Biology › Animal transport and cardiovascular biology › The heart and the cardiac cycle: pressure decides everything

The heart and the cardiac cycle: pressure decides everything

Four chambers, four valves, two circuits and about 0.8 seconds a beat. Once you can say which pressure is higher than which at any moment, every valve movement, every curve on the graph and every sound the heart makes follows from that.

Before this Surface area to volume ratio and diffusion distance · Muscle as a tissue that shortens when stimulated

What you should be able to do

• Explain why a large, active animal needs a mass transport system when a flatworm does not.
• Say what double circulation gives a mammal that single circulation cannot.
• Label the mammalian heart and account for the thickness of the left ventricle wall in terms of the pressure it generates.
• Describe the three stages of the cardiac cycle, giving the pressure change responsible for every valve movement.
• Read a pressure-against-time graph: identify each curve and mark where each valve opens and shuts.
• Calculate cardiac output, stroke volume or heart rate from the other two.
• Trace an impulse from the SAN to the ventricle walls and explain why the delay at the AVN matters.

Small animals get away with it; you do not

A flatworm has no heart, no blood and no vessels, and it manages perfectly well. It is flat and thin, no cell in it sits more than a fraction of a millimetre from the surface, and oxygen diffusing in reaches everything that needs it. Scale that animal up and two separate things go wrong.

The first is the ratio you can see above. 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 worse and is often left out of answers: diffusion is hopeless over distance. Doubling the distance does not double the time taken, it roughly quadruples it. Over a few micrometres diffusion is effectively instant; over a few centimetres it takes days.

Now add the third pressure on the system. Mammals and birds are endothermic and metabolically expensive, so their demand for oxygen and glucose per gram of tissue is high and their production of carbon dioxide is high with it. Large, active and warm is the combination that makes a mass transport system unavoidable.

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.

Two circuits, and why one is not enough

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 carries straight on to the body before returning. That sounds efficient until you notice what capillaries do to pressure. Squeezing blood through the gill capillaries costs almost all of it, so blood leaves the gills at low pressure and dawdles round the rest of the fish. It works because a fish is not trying to hold its body temperature 20 °C above its surroundings.

In a double circulation the blood comes back to the heart in between. The right side sends it to the lungs and it returns to the left side, which sends it out again at a pressure the gas exchange surface never had to survive. Each circuit gets the pressure that suits it, and that is the whole argument.

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.

What is actually inside the heart

Two pumps side by side, sharing a wall and beating together. The right side handles blood coming back from the body and sends it to the lungs; the left side handles blood coming back from the lungs and sends it everywhere else. The septum between them keeps oxygenated and deoxygenated blood from mixing, which matters because mixed blood would deliver less oxygen per unit volume.

Chase a red blood cell round it. Vena cava → right atrium → atrioventricular valve → right ventricle → semilunar valve → pulmonary artery → lungs → pulmonary vein → left atrium → atrioventricular valve → left ventricle → semilunar valve → aorta. That sequence is worth being able to write out cold, because questions about holes in the septum, faulty valves or blocked coronary arteries all assume you have it.

Two names catch people out. The pulmonary artery carries deoxygenated blood and the pulmonary vein carries oxygenated blood. An artery is a vessel taking blood away from the heart; a vein is one bringing it back. Oxygenation has nothing to do with the definition, it is simply true of every other artery in your body.

Now the wall thicknesses, which examiners like because they are pure structure-and-function. Atria have thin walls: they only have to 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, enough to drive blood through the lungs and no more — any harder and the pressure would force fluid out of the delicate alveolar capillaries. The left ventricle wall is around 15 mm, three times as thick, with far more cardiac muscle, and generates about 120 mmHg because its blood has to reach your toes and your brain and come back.

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 tethering the atrioventricular valve flaps to the ventricle wall, so that ventricular pressure cannot turn them inside out.

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.

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 slams 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 at all. About 0.46 s, so the ventricles spend more of every cycle relaxed than contracting — which is how cardiac muscle gets away with never resting.

Those three figures add to the 0.8 s the whole cycle takes, and none of them is a number to memorise: they are read off the pressure graph in the next section, where each boundary is a crossing of two curves.

Reading the pressure graph properly

This graph appears in every specification and most papers. There are three curves and the first job is always to identify them, which you can do without being told.

The ventricle is the curve with the enormous swing: near zero, up to about 120 mmHg, back to near zero. Nothing else in the chest does that. 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 flat one along the bottom, never above about 15 mmHg. Say which is which before you answer anything.

Then the four events, in order, each at a crossing:

Look at the gap between 0.10 s and 0.18 s. The ventricle is contracting hard for those 80 milliseconds, but both valves are shut and not one cubic centimetre of blood moves. Pressure climbs, volume stays put. If a question gives you a volume curve alongside the pressure curves, that flat stretch is the giveaway.

Cardiac output, and the arithmetic that goes with it

Two quantities describe how hard the heart is working. 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 — the rate a 0.8 s cycle gives you, and the two figures have to agree. Multiply them and you have the volume leaving the heart each minute.

cardiac output = stroke volume × heart ratecm³ min⁻¹ = cm³ × min⁻¹, so watch the units before you write anything down

At rest that comes to roughly 70 × 75 = 5250 cm³ min⁻¹, or 5.25 dm³ every minute — your entire blood volume, once a minute, at rest. 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 exactly why a very fit person can have a resting heart rate in the forties and still deliver the same 5 dm³.

Who tells the heart when to beat

Cut a heart out and it keeps beating for a while, which tells you something important: the signal starts inside the muscle, not 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 pause is not a flaw, it is the design: it gives the atria time to finish emptying before the ventricles start to contract. Without it, atria and ventricles would squeeze at once and the ventricles would fill badly.

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. Starting at the apex is the second piece of clever design: the ventricles contract from the bottom upwards, so blood is pushed towards the arteries at the top rather than being squashed against a closed floor.

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.

In the exam

• Name the pressure difference, every single time. 'The valve opens' scores nothing. 'Ventricular pressure rises above aortic pressure, so the semilunar valve is forced open' scores the mark and takes four more words.
• Never write that a valve contracts, relaxes, pulls or is pulled. Valves are flaps moved by the blood either side of them.
• On a pressure graph, identify the curves before you answer: the huge swing is the ventricle, the one that never drops below about 80 mmHg is the aorta, the flat one at the bottom is the atrium.
• Cardiac output questions almost always hide a conversion. 1 dm³ = 1000 cm³, and 'per minute' means the heart rate has to be per minute too.
• If you are asked about the AVN delay, say what it achieves — atria finish emptying before the ventricles contract — not just that it exists.
• The pulmonary artery carries deoxygenated blood and the pulmonary vein carries oxygenated blood. Define artery and vein by direction of travel.

Questions

Written to the command words the boards use. Try them on paper before opening a scheme: the marks go to points made, not to length.

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