The Heart - Biofluidmechanics: An Introduction to Fluid Mechanics Macrocirculation and Microcirculation - page 118

closes. Immediately after peak systole, the ventricular muscles begin to relax and the intra-
ventricular pressure begins to reduce from approximately 115 mmHg. However, due to
the viscoelastic mechanical properties of the aorta, the pressure in the aorta does not fall
rapidly but continues to increase for a few milliseconds (reaching approximately
120 mmHg under normal conditions). This causes a backward flow pressure gradient; the
pressure is higher at the tip of the aortic valve leaflet and is lower at the valve orifice. Due
to this backward pressure gradient, the blood that was already in the sinus prevents the
tip of the aortic valve from opening more fully in response to the reverse pressure gradi-
ent. The reverse pressure gradient also causes more blood to flow into the aortic valve
sinus, which aids in forcing the leaflet into the secure closed position. Once the valve is
completely closed, the blood in the sinus enters the aortic mainstream flow and is deliv-
ered to the body. The aortic valve is open for approximately 25% of the cardiac cycle.
The Gorlin equation is an approximation to the aortic valve opening area, as a function
of the cardiac output, the heart rate, the hydrostatic pressure, and the ejection period. The
cardiac output, the heart rate, and the ejection period will approximate the flow across the
valve leaflet during ejection. This formulation takes the form of

Cardiac Output

Valve Area 5 p

Mean Pressure Gradient

44 : 3 ð Heart Rate Þð Systolic Ejection Period Þð Þ

If the cardiac output is measured in mL/min, the heart rate is measured in beats/min, the
ejection period is measured in seconds, and the mean gradient is measured in mmHg,
then the valve area will be in units of cm 2 . Typically, the ejection period is 0.33 s, and the
mean pressure gradient is 50 mmHg.

4.6 DISEASE CONDITIONS

In most of the following chapters, there is a section for disease conditions. The authors
want to preface this section by stating that most of the diseases of the cardiovascular sys-
tem are related to each other, but we will discuss some of them in particular chapters,
although disease characteristics may belong to multiple chapters. We will try to mention
them again in other locations. For instance, one of the major causes of coronary artery dis-
ease is atherosclerosis, but atherosclerosis is a disease that can affect the entire cardiovas-
cular system. Due to the devastating downstream effects of an atherosclerotic lesion
within the coronary blood vessel, we include the first discussion on coronary artery dis-
ease here and discuss atherosclerosis in Chapter 5.

4.6.1 Coronary Artery Disease
The coronary arteries supply the cardiac muscle with blood ( Figure 4.14 ). The left main
coronary artery and the right main coronary artery branch off from the ascending aorta
approximately 0.5 to 1 cm above the aortic valve. The left coronary artery is responsible
for providing the left ventricular muscle mass with blood (along with sections of the right
ventricle and the left atrium). The left coronary artery delivers blood to approximately