HeartPrint10 - The Heart: The fundamentals The Heart: The...

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The Heart: The fundamentals
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The Heart: The fundamentals Figure 1 Systolic and Diastolic Pressures: We have discussed the basic design of the cardiovascular system and that its major function is to generated pressures. This module will discuss the pressure generating characteristics of the cardiovascular system. Before the left ventricle ejects blood into the coronary arteries and the aorta, the pressure rises. The pressure generated during this ejection phase is called the systolic pressure. As the left ventricle relaxes, the pressure in the chamber decreases. The lowest pressure in the left ventricle before the ventricle begins to contract is called the diastolic pressure. In clinical practice, the patient is evaluated based on both the systolic and diastolic pressures. The values are presented as Systolic/Diastolic in terms of mm of Hg pressure. Normal systolic pressure is 120 whereas diastolic is 80 . Thus the designation would be 120/80 mm/Hg. Most importantly, it is the difference between systolic and diastolic pressure that determines blood flow. This value is called the pulse pressure referring to the net pressure generated with each pulse or cardiac contraction.
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Figure 2 However, pulse pressure is determined by two other factors. As the blood is ejected from the left ventricle it pushes against the aorta. Subsequently the aorta expands followed by a compression. This compression is due to the elastic fibers in the aorta. Thus the aorta balloons out and then returns to its normal circumference. The expansion of the aorta is called compliance. The aorta has the highest compliance because it has the greatest amount of elastic tissue fibers. The more compliant the aorta is, the less the pressure change during ventricular ejection. The other important consequence of aortic compliance is that it minimizes a large pressure difference being transmitted throughout the body. As the aorta compresses the blood volume, it neutralizes the systolic and diastolic pressure differences. Blood flow follows this simple but important formula. Blood Flow= Change in Pressure/resistance What is most important is the amount of blood that leaves the heart to supply the metabolic demands of the body. This value is called the cardiac output and is determined by the heart rate and the stroke volume. CO=SV x HR. The heart rate is determined by neural and endocrine factors. The sympathetic nervous system causes an increase in heart rate but no change in coronary artery diameter. The coronary arteries must not constrict as heart rate increases, otherwise, the amount of blood will decrease resulting in less oxygen getting to the cardiac cells. Although the coronary arteries do not constrict, other arterial beds do with increased sympathetic activity. The blood vessels that supply the skin and intestinal tract will constrict (e.g. their diameter will decrease) thereby directing more blood to the skeletal muscle. The parasympathetic nervous system causes a decrease in heart rate and has
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This note was uploaded on 04/21/2008 for the course BIOL 100 taught by Professor Lee during the Winter '07 term at San Diego State.

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HeartPrint10 - The Heart: The fundamentals The Heart: The...

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