Circulatory System Basics


This page is an review or introduction of the preliminary basics about the circulatory system.

Chambers and Vessels Connected to the Heart

First, recall that the heart has two chambers on the right side and two chambers on the left side. Each side has a thin-walled, distensible atrium , which accumulates bloods as it flows back the heart. Blood flows from each atrium into the corresponding ventricle, which is thick-walled, and supplies the energy that moves the blood through the blood vessels.

The right ventricle pumps blood to the lungs in pulmonary circulation. The left ventricle pumps blood to the systemic circulation, which includes all regions except the lungs.


The pressures created by the right ventricle are only about one-seventh as high as those created by the left ventricle. This is because the lungs are close to the level of the heart, because the extremely extensive network of pulmonary capillaries allows blood to move easily through the lungs, and because low pressures in the pulmonary circulation help prevent a net loss of fluid into the alveoli. These are the tiny, air-filled sacs where gas exchange occurs.

Thus the two ventricles have quite different jobs and their structures correspond to their specific roles.


The figure to the left shows a cross section through the ventricles. Notice that the right ventricle has much thinner walls and is crescent-shaped. You can think of it working more or less like a bellows. It is sometimes referred to as a volume pump, because the shortening of the muscle fibers efficiently ejects volumes of blood.

The left ventricle on the other hand has very thick walls -- at least three times that of the right ventricle -- and is circular in cross section. It works more like a clenching fist and is referred to as a pressure pump. This type of configuration is ideal for developing pressure through muscle fiber shortening.



Refer to the figure to the right and be sure you can trace blood around the circulatory system in this fashion. The vessels connected to the heart are:




Valves


The schematic diagram to the left adds the four valves found in the heart. Between the atrium and ventricle on each side is an atrioventricular valve. The one of the right is usually called the tricuspid valve; the one of the left the mitral valve.

Between the right ventricle and the pulmonary trunk is the pulmonary valve (pulmonic valve). Between left ventricle and aorta is the aortic valve.




All four valves lie in a plane in the heart. Observe the location of this plane at the junction between the atria and ventricles. The figure to the right shows a top view of this slice through the heart and illustrates how the valves are arranged in this plane. In the figure to the right, anterior is down.




Surrounding all the valves in the plane is dense connective tissue, which supports the structure of the valves. But at least as important, this layer of connective tissue also electrically separates the atria and ventricle. You are going to find this is crucial to the sequence through which the heart is activated to contract.




Finally, let's look briefly at the structure of the valves, although we will explore this much further in lecture and lab. A schematic diagram of the valves is shown on the right.

The top level of the figure shows the structure of the aortic and pulmonary valves, both in top view and from the side. The three valve leaflets (cusps) are symmetrical. As blood attempts to flow backwards through these valves, each leaflet balloons downwards, which causes the edges to be pressed together.

The bottom level of the figure shows the layout of the atrioventricular valves. The cross sectional area, of course, is much larger than that of the aortic or pulmonary (see previous figure for comparison). The valve leaflets create a funnel-like structure pointing into the ventricle.

In order to prevent these large leaflets from blowing backwards into the atrium when the ventricle contracts, the leaflets are secured at their edges by strands of connective tissue called chordae tendineae. These are attached on their other ends to papillary muscles, which are some of the first ventricular muscle fibers to contract.





Quick Quiz

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Correct Answer

What chamber receives blood from the pulmonary veins?

What vessel receives blood from the right ventricle?

Is the cross sectional area of the mitral valve or the aortic valve larger?

What valve lies between the left ventricle and the aorta?

What is the name of the small blood vessels that receive blood directly from capillaries?

What structure receives blood from the coronary sinus?


(Spelling must be correct)