Final answer:
The sudden decrease in blood pressure after an acute hemorrhage is due to decreased blood volume. This leads to sympathetic responses such as vasoconstriction and increased heart activity, as well as ADH release to raise blood pressure and conserve water.
Step-by-step explanation:
The primary cause of a sudden decrease in blood pressure after an acute hemorrhage is decreased blood volume. When hemorrhage occurs, there is an immediate loss of blood, which reduces the volume of blood within the circulatory system. This leads to a drop in blood pressure since there is less blood to exert pressure against the walls of the blood vessels. The body responds to this by triggering an increase in sympathetic stimulation, resulting in both vasoconstriction and an increase in the rate and strength of heart contractions. Additionally, the hormone ADH (antidiuretic hormone) is released, which causes immediate vasoconstriction to increase blood pressure and activates aquaporin channels in the kidneys to conserve water and thereby help restore vascular volume.