PHSI 208 Lecture 3: PHSI 208.6 SG Cardiovascular 3
Document Summary
The heart contracts and relaxes during a cardiac cycle. A cycle of one complete contraction and relaxation has two phases: diastole and systole. Diastole: the time during which cardiac muscle relaxes (~500ms). Systole: the time during which cardiac muscle contracts (~300ms). *because the atria and ventricles do not contract and relax at the same time, the events are discussed separately. A single cardiac cycle is divided into 5 phases: the heart at rest (atrial and ventricular diastole, late diastole). Both sets of chambers are relaxed and ventricles fill passively: completion of ventricular filling (atrial systole). Atrial contraction forces a small amount of additional blood into ventricles. Most blood enters ventricles passively, but the last 20% enters when the atria contract: early ventricular contraction (isovolumetric ventricular contraction). The first phase of ventricular contraction pushes the av valves closed, but does not create enough pressure to open semilunar valves: the heart pumps (ventricular ejection).
