Anatomy & Physiology I and II

valve closes when the right ventricle contracts, preventing the backflow of blood into the right atrium. The internal surface of the ventricle also includes a series of muscular ridges: the trabeculae carneae . The conus arteriosus forms the superior end of the right ventricle and ends at the pulmonary valve. The pulmonary valve consists of three semilunar cusps of thick connective tissue. Deoxygenated blood travels through the valve to the pulmonary arteries, which carry blood to the lungs for oxygenation by way of diffusion at the level of the pulmonary capillaries. The Left Atrium From the pulmonary capillaries, blood collects into small veins that ultimately unite to form the four pulmonary veins. The left atrium receives blood through entrances in its posterior wall from two right and two left pulmonary veins. The left atrium has an auricle like the right atrium. A valve, the left atrioventricular (AV) valve, also called bicuspid or mitral valve, guards the entrance to the left ventricle. As the name bicuspid implies, the left AV valve contains a pair, not a trio, of cusps. The bicuspid valve permits blood to flow from the left atrium into the left ventricle but prevents backflow during ventricular contraction. The Left Ventricle The two ventricles hold equal amounts of blood; however, the left ventricle has thicker walls and is much larger than the right ventricle. Its thick, muscular walls enable the left ventricle to develop pressure sufficient to push blood through the large systemic circuit, whereas the right ventricle needs to pump blood, at lower pressure, only about 15 cm (6 in.) to and from the lungs. The internal organization of the left ventricle resembles that of the right ventricle, except for the absence of a moderator band. The aortic valve prevents the backflow of blood into

the left ventricle once the blood has been out of the heart and into the systemic circuit. From the ascending aorta, blood flows through the aortic arch and into the descending aorta. The ligamentum arteriosum connects the aortic arch to the pulmonary trunk. It is a fibrous band that is a remnant of a fetal blood vessel that once linked the pulmonary and systemic circuits to bypass the lungs before birth. The Heart Valves The heart has two pairs of one-way valves that stop the backflow of blood as the chambers contract. The atrioventricular (AV) valves prevent the backflow of blood from the ventricles to the atria when the ventricles are contracting.

Anatomy & Physiology Study Guide

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