Major Hemorrhage - Cardiovascular Homeostasis

Overview
  • Sudden, non-minor reductions in blood volume due to a major hemorrhage of less than 20% of the total blood volume can be accommodated by the cardiovascular system of a healthy individual by substantial modulation of cardiovascular parameters. The basic goal of this response is to maintain systemic arterial pressure at a sufficient level to maintain consciousness until additional blood volume can be generated from fluid ingestion.
Response Initiation
  • Sudden reductions in total blood volume reduce the venous return to the heart and thus the Ventricular End Diastolic Volume (VEDV) and preload of the heart. Because of the Frank-Starling Relationship, reduced VEDV will result in a drop in the cardiac output. Given a constant systemic vascular resistance, the sudden drop in cardiac output will in turn cause a sudden drop in the systemic arterial pressure as explained in systemic arterial pressure regulation.
Short-term Response
  • Overview
  • Cardiac Effects
    • Increased SNS tone will increase cardiac contractility and heart rate which together boost cardiac output.
  • Venous Effects
    • SNS tone will also induce venoconstriction which will push the venous blood reservoir into the heart, thus increasing venous return. Increased venous return to the heart will boost the VEDV which, courtesy of the Frank-Starling Relationship, will also increase cardiac output.
  • Arterial Effects
    • SNS tone throughout the vascular system will cause vasoconstriction of arterioles and thus boost the systemic vascular resistance. The increased cardiac output of the heart against a higher systemic vascular resistance will synergize to return the systemic arterial pressure largely back to normal.
Long-term Response
  • The above short-term responses will largely return the systemic arterial pressure to normal; however, they do not represent a long-term solution to the loss of blood volume as they require unsustainable changes in cardiovascular function. The long-term solution to major hemorrhage must be a restoration of the total blood volume which can only be achieved by ingestion of salt and water combined with a changes renal physiology which prevent additional loss of fluid volume in the urine. As discussed in Systemic Arterial Pressure - Long-term Regulation, the kidneys respond to drops in arterial pressure by reducing their urinary excretion of salt and water. These processes depend on the regulatory mechanisms of pressure natriuresis and the RAAS System and guarantee that any additional salt and water ingested by the individual is added to the extracellular fluid volume and is not lost in the urine.