Porth's Essentials of Pathophysiology, 4e

386

Circulatory Function

U N I T 5

Backflow of blood due to recoil of elastic aorta (closes valve and causes filling of coronary arteries when myocardium is relaxed)

Beginning of ascending aorta Posterior aortic sinus

Bloodflow

Lunule

Nodule

Left coronary artery

Right coronary artery

Left aortic sinus

Right aortic sinus

To heart muscle (myocardium)

To heart muscle (myocardium)

A Anterior view of aortic valve

B Valve open

C Valve closed

FIGURE 17-10. Aortic valve, aortic sinuses, and coronary arteries. (A) Like the pulmonary valve, the aortic valve has three semilunar cusps: right, posterior, and left. (B) Blood ejected from the left ventricle forces the cusps open. (C) When the valve closes, the valve edges and nodules meet in the center. (From Moore KL, Dalley AF, Agur AMR. Clinically Oriented Anatomy. 6th ed. Philadelphia, PA: Wolters Kluwer Health | Lippincott Williams &Wilkins; 2010:144.)

facilitated by a nodular thickening at the apex of each leaflet (Fig. 17-10C). Immediately behind each of the semilunar cusps, the walls the pulmonary trunk and aorta are slightly dilated, forming a sinus (Fig. 17-10A). In these sinuses, eddy currents develop that tend to keep the valve cusps away from the vessel wall. The opening for the right coronary artery is located in right aortic sinus, and the opening for the left coronary artery is located in left aortic sinus. Were it not for the presence of the sinuses and eddy currents, the coronary artery openings would be blocked by the valve cusps. There are no valves at the atrial sites (i.e., venae cavae and pulmonary veins) where blood enters the heart (see Fig. 17-8). This means that excess blood is pushed back into the veins when the atria become distended. For example, the jugular veins typically become distended when inflow into the right atria is impeded in right-sided heart failure, whereas normally they are flat or col- lapsed. Likewise, the pulmonary venous system becomes congested when outflow from the left atrium is impeded. Electrical Activity of the Heart Heart muscle is unique among other muscles in that it is capable of generating and rapidly conducting its own electrical impulses or action potentials. These action potentials result in excitation of muscle fibers through- out the myocardium. Impulse formation and conduction result in weak electrical currents that spread through the entire body. Cardiac Conduction System In certain areas of the heart, the myocardial cells have been modified to form the specialized cells of the con- duction system (Fig. 17-11). Although most myocardial

cells are capable of initiating and conducting impulses, it is this specialized conduction system that maintains the pumping efficiency of the heart. Specialized pace- maker cells generate impulses at a faster rate than other myocardial cells, and the cells of the conduction system transmit impulses at a faster rate than other myocardial cells. Because of these properties, the conduction system normally controls the rhythm of the heart. The conduction system consists of (1) the sinoatrial (SA) node, where the rhythmic impulse is generated; (2) the internodal pathways, which conduct the impulse from the SA node to the AV node; (3) the AV node, in which the impulse from the atria is delayed before pass- ing to the ventricles; (4) the AV bundle, which conducts the impulse from the atria to the ventricles; and the (5) left and right bundles of the Purkinje system, which con- duct the impulses to all parts of the ventricles. The SA node has the fastest intrinsic rate of firing (60 to 100 beats per minute) and normally functions as the pacemaker of the heart. From the SA node, the impulse travels radially throughout the right atrium, ultimately reaching the AV node. A special pathway, the anterior interatrial pathway, conducts the impulse to the left atrium. The heart essentially has two conduction systems: one that controls atrial activity and one that controls ventricular activity. The AV node connects the two sys- tems and normally provides for a one-way conduction between the atria and ventricles. Within the AV node, atrial fibers connect with very small junctional fibers in the node itself. Because of these connections, the zone surrounding and including the AV node and the adja- cent atrial and ventricular conduction pathways is often referred to as the AV junctional area . The velocity of con- duction through these fibers is very slow (approximately one-half that of normal cardiac muscle), which greatly