Circulation of the blood

• Function of the cardiovascular system*Maintain an adaptable supply of blood to tissue in order to supply nutrients

  and signalling molecules and remove waste products*Achieve through generating pressure differentials across tissue to

  enable capillary exchange
• Haemodynamics

  • Blood flow (F) from any two points its determined by two factors
  • The pressure difference between to points (ΔP)
  •  Resistance to flow (R)
  •  Darcy’s law: F = ΔP/R

  ○ Describes flow of a fluid through a porous membrane*Higher pressure = faster flow, pressure must be higher at point A or blood

  flow can go backwards
• Vascular resistance

  • Factors affecting flow
  • Vessel length (L):

  *The longer the vessel, the greater the friction

  • Blood viscosity (η); Low viscosity = low resistance, higher flow○ High viscosity = high resistance, lower flow
  • Vessel radius (R)Ie. vessel A, r =1 vessel b, r = 2. Radius is bigger in B thereforeresistance is 16x less and flow is 16x greater
  • Flow is proportional to r4
• $Q=$$πPR^4/8Ln$

  Q= blood flow

  P= pressure gradient

  R= vessel radius

  L= vessel length

  n= viscosity
• Cardiovascular pressure gradients

  • The left side of the heart (systemic circuit) produces higher pressure as it hasthicker muscles as it has to travel a great distance towards to body
  • Elastic arteries act as pressure reservoirs to maintain blood flow duringdiastole
  • Ventricular systole = heart contracts
  • Ventricular diastole = elastic rebound in arteries so that during relaxation blood can continue to flow
  • *Pressure gradients move blood through the circulation
• Blood pressure values

  • Arterial pressure is measured at the brachial artery
  • Ideal values: systolic = 90-120 mmHg, diastolic = 60 - 80 mmHg
  • Mean arterial pressure (MAP) =
  • diastolic blood pressure (DBP) - 1⁄3 pulse pressure
  • Pulse pressure = systolic - diastolic pressure
• The cardiac cycle

  • Series of electrical and mechanical events determining blood flow through theheart and into the circulation during one heartbeat
  • Systole = contraction → ejection
  • Diastole = relaxation → filling
• The cardiac conduction system

  *Electrical impulses across the heart that initiate contraction and relaxation

  1. SA node
  2. Pacemaker cells depolarise spontaneously
  3. Cell-to-cell conduction across atria
  4. AV node
  5. 100 msec delay through AV node to allow atrial contraction
  6. Impulse through AV bundle into intraventricular septum through right andleft bundle branches to reach heart aped
  7. Bundle of His
  8. Rapid conduction for coordinated ventricular contraction
  9. Impulses through myocardium through Purkinje fibres
  10. Ventricular myocardium
• ECG

  • Detects electrical activity (ionic movement) across the heart
  • Amalgamation of all action potentials
  • P-wave = atrial systole (contract)
  • QRS complex = ventricular systole (masks atrial repolarization)
  • T wave = ventricular diastole (relax)
  • P wave: (Atrial depolarisation, originiates from SA node, membrane potentials increase and +ve deflection towards electrodes)
  •  QRS complex (Ventricular depolarisation , Septum electrical events = R, Myocardium = R → S, Goes through bundle branches and Purkinje fibres)
  • T wave (ventricular repolarization)
• Mechanical events of the cardiac cycle pt 1

  1. Atrial systole (Atrial contraction forces small amount of additional blood into relaxed ventricles)
  2. Atrial systole ends, atrial diastole begins
  3. Ventricular systole (1st phase)
  4. Ventricular contraction pushes AV valves closed
  5. Not enough pressure to open semilunar valves
  6. Ventricular systole (2nd phase)
  7. Ventricular pressure rises
  8. Exceeds pressure in arteries
  9. Semilunar valves open and blood is ejected
• Mechanical events of the cardiac cycle pt2

  1. Ventricular diastole (early)
  2. Ventricles relax
  3. Pressure drops
  4. Blood forces back against cusps of semilunar valves and forces themclosed
  5. Blood flows into relaxed atria
  6. Ventricular diastole (late)
  7. All chambers are relaxed
  8. Ventricles fill passively

  ● Process repeats
• Pressure-volume changes during the cardiac cycle

  *Isovolumetric contraction = causes left ventricular pressure to rise above atrial pressure