Pulmonary circulation

From Iusmphysiology

  • started here on 02/15/11 at 11AM.


Contents

[edit] Pulmonary circulation

  • He will be clear about which vessel blood he means: venous or arterial.

[edit] Pulmonary circulation

  • Lung tissue needs oxygen and a blood supply.
  • Lung gets blood from two sources:
    • from bronchial circulation: arterial blood supply from aorta and intercostal arteries to perfuse mostly the upper airways (down to terminal bronchials)
    • pulmonary circulation
    • pulmonary flow = cardiac output of right heart
    • there are 300 million alveoli and 280 billion capillaries
    • total blood volume in the right system is 500 ml
      • Serves to buffer changes in LV filling with changes in venous return
  • Venous drainage is into the pulmonary veins (which have oxygenated blood) to mix deoxygenated with oxygenated blood.

[edit] Image

  • Bronchial arteries come off aorta and intercostals, perfuses airway down to terminal bronchiole.
    • These arteries drain into veins that drain into pulmonary veins.
  • Blood flow in the pulmonary artery goes to the alveoli and ...

[edit] Right-left shunt

  • The pulmonary circulation is a normal right-left shunt.
  • Rgith side of the heart is deoxygenated and left is oxygenated.
  • When lung's blood supply dumps into the pulmonary veins, the cocnetration of oxgyen will be lower.
  • Thebesian drainage:
    • Some direct venous drainage into the left heart.
    • Can be bad when these get big.
  • Rigth to left shunts "waste ventilation".
  • Left to right shunt:
    • Septal defect
    • Usually congenital issues
    • Will not affect PO2 at all

[edit] Functional anatomy

  • Right ventricle has a shorter distance (less resistance) so less pressure.
  • Only has to pump through lungs.
  • Pulmonary artery is much thinner to aorta.
    • Thin walled in genreal on right side.

[edit] Vascular resistance

  • Ohm's law:
    • Flow = change in pressure / resistance
    • Pulmonary resistance = mean pulmonary artery pressure - mean lefta atrial pressure / pulmonary flow.
  • SVR = AoP - RAP / CO
    • = 93 -2 / 5 liters / min
    • SVR = 18.2 mmHg per liter / min
  • PVR = PAP - LAP / co
    • = 15 - 5 / 5
    • = 2mmHg per liter / min
  • PVR is approximately 1/10th of systemic vascular resistance (SVR)

[edit] What affects pulmonary vascular resistance PVR?

  • Lung volume
  • Right ventricular output
  • Gravity
  • Alveolar hypoxia
  • Vasoactive factors

[edit] Lung volume

  • The pulmonary vascular resistance is lowest at FRC.
    • Memorize it! Know it! Understand it!
  • Either way you deviate form FRC, resistance goes up but for different reasons.
  • In a higher total volume state:
    • Interpleural
    • 1000 caps over each alveoli, get stretched as volume goes up.
    • So resistance goes up.
    • So the alveoli affects on caps are the reason resistance goes up as lung volume goes up.
  • In a lower residual volume state:
    • When breathing out, pleural pressure is very positive.
    • So blood vessels are pushed on.
    • So resistance goes up because the vessels are being closed.

[edit] Flow and pulmonary vascular resistance

  • AKA right ventricular output as a factor in resistance
  • AS blood flow goes up, resistance increases.
  • Blood flow is directly proportional to pressure.
  • As flow goes up, more caps open.
  • Increase in flow decreases resistance because there are more pathways opened.
    • This reduces the resistance? 
Where is this in the equation?
  • In addition, the vessels that are open, are distending (they are very compliant compared to systemic arteries).
  • Recruitment and distension generate an exponential fall in resistance as cardiac output increases.

[edit] Distribution of pulmonary blood flow - Gravity

  • Gravity also affects blood flow.
  • At TLC, blood flow is highest at the base and lowest at the apex.
  • One yreason for this is the distribution of gravity on the lung and the way it generates a hydrostatic column pressure on the blood.
  • What determines where blood flows?
    • Gradient of pressure between arterial pressure and venous pressure.
  • REsistance is affected by volume, remember. 
Missed some logic here.
  • We tlak about zones of the lung.
    • Zone 1: Apex
      • If PA > Pa > Pv there is no flow.
      • This is physiological deadspace
      • NOrmally Pa > PA so there is little to no zone 1.
    • Zone 2: Upper 1/3
      • Pa > PA > Pv
        • Recall that PA is alveolar
      • Primary determinant of flow is the downstream alveolar pressure
    • Zone 3: Lower 1/3
      • Pa > Pv > PA
      • Increased transmural pressure 
These will be on the exam
[edit] Conditions that change the zones
  • Hemorrhage:
    • Blood lost
    • BP drops
    • Lost of the lung may turn to zone 1; very low Pa
    • Same with aenesthesia
  • Exercise:
    • Pa goes way up b/c CO is up
    • Flow more evenly distributed.
    • Very little Zone 1
[edit] Continuity of zones
  • Note that the zones are not hard and fast but along a continuum.
  • Blood flow to the base is greater, as we said
  • But the variation at each level in the lung is highly variable: heterogeneity.
  • Gravity accounts for 25% of the variablity.
  • Other factors include branching orders, etc.

[edit] ACtive regulation of pulmonary blood flow

  • These are factors that affect PVR.
  • The primary active regulator of PVR is PAO2
    • Pulmonary smooth muscle cells are bathed in alveolar O2.
    • In systemic circulation, flow goes up when oxygen is low.
    • Opposite in lung: vessels will constrict as PAO2 goes down. Called pulmonary hypoxic vasoconstriction.
  • If oxygen tension is low in that alveoli area, then ventilation in that area must not be so well, so we should send the blood elsewhere.
    • This increases the chance that the blood will be highly oxygenated via exchange.

[edit] Hypoxic pulmonary vasoconstriction

  • If PAO2 is low in one alveoli, the vessels will constrict so as to shunt blood to better ventilated alveoli.
    • This is the case of regional hypoxia.
  • General hypoxia:
    • Climb a mountain, barometric pressure is falling as you ascend.
    • So alveolar PAO2 is falling.
    • So none of the alveoli have high oxygen so all caps constrict.
    • This increases resistance everywhere.
[edit] Other active regulators
  • Remember: oxygen is the major active regulator.
  • Dilators:
    • Ach
    • NO
    • Prostaglandins 
Get the constrictors

[edit] Questions

  • A: Correct
  • B:
  • C:
  • D:
  • E:

[edit] Questions

  • A: no, generaly hypoxic ...
  • B: STrethcing of alveoli makes resistance higher
  • C: correct
  • D: increased alveolar pressure, increased cap stretching, increased resistance
  • E:


  • stopped here on 02/15/11 at 12PM.
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