Cambridge O Level Biology · Syllabus 5090 · Transport in Humans
Heart
What is Heart?
The heart is the muscular pump of the human circulatory system. It is divided by a septum into a right side, which receives deoxygenated blood from the body and pumps it to the lungs, and a left side, which receives oxygenated blood from the lungs and pumps it to the rest of the body. Each side has a thin-walled atrium above and a thick-walled ventricle below, separated by an atrioventricular valve, and each ventricle empties through a semilunar valve into a great artery.
This definition is part of the Transport in Humans chapter in Cambridge O Level Biology.
Heart in context
The human circulatory system is a muscular pump (the heart), a network of blood vessels and the blood itself, with one-way valves that keep blood moving in a single direction. Blood passes through the heart twice during one complete circuit of the body, which is what makes the system a double circulation: a low-pressure pulmonary circuit to the lungs and a high-pressure systemic circuit to the rest of the body.
Common mistakes with Heart
- “Double circulation means the blood goes round the body twice.” CorrectBlood passes through the heart twice during one complete circuit of the body: once on the way to the lungs and once on the way to the rest of the body.
- “Arteries carry oxygenated blood; veins carry deoxygenated blood.” CorrectArteries carry blood away from the heart and veins carry blood towards it. The pulmonary artery carries deoxygenated blood and the pulmonary vein carries oxygenated blood, and both are named correctly.
- “The right side of the heart is on the right of the diagram.” CorrectIn a standard front-facing diagram you are looking at the person from the front, so their anatomical right appears on your left. Check which side the aorta arches away from before you label anything.
- “Smoking causes coronary heart disease, so a smoker will get it.” CorrectSmoking is a risk factor: it raises the probability of the disease across a population. It does not guarantee the disease in any individual, and non-smokers can develop it too.
- “Double circulation means the blood goes round the body twice.” Why wrongIt confuses the number of trips round the body with the number of visits to the pump. WriteBlood passes through the heart twice during one complete circuit of the body: once on the pulmonary circuit and once on the systemic circuit.
- “Arteries always carry oxygenated blood.” Why wrongIt takes a pattern that happens to hold in the systemic circuit and treats it as a definition. WriteAn artery carries blood away from the heart. The pulmonary artery carries deoxygenated blood to the lungs.
- “Veins always carry deoxygenated blood.” Why wrongSame error, the other way round. WriteA vein carries blood towards the heart. The pulmonary vein carries oxygenated blood from the lungs.
- “The left side of the heart contains deoxygenated blood.” Why wrongIt usually comes from reading the diagram the wrong way round. WriteThe left side receives oxygenated blood from the lungs through the pulmonary veins and pumps it to the body.
- “The right side of the heart is on the right of the diagram.” Why wrongA heart diagram is drawn as if you are facing the person, so their right is opposite your right. WriteThe anatomical right side appears on the viewer's left. Confirm it by finding the thickest ventricle wall, which is always the left ventricle.
- “Coronary arteries carry blood into the heart chambers.” Why wrongIt confuses supplying the muscle with filling the pump. WriteCoronary arteries branch from the aorta and supply the cardiac muscle itself with oxygen and glucose. The chambers are filled by the vena cava and the pulmonary veins.
- “Veins have thick muscular walls because they have to push the blood back.” Why wrongVeins do no pushing; the heart and the skeletal muscles do. WriteVeins have thin walls and a wide lumen because the blood in them is at low pressure, and valves stop it flowing backwards.
- “Smoking causes coronary heart disease, so every smoker gets it.” Why wrongIt treats a change in probability as a certainty for an individual. WriteSmoking is a risk factor: it increases the probability of coronary heart disease. It does not guarantee it, and people who have never smoked can develop the disease too.
Examiner tips on Heart
- The single most valuable habit in this topic. Whenever you state a structural feature, immediately state what it does. “The wall of an artery is thick” is worth nothing on its own. “The wall of an artery is thick, so it can withstand the high pressure of blood leaving the heart without bursting” is the mark.
- Orientation first, labels second. A heart diagram is drawn as if you are looking at the person from the front. Their anatomical right side therefore appears on your left. Before you write a single label, find the thickest ventricle wall: that side is always the left ventricle, whichever way the diagram is printed.
- Two words that must appear: oxygen and respiration. A great many answers say “the heart does not get enough blood” and stop. Say what the blood was carrying, and say what the muscle cells needed it for.
- The naming rule. “Hepatic” means of the liver; “renal” means of the kidney; “pulmonary” means of the lung. Put those together with artery = away from the heart and vein = towards the heart, and eight of the nine names build themselves. Only the hepatic portal vein has to be learned as a special case.
- Arteries do not push blood. The heart provides the push. An artery withstands the pressure and, through elastic recoil, maintains it. Writing that the artery wall “squeezes each blood cell forward” loses the mark.
Questions students ask about Heart
What is double circulation in humans?
Double circulation means that blood passes through the heart twice during one complete circuit of the body: once through the right side on the low-pressure pulmonary circuit to the lungs, and once through the left side on the high-pressure systemic circuit to the rest of the body.
Do arteries always carry oxygenated blood?
No. An artery is defined by direction: it carries blood away from the heart. The pulmonary artery carries deoxygenated blood from the right ventricle to the lungs, and the pulmonary vein carries oxygenated blood from the lungs back to the heart.
How do heart valves open and close?
By differences in pressure. A valve is pushed open when the pressure behind it is greater than the pressure in front of it, and pushed shut when the pressure in front becomes greater. Valves are passive flaps of tissue and never contract.
What do the coronary arteries do?
They branch from the aorta just above the aortic valve and supply the cardiac muscle itself with oxygen and glucose. They do not deliver blood into the heart's chambers. If one narrows or becomes blocked, that region of muscle receives less oxygen and glucose, aerobic respiration is reduced, and the muscle may be damaged.
What is the difference between the hepatic vein and the hepatic portal vein?
The hepatic vein carries blood away from the liver to the vena cava, and so back towards the heart. The hepatic portal vein carries blood from the gut to the liver, delivering the products of digestion for processing before they reach the rest of the body.
Is a risk factor the same as a cause?
No. A risk factor increases the probability that a person will develop a disease; it does not guarantee it. A person with several risk factors for coronary heart disease may never develop it, and a person with none of the modifiable risk factors still may.