The heart

1. Overview

The mammalian heart is a muscular double pump that maintains a double circulation: blood passes through the heart twice for every complete circuit of the body, once around the pulmonary (lung) circuit and once around the systemic (body) circuit. It has four chambers, two thin-walled atria on top and two thick-walled ventricles below, separated into right and left sides by the septum so that oxygenated and deoxygenated blood never mix. Valves keep blood moving in one direction, and these open and close because of pressure differences generated as the muscle contracts (systole) and relaxes (diastole). The whole sequence is coordinated by the heart's own electrical conducting system: the sinoatrial node, the atrioventricular node and the Purkyne tissue.

Key Definitions

• Atrium: an upper, thin-walled chamber of the heart that receives blood returning from the veins and pumps it into the ventricle below it.
• Ventricle: a lower, thick-walled chamber of the heart that pumps blood out into the arteries.
• Septum: the wall of muscle that separates the right side of the heart from the left side, keeping oxygenated and deoxygenated blood apart.
• Atrioventricular valve: a valve between an atrium and a ventricle that prevents backflow of blood into the atrium; the right one is the tricuspid valve and the left one is the bicuspid (mitral) valve.
• Semilunar valve: a valve at the base of the pulmonary artery and the aorta that prevents blood flowing back into the ventricles.
• Cardiac cycle: one complete sequence of contraction (systole) and relaxation (diastole) of the heart that pumps blood through the chambers.
• Systole: the phase of the cardiac cycle when heart muscle contracts and pressure in the chambers rises.
• Diastole: the phase of the cardiac cycle when heart muscle relaxes and the chambers refill with blood.
• Sinoatrial node: the pacemaker in the wall of the right atrium that initiates each heartbeat by sending out waves of electrical excitation.
• Purkyne tissue: specialised conducting fibres that carry electrical excitation rapidly through the ventricle walls so the ventricles contract together.

Content

External structure

Seen from the outside, the heart is a cone whose wall is mostly cardiac muscle (myocardium), enclosed in a tough membrane called the pericardium. The coronary arteries branch from the aorta just above the heart and run across its surface, supplying the heart muscle itself with oxygenated blood; if these become blocked the muscle is starved of oxygen.

The large vessels enter and leave at the top of the heart. Each one connects to a particular chamber and carries blood in a particular direction:

Vessel	Connects to	Carries	Direction
Vena cava	Right atrium	Deoxygenated blood	From the body into the heart
Pulmonary artery	Right ventricle	Deoxygenated blood	From the heart to the lungs
Pulmonary vein	Left atrium	Oxygenated blood	From the lungs into the heart
Aorta	Left ventricle	Oxygenated blood	From the heart to the body

Notice the pattern: the two veins (vena cava, pulmonary vein) bring blood into atria; the two arteries (pulmonary artery, aorta) carry blood out of ventricles.

Internal structure

Inside, the heart is divided into four chambers. The two atria sit above the two ventricles, and the muscular septum runs down the middle, separating the right side (which handles deoxygenated blood) from the left side (which handles oxygenated blood). Between each atrium and the ventricle below it is an atrioventricular (AV) valve: the tricuspid valve (three flaps) on the right and the bicuspid valve (two flaps, also called the mitral valve) on the left. These valves are anchored by tendinous cords ("heart strings") that run to the ventricle walls, preventing the valves from turning inside out when the ventricles contract at high pressure. At the exits from the ventricles, semilunar valves guard the openings into the pulmonary artery and the aorta.

Why the wall thicknesses differ

The thickness of each chamber's wall matches the distance the blood must be pushed and the pressure needed.

• Atria vs ventricles: the atria have thin walls because they only pump blood a short distance into the ventricles directly below them, which needs little force. The ventricles have thick walls because they must generate enough pressure to push blood out of the heart and around a whole circuit, so their thicker muscle can contract more powerfully.
• Left ventricle vs right ventricle: the left ventricle wall is much thicker than the right. The left ventricle pumps blood through the systemic circuit to the entire body, which has high resistance, so it must produce a high pressure. The right ventricle only pumps blood the short distance to the nearby lungs (the pulmonary circuit), at a lower pressure that protects the delicate lung capillaries, so a thinner wall is sufficient.

