few centimetres into a ventricle that is already relaxed and filling. The right ventricle wall is around 5 mm thick and generates about 24 mmHg, sufficient to drive blood through the lungs. A higher pressure would force fluid out of the thin-walled alveolar capillaries. The left ventricle wall is around 15 mm, three times as thick, with far more cardiac muscle, and generates about 120 mmHg, which is required to drive blood around the whole systemic circuit.

Atrioventricular valves

The valves between each atrium and its ventricle; tricuspid on the right, bicuspid or mitral on the left.

Semilunar valves

The valves at the base of the aorta and the pulmonary artery, preventing backflow into the ventricles.

Tendinous cords

Inelastic strands running from the atrioventricular valve flaps to papillary muscles projecting from the ventricle wall. The muscles tense as the ventricle contracts, so ventricular pressure cannot turn the flaps inside out; the pressure does the opening and shutting, the cords only stop the flaps going too far.

Coronary arteries

The vessels branching from the base of the aorta that supply the heart muscle itself with oxygenated blood.

One beat, three stages

At rest a cycle takes about 0.8 s, which is 75 beats a minute. Systole means a chamber is contracting; diastole means it is relaxing. The stages below are described by what the pressure is doing, because that is what the valves respond to.

Describe the drawing

Three rows, one for each stage of the cardiac cycle. Each row draws an atrium above a ventricle with an artery leaving to the right, and the valves are drawn open or shut. In atrial systole the atrioventricular valve hangs open and the semilunar valve is a closed bar. In ventricular systole the atrioventricular valve is a closed bar and the semilunar valve is open with blood leaving. In diastole the atrioventricular valve is open again and the semilunar valve shut. Beside each drawing is the pressure change that moves those valves.

In atrial systole the atria contract, atrial pressure rises above ventricular, and the atrioventricular valves, already open, stay open while the last 20 to 30 per cent of the ventricles' filling is completed. Ventricular pressure is nowhere near aortic, so the semilunar valves stay shut. This stage lasts about 0.1 s.

In ventricular systole the ventricles contract from the apex upwards. Ventricular pressure shoots past atrial pressure and forces the atrioventricular valves shut, which is the first heart sound. For a moment both sets of valves are closed and the blood has nowhere to go, so the pressure rises without any change in volume. The instant ventricular pressure passes aortic pressure the semilunar valves are forced open and blood is ejected. About 0.24 s, measured from the first heart sound to the second.

In diastole the ventricles relax and their pressure falls. As soon as it drops below aortic pressure, blood in the aorta pushes back on the semilunar valves and shuts them: the second heart sound. Pressure keeps falling until it is below atrial pressure, at which point the atrioventricular valves open and blood flows in from the veins through the relaxed atria and straight into the ventricles. Roughly 70 per cent of ventricular filling happens here, before the atria contract. About 0.46 s, so the ventricles spend more of every cycle relaxed than contracting, which allows cardiac muscle to be perfused between contractions.

Those three intervals total the 0.8 s of the whole cycle. They need not be memorised: the end of atrial systole and the end of ventricular systole can be read from the pressure graph as the points where two curves cross, at 0.10 s and 0.34 s.

Reading the pressure-time graph

The graph shows three curves, and the first step is to identify each of them from its shape.

Describe the drawing

A graph of pressure in millimetres of mercury against time in seconds for one heartbeat. The left ventricle curve starts low, rises steeply to a peak of 120, then falls back. The aorta curve never drops below about 80 and rides with the ventricle while blood is being ejected. The left atrium curve stays under 15 throughout. Four points are ringed where the curves cross, numbered one to four, and each is the moment a valve opens or shuts.

The ventricle is the curve with the largest range: near zero, up to about 120 mmHg, and back to near zero. The aorta is the curve that never falls below about 80 mmHg, because elastic recoil in the arterial walls holds pressure up between beats. The atrium is the low, flat curve, never above about 15 mmHg. Identify all three before answering.

The four valve events follow in order, each at a crossing of two curves:

Between 0.10 s and 0.18 s the ventricle is contracting while both valves are closed, so no blood leaves it. Pressure rises and volume is unchanged.

