Anatomy and Physiology - Introduction to Biomedical Engineering - page 99

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 to 70 ml of blood for the left ventricle. Thus,
the ventricles normally empty about half their contained volume with each heartbeat, the
remainder being termed the cardiac reserve volume . More generally, the difference between

the actual EDV and the actual ESV, called the (SV), is the volume of blood

expelled from the heart during each systolic interval, and the ratio of SV to EDV is called the
cardiac ejection faction

stroke volume

(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 less than 0.25 warns of severe dam-
age to the heart's pumping ability). If the stroke volume is multiplied by the number of
systolic intervals per minute, or heart rate (HR), one obtains the total cardiac output (CO):
CO

,or ejection ratio

¼ HR ð EDV ESV Þ

where EDV-ESV to the stroke volume.
Several investigations have suggested that the cardiac output (in milliliters per minute) is
proportional to the weight W (in kilograms) of an individual according to the equation

224W 3=4
and that “normal” heart rate obeys very closely the relation

CO

229W 1=4 HR ¼

For a “typical” 68.7-kg individual (blood volume ¼ 5,200 ml), these equations yield CO

¼ 5,345 ml/min, HR ¼ 80 beats/min (cardiac cycle period ¼ 754 ms) and SV ¼ CO/HR

229W 1/4 /CO-224W 3/4 ¼ 67.2 ml/beat, which are very reasonable values.

Furthermore, assuming this individual lives to be about 75 years old, his or her heart will
have cycled over 3.1536 billion times, pumping a total of 0.2107 billion liters of blood
(55.665 million gallons, or 8,134 quarts per day) within their lifetime.
In the normal heart, the cardiac cycle, which refers to the repeating pattern of contraction
(systole) and relaxation (diastole) of the chambers of the heart, begins with a self-generating
electrical pulse in the pacemaker cells of the sinoatrial node (Figure 3.20). This rapid electri-
cal change in the cells is the result of the movement of ions across their plasma membranes.
The permeability of the plasma membrane to Na þ changes dramatically and allows these
ions to rush into the cell. This change in the electrical potential across the plasma membrane
from one in which the interior of the cell is more negative than the extracellular fluid
(approximately -90 mV) to one in which the interior of the cell is more positive than the
extracellular fluid (approximately 20 mV) is called depolarization. After a very short period
of time (

¼ 0.978W ¼

0.3 s), changes in the membrane and activation of the sodium-potassium pumps
result in repolarization, the restoration of the original ionic balance in the cells. The entire
electrical event in which the polarity of the potential across the plasma membrane rapidly

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