Transport in Animals: Question 8
Syllabus 9.2
A physiology student records an ECG trace and a pressure trace from inside a mammal's left atrium and left ventricle during a single heartbeat, and wants to relate the changes in pressure to the movement of the heart valves.
(a) State what happens to the atria, and to the blood inside them, during atrial contraction. [2]
(b) Describe what happens to the ventricles, and to the atrioventricular and semilunar valves, during ventricular contraction. [3]
(c) Explain why the atrioventricular valves must close before the semilunar valves open during each heartbeat. [2]
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Worked solution
Part (a): Atrial contraction
During atrial contraction, the two atria contract together. This raises the pressure of the blood inside them, and because the atrioventricular valves are open at this point, the blood is pushed down through these valves into the two ventricles below.
Part (b): Ventricular contraction
Once the ventricles are filled, they contract during ventricular contraction. As the ventricle walls squeeze inwards, the pressure of the blood inside the ventricles rises rapidly. As soon as ventricular pressure rises above atrial pressure, the atrioventricular valves are pushed shut, so blood cannot flow back up into the atria. The ventricles keep contracting, and once the rising ventricular pressure exceeds the pressure in the pulmonary artery and the aorta, it pushes the semilunar valves open, and blood is forced out of the heart into these arteries.
Part (c): Why the atrioventricular valves must close first
If the atrioventricular valves stayed open while the ventricles contracted, the rising pressure inside the ventricles would simply push blood back up into the atria, since this is the path of least resistance. By closing first, the atrioventricular valves force all of the pressure generated by the contracting ventricle to build up inside a sealed chamber. This pressure keeps rising until it is high enough to overcome the pressure in the arteries, at which point it pushes the semilunar valves open instead, so that blood can only be ejected forwards, out of the heart and into the circulation.
Final answers
- (a) Atria contract, pushing blood through the open atrioventricular valves into the ventricles.
- (b) Ventricles contract; rising pressure closes the atrioventricular valves, and once pressure exceeds arterial pressure it opens the semilunar valves, ejecting blood into the pulmonary artery/aorta.
- (c) Closing the atrioventricular valves first stops blood being pushed back into the atria, forcing ventricular pressure to build up until it is high enough to force the semilunar valves open, ensuring blood only flows forward.