The goal of this chapter is to review the structural and functional properties of the cardiac microcirculation. The primary focus will be the endothelial lining of the microves-sels. The endothelium may be viewed as a subsystem of the blood vessel wall. The blood vessel, in turn, may be considered a subsystem of the cardiovasculature. In serving as a frame of reference, the endothelium readily lends itself to a consideration of other cellular and noncellular components of the cardiac microvasculature.
any one time. During exercise, coronary vasodilation results in increased flow and capillary recruitment. Blood drains from the capillaries into postcapillary venules, cardiac veins, and ultimately the coronary sinus and right atrium. The microcirculation consists of vessels smaller than 300 mm in diameter, and includes small arteries, arterioles, capillaries, and postcapillary venules. The endocardium, which represents the endothelial lining of the atria and ventricles and heart valves (endocardium), is a developmentally and phenotypically distinct vascular bed and is not discussed in this chapter.
The coronary circulation, like all vascular beds, has a treelike or fractal structure. Two major arteries arise from the aorta, the left main coronary artery (which divides into the left anterior descending and the circumflex arteries) and the right coronary artery. These large vessels lie on the epicardial surface of the heart and function as distribution or conduit vessels, providing rapid mass transport of blood to the myocardium. Once the large arteries penetrate the myocardium, they branch into resistant vessels or arterioles. The small arteries and arterioles (75 to 200 mm in diameter) are the primary sites of coronary vascular resistance, and thus the principal regulators of coronary blood flow. These arterioles give rise to a densely packed capillary network, which runs parallel to sheets of cardiomyocytes. Capillaries serve as exchange vessels, transferring oxygen and nutrients to the underling tissue. The mean intercapillary distance is 17 mm at rest, thus providing a mean maximal diffusion distance of 8.5 mm. There are approximately 2,500 cardio-myocytes and 2,500 capillaries per mm2. In rodents, the capillary-cardiomyocyte ratio ranges between 0.91 and 1.12. At rest only a third of the capillaries are perfused at
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This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.