Final answer:
Dr. Nelson's figure likely illustrates the cardiac cycle showing that between C and D, ventricular pressure is below aortic or pulmonary pressure, corresponding with the second heart sound (S2) and the closure of the semilunar valves during the isovolumic ventricular relaxation phase.
Step-by-step explanation:
Dr. Nelson's figure pertaining to cardiac physiology is likely illustrating the changes in pressure within the heart during the cardiac cycle, particularly between points C and D on a diagram that likely represents the phases of ventricular systole and ventricular diastole.
Between points C and D we would observe several events:
- Ventricular pressure rises above atrial pressure, preventing backflow of blood into the atria.
- The second heart sound (S2), described as 'dub,' is heard when the semilunar valves close, which occurs at the onset of ventricular diastole.
- Ventricular pressure is now below the pressure in the aortic and pulmonary arteries, leading to semilunar valve closure.
- The volume in the ventricles remains constant during the isovolumic relaxation phase.
- Ventricular pressure does not exceed aortic or pulmonary pressure during the isovolumetric relaxation phase, so it must be below these pressures between C and D.
Therefore, the correct interpretation for what Dr. Nelson is trying to show is that between letters C and D, ventricular pressure is below the aortic or pulmonary pressure (3), coinciding with the closure of the semilunar valves and the second heart sound (S2).