Cardiac Cycle and Heart Structure — GCSE Biology

The heart is a muscular pump divided into four chambers that work in a coordinated cycle to push blood through two circuits simultaneously — the pulmonary circuit to the lungs and the systemic circuit to the rest of the body. Each complete sequence of filling and emptying is one cardiac cycle, occurring about 70 times per minute at rest.

What are the four chambers of the heart and what does each do?

The heart is divided vertically by the septum into a right side and a left side, which never mix blood. Each side has two chambers:

• Right atrium — receives deoxygenated blood from the body via the vena cava (superior and inferior).
• Right ventricle — pumps deoxygenated blood to the lungs via the pulmonary artery.
• Left atrium — receives oxygenated blood from the lungs via the pulmonary veins.
• Left ventricle — pumps oxygenated blood to the entire body via the aorta.

The left ventricle has a much thicker muscular wall than the right because it must generate sufficient pressure to push blood through the full systemic circuit — a much longer and higher-resistance pathway than the short route to the lungs.

What are the stages of the cardiac cycle?

The cardiac cycle has three main phases:

1. Diastole (relaxation and filling) All four chambers relax. Blood flows passively from the vena cava and pulmonary veins into the atria, and then on into the ventricles through the open atrioventricular valves. The semi-lunar valves (aortic and pulmonary) are closed.

2. Atrial systole (atrial contraction) Both atria contract simultaneously. This pushes the remaining blood (about 25 % of total stroke volume) from the atria into the ventricles, ensuring the ventricles are completely filled. Duration: approximately 0.1 seconds.

3. Ventricular systole (ventricular contraction) Both ventricles contract simultaneously. Rising pressure in the ventricles:

• Pushes the atrioventricular valves shut (producing the "lub" heart sound).
• Forces the semi-lunar valves open.
• Ejects blood into the aorta (left) and pulmonary artery (right).

When ventricular pressure falls as contraction ends, the semi-lunar valves snap shut (producing the "dub" heart sound) to prevent backflow.

What do heart valves do and where are they?

Valves ensure blood flows in one direction only through the heart.

Atrioventricular (AV) valves — between the atria and ventricles:

• Tricuspid valve — right side, three cusps.
• Bicuspid (mitral) valve — left side, two cusps. These open when atrial pressure exceeds ventricular pressure, and slam shut when ventricles contract to prevent backflow into the atria. Fibrous chords (chordae tendineae) tether the valve cusps to the ventricular wall so they cannot invert.

Semi-lunar valves — at the exit of each ventricle:

• Pulmonary valve — at the base of the pulmonary artery.
• Aortic valve — at the base of the aorta. These open when ventricular pressure exceeds arterial pressure, then close when the ventricles relax to prevent blood flowing back from the arteries into the heart.

What are the coronary arteries and why do they matter?

The heart muscle itself requires a continuous blood supply. The coronary arteries branch off the aorta just above the aortic valve and form a network across the surface of the heart, supplying it with oxygenated blood.

If a coronary artery becomes narrowed by fatty deposits (atherosclerosis), blood flow to part of the heart muscle is restricted — causing angina (chest pain on exertion). Complete blockage causes a myocardial infarction (heart attack): the deprived muscle cells die and the heart may stop pumping effectively.

Risk factors for coronary heart disease include smoking, high blood pressure, high LDL cholesterol, obesity, physical inactivity, and type 2 diabetes.

How does the heart rate change with exercise?

At rest, the heart beats approximately 60–80 times per minute. During exercise, the cardiovascular system must supply muscles with more oxygen and remove more carbon dioxide. Heart rate increases because:

• Rising CO₂ levels in the blood are detected by chemoreceptors, which signal the medulla oblongata (in the brain stem).
• The medulla sends signals via the sympathetic nervous system to the sinoatrial node (the heart's natural pacemaker, located in the right atrium).
• The sinoatrial node fires more frequently, increasing heart rate.
• Stroke volume (volume pumped per beat) also increases, so cardiac output rises rapidly.

Cardiac output = heart rate × stroke volume

For example: 70 beats/min × 70 mL/beat = 4,900 mL/min ≈ 5 litres per minute at rest. During intense exercise this can rise to 20–25 litres per minute in a trained athlete.

Frequently asked questions

What is the cardiac cycle in GCSE biology?

The cardiac cycle is the sequence of events in one complete heartbeat. It has three phases: diastole (all chambers relax and fill with blood), atrial systole (atria contract to push blood into the ventricles), and ventricular systole (ventricles contract to pump blood into the aorta and pulmonary artery). The two heart sounds — "lub" and "dub" — are produced by the atrioventricular and semi-lunar valves closing respectively. The cycle takes about 0.8 seconds at rest.

Why does the left ventricle have a thicker wall than the right?

The left ventricle pumps oxygenated blood through the systemic circuit — to the brain, kidneys, muscles, and every organ in the body. This requires generating far higher pressure than pumping blood the short distance to the lungs (pulmonary circuit). A thicker, more muscular wall allows the left ventricle to produce the force needed. The thicker wall generates higher blood pressure in the aorta, which is why you can feel a strong pulse in systemic arteries.

What happens when a heart valve stops working properly?

A faulty valve may either fail to close fully (regurgitation), allowing blood to flow backwards, or fail to open properly (stenosis), restricting blood flow. Either problem forces the heart to work harder to maintain cardiac output. Symptoms include fatigue, breathlessness, and abnormal heart sounds (murmurs). Severely damaged valves can be replaced surgically with mechanical or biological prosthetic valves.

What is the role of the sinoatrial node?

The sinoatrial (SA) node is a small region of specialised muscle tissue in the right atrium that acts as the heart's natural pacemaker. It generates electrical impulses at a regular rate — about 70 per minute at rest — that spread across the atria, causing them to contract. The impulse is then passed to the atrioventricular (AV) node, which delays it briefly before conducting it down the bundle of His to the ventricles, causing them to contract from the base upwards. This coordination ensures the ventricles eject blood efficiently.

For Socratic GCSE biology with Professor Darwin — reasoning from cell to chamber to cardiac output so the heart's design becomes genuinely intuitive — visit aitutors.me.