A ventricular volume curve follows from the same four events. Volume rises through diastole as blood flows in through the open atrioventricular valves, and a little more in atrial systole, to about 130 cm³ when those valves shut at 0.10 s. It stays level until the aortic valve opens at 0.18 s, falls to about 60 cm³ during ejection, and is level again from 0.34 s to 0.44 s, while every valve is shut and the ventricle relaxes. The fall between the two level sections is the stroke volume, about 70 cm³.

Cardiac output

Two quantities describe the output of the heart. Stroke volume is the volume pushed out by one ventricle in one beat, about 70 cm³ at rest. Heart rate is beats per minute, about 75 at rest for an adult, which corresponds to a 0.8 s cycle. The product of the two gives the volume leaving one ventricle each minute.

cardiac output = stroke volume × heart ratecm³ min⁻¹ = cm³ × min⁻¹; check the units before substituting

At rest that gives roughly 70 × 75 = 5250 cm³ min⁻¹, or 5.25 dm³ per minute, which is approximately the whole blood volume once a minute. A trained athlete at full effort can reach 30 dm³ min⁻¹ or more, partly by raising heart rate towards 190 and partly by raising stroke volume well beyond 100 cm³. Endurance training raises resting stroke volume, which is why a very fit person can have a resting heart rate in the forties and still deliver the same 5 dm³.

Myogenic control and the conduction pathway

An isolated heart continues to beat for a period, which shows that the impulse originates within the cardiac muscle rather than in the brain. Cardiac muscle is myogenic. Nerves from the medulla and hormones such as adrenaline change the rate, but they do not create the beat.

The sinoatrial node (SAN) in the wall of the right atrium depolarises spontaneously about 75 times a minute and is the pacemaker. The wave of depolarisation spreads across both atria through the muscle itself, so both atria contract together, top down.

Between the atria and the ventricles is a band of connective tissue that will not conduct. The only way through is the atrioventricular node (AVN), and the AVN holds the impulse for roughly 0.13 s before passing it on. That delay allows the atria to finish emptying before the ventricles contract. Without it, atria and ventricles would contract simultaneously and ventricular filling would be incomplete.

From the AVN the impulse runs down the bundle of His through the septum to the apex of the heart, then out into the Purkyne fibres that carry it up through the ventricle walls. Because contraction begins at the apex, the ventricles contract from the apex upwards, so blood is driven towards the arteries leaving the top of the heart.

Myogenic

Contracting from within the muscle itself, without needing a nerve impulse to initiate the beat.

Sinoatrial node

The patch of tissue in the right atrium wall that sets the rhythm of the heart.

Atrioventricular node

The only electrical connection between atria and ventricles; it delays the impulse so the atria empty first.

Purkyne fibres

Conducting fibres running from the bundle of His up through the ventricle walls, so contraction begins at the apex.

Reading an electrocardiogram

That wave of depolarisation is an electrical event, and electrical events in a large mass of muscle produce small currents that spread through the tissues and reach the skin. Put electrodes on the skin, measure the potential difference between them, and plot it against time, and the result is an electrocardiogram. State what the trace records: the electrical activity of the heart muscle, not the movement of the walls, the pressure in a chamber or the flow of blood. Contraction follows each deflection a fraction of a second later. Because the trace records electrical activity only, it can appear close to normal in a heart with impaired pumping.

Describe the drawing

A normal electrocardiogram drawn twice over on axes of potential difference in millivolts against time in seconds. Each beat shows a small P wave, a tall narrow QRS complex reaching one millivolt, and a broader T wave, and the gap between the two R peaks is marked as nought point eight zero seconds. Below it, four further traces: tachycardia with the beats crowded together, bradycardia with only two beats in the same time, atrial fibrillation with an unsteady baseline, no P waves and unevenly spaced QRS complexes, and an ectopic beat, where an otherwise regular rhythm is broken by a third complex that arrives early, has no P wave in front of it, is broader than the others and has a T wave pointing the other way, and is followed by a gap longer than the regular interval before the rhythm resumes.

