Final answer:
The heart pumps out the same amount of blood that enters it, ensuring a continuous flow of blood through the body. Cardiac output is influenced by factors such as heart rate and stroke volume. By increasing heart rate or stroke volume, the heart can increase cardiac output to meet the body's demands.
Step-by-step explanation:
The statement 'what enters the heart, the heart pumps out' refers to the regulation of cardiac output by the heart itself. The heart pumps out the same amount of blood that enters it, ensuring a continuous flow of blood through the body. Cardiac output is influenced by factors such as heart rate and stroke volume.
Heart rate refers to the number of times the heart beats per minute, while stroke volume is the amount of blood pumped by each ventricle in one contraction. By increasing heart rate or stroke volume, the heart can increase cardiac output to meet the body's demands. For example, during exercise, the heart beats faster and pumps out more blood to supply oxygen and nutrients to the muscles.