known as collateral circulation. However, there are situations, such as in the heart and
the brain, where a single artery supplies a specific part of a structure. Such an artery is
called an end artery. When an end artery is damaged, that area supplied by it will
usually die, as in the case of the coronary artery above, or the cause of a "stroke" in the
b. Pulmonary Cycle. The pulmonary cycle begins in the right ventricle of the
heart. Contraction of the right ventricular wall applies pressure to the blood. This forces
the tricuspid valve closed, and the closed valve prevents blood from going back into the
right atrium. The pressure forces blood past the semilunar valve into the pulmonary
trunk. Upon relaxation of the right ventricle, backpressure of the blood in the pulmonary
trunk closes the pulmonary semilunar valve. The blood then passes into the lungs
through the pulmonary arterial system. Gases are exchanged between the alveoli. This
blood, now saturated with oxygen, is collected by the pulmonary veins and carried to the
left atrium of the heart. This completes the pulmonary cycle.
Figure 2-4. Cardiovascular circulatory pattern.