Question

Draw a pressure/volume loop of the cardiac cycle and label the axes and phases clearly. For each phase (numbered 1 through 4), Write a summary EXPLAINING the movement of the line on the graph for each phase. Be sure to note when systole and diastole begin/end as well as to include all valve behaviour.

Answer 1

Cardiac cycle.pressure/volume loop

The cardiac cycle can be divided into four stages:

1. Ventricular filling phase – during this phase the ventricles fill  

2. Isovolumetric contraction – the ventricles contract, building up pressure ready to pump blood into the aorta/pulmonary trunk

3. Ejection  – the ventricles continue to contract, LV volume decreases as LVP increases to a peak value pushing blood into the aorta and the pulmonary trunk.  

4. Isovolumetric relaxation – the ventricles relax, ready to re-fill with blood in the next filling phase.

Graph explanation

1is  the point where the mitral valve opens and ventricular filling begins. Moving to the right, there is little pressure change in the ventricle with the normal filling. Immediately before the mitral valve closes (2), there is a slight bump in pressure because of atrial systole. The volume at 2 is the end-diastolic volume. The 2-3line represents the period of isovolumetric contraction after the mitral valve closes, where pressure rises with ventricular contraction before the aortic valve opens at 3. The curved 3-4 line is the period of ejection. Point 4 represents the closure of the aortic valve, where the pressure drops because of ventricular relaxation. The volume at 4 is the end-systolic volume. After the aortic valve closes (4), there is a time when pressure falls while both valves are closed (4-1 line, the period is isovolumetric relaxation. When the ventricular pressure falls below the atrial pressure (1), the mitral valve opens. Stroke volume is shown between the double-headed arrow.