The various parts of the heart, including its chambers, valves and arteries are shown in the figures displayed below.

  The diagram on the right shows a longitudinal (cut from top to bottom) section of the heart. The flow of blood is represented by arrows. The narration will describe different phases of the cardiac circulation.
    The animation will serve as a revision or reinforcement of the various stages or steps of the circulation, with arrows and labels serving as a reminder of what takes place. You may click on the stop, rewind and play buttons to control the animation.

    The pictures below represent a heart that is cut along the horizontal axis. The picture on the left shows the plane along which the heart is cut. That is, the top of the heart, including the right and left atria (atria is plural for atrium), the pulmonary artery and aorta are removed on the picture on the right (below). It shows the heart as you would look down at it from the front. The tricuspid and mitral valves are represented right and left, respectively (you can see the right and left ventricles through the two valves). The aortic and pulmonic valves are shown up and down, respectively, in the bottom half of the picture. The heart size increases and decreases during the filling (DIASTOLE, pronounced die-as-tull-ee) and contraction or emptying (SYSTOLE, pronounced sis-tull-ee) of the heart chambers.

The animation on the bottom left shows a longitudinal section of the beating heart, together with valve structures that open and shut to let blood pass through the atria, ventricles and the great vessels. The animation on the right shows a cross-sectional view of the heart.

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    The mitral and tricuspid valves open and the aortic and pulmonic valves are shut while the ventricles fill during diastole. In contrast, the mitral and tricuspid valves shut while the aortic and pulmonic valves open during ventricular systole. This sequence ensures that the ventricles are filled to capacity before the aortic and pulmonic valves are opened. At this time, the mitral and tricuspid valves are shut so that blood does not leak back into the the two atria. Yes, the heart is an ingenious device that could have inspired design of the modern day mechanical pump and integrated valves.

Did you stop and wonder why each side of the heart has two pumping chambers (atrium and ventricle)? Why not just have a ventricle to receive blood and then pump it straight out? The reason is that the atrium serves as a "booster pump" that increases the filling of the ventricle. Filling a normal ventricle to capacity translates to more vigorous contraction or emptying. You can compare this to a strong spring, and imagine that the heart muscle is made up of tiny little "springs" known as ACTIN and MYOSIN. Within reasonable limits, the more you stretch a spring, the more vigorously will be its contraction or recoil. In medical terms, this is known as "Frank-Starling's" law.