Venous Return and Pulmonary Circulation

• Venous side:

  • return blood to right heart
  • from peripheral
  • venous ends at terminal end of capillaries
  • *** veins = reservoir for blood
• Venous return and cardiac output:

  • VR = volume of blood from veins to atria per minute
  • VR, right ventricle Q, and left ventricle Q = all have the same Q
• Example:

  • increase VR = increase RV and increase LV filling
• Increasing venous return…:

  • *** VR = P / TVR
  • P = Pvenous - Pra
  • Pvenous —> peripheral venous pressure
  • Pra —> central venous pressure
• Driving pressure (P):

  • arterial = high pressure
  • as we move farther from heart, pressure decreases
  • this is due to the structure of arterial walls
  • *** Q = (Parterial - Pcap) / TPR
• Driving pressure (P):

  • venous = low pressure
  • left ventricle must equal venous return flow
  • to achieve flow with low pressure differential, we must decrease the TVR/resistance
• Low resistance in venous circulation:

  • *** compliance = volume / pressure
  • compliance = amount of pressure needed to increase volume
  • veins have higher compliance and more volume
  • this is why there is more blood in venous circulation compared to arterial
• Increase VR = increase Q:

  afterload:

  • resistance in circulation that heart must pump against
  • we want afterload to be low
• Increase VR = increase Q:

  inotropy/contractility:

  • how strong the cardiac muscles of ventricle can contract
  • how much pressure cardiac muscles can produce
• Increase VR = increase Q:

  • volume of blood received by ventricle during diastole
  • *** manipulate by manipulating VR
• Increase VR = increase Q:

  frank-starling:

  • more blood in ventricles = stretches walls more = more contractile force = more elastic effort to help increase amount of blood pumped
• End diastolic filling:

  • during exercise… increase VR = increase EDV (preload)
  • therefore —> increase SV
• Posture:

  • standing = gravity pulls venous blood
  • lower extremity must work harder to pump blood up
  • compliance of veins is an issue if we cannot overcome gravity
  • cannot increase VR
• How to increase VR:

  • venous valves
  • skeletal muscle pump
  • respiratory pump
• Valves:

  • allow blood in veins to flow in one direction
  • pressure in large veins is low
  • vessels within muscles of veins are tethered to surrounding tissue = transmission of forces
  • upright posture, gravity opposes upward flow
  • active mechanisms oppose gravity
• Skeletal muscle pump:

  • if there is no muscle pump, high flow of blood would pool in compliant vasculature (due to gravity)
  • muscle pump prevents this
  • pump maintains a low volume of blood within muscle veins
  • pump also increases driving pressure for blood flow
  • tether = negative venous pressure to suck blood through muscle veins
• Respiratory pump:

  • inspiration = decrease pressure and increase volume (of lungs AND heart)
  • causes translocation of blood to heart
  • squeezing blood from lower veins to chest = increase VR
• Driving pressures:

  left ventricle:

  • Q = (Parterial - Pcap) / TPR

  right ventricle:

  • VR = (Pvenous - Pra) / TVR
  • Pvenous —> muscle pump
  • Pra —> respiratory pump (adds pressure)
  • both pumps increase VR
• Both sides:

  • right and left heart both circulate the same volume of blood at the same frequency
• Pulmonary circulation:

  • massive surface area for gas exchange to occur
  • small amount of tissue where O2 loading and CO2 offloading occur
• Pulmonary vasculature:

  • low resistance circulation
  • high pressure circulation
  • veins lack SNS innervation and smooth muscle contraction because we don’t want to constrict the veins in the lungs
  • veins are built different
  • right ventricle produces less pressure than left ventricle
• Ventricular dimensions:

  • wall mass = right ventricle has less mass because pressure in systemic is greater than in pulmonary
  • properties that make up equal flows are different
• Aerobic fitness:

  • increase resistance = decrease VO2max
  • decrease resistance = increase VO2max
  • beneficial to absorb higher right ventricle SV
  • optimize gas exchange