Which of the following is NOT true for ventricular systole?
A. The ventricles contract during ventricular systole.
B. The atria contract during ventricular systole.
C. The semilunar valves open during ventricular systole. The atrioventricular valves close at the beginning of ventricular systole.
D. Blood flows through the aortic and pulmonary semilunar valves during ventricular systole.
The Correct Answer and Explanation is:
The statement that is NOT true for ventricular systole is B. The atria contract during ventricular systole.
Explanation:
Ventricular Systole refers to the phase of the cardiac cycle when the ventricles contract, ejecting blood into the systemic and pulmonary circulation. During this phase, several important events occur, and understanding the timing of these events is crucial.
- Ventricular Contraction: The primary action in ventricular systole is the contraction of the ventricles. This contraction generates pressure that propels blood into the aorta and pulmonary artery. Thus, statement A is true.
- Atrial Activity: While the ventricles contract, the atria are actually in a state of relaxation (diastole). The atrial contraction occurs just before ventricular systole, during the end of ventricular diastole, when the atria fill with blood and contract to push blood into the ventricles. Therefore, statement B is false, as the atria do not contract during ventricular systole.
- Valvular Changes: At the beginning of ventricular systole, the atrioventricular (AV) valves (the mitral and tricuspid valves) close to prevent backflow into the atria. This closure is crucial for maintaining unidirectional blood flow. Concurrently, the semilunar valves (the aortic and pulmonary valves) open in response to the increased pressure in the ventricles. Thus, statement C is accurate.
- Blood Flow: Blood flows through the opened semilunar valves into the aorta and pulmonary artery during ventricular systole. This ejection of blood is a hallmark of this phase. Therefore, statement D is also true.
In summary, during ventricular systole, the ventricles contract, the atria relax, the semilunar valves open, and blood is ejected from the heart. Understanding these events clarifies the sequence of the cardiac cycle and highlights why option B is the correct answer as the statement that is not true.