The cardiovascular system is the system mainly affected in an individual who has lost a large quantity of blood. Arterial systolic, diastolic, and pulse pressures decrease, and the arterial pulse is rapid and feeble. The cutaneous veins collapse, and they fill slowly when compressed centrally. The skin is pale, moist, and slightly cyanotic. Respiration is rapid but may be shallow or deep.
Course of Arterial Blood Pressure Changes
Cardiac output decreases as a result of blood loss. The amount of blood removed when it is donated is approximately 10% of total blood volume; its removal is well tolerated, and mean arterial blood pressure changes little. This is not the case when greater amounts are lost from the circulation. The changes in Pa evoked by acute hemorrhage are illustrated in
Fig. 19.19 . If sufficient blood is rapidly withdrawn to decrease Pa to 50 mm Hg, the pressure then tends to rise spontaneously toward the control level over the next 20 or 30 minutes. In some individuals (curve A in
Fig.
19.20 ), this trend continues, and normal pressure is regained within a few hours. In others (curve B in
Fig. 19.20 ), the pressure rises initially after the cessation of hemorrhage. The pressure then begins to decline, and it continues to fall at an accelerating rate until death ensues. This progressive deterioration in cardiovascular function is termed hemorrhagic shock. At some time after the hemorrhage, the deterioration in the cardiovascular system becomes irreversible. A lethal outcome in patients with hemorrhagic shock can be prevented only temporarily by any known therapy, including massive transfusions of donor blood.
•Fig. 19.20 Changes in Mean Arterial Pressure After Rapid Hem-orrhage. Attime0,rapidlossofbloodcausesareductioninthemeanarterialpressureto50mmHg.Afteraperiodinwhichthepressurereturnstowardthecontrollevel,someindividualscontinuetoimprove(curveA)untilthecontrolpressureisattained.However,inotherindividuals,thepressurebeginstodecline(curveB)untildeathensues.–2 –1 0 0 25 50 75 100 1 2 3 4 5 A B 6 7 Time (hr) Mean arterial pressure (mm Hg)
The changes in arterial pressure immediately after acute blood loss (see
Fig. 19.20 ) indicate that certain compensatory mechanisms must be operative. Any mechanism that raises arterial blood pressure toward normal in response to a reduction in pressure is designated a negative feedback mechanism. This mechanism is termed negative because the direction of the secondary change in pressure is opposite the direction of the initiating change after the acute blood loss. The following negative feedback responses are evoked: baroreceptor reflexes, (2) chemoreceptor reflexes, (3) cerebral ischemia responses, (4) reabsorption of tissue fluids, release of endogenous vasoconstrictor substances, and renal conservation of salt and water.