The cardiovascular system, including the heart and blood vessels, is responsible for the circulation of blood to all tissues of the body while carrying away carbon dioxide and waste products. Cardiac muscle shares many attributes with skeletal and smooth muscles but represents a separate muscle group with distinct structure, function, and regulation. Key in this function is the self-activating nature of particular cardiac cells that normally provide an ordered contraction of the cardiac chambers. Blood vessels contain smooth muscle cells regulated by a variety of signal molecules and also play an important role in maintenance of blood pressure and oxygen/nutrient distribution. Diseases of these blood vessels, especially hypertension and atherosclerosis, cause much of the illness and most of the deaths in the developed world.
CARDIAC MUSCLE STRUCTURE AND FUNCTION
The heart is derived from the mesoderm and provides the force that propels oxygenated blood to all cells of the body and returns deoxygenated blood back to the lungs (Figure 16-1). The heart comprises specialized striated muscle termed cardiac muscle.
Sectional View of Heart. Basic anatomy of the heart is indicated, including chambers, major blood vessels, and conducting system (yellow), including the sinoatrial node, atrioventricular node, and Purkinje fibers. [Reproduced with permission from Barrett KE, et al.: Ganong’s Review of Medical Physiology, 23rd edition, McGraw-Hill, 2010.]
Cardiac muscle differs from skeletal and smooth muscle in several important ways:
Increased numbers of mitochondria allow continuous high-level production of adenosine triphosphate (ATP) to support continuous function without fatigue. During periods of low oxygen, anaerobic respiration can provide enough energy for contraction to continue.
Utilizes fatty acids as the major energy source (60%) followed by carbohydrates (35%), amino acids, and ketone bodies (5%).
Contains fewer but larger T tubules and intercalated discs that allow the rapid and synchronous spread of action potentials between adjacent cells, allowing the coordinated contraction of cardiac muscle.
Relies on the rapid release and re-uptake of intracellular calcium stores to convert the electrical signal of the action potential into the mechanical work of contraction.
Because cardiac cells and smooth muscle cells rely strongly on an L-type (“longlasting”) calcium channel for contraction, the medication class of calcium channel blockers (CCBs) are used to treat high blood pressure and certain cardiac disorders. In the heart, CCBs decrease the total calcium released and both the heart rate and force of the contraction, thereby reducing oxygen demand. This effect of CCBs is particularly useful in helping to control abnormal heart rhythms such as atrial fibrillation. In blood vessels, the decreased calcium-induced contraction results in dilation of the blood vessels (by decreasing the ability of the smooth muscle to contract) and lowers the resistance of those vessels. Additional cardiovascular uses of CCBs include reducing the outflow gradient in hypertrophic cardiomyopathy ...