Question 1

Describe the cardiovascular changes that occur following the loss of  1000ml of blood in an adult.

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College Answer

A structured approach that included mentioning that 1000mls of blood was 
substantial – being approximately 20% of the blood volume of a 70 kg person was 
required for a good answer. Candidates were expected to also include changes in 
systolic and diastolic blood pressure, pulse pressure, heart rate, cardiac output and 
the neuronal (eg sympathetic nervous system response on the various circulations) 
and hormonal responses (eg rennin aldosterone, Anti-Diuretic Hormone, 
catecholamines, etc). Candidates were also expected to discuss differences in 
responses according to rate of blood loss. Flow diagram could have been used to 
illustrate some of these concepts. 
Syllabus: C1e
References: Textbook of Medical Physiology, Guyton pg 278 – 282, Principles of 
Physiology for the Anaesthetist, Power & Kam pg 154


Flow diagram could have been used, but 

  • The loss of 1000ml of blood corresponds to 20% of the total circulating volume in a 70kg subject, which represents most of the stressed volume.
  • This results in autonomic and neurohormonal effects:
  • Autonomic effects
    • Arterial hypotension causes baroreflex activation.
    • Decreased cardiac output causes chemoreceptor activation.
    • Both reflexes result in autonomic phenomena:
      • Decreased vagal stimulus; thus increased heart rate
      • Sympathetic activation, which has multiple effects:
        • Increased peripheral vascular resistance
        • Redistribution of blood flow away from the cutaneous and splanchnic circulation
        • Stimulation of systemic catecholamine release from adrenal glands, which produces an increased systemic effect in addition to the peripheral sympathetic nervous system effects
        • Stimulation of vasopressin release via the projections from the nucleus of the solitary tract to the hypothalamus
        • Stimulation of renin release by sympathetic stimulation of the juxtaglomerular cells, and due to lower renal perfusion
  • Neurohormonal effects
    • Renin secretion causes:
      • Vasoconstriction (by angiotensin)
      • Increased sodium retention (by aldosterone)
    • Vasopressin release causes:
      • Vasoconstriction (by V1 receptors)
      • Increased water retention (by V2 receptors)
    • Venous hypotension decreases atrial natriuretic peptide secretion,
      which causes:
      • Decreased renal blood flow
      • Decreased urinary water and sodium excretion
    • The net effect is decreased urine output and increased retention of sodium and water
  • Effect of the rate of blood loss
    • A more rapid rate of blood loss places increased stress on the cardiovascular system to maintain haemodynamic homeostasis
    • Healthy individuals will be better able to compensate for more rapid rates of blood loss by increasing their heart rate and cardiac contractility
    • Patients with compromised cardiac function (eg. ischaemic heart disease or heart failure) will have impaired compensatory mechanisms and will not be able to compensate for even relatively slow blood loss


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