Feedback Control in the Cardiovascular System

Blood pressure

  • Blood pressure: the outwards (‘hydrostatic’) pressure exerted by the blood on the blood vessel walls

Systolic BP

  • The pressure exerted by the blood on the walls of the aorta and systemic arteries when the heart contracts
  • Normally <140 mmHg (resting)

Diastolic BP

  • The pressure exerted by the blood on the walls of the aorta and systemic arteries when the heart relaxes
  • Normally <90 mmHg (resting)

Pulse pressure

  • The difference between systolic BP and diastolic BP
  • Normally 30 - 50 mmHg

Mean arterial blood pressure (MAP)

  • The average arterial blood pressure during a single cardiac cycle
  • Normally 70 - 105 mmHg
  • Must be regulated in a narrow range
    • MAP of at least 60 mmHg is needed to perfuse vital organs
    • If MAP is too high, it puts strain on the heart/blood vessels
Calculating MAP
  • MAP = [(2 x Diastolic) + Systolic] / 3 OR
  • MAP = Diastolic + (pulse pressure/3)

Hypertension

  • Clinic blood pressure of 140/90 mmHg or higher and a daytime average of 135/85 mmHg or higher

Control of blood pressure

Baroreceptors

  • Sense the change in blood pressure (sensitive to stretch)
  • Firing rate increases when MAP increases and decreases when MAP decreases
  • Medulla receives afferent signals (via CN IX and X) and sends instructions to the effectors – heart and blood vessels
  • Only respond to acute changes – firing decreases if high blood pressure is sustained

Important relationships

  • MAP = Cardiac Output (CO) x Systemic Vascular Resistance (SVR)
    • Cardiac output: volume of blood pumped by each ventricle of the heart per minute
    • SVR: the sum of all resistance in the systemic circulation
  • CO = Stroke Volume (SV) x Heart Rate (HR)
    • Stroke volume: volume of blood pumped by each ventricle of the heart per heartbeat
  • Therefore, MAP also = SV x HR x SVR

Regulation of MAP

  • MAP can be regulated by regulating HR, SV and SVR
  • SV regulated by sympathetic nerves (ANS) - stimulation increases force of contraction
  • HR is increased by sympathetic stimulation (ANS) - noradrenaline acts on β1 receptors
  • SVR regulated by vascular smooth muscles - sympathetic nerve fibres (ANS)
    • Arterioles are the main resistance vessels
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Vasomotor tone

  • Vascular smooth muscles partially constricted at rest - caused by tonic discharge of sympathetic nerves
  • Increased sympathetic discharge → vasoconstriction