Transport in Mammals: Question 9
Syllabus 8.3
The table below shows the pressure of blood in the left atrium, the left ventricle and the aorta at four points, T1 to T4, during one cardiac cycle in a resting mammal.
| Time point | Left atrial pressure / kPa | Left ventricular pressure / kPa | Aortic pressure / kPa |
|---|---|---|---|
| T1 | 1.0 | 0.9 | 11.0 |
| T2 | 1.0 | 6.0 | 11.0 |
| T3 | 1.0 | 15.0 | 15.0 |
| T4 | 1.0 | 8.0 | 10.0 |
(a) Using the pressure values at T2, state whether the atrioventricular (bicuspid) valve and the aortic (semilunar) valve are open or closed at this point in the cardiac cycle, and explain your answer by reference to the pressure data. [3]
(b) At T4, left ventricular pressure has fallen below the aortic pressure recorded at T3. State the name given to this phase of the cardiac cycle, and explain, in terms of valve action, what is happening to the volume of blood inside the left ventricle at T4 despite the ventricle wall no longer actively contracting. [3]
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Worked solution
Part (a): Valve states at T2
To decide whether a valve is open or closed, compare the pressure on each side of it.
- Atrioventricular valve (between left atrium and left ventricle): at T2, left ventricular pressure is 6.0 kPa and left atrial pressure is 1.0 kPa. Since ventricular pressure exceeds atrial pressure, blood is pushed back against the underside of the valve cusps, forcing this valve closed.
- Aortic (semilunar) valve (between left ventricle and aorta): at T2, left ventricular pressure is 6.0 kPa, still lower than the aortic pressure of 11.0 kPa. Since ventricular pressure has not yet risen high enough to exceed aortic pressure, this valve remains closed too.
With both valves closed, blood can neither enter nor leave the left ventricle, even though the ventricle wall is contracting and pressure is rising rapidly. This is the isovolumetric contraction phase of the cardiac cycle.
Part (b): Isovolumetric relaxation at T4
Comparing T3 (left ventricular pressure 15.0 kPa, aortic pressure 15.0 kPa, during ejection) with T4 (left ventricular pressure 8.0 kPa, aortic pressure 10.0 kPa), left ventricular pressure has fallen below aortic pressure between these two points. This drop in ventricular pressure, occurring as the ventricle wall relaxes after systole, marks the start of the phase called isovolumetric relaxation.
At T4:
- Left ventricular pressure (8.0 kPa) is still greater than left atrial pressure (1.0 kPa), so the atrioventricular valve remains closed.
- Aortic pressure (10.0 kPa) is now greater than left ventricular pressure (8.0 kPa), so the aortic (semilunar) valve has closed, a small volume of blood briefly flows backward toward the ventricle as ventricular pressure drops below aortic pressure, and this backflow fills and closes the pocket-like semilunar valve cusps.
Because both valves are closed at T4, no blood can flow into or out of the left ventricle. Its volume therefore stays constant (isovolumetric) even though the ventricle wall is actively relaxing and its internal pressure keeps falling. This phase continues until ventricular pressure eventually drops below atrial pressure, at which point the atrioventricular valve is forced open again and ventricular filling begins for the next cardiac cycle.
Final answers
- (a) At T2 both the atrioventricular valve (ventricular pressure > atrial pressure) and the aortic valve (ventricular pressure < aortic pressure) are closed, isovolumetric contraction.
- (b) T4 is isovolumetric relaxation: the atrioventricular valve stays closed (ventricular pressure still exceeds atrial pressure) and the aortic valve has just closed (aortic pressure now exceeds ventricular pressure), so with both valves shut, ventricular volume stays constant as the wall relaxes and pressure keeps falling.