The heart, double circulation and the heartbeat
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What this lecture covers
- A pump that never rests
- Closed double circulation
- Chambers and valves
- Wall thickness
Read the transcript of the teaching part
Last time
Before we start: last time, in The oxygen dissociation curve, the Bohr shift and carbon dioxide transport, you learned these points. Here they are again on one card. Keep them in mind, because today builds on them.
A pump that never rests
Your heart beats about seventy times a minute, every minute, for your whole life. It has to push blood to the lungs and round the whole body, at two different pressures. To manage that, it is really two pumps side by side, and its beat is started by the heart itself.
Closed double circulation
Mammals have a closed circulation: blood stays inside vessels. This keeps pressure high, and lets blood travel fast. It is double because blood passes through the heart twice on each complete trip. The pulmonary circuit goes heart, lungs, heart. The systemic circuit goes heart, body, heart. Blood returns to the heart after the lungs, to be boosted before it goes to the body. This keeps the pressure high enough to push blood to every organ, which a single circuit cannot do after the narrow lung capillaries.
Chambers and valves
The heart has four chambers. The two atria receive blood, and the two ventricles pump it out. The left and right sides are separated by the septum. The right side receives deoxygenated blood from the body through the vena cava, and pumps it to the lungs through the pulmonary artery. The left side receives oxygenated blood from the lungs through the pulmonary vein, and pumps it round the body through the aorta. Atrioventricular valves between atria and ventricles, and semilunar valves in the aorta and pulmonary artery, stop backflow.
Wall thickness
The left ventricle has a thicker muscular wall than the right. It must generate enough pressure to push blood all the way round the body. The right ventricle only pumps blood to the nearby lungs. A thin wall is enough, and the lower pressure protects the delicate lung capillaries. The atria have thin walls because they only push blood a short way into the ventricles.
How often it comes up
How often Cambridge has asked each part, counted by ExamYard from every past paper in its question bank: Structure of the heart.
The whole chapter: Transport in Mammals, Transport in Mammals.
Chapter 8: Transport in mammals
- The heart, double circulation and the heartbeat 5 min
- The cardiac cycle and blood cells 5 min
- Blood vessels, tissue fluid and lymph 6 min
- The oxygen dissociation curve, the Bohr shift and carbon dioxide transport 6 min
Also for A Level Biology 9700
Which topics come up most in A Level Biology 9700 past papers, counted from every paper since 2004: Paper 1 topic frequency, Paper 2 topic frequency, Paper 4 topic frequency.