Page 30 - Cardiac Nursing

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6 PA R T I / Anatomy and Physiology

in an anterior, leftward, and inferior direction. Blood flows out of the normal heart, blood is prevented from flowing backward into
the ventricle from the apex toward the aorta in a superior and right- the atria despite the high systolic ventricular pressures.
ward direction (Fig. 1-8).
Thus, blood flows from posterior orifices into both ventricles Semilunar Valves
in a leftward direction and is ejected superiorly toward the center The two semilunar (pulmonary [or pulmonic] and aortic) valves
of the heart. The right ventricular outflow tract is more tubular; are each composed of three cup-shaped cusps of approximately
the left ventricular outflow tract more conical (Fig. 1-8). equal size that attach at their base to the fibrous skeleton. The
valve cusps are convex from below, with thickened nodules at the
Valves center of the free margins.
The cusps are composed of fibrous connective tissue lined with
Atrioventricular Valves endothelium. The endothelial lining on the nonventricular side of
The AV tricuspid and bicuspid (mitral) valve complexes are com- the valves closely resembles and merges with that of the intima
posed of six components that function as a unit: the atria, the valve of the arteries beyond the valves. The aortic cusps are thicker than
rings or annuli fibrosi of the fibrous skeleton, the valve cusps or the pulmonic; both are thicker than the AV cusps.
leaflets, the chordae tendineae, the papillary muscles, and the ven- The pulmonary valve orifice is approximately 8.5 cm in cir-
tricular walls (Fig. 1-6). The mitral and tricuspid valve cusps are cumference. The pulmonic valve cusps are termed right anterior
composed of fibrous connective tissue covered by endothelium. (right), left anterior (anterior), and posterior (left). The aortic valve
They attach to the fibrous skeleton valve rings. Fibrous cords called is approximately 7.5 cm in circumference. The sinuses of Valsalva
chordae tendineae connect the free valve margins and ventricular sur- are pouch-like structures immediately behind each semilunar
faces of the valve cusps to papillary muscles and ventricular walls. cusp. The coronary arteries branch from the aorta from two of the
The papillary muscles are trabeculae carneae muscle bundles ori- pouches or sinuses of Valsalva. The aortic cusps are designated by
ented parallel to the ventricular walls, extending from the walls to the the name of the nearby coronary artery: right coronary (right or
chordae tendineae (Fig. 1-6). The chordae tendineae provide many anterior) aortic cusp, left coronary (left or left posterior) aortic
cross-connections from one papillary muscle to the valve cusps or cusp, and noncoronary (posterior or right posterior) aortic cusp.
from trabeculae carneae in the ventricular wall directly to valves. The aortic and pulmonic semilunar valves are approximately at
In the adult, the tricuspid orifice is larger (approximately right angles to each other in the closed position. The pulmonic
11 cm in circumference, or capable of admitting three fingers) valve is anterior and superior to the other three cardiac valves.
than the mitral orifice (approximately 9 cm in circumference, or When closed, the semilunar valve cusps contact each other at the
capable of admitting two fingers). The combined surface area of nodules and along crescent arcs, called lunulae, below the free
the AV valve cusps is larger than the surface area of the valvular margins. During systole, the cusps are thrust upward as blood
orifice because the cusps resemble curtain-like, billowing flaps. flows from an area of greater pressure in the ventricle to an area of
Most commonly, there are three tricuspid valve cusps: the large lesser pressure in the aorta or the pulmonary artery. The effect of
anterior, the septal, and the posterior (inferior). There are usually the deceleration of blood in the aorta during late systole on small
two principal right ventricular papillary muscles, the anterior and circular currents of blood in the sinuses of Valsalva helps passively
the posterior (inferior), and a smaller set of accessory papillary to close the semilunar valve cusps. Backflow into the ventricles
muscles attached to the ventricular septum. during diastole is prevented because of the cusps’ fibrous strength,
The arrangement of the two triangular bicuspid valve cusps their close approximation, and their shape.
has been compared to a bishop’s hat, or miter; hence the structure
is called the “mitral” valve. The smaller, less mobile posterior cusp
is situated posterolaterally, behind, and to the left of the aortic CARDIAC TISSUE
opening. The larger, more mobile anterior cusp extends from the
anterior papillary muscle to the ventricular septum. The heart wall is composed mainly of a muscular layer, the my-
The left ventricle most commonly has two major papillary ocardium. The epicardium and the pericardium cover the external
muscles: the posterior papillary muscle attached to the diaphrag- surface. Internally, the endocardium covers the surface.
matic ventricular wall and the anterior papillary muscle attached
to the sternocostal ventricular wall. Thus, the posteromedial pap- Epicardium and Pericardium
illary muscle extends to the posterolateral valve leaflet, and the an-
terolateral papillary muscle extends to the anteromedial valve The epicardium is a layer of mesothelial cells that forms the vis-
leaflet. Chordae tendineae from each papillary muscle go to both ceral or heart layer of the serous pericardium. Branches of the
mitral cusps. coronary blood and lymph vessels, nerves, and fat are enclosed in
During diastole, the AV valves open passively when pressure in the epicardium and the superficial layers of the myocardium.
the atria exceeds that in the ventricles. The papillary muscles are The epicardium completely encloses the external surface of the
relaxed. The valve cusps part and project into the ventricle, form- heart and extends several centimeters along each great vessel, en-
ing a funnel and thus promoting blood flow into the ventricles circling the aorta and pulmonary artery together. It merges with
(Fig. 1-8). Toward the end of diastole, the deceleration of blood the tunica adventitia of the great vessels, at which point it doubles
flowing into the ventricles, the movement of blood in a circular back on itself as the parietal pericardium. This continuous mem-
motion behind the cusps, and the increasing pressures in the ven- brane thus forms the pericardial sac and encloses a potential space,
tricle compared with lessening pressures in the atria, help to close the pericardial cavity (Fig. 1-1). The serous parietal pericardium
each valve. During systole, the free edges of the valve cusps are lines the inner surface of the thicker, tougher fibrous pericardial
prevented from being everted into the atria by contraction of the membrane. The pericardial membrane extends beyond the serous
papillary muscles and tension in the chordae tendineae. Thus, in pericardium and is attached by ligaments and loose connections to