Coordinated responses of the CVS - Gravity & Exercise

• What is orthostasis and how does gravity affect blood distribution when a person stands up?

  • Refers to the transition from lying down to standing up.
  • Gravity causes ~500–1000 mL of blood to pool in the lower extremities and splanchnic circulation, reducing venous return to the heart and central blood volume.
• What immediate effect does orthostasis have on venous return and preload?

  Venous return decreases due to gravitational pooling of blood, reducing right atrial filling (preload), which leads to a fall in stroke volume and cardiac output.
• How does orthostasis affect arterial blood pressure initially?

  Arterial blood pressure transiently drops (orthostatic hypotension) due to reduced cardiac output following decreased preload.
• What is the compensatory response to the drop in arterial pressure during orthostasis?

  The baroreceptor reflex is activated by reduced arterial pressure sensed by baroreceptors (mainly in the carotid sinus and aortic arch). It triggers:

  • Increased sympathetic tone
  • Decreased parasympathetic tone
  • Resulting in:
  • Increased heart rate (tachycardia)
  • Increased contractility
  • Vasoconstriction → Restores arterial pressure and venous return.
• How does venous pressure in the lower limbs change with standing?

  Venous pressure in the legs increases due to gravity, rising from ~10 mmHg when lying down to ~80 mmHg when standing, promoting capillary filtration and potential oedema if uncorrected.
• Why does cerebral perfusion risk being compromised during orthostasis?

  Reduced arterial pressure and venous return can lower cerebral perfusion pressure, potentially causing dizziness or fainting (syncope) if compensatory mechanisms are inadequate.
• How long does it typically take for the CVS to stabilize after standing up?

  Within ~10–20 seconds, the baroreceptor reflex compensates, stabilising blood pressure and cerebral perfusion through vasoconstriction and increased heart rate.
• What factors can impair the cardiovascular response to orthostasis?

  • Aging
  • Dehydration
  • Autonomic dysfunction
  • Medications (e.g., antihypertensives)
  • Prolonged bed rest can impair reflex compensation, increasing the risk of orthostatic hypotension.
• What immediate cardiovascular effects occur when moving from lying down to standing up?

  • Blood pools in the lower extremities due to gravity.
  • Decreased venous return (central venous filling).
  • Reduced preload and stroke volume (Starling’s Law).
  • Decreased cardiac output and arterial pressure.
  • This activates compensatory mechanisms like the baroreceptor reflex.
• How does gravity affect central venous filling upon standing?

  Upon standing, gravity causes blood to pool in the lower limbs, decreasing central venous pressure and venous return to the heart. This reduces central venous filling and preload.
• What happens to cardiac output immediately after standing?

  It transiently decreases due to reduced venous return and stroke volume. If uncompensated, this can lead to dizziness or syncope. Compensation restores cardiac output via increased heart rate and vasoconstriction.
• What is the role of the skeletal muscle pump in postural changes?

  Contraction of leg muscles during movement compresses veins, pushing blood back toward the heart and counteracting the pooling effect of gravity, helping restore venous return and cardiac output.