The mammalian heart and cardiac cycleEdexcel International A Level Biology: Revision notes
Section 1
Structure of the heart
The mammalian heart has four chambers: two thin-walled atria that receive blood and two thick-walled ventricles that pump blood out. A muscular wall called the septum separates the left and right sides, so oxygenated and deoxygenated blood do not mix.
The left ventricle has a much thicker muscular wall than the right ventricle. It pumps blood at high pressure to the whole body, whereas the right ventricle pumps blood only to the nearby lungs, where lower pressure protects the delicate capillaries. The atria have thin walls because they only push blood a short distance into the ventricles.
The heart muscle is supplied with oxygenated blood by the coronary arteries, which branch from the aorta over the surface of the heart.
Section 2
Major blood vessels
- Vena cava: brings deoxygenated blood from the body to the right atrium
- Pulmonary artery: takes deoxygenated blood from the right ventricle to the lungs
- Pulmonary vein: brings oxygenated blood from the lungs to the left atrium
- Aorta: takes oxygenated blood from the left ventricle to the body
Notice that the pulmonary artery carries deoxygenated blood and the pulmonary vein carries oxygenated blood. This is an exception to the pattern of arteries and veins.
Do not say all arteries carry oxygenated blood. The pulmonary artery carries deoxygenated blood.
Section 3
Valves
Valves keep blood flowing one way.
- Atrioventricular (AV) valves lie between the atria and ventricles: the bicuspid valve on the left and the tricuspid valve on the right. Tendons (valve tendons) attach them to the ventricle wall and stop them turning inside out.
- Semilunar valves lie at the base of the aorta and pulmonary artery and prevent blood flowing back into the ventricles.
Valves open and close passively, depending on the pressure on either side.
Section 4
The cardiac cycle
One heartbeat is one cardiac cycle, made of three stages.
- Atrial systole: the atria contract, the atrial pressure rises above the ventricular pressure and blood is forced through the open AV valves into the ventricles. Semilunar valves are closed.
- Ventricular systole: the ventricles contract, the ventricular pressure rises above the atrial pressure and the AV valves close (first heart sound). When the ventricular pressure exceeds that in the aorta and pulmonary artery, the semilunar valves open and blood is ejected.
- Cardiac diastole: the atria and ventricles relax, the pressure in the ventricles falls below that in the arteries and the semilunar valves close (second heart sound). The atria fill with blood from the veins, and the AV valves open again.
The length of a cardiac cycle in seconds is 60 ÷ heart rate (beats per minute). At 75 beats per minute it is 0.8 s.
For every valve event, name the two pressures being compared. A valve opens when pressure behind it is greater than in front, and closes when it is less.
Section 5
Heart dissection
In a dissection of a sheep's or pig's heart, you can:
- identify the four chambers and the vessels (aorta, vena cava, pulmonary artery and vein)
- cut open the ventricles and compare wall thickness: the left is about three times as thick as the right
- see the AV valves and tendons and the semilunar valves
- find the coronary arteries on the surface
Take care with the scalpel, cut away from your body and wash your hands afterwards. Draw what you see and label the features.
Must Know
- Four chambers, septum, thick left ventricle wall
- Aorta, vena cava, pulmonary artery and pulmonary vein carry blood to and from the heart
- AV valves with tendons; semilunar valves
- Atrial systole, ventricular systole and cardiac diastole
- Valves open and close due to pressure differences
That's the notes covered.
Carry on to the next subtopic.
Exam questions on The mammalian heart and cardiac cycle
- A student dissects a sheep's heart. She identifies the four chambers and the major blood vessels attached to the heart, and notes the difference in thickness between the walls of the two ventricles.Describe two features the student could use to identify the left ventricle in the dissected heart.2 marks
- Valves in the heart make sure that blood flows in one direction only. A doctor listening to the heart of a patient with a stethoscope hears two sounds in every cardiac cycle, which are made by valves closing.The atrioventricular valves are attached to the ventricle walls by tendons (valve tendons). Explain the function of these tendons.2 marks
- A resting person has a heart rate of 75 beats per minute. In each cardiac cycle, atrial systole lasts 0.1 s and ventricular systole lasts 0.3 s. As the left ventricle contracts, the pressure in it rises from about 0.5 kPa to about 16 kPa. At the start of this contraction, the pressure in the left atrium is about 1 kPa and the pressure in the aorta is about 10 kPa.Calculate the length of the cardiac diastole in each cycle, in seconds. Show your working.3 marks
Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).