When an individual loses a significant amount of blood, the cardiovascular system is primarily affected. This results in a decrease in arterial systolic, diastolic, and pulse pressures, and a rapid and feeble arterial pulse. The cutaneous veins collapse and refill slowly when compressed centrally. The skin appears pale, moist, and slightly cyanotic. Respiration becomes rapid, but it may be shallow or deep.

The course of arterial blood pressure changes as a consequence of blood loss. When blood is donated, approximately 10% of the total blood volume is removed, which is well tolerated and causes little change in mean arterial blood pressure. However, when larger amounts are lost from the circulation, the effects are different. The changes in arterial pressure caused by acute hemorrhage are depicted in Figure 19.19. If enough blood is rapidly withdrawn, causing the arterial pressure (Pa) to decrease to 50 mm Hg, the pressure then tends to spontaneously rise back towards the control level within the next 20 or 30 minutes. In some individuals (curve A in Figure 19.20), this trend continues and normal pressure is regained within a few hours. However, in others (curve B in Figure 19.20), the pressure initially rises after hemorrhage cessation but then begins to decline, falling at an accelerating rate until death occurs. This progressive deterioration in cardiovascular function is known as hemorrhagic shock. At some point after the hemorrhage, the cardiovascular system's deterioration becomes irreversible. Any known therapy, including massive transfusions of donor blood, can only temporarily prevent a lethal outcome in patients with hemorrhagic shock.

Figure 19.20 illustrates the changes in mean arterial pressure after rapid hemorrhage. At time 0, a rapid loss of blood causes a reduction in mean arterial pressure to 50 mm Hg. After a period in which the pressure returns towards the control level, some individuals continue to improve (curve A) until the control pressure is attained. However, in other individuals, the pressure begins to decline (curve B) until death occurs.

The immediate changes in arterial pressure after acute blood loss (see Figure 19.20) indicate that certain compensatory mechanisms must be in place. Any mechanism that raises arterial blood pressure towards normal in response to a reduction in pressure is termed a negative feedback mechanism. This mechanism is referred to as negative because the secondary change in pressure is opposite to the initiating change after acute blood loss. The following negative feedback responses are triggered: baroreceptor reflexes, chemoreceptor reflexes, cerebral ischemia responses, reabsorption of tissue fluids, release of endogenous vasoconstrictor substances, and renal conservation of salt and water.
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.