What are the compensatory mechanisms that acting to maintain blood pressure in face of loss?
Several compensatory mechanisms act to maintain blood pressure in the face of blood loss or hypovolemia:
1. Tachycardia: Heart rate increases to pump more blood per minute, thereby increasing cardiac output and maintaining blood pressure.
2. Vasoconstriction: Blood vessels constrict, especially in non-essential organs, to redirect blood flow to vital organs such as the brain, heart, and kidneys. This helps to maintain blood pressure by increasing systemic vascular resistance.
3. Renin-angiotensin-aldosterone system (RAAS) activation: The kidneys release renin in response to reduced blood flow and decreased sodium delivery to the distal tubules. Renin converts angiotensin I to angiotensin II, which causes vasoconstriction and stimulates the adrenal glands to release aldosterone. Aldosterone promotes sodium and water reabsorption in the kidneys, increasing blood volume and blood pressure.
4. Sympathetic Nervous System Activation: Activation of the sympathetic nervous system leads to increased heart rate and force of contraction, as well as vasoconstriction. These effects increase blood pressure.
5. Baroreflex and Carotid Sinus Reflex: These reflexes sense changes in blood pressure and trigger compensatory mechanisms. For example, when blood pressure drops, the baroreflex causes increased heart rate and vasoconstriction, which help to raise blood pressure.
6. Shift of Fluid from Interstitial Spaces: As blood pressure decreases, fluid moves from the interstitial spaces into the vascular compartment, increasing blood volume and pressure.
7. Increased Vasopressin (Antidiuretic Hormone) Release: Vasopressin promotes water reabsorption in the kidneys, reducing urine output and conserving fluid, which can help to maintain blood pressure.
These compensatory mechanisms work together to maintain blood pressure within a narrow range despite changes in blood volume. However, if the blood loss is severe or prolonged, these mechanisms may not be sufficient, and blood pressure may drop to critically low levels, leading to shock and organ damage.