heart valves also play a role in fill-volume and cardiac output. Passive flow of blood fills the right/left atrium. As the volume of blood that fills the atrium goes up, the atrial cardiac tissue contracts, thereby decreasing the "volume" of the atrium and driving up the pressure. Blood moves from high pressure to low pressure, and pushes through the atriventricular valve The cardiac muscle that envelopes the ventricle examples slightly with the stretch, and forces of expulsion are maximized ("stirlings" law in physiology) because actin/myosin overlap in the sarcomeres are maximized by a "slight stretch" of the cardiac tissue. This means that a greater force can eject the blood from the ventricle with each pump.
valves play a major role in directionality of blood flow by regulating pressure. Blood is pushed from regions of high pressure to low pressure. If ventricle is pressurized, the atriventricular valves are crucial to present it from moving back "up". In the case of the ventricles, the semilunar valves ensure that pressure is built up in the ventricles prior to expulsion of blood through the systemic/pulmonary circuits. When the pressure in the ventricle exceeds the pressure in the aorta, blood flows out into the system.
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