The mammalian heart is two pumps side by side. Explore its chambers, walls and valves, then play the cardiac cycle to see how pressure changes open and close the valves and how the SAN, AVN and Purkyne tissue coordinate each beat.
The heart has four chambers: thin-walled atria receive blood from the vena cava (right) and pulmonary vein (left) and pump it a short distance into the ventricles. The left ventricle has a thicker muscular wall than the right because it pumps blood at high pressure all round the body, while the right ventricle pumps only to the nearby lungs at lower pressure. The septum keeps oxygenated and deoxygenated blood apart. Atrioventricular valves stop backflow into the atria; semilunar valves stop backflow from the aorta and pulmonary artery. Valves open and close only because of pressure differences. In the cardiac cycle, atrial systole tops up the ventricles, ventricular systole closes the AV valves and opens the semilunar valves, and in diastole the semilunar valves close and the AV valves open as the heart refills. The sinoatrial node starts each wave of excitation, the atrioventricular node delays it, and the Purkyne tissue carries it to the apex so the ventricles contract from the bottom up.
Click each part of the heart: the atria, ventricles and septum, the two kinds of valve, the four vessels, then the SAN, AVN and Purkyne tissue. Then press Play the cardiac cycle and watch the PHASE and VALVES readouts and the pressure graph.
| Part | Structure | Why |
|---|---|---|
| atria | thin muscular walls | pump blood only a short distance, into the ventricles, at low pressure |
| right ventricle | thicker than the atria, thinner than the left ventricle | pumps blood only to the nearby lungs; lower pressure avoids damaging the lung capillaries |
| left ventricle | the thickest wall | must generate a high pressure to push blood through the whole systemic circulation |
| septum | muscular wall between left and right sides | keeps oxygenated and deoxygenated blood apart; carries the Purkyne tissue |
| atrioventricular valves | between each atrium and ventricle (tricuspid on the right, bicuspid on the left), held by tendons | prevent backflow into the atria when the ventricles contract |
| semilunar valves | at the base of the aorta and pulmonary artery | prevent backflow into the ventricles when they relax |
External structure: the aorta and pulmonary artery leave the top of the heart, the vena cava and pulmonary veins enter the atria, and coronary arteries on the surface supply the heart muscle itself. Diagrams show the heart as if facing the person, so the left side is on your right.
The ventricles have thicker walls than the atria because of the distance and resistance they pump against, not because they hold more blood. Both ventricles pump the same volume per beat.
Use the graph in the lab and Fig. 1. A valve opens when the pressure behind it is higher than the pressure in front of it, and shuts when the pressure reverses.
Four quick checks. Each correct answer earns XP and records this skill on your profile.
In the heart model, which chamber is described as having the thickest wall?
During the phase labelled ventricular systole (ejection), what does the VALVES readout show?
On the lab's pressure graph, about what is the peak pressure in the left ventricle?
According to the lab, how long does the AVN delay the wave of excitation?
Heart muscle is myogenic: it starts its own beat. Three specialised regions make the chambers contract in the right order.
The AVN does not start the heartbeat; the SAN does. The AVN's job is the delay.
Unlocks once the four checks above are done. Worth more XP, written to AS Paper 1 and Paper 2 standard.
What causes the atrioventricular valves to close at the start of ventricular systole?
Why is the wall of the right ventricle thinner than the wall of the left ventricle?
What is the role of the Purkyne tissue?
A cardiac cycle lasts 0.8 s. What is the heart rate?
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