Cardiac Cycle

During a cardiac cycle, the pressure within the chambers rises and falls. When the atria contract (atrial systole), the ventricles relax (ventricular diastole). When the ventricles contract (ventricular systole), the atria are relaxed (atrial diastole). Then the atria and ventricles both relax for a brief interval (fig. 15.15).

Pressure in the ventricles is low early in diastole, and the pressure difference between atria and ventricles causes the A-V valves to open and the ventricles to fill. About 70% of the returning blood enters the ventricles prior to contraction, and ventricular pressure gradually increases. When the atria contract, the remaining 30% of returning blood is pushed into the ventricles, and ventricular pressure increases a bit more. Then, as the ventricles contract, ventricular pressure rises sharply, and as soon as the ventricular pressure exceeds atrial pressure, the A-V valves close. At the same time, the papillary muscles contract, and by pulling on the chordae tendi-nae, they prevent the cusps of the A-V valves from bulging too far into the atria.

During ventricular contraction the A-V valves remain closed. The atria are now relaxed, and pressure in the atria is quite low, even lower than venous pressure. As a result, blood flows into the atria from the large, attached veins. That is, as the ventricles are contracting, the atria are filling, already preparing for the next cardiac cycle.

As ventricular systole progresses, ventricular pressure continues to increase until it exceeds the pressure in the pulmonary trunk (right side) and aorta (left side). At this point, the pressure differences across the semilunar valves causes the pulmonary and aortic valves to open, and blood is ejected from each valve's respective ventricle into these arteries.

As blood flows out of the ventricles, ventricular pressure begins to drop, and it drops even further as the ventricles begin to relax. When ventricular pressure is lower than blood pressure in the aorta and pulmonary trunk, the pressure difference is reversed, and


Right coronary artery

Left coronary artery

Marginal artery

Circumflex artery

Anterior interventricular artery

Essentials of Human Physiology

Essentials of Human Physiology

This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.

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