GAS EXCHANGE: DIFFUSION AND PERFUSION

 

Normal gas exchange requires normal DIFFUSION, the movement of gases from the alveolar sac across the alveolar and capillary walls into the capillary blood, as well as normal PERFUSION, blood flow through the pulmonary arterioