Cardiovascular Responses — 1-L Blood Loss

Distribution of water

• TBW = 60% body weight ~ 42L
• Intracellular water = 66% of TBW ~ 28L
• Extracellular water = 33% of TBW ~ 14L
• Plasma volume = 25% Extracellular water ~3.5L
• Blood volume (plasma + RBC intracellular water) = 7% of total body water ~5L

1000ml blood loss = approx 20% blood loss = Stage II shock

Cardiovascular changes

Immediate

• Decreased venous return due to loss of blood volume
  • Frank-Starling Curve à Cardiac output decreased
  • Systolic and mean arterial blood pressure decreases (MAP = CO x SVR)
• Baroreflex
  • Decreased MAP
    • Decreased stretch of aortic arch and carotid sinus baroreceptors
    • Decrease firing of baroreceptors
    • Decreased signalling to sensory area of medullary vasomotor centre in NTS
    • Decreased inhibition of vasomotor area
    • Increased sympathetic and decreased parasympathetic output
  • Heart
    • Increased heart rate and contractility
  • Vasculature
    • Vasoconstriction
      • Blood diverted from peripheries to essential organ function (coronary and cerebral circulation)
      • Increased diastolic blood pressure
  • Leads to increased MAP
• Barcroft-Edholm reflex
  • If venous return severely impaired
    • Mechanoreceptors in R atrium/ventricle sense collapsing of chambers in systole
    • Increased vagal signal to NTS
    • Bradycardia (and hypotension) in order to allow greater diastolic filling time
• As all blood constituents lost → [Hb] and haematocrit unaltered
• At >10% blood volume loss, low pressure (cardiopulmonary) baroreceptors cause secretion of vasopressin from posterior pituitary
  • Vasoconstriction via V₁ receptor
  • Renal water retention via V₂ receptor (also assists in coagulation via vWF release)

Acute Changes

• Decreased intravascular hydrostatic pressure à transcellular shift of extracellular fluid into intravascular space and intracellular water into extracellular space
  • Decrease in plasma osmolality (usually 275~290mOsm/L)
    • Sensed by Organum Vascularis of the Lamina Terminalis (OVLT)
    • Increased secretion of vasopressin from posterior pituitary
  • Decreased [Hb] due to transcellular shift of water decreases blood viscosity
    • Decreased resistance to laminal flow
    • Promotes cardiac output
• Decreased MAP → Decreased renal perfusion pressure → Decreased glomerular filtrate (although some increase in filtration fraction to compensate)
  • Decreased [Na] and [Cl] in DCT
    • Activation of RAAS
  • Angiotensin II
    • Potent vasoconstriction
    • Stimulates secretion
      • ACTH
      • Vasopressin
      • Aldosterone
    • Potentiates secretion of noradrenaline from post-ganglionic sympathetic fibres
    • Stimulates thirst
  • Aldosterone
    • Increases cellular Na/K ATPase
    • Inserts ENaC into Collecting Duct tubular cells
      • Increased Na reabsorption
      • Water reabsorption
    • Inserts UT-A1 into Medullary Collecting Duct
      • Increased Urea reabsorption
      • Increased osmolarity of medullary interstitium
      • Increased water reabsorption

Chronic

• Decreased DO₂ to renal interstitium → activation of hypoxia inducible factors (HIF)
  • Synthesis of secretion of erythropoietin (EPO)
  • Stimulation of erythropoiesis by haematopoietic stem cell
• Decreased DO₂ to tissues → angiogenesis

Sakurai 2016

Answer by Sakurai