Several diseases affect the valves of the heart, causing their degeneration (e.g., calcification, fibrosis) and resulting in heart malfunction because of insufficiency or stenosis of valvular orifices. These conditions, known collectively as valvular heart disease, include rheumatic heart disease, vegetative endocarditis, degenerative calcific aortic valve stenosis, and mitral annular calcification. For example, rheumatic fever causes inﬂammation of the heart valves (valvulitis). Inﬂammation induces angiogenesis in the valve and vascularization in the normally avascular layers of the valve. These changes most commonly affect the mitral valve (65% to 70%) and aortic valve (20% to 25%). This inﬂammation can lead to progressive replacement of elastic tissue by irregular masses of collagen fibers, causing the valve to thicken. The valves become rigid and inﬂexible, which affects their ability to open and close.
Intrinsic Regulation of Heart Rate
Contraction of the heart is synchronized by specialized cardiac conducting cells.
Cardiac muscle can contract in a rhythmic manner without any direct stimulus from the nervous system. For the heart to be an effective pump, it is necessary for the atria and ventricles to contract in a coordinated rhythmic manner. The electrical activity (impulses) that results in the rhythmic pulsations of the heart is initiated and propagated by the conducting system of the heart. The rate of depolarization of cardiac muscle
FIGURE 13.9 • Mitral valve in the human heart. a. This photograph shows a sagittal section of the posterior wall of the left ventricle and the posterior cusp of the mitral valve. The chordae tendineae extend from the papillary muscle to the ventricular side of the mitral valve cusp. Note the thickness of the myocardium in the left ventricle. Glistening inner surface of the heart represents the endocardium, the outer surface of the myocardium is covered by the epicardium. 2. (Courtesy of Dr. William D. Edwards, Mayo Clinic, Rochester, MN.) b. Photomicrograph of a mitral valve. This photomicrograph shows a section through one of the two cusps of the mitral valve. Both sides of the cusp are covered by the endothelium. Note that the valve exhibits a layered architecture. Beginning at the atrial side (top of the image), the first layer underlying the endothelium is the spongiosa—not well developed in this part of the cusp. The second layer is the fibrosa, which forms the majority of the dense connective tissue in the core of the valve. The third layer, the ventricularis, is formed by dense connective tissue containing layers of elastic and collagen fibers. 125.
varies in different parts of the conducting system; the fastest is in the atria, the slowest in the ventricles. The contraction cycle of the heart is initiated in the atria, forcing blood into the ventricles. A wave of contraction in the ventricles then begins at the apex of the heart, forcing blood from the heart into the aorta and pulmonary trunk.