From the AV node, the wave of depolarization passes into and through the ventricle (as signified by the QRS complex of the ECG), and this triggers ventricular contraction. Remember that just before the contraction, the aortic valve was closed and the AV valve was open. As the ventricle contracts, ventricular pressure rises very rapidly, and almost immediately this pressure exceeds the atrial pressure, closing the AV valve and thus preventing backflow of blood into the atrium. Since the aortic pressure still exceeds the ventricular pressure, the aortic valve remains closed, and the ventricle cannot empty despite its contraction.
Vander et al.: Human Physiology: The Mechanism of Body Function, Eighth Edition
Circulation CHAPTER FOURTEEN
This brief phase of isovolumetric ventricular contraction ends when the rapidly rising ventricular pressure exceeds aortic pressure. The aortic valve opens, and ventricular ejection occurs. The ventricular volume curve shows that ejection is rapid at first and then tapers off. Note that the ventricle does not empty completely. The amount of blood remaining after ejection is called the end-systolic volume (ESV). Thus:
As shown in Figure 14-25, normal values for an adult at rest are stroke volume = 70 ml, end-diastolic volume = 135 ml, and end-systolic volume = 65 ml.
As blood flows into the aorta, the aortic pressure rises along with the ventricular pressure. Throughout ejection, only very small pressure differences exist between the ventricle and aorta because the aortic valve opening offers little resistance to flow.
Note that peak ventricular and aortic pressures are reached before the end of ventricular ejection; that is, these pressures start to fall during the last part of systole despite continued ventricular contraction. This is because the strength of ventricular contraction and rate of blood ejection diminish during the last part of systole as shown by the ventricular volume curve. Therefore the ejection rate becomes less than the rate at which blood is leaving the aorta. Accordingly, the volume and therefore the pressure in the aorta begin to decrease.
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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.