03. Pulmonary Circulation

• pulmonary vessels are thin walled and highly branches

  • where o2 and co2 exchange occurs
  • act as capacitance vessels
• pulmonary vessels have lower pressure than systemic circulation as:

  • vessels are shorter and wider
  • right ventricle has thin walls

  pulmonary arterial vessels act like systemic venous vessels - both function as capacitance vessles
• there is 15cm from base or apex of lungs to the output of the right ventricle

  • this means an added or minused 12mmHg of pressure in these blood vessels
• at the base of lungs the vessels are distended due to gravity, while at the apex during diastole the vessels collapse as the pressure is too low
• when standing upright, 400ml is redistributed from the pulmonary to the systemic circulation
• increased cardiac output during exercise leads to increased perfusion in the apex of the lungs which leads to increased gas exchange
• sympathetic activity leads to vasoconstriction

  • therefore during exercise, vasoconstriction occurs leading to reduced pulmonary capacity and therefore increased cardiac output
  • also increases pressure to increase perfusion to apex
• hypoxia leads to pulmonary vasoconstriction, and systemic vasodilation

  • individual alveoli = redistributes blood to functioning alveoli
  • widespread = increased pressure leading to pathology and decreased gas exchange
• pulmonary vasoconstriction due to hypoxia may occur due to increased or decreased reactive oxidative species

  • or increased Ca2+ in vascular smooth muscle
• normally in the pulmonary system there is net outward fluid movement with excess fluid being taken away by lymphatics

  • due to low hydrostatic pressure in interstitium
• there is a smaller oncotic pressure difference in pulmonary system than systemic system
• increased capillary hydrostatic pressure leads to pulmonary oedema

  • occurs in hypoxia, inflammation and left ventricle failure
• microthrombi = small red blood cell particles, platelets and fibrin that lodge in pulmonary circulation and get lysed
• pulmonary embolism = blood clot/air/fat blocking pulmonary vessels
• pulmonary embolism leads to increased dead space
• pulmonary circulation regulates levels of vasoactive substances and hormones
• pulmonary endothelial cells synthesise:

  • nitric oxide due to shear stress - leads to vasodilation
  • prostacyclin - leads to vasodilation
  • angiotensin converting enzyme
• pulmonary endothelial cells degrade:

  • serotonin
  • noradrenaline