CELLULAR CARDIAC PHYSIOLOGY

Cellular Anatomy

At the cellular level, the heart consists of three major components: cardiac muscle tissue (contracting cardiomyocytes), conduction tissue (conducting cells), and extracellular connective tissue. A group of cardiomyocytes with its connective tissue support network or extracellular matrix make up a myofiber ( Fig. 18-9 ). Adjacent myofibers are connected by strands of collagen. The extracellular matrix is the synthetic product of fibroblasts and is made up of collagen, which is the main determinant of myocardial stiffness, and other major matrix proteins. One of the matrix proteins, elastin, is the chief constituent of elastic fibers. The elastic fibers account for, in part, the elastic properties of the myocardium. ^[6] Other matrix proteins include the glycoproteins or proteoglycans and matrix metalloproteins. Proteoglycans are proteins with short sugar chains, and

Figure 18-8 Illustration demonstrating the principle of determination of cardiac output by using the indicator dilution technique. This model assumes that there is no recirculation. A known amount of dye (q) is injected at point A into a stream flowing at (mL/min). A mixed sample of the fluid flowing past point B is withdrawn at a constant rate through a densitometer. The change in dye concentration over time is depicted in a curve. Flow may be measured by dividing the amount of indicator injected upstream by the area under the downstream concentration curve. (From Berne RM, Levy MN: The cardiac pump. In Cardiovascular Physiology, 8th ed. St Louis, CV Mosby, 2001, pp 55–82.)