The human heart: structure, cardiac cycle, heartbeat and double circulation
The human heart has four chambers and four valves. Each beat (about 0.8 s) is started by the SA node, and blood passes through it twice per round.
Parts of the heart
| Part | Feature and job |
|---|---|
| Position and cover | In the chest between the lungs, tilted to the left, about the size of your fist. A double membrane, the pericardium, covers it; the fluid inside reduces friction |
| Right auricle | Receives deoxygenated blood from the body through the superior and inferior vena cava |
| Left auricle | Receives oxygenated blood from the lungs through four pulmonary veins |
| Right ventricle | Pumps deoxygenated blood to the lungs through the pulmonary artery |
| Left ventricle | Has the thickest wall; pumps oxygenated blood to the whole body through the aorta |
| Septa | The interatrial and interventricular septa keep the right and left sides apart |
| Valves | Tricuspid (right auricle to ventricle), bicuspid or mitral (left side), and semilunar valves at the openings of the pulmonary artery and the aorta. Chordae tendineae hold the flaps. Valves let blood flow one way only |
| Coronary arteries | Supply oxygen to the heart muscle itself |
Double circulation
- back to Right auricle
**Pulmonary circulation**: right ventricle → lungs → left auricle. **Systemic circulation**: left ventricle → body → right auricle. Blood passes through the heart **twice** in one round, so oxygenated and deoxygenated blood never mix.
Cardiac cycle and heart sounds
One **cardiac cycle** is one complete heartbeat: a contraction (**systole**) and a relaxation (**diastole**) of the chambers. At 72 beats per minute, it lasts about **0.8 second** (60 ÷ 72 = 0.83 s, about 0.8 s).
- 1Auricular (atrial) systole, about 0.1 s: the auricles contract and push blood into the ventricles.
- 2★Ventricular systole, about 0.3 s: the ventricles contract. The tricuspid and bicuspid valves close, making the first sound, 'lubb'. Blood is pushed into the pulmonary artery and the aorta.
- 3★Ventricular diastole (joint diastole), about 0.4 s: all chambers relax. The semilunar valves close, making the second sound, 'dupp'. The heart refills with blood.
★ key step
Heartbeat: the pacemaker
The human heart is myogenic: each beat starts in the heart muscle itself, not in a nerve. The SA node (sinoatrial node) is a small patch of special muscle in the wall of the right auricle. It sends out a wave of excitation about 72 times a minute. It is the heart's pacemaker. The wave spreads over both auricles to the AV node, then through the AV bundle (bundle of His) and the Purkinje fibres to the ventricles, which contract. A doctor listens to the heart sounds with a stethoscope, and you can feel each beat as the pulse at your wrist.
How to draw the L.S. of the human heart
- 1Draw a cone-shaped outline, broad at the top and pointed at the bottom left. Divide it with a vertical line (septum) and a slanting line into four chambers.
- 2Make the two upper chambers smaller: the right auricle (on the left side of your page, as if facing the person) and the left auricle. Make the lower ones larger: the right ventricle and the left ventricle, with the left wall thickest.
- 3Draw flaps between each auricle and ventricle: the tricuspid valve (right) and the bicuspid valve (left), with thin threads (chordae tendineae) to the ventricle walls.
- 4Draw the vessels. The superior and inferior vena cava enter the right auricle, and the pulmonary artery leaves the right ventricle. The pulmonary veins enter the left auricle, and the arched aorta leaves the left ventricle.
- 5Mark small cups at the bases of the pulmonary artery and aorta: the semilunar valves. Add arrows for blood flow, and shade the deoxygenated side blue and the oxygenated side red.
Count your pulse at the wrist for one minute while sitting, then again after skipping: it rises because your muscles need more oxygen. Regular exercise, less salt and oily food, and no tobacco keep the heart healthy.
2 marks: “Why is the SA node called the pacemaker?” It starts each heartbeat by sending out regular waves of excitation, and so sets the rate of the heart.
4 marks: “What is double circulation? Why is it useful?” Blood passes through the heart twice in one round: pulmonary (heart, lungs, heart) and systemic (heart, body, heart). Oxygenated and deoxygenated blood do not mix, so tissues get more oxygen.
7 marks: “Describe the structure of the human heart with a labelled diagram.” Labelled L.S.; position and pericardium; four chambers with their vessels; septa; tricuspid, bicuspid and semilunar valves; thick left ventricle; coronary arteries.
'Tri before you Bi': blood meets the tricuspid (right) first, then the bicuspid (left). Lubb = the valves between auricles and ventricles shut; Dupp = the semilunar valves shut.