What happens to blood during systole?

During systole, the heart muscle contracts, specifically the ventricles, which drives the blood out of the heart and into the arteries. This phase is crucial for pumping oxygenated blood from the left ventricle into systemic circulation and deoxygenated blood from the right ventricle into the pulmonary circulation. This expulsion of blood creates the pressure necessary for circulation throughout the body, ensuring that tissues receive the necessary oxygen and nutrients delivered by the blood.

The other options present processes that do not occur during systole. The flow of blood back into the atria happens during diastole when the heart relaxes. Blood is not collected by capillaries during systole; rather, capillaries are sites of exchange for gases and other substances that occur continuously but are unrelated to the contraction of the heart. Filtering through the kidneys is also a separate process that happens when blood passes through the renal system, which is not directly related to the contracting phase of the cardiac cycle.