The first reactionary change to occur after vessel injury and haemorrhage is: September 2011
Correct Answer: Vasoconstriction
Description: Ans. A: VasoconstrictionWhen a small blood vessel is transected or damaged, the injury initiates a series of events that lead to the formation of a clot. This seals off the damaged region and prevents fuher blood loss.The initial event is constriction of the vesselPhysiological response to hemorrhageIt is a three-pa process. The principal features are:- Rapidly responding neural and humoral mechanisms direct available blood flow toward vital organs.- More slowly evolving salt and water retention by the kidneys replaces the lost plasma.- Erythropoiesis gradually replaces the lost red blood cells.Acute response to hemorrhageThe acute response includes a primary decrease in cardiac output, a secondary decrease in aerial pressure and compensatory increases in hea rate and vascular resistance.Autonomic involvementIncreased hea rate and peripheral vasoconstriction signal the autonomic nervous system's paicipation in the acute response to hemorrhage.Autonomic dysfunction decreases the body's tolerance to blood loss.Selective vasoconstrictionSympathetic vasoconstriction during hemorrhage is selective.Many organs show intense vasoconstriction, while the cerebral and coronary circulations show little response to increased sympathetic outflow.The benefit of selectivity is that available blood flow, as meager as it may be, is preferentially directed to the brain and hea -- the vital organs.Suppo from Renin-Angiotensin systemThe renin-angiotensin system suppos aerial pressure in hemorrhage by constricting nonvital organs.This response is slower than the response of the sympathetic nervous system, but it is still an impoant pa of the acute circulatory response to hemorrhage.Salt and water retentionAfter hemorrhage, with no intervention, salt and water retention over several days will increase blood volume to normal or above.Erythrocytes are replaced much more slowly.
Category:
Physiology
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