Distribution of Perfusion

General

pulmonary system is high flow, high SA; low pressure; low resistance; highly autoregulated

pulmonary arteries run very close to airways, quickly get less muscular

extensive capillary network gives 70 m² of surface area ~ 75 ml of blood at rest

50% of the pulmonary resistance is in caps; 20% in systemic
caps are very distensible, autoregulated
many caps are collapsed at rest, so lots of reserve volume
exercise leads to distension and recruitment, so get ~ 200 ml of blood volume
this compensates pressure from greater CO₂ to lungs (6L/min rest to 18L/min exercise)

50% of lung is blood by weight

large vascular volume provides buffer system for left atrium; changes in flow are buffered

pulmonary vascular resistance is less than 10% of SVR

Distribution of Blood Flow

gravity: effects are relatively greater for pulmonary circulation than for systemic

because of lower overall pressures in pulmonary system (.74 mm/cm of body height)
higher pressures in bottom of lung distend the vessels and decrease resistance so increase flow
as arterial pressure increases, get relatively more flow goes to the top
effects of gravity divided into zones

zone 1: Palv > Pa > Pv = no flow – capillaries collapsed
zone 2: Pa > Palv > Pv = minimal flow – Starling resister
zone 3: Pa > Pv > Palv = high flow – open or distended

hydrostatic forces

chemical forces

Alveolar and Extra-Alveolar Vessels

alveolar: mostly caps, pressure on them is the alveolar pressure, usually atmospheric

inhalation causes collapse

extra-alveolar: arteries and veins, pressure is usually less (related to pleural pressure)

inhalation causes expansion

overall, see little change in resistance due to breathing because effects offset each other in normal range

Passive Regulation of resistance

pulmonary artery pressure: increase pressure leads to decreased resistance

left atrial pressure: causes backup in pulmonary

transpulmonary pressure: see figure below

viscosity of blood

blood volume

Active Regulation of resistance

small amount of smooth muscle constriction is enough to alter resistance greatly

hypoxia actually causes vasoconstriction in arterioles near alveoli (acts directly on smooth muscle)

result: as oxygen decreases in one part of the lung, it is diverted to other parts
can shift up to 20% of bloodflow in lung without altering PVR
global reduction in oxygen results in increased PVR

acidosis

hypercapnea (Ý CO₂)

circulating vasodilators (bradykinin, Ach, aminophyline, isoproterenol, PGE2, prostacyclin) and vasoconstrictors (Epi, NorEpi, AII, histamine, prostaglandin F, thromboxane)