Final answer:
In the cardiac cycle, valves within the heart do prevent backflow of blood, making A true. Ejection fraction (EF) refers to blood ejected during systole, not diastole, rendering B false. Cardiac output (CO) is the product of EF and HR, affirming C as true. Diastole is when the heart relaxes, making D false.
Step-by-step explanation:
The cardiac cycle includes a series of events that occur from the beginning of one heartbeat to the beginning of the next. During the cycle, the heart muscle undergoes periods of contraction (systole) and relaxation (diastole). Regarding the true statements about the cardiac cycle:
- Valves within the heart prevent the flow of blood backward, ensuring unidirectional blood flow through the heart's chambers, which is a statement consistent with cardiac physiology.
- EF, or ejection fraction, is the proportion of blood that is ejected from the ventricles during systole, not diastole, thus making the statement regarding EF being defined as the amount of blood ejected during diastole false.
- Cardiac output (CO) is indeed equal to the ejection fraction (EF) times the heart rate (HR), representing the volume of blood being pumped by the heart per minute.
- Diastole is the phase when the heart muscle is relaxed and the chambers are filling with blood, in contrast to systole when the heart is contracting.
It's important to note that both atrial and ventricular diastole occur as part of the cardiac cycle, allowing the chambers to fill with blood. When the atria contract (atrial systole), they push blood into the ventricles. Conversely during ventricular systole, the ventricles contract, pumping blood out of the heart.