One beat gives three deflections, and each one is a piece of the conduction pathway from the previous section. The P wave is the depolarisation of the atria, spreading out from the SAN, and atrial contraction follows it. The QRS complex is the depolarisation of the ventricles, carried down the bundle of His and out along the Purkyne fibres, and ventricular contraction follows it. It is much the largest deflection because there is far more muscle in the ventricle walls than in the atria, so far more tissue depolarises at once. The T wave is the repolarisation of the ventricles, the muscle returning to its resting state before the next beat. There is no separate wave for atrial repolarisation; it happens underneath the QRS complex and is hidden by it.

The flat section between the end of the P wave and the start of the QRS complex corresponds to the AVN delay. No large mass of muscle is depolarising during it, because the impulse is held at the node while the atria finish emptying.

Electrocardiogram

A recording of the potential difference measured at the skin against time, produced by the wave of depolarisation spreading through the heart muscle.

P wave

The deflection produced by depolarisation of the atria, which atrial contraction follows.

QRS complex

The large deflection produced by depolarisation of the ventricles, large because the ventricle walls hold most of the heart's muscle.

T wave

The deflection produced by repolarisation of the ventricles.

Obtain a heart rate from a trace by measurement rather than by counting. Find two R peaks, which are the easiest points to locate because they are the tallest and the narrowest. Read the time of each off the horizontal axis and subtract, giving the interval between beats in seconds. Then heart rate in beats per minute is 60 divided by that interval. In the figure the R peaks are 0.80 s apart, so the rate is 60 ÷ 0.80 = 75 beats per minute. Measure across several intervals and divide rather than taking one, because a millimetre's error in reading one peak matters much less when it is spread over four intervals than over one.

Four abnormal traces are named on the specifications, and each is recognised by one feature.

Distinguish two of those carefully. Bradycardia is a slow but regular rhythm: the intervals are long and equal, and every QRS complex is preceded by a P wave. Atrial fibrillation is irregular rather than slow: the intervals differ from beat to beat, and the P waves are absent, because the atrial muscle depolarises in an uncoordinated way rather than as one wave. When distinguishing two traces, name the feature used.

The interpretation of a bradycardia depends on the individual. A resting rate of 45 beats per minute in an endurance athlete is a sign of a large stroke volume, not of disease: the same cardiac output is being delivered in fewer, bigger beats. The same rate in someone untrained who is dizzy on standing is a different matter.

Control of heart rate

Myogenic means the heart does not require a nerve impulse to initiate a beat. It does not mean the rate is fixed. Nothing outside the heart initiates a beat, but nervous and hormonal signals from outside it adjust the frequency.

Heart rate is controlled from the cardiac centre in the medulla oblongata (the cardiovascular control centre in some specifications), which acts on the SAN through the autonomic nervous system. Two nerves reach the node with opposite effects. Impulses along the sympathetic nerve raise the rate; impulses along the parasympathetic branch of the vagus nerve lower it. At the node, sympathetic nerve endings release noradrenaline and vagus nerve endings release acetylcholine. Both operate continuously at some level, so the rate at any moment is set by the balance between them rather than by one of them being switched on.

The medulla receives input from two kinds of receptor, which detect different changes.

Chemoreceptors, in the carotid bodies and the aortic body and the medulla itself, monitor the pH of the blood. Exercise makes muscle respire faster, carbon dioxide concentration rises, more carbonic acid forms and pH falls. The chemoreceptors detect the fall and the medulla increases sympathetic impulses to the SAN, so heart rate rises, so carbon dioxide is carried to the lungs and removed faster, so pH returns towards normal and the frequency of impulses from the receptors falls. Set that out as a negative feedback loop rather than as a list of separate statements.

Pressure receptors, also called baroreceptors, sit in the walls of the carotid sinus and the aorta and monitor blood pressure. High pressure means more parasympathetic impulses and a slower, weaker beat, which brings the pressure down; low pressure means more sympathetic impulses and a faster, stronger beat, which brings it up. State the direction carefully: the receptor that detects a rise in pressure triggers a fall in cardiac output, not an increase.

Chemoreceptor

A receptor that detects a chemical change; those controlling heart rate detect the fall in blood pH caused by dissolved carbon dioxide.

Pressure receptor

A receptor in an artery wall that detects a change in blood pressure; also called a baroreceptor.

Medulla oblongata

The region of the brain that receives impulses from the chemoreceptors and pressure receptors and adjusts heart rate through the autonomic nervous system.