A useful index of the cardiac oxygen consumption is the product of aortic pressure and heart rate—the double product. This index includes one of the determinants of external work (pressure) and the determinant of energy use as a function of time, heart rate. The double product does not include the effect of changes in stroke volume on energy consumption, but these are less significant than changes in pressure. In addition, the double product does not take into account effect of changes in radius of the ventricle on energy consumption. The extra energy required by pathologically dilated hearts is not reflected in the double product.
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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.