The cardiac cycle and the valves

The cardiac cycle is one full heartbeat, controlled entirely by changing pressures that open and close the valves. A valve opens when the pressure behind it is higher than the pressure in front of it, and closes (snapping shut) when the pressure in front becomes higher.

1. Atrial systole: both atria contract. Atrial pressure rises just above ventricular pressure, so the AV valves are open and blood is pushed down into the ventricles. The semilunar valves remain shut.
2. Ventricular systole: the ventricles contract. Ventricular pressure rises sharply and quickly exceeds atrial pressure, so the AV valves close (giving the first heart sound and preventing backflow into the atria). For a brief moment after this all four valves are closed - the AV valves have snapped shut but ventricular pressure has not yet climbed above arterial pressure - so the ventricle contracts on a fixed volume of blood and pressure rises steeply (this is the steepest part of the ventricular curve on the pressure trace). When ventricular pressure rises above the pressure in the aorta and pulmonary artery, the semilunar valves open and blood is forced out of the heart.
3. Diastole (relaxation): all the chambers relax. Ventricular pressure falls below the pressure in the arteries, so the semilunar valves close (the second heart sound), stopping blood flowing back into the ventricles. As ventricular pressure also falls below atrial pressure, the AV valves open again and blood flows from the veins through the relaxed atria into the ventricles, refilling the heart ready for the next cycle.

The single most important idea is that each valve event is caused by one pressure overtaking another, not by the chambers "deciding" to contract. The table below summarises the whole cycle on the left side of the heart:

Phase	Pressure comparison	AV (bicuspid) valve	Semilunar (aortic) valve
Atrial systole	Atrial > ventricular	Open	Closed
Ventricular systole	Ventricular > atrial, then ventricular > aortic	Closes, then stays closed	Opens once ventricular > aortic
Diastole	Ventricular < aortic, then ventricular < atrial	Opens once ventricular < atrial	Closes once ventricular < aortic

The pressure trace below makes this visible: it illustrates the shape of the left-atrial, left-ventricular and aortic pressure curves through one beat. The aortic valve opens at the moment the ventricular curve rises above the aortic curve, and closes again when the ventricular curve drops back below it. Reading any cardiac-cycle graph comes down to spotting these crossover points between the curves rather than reading off exact pressure values.

The conducting system: SAN, AVN and Purkyne tissue

Cardiac muscle is myogenic, meaning it can contract on its own without a signal from the nervous system. The rhythm is set and coordinated by specialised tissue (knowledge of nervous and hormonal control is not required here).

• The sinoatrial node (SAN), in the wall of the right atrium, acts as the pacemaker. It generates a wave of electrical excitation that spreads across both atria, causing them to contract together (atrial systole). This is a wave of electrical activity passing through the muscle itself - it is not a nerve impulse sent down a nerve.
• A band of non-conducting tissue between the atria and ventricles stops the excitation passing straight down. Instead it reaches the atrioventricular node (AVN), which delays the impulse for a fraction of a second. This delay is essential: it allows the atria to finish emptying their blood into the ventricles before the ventricles contract.
• The AVN then passes the excitation into the Purkyne tissue, conducting fibres that run down the septum to the apex (the bottom tip of the heart) and then spread up through the ventricle walls. Because the impulse is carried quickly to the apex first, the ventricles contract from the bottom upwards, squeezing blood efficiently up and out into the aorta and pulmonary artery.

Worked example

Exam-style question: During the cardiac cycle, explain why the impulse from the sinoatrial node is delayed at the atrioventricular node, and why the Purkyne tissue then carries the impulse to the apex of the heart before the ventricles contract. [3]

Model answer:

• The delay at the AVN gives the atria time to finish contracting and push all their blood into the ventricles before the ventricles begin to contract.
• The Purkyne tissue conducts the impulse rapidly to the apex (the bottom tip) of the heart so that the ventricles contract from the base of the ventricles upwards.
• Contracting from the bottom up forces blood upwards towards the arteries (the aorta and pulmonary artery), so the ventricles empty efficiently.

