THE PUMPING STATION-HEART

Barely the size of the clenched fist of the individual in whom it resides—an inverted, conically shaped,hollow muscular organ measuring 12 to 13 cm from base(top) to apex (bottom) and 7 to 8 cm at its widest point and weighing just under 0.75 lb (about 0.474% of the individual’s body weight, or some 325 g)—the human heart occupies a small region between the third and sixth ribs in the central portion of the thoracic cavity of the body.It rests on the diaphragm, between the lower part of the twolungs, its base–to–apex axis leaning mostly toward the left side of the body and slightly forward. The heart is divided by a tough muscular wall—the interatrial-interventricular septum—into a somewhat crescent-shaped right side and cylindrically shaped left side, each being one self-contained pumping station, but the two being connected in series. The left side of the heart drives oxygen-rich blood through the aortic semilunar outlet valve into the systemic circulation, which carries the fluid to within a differential neighborhood of each cell in the body—from which it returns to the right side of the heart low in oxygen and rich in carbon dioxide. The right side of the heart then drives this oxygenpoor blood through the pulmonary semilunar (pulmonic) outlet valve into the pulmonary circulation,which carries the fluid to the lungs—where its oxygen supply is replenished and its carbon dioxide content is purged before it returns to the left side of the heart to begin the cycle all over again.Because of the anatomic proximity of the heart to the lungs, the right side of the heart does not have to work very hard to drive blood through the pulmonary circulation, so it functions as a low-pressure (P = 40mmHg gauge) pump compared with the left side of the heart, which does most of its work at a high pressure (up to 140 mmHg gauge or more) to drive blood through the entire systemic circulation to the furthest extremes of the organism.

Each cardiac (heart) pump is further divided into two chambers: a small upper receiving chamber, or atrium (auricle), separated by a one-way valve from a lower discharging chamber, or ventricle, which is about twice the size of its corresponding atrium. In order of size, the somewhat spherically shaped left
atrium is the smallest chamber—holding about 45 ml of blood (at rest), operating at pressures on the order of 0 to 25 mmHg gauge, and having a wall thickness of about 3 mm. The pouch-shaped right atrium is next (63 ml of blood, 0 to 10 mmHg gauge of pressure, 2-mm wall thickness), followed by the conical/cylindrically shaped left ventricle (100 ml of blood, up to 140 mmHg gauge of pressure, variable
wall thickness up to 12 mm) and the crescent-shaped right ventricle (about 130 ml of blood, up to 40 mmHg gauge of pressure, and a wall thickness on the order of one-third that of the left ventricle, up to about 4 mm). All together, then, the heart chambers collectively have a capacity of some 325 to 350 ml, or about 6.5% of the total blood volume in a “typical” individual—but these values are nominal, since the organ alternately fills and expands, contracts, and then empties as it generates a cardiac output.

During the 480-ms or so filling phase—diastole—of the average 750-ms cardiac cycle, the inlet valves of the two ventricles (3.8-cm-diameter tricuspid valve from right atrium to right ventricle; 3.1-cm-diameter bicuspid or mitral valve from left atrium to left ventricle) are open, and the outlet valves(2.4-cm-diameter pulmonary valve and 2.25-cm-diameter aortic semilunar valve, respectively) are closed—the heart ultimately expanding to its end-diastolic-volume (EDV), which is on the order of 140 ml of blood for the left ventricle. During the 270-ms emptying phase—systole—electrically induced vigorous contraction of cardiac muscle drives the intraventricular pressure up, forcing the one-way inlet valves closed and the unidirectional outlet valves open as the heart contracts to its end-systolic-volume (ESV),which is typically on the order of 70 ml of blood for the left ventricle. Thus the ventricles normally empty about half their contained volume with each heart beat, the remainder being termed the cardiac reserve volume. More generally, the difference between the actual EDV and the actual ESV, called the stroke volume (SV), is the volume of blood expelled from the heart during each systolic interval, and the ratio

Anterior view of the human heart showing the four chambers, the inlet and outlet valves, the inlet and outlet major blood vessels, the wall separating the right side from the left side, and the two cardiac pacing centers—the sinoatrial node and the atrioventricular node. Boldface arrows show the direction of flow through the heart chambers, the valves, and the major vessels.1-6 Biotechnology for Biomedical Engineers of SV to EDV is called the cardiac ejection fraction, or ejection ratio (0.5 to 0.75 is normal, 0.4 to 0.5 signifies mild cardiac damage, 0.25 to 0.40 implies moderate heart damage, and <0.25 warms of severe damage to the heart’s pumping ability). If the stroke volume is multiplied by the number of systolic intervals per minute, or heart (HR), one obtains the total cardiac output (CO).