Hypovolemia may be defined as a volume-capacitance mismatch. That is, the blood volume is less than is required to fill the vascular system to a normal mean circulatory filling pressure. The vascular system is normally filled so that there is an intravascular pressure of about 7 mmHg during circulatory arrest, the “mean circulatory filling pressure”, and it is this pressure which is the principal determinant of the rate at which right ventricular filling occurs during diastole, and hence cardiac output . Decreased blood volume or vasodilation may reduce the mean circulatory filling pressure to such a degree that the cardiac output falls because of reduced venous return. From this it is apparent that two types of hypovolemia may be defined, relative and absolute.
KeywordsCardiac Output Blood Volume Central Blood Volume Shock Index Systolic Arterial Blood Pressure
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