Worked example

Exam-style question: A pressure-time trace plots how the pressure in the left atrium, left ventricle and aorta changes during one cardiac cycle. On that trace, point X is where the ventricular curve crosses above the atrial curve and the bicuspid (AV) valve closes, and point Y is where the ventricular curve crosses above the aortic curve and the aortic (semilunar) valve opens. Using the curves, explain what is happening to the pressures at points X and Y to cause each valve event. [4]

Model answer:

• At X, the ventricular pressure has just risen above the atrial pressure. Because the pressure in front of the AV valve (ventricle) is now higher than the pressure behind it (atrium), the valve is forced shut, stopping backflow into the atrium.
• This happens at the start of ventricular systole, as the ventricle muscle begins to contract and pressure climbs steeply.
• At Y, the ventricular pressure has now risen above the aortic pressure. Once the pressure behind the semilunar valve (ventricle) exceeds the pressure in front of it (aorta), the valve is pushed open and blood is ejected into the aorta.
• In both cases the valve event is identified by reading off which curve has just crossed above the other - the valve always responds to the direction of the pressure difference, never the other way round.

Key Equations

This topic is qualitative; it is understood through structure, pressure changes and the sequence of electrical events rather than calculations.

Common Mistakes to Avoid

• Defining double circulation without mentioning the heart. Do not just say "blood goes to the lungs and the body". State that blood passes through the heart twice for each complete circuit, or name the pulmonary and systemic circuits.
• Saying valves open or close "because the heart tells them to". Valves are passive: they open and close only because of pressure differences. Always state which pressure is higher than which.
• Naming a valve event on a graph without saying which pressure exceeds which. When you read a pressure-time trace, never just write "the AV valve closes here". You must link the event to the direction of the pressure difference - for example, "ventricular pressure has risen above atrial pressure, so the AV valve closes". This is the most common slip in cardiac-cycle questions, and the marks are for the pressure reasoning, not for naming the valve.
• Claiming the left ventricle is thicker "because it pumps oxygenated blood". The oxygen content is irrelevant. The left wall is thicker because it must generate a higher pressure to pump blood around the whole body, against greater resistance, than the right ventricle needs for the nearby lungs.
• Confusing the apex with the base of the heart. The apex is the pointed bottom tip; the Purkyne tissue carries the impulse there so the ventricles contract from the bottom upwards. Do not call the apex "the top of the heart".
• Mixing up the AV valves. The tricuspid valve is on the right and the bicuspid (mitral) valve is on the left. A simple memory aid is "tri before bi, right before left".
• Writing that the SAN "sends a nerve impulse". The SAN sets off a wave of electrical excitation that passes through the cardiac muscle itself - it is not a nerve and it does not send a signal along a nerve. Cardiac muscle is myogenic, so the beat starts in the heart; nerves and hormones only adjust the rate and are not needed for this section.

Exam Tips

• For pressure-trace graphs, identify each valve event by finding where two pressure lines cross over: an AV valve closes when ventricular pressure rises above atrial pressure, and a semilunar valve opens when ventricular pressure rises above arterial pressure.
• When asked to compare wall thicknesses, always finish the logic chain: thicker wall to more muscle to greater force to higher pressure to blood pushed further. A bare statement like "it is thicker" earns nothing.
• Use the precise sequence SAN to atria to AVN (delay) to Purkyne tissue to ventricles when describing coordination, and explain why the AVN delay matters.
• Spell the conducting terms carefully and use the correct names: sinoatrial node, atrioventricular node, Purkyne tissue, tricuspid and bicuspid (mitral) valves.
• Keep oxygenated and deoxygenated blood on the correct sides: deoxygenated blood is on the right, oxygenated blood is on the left, kept apart by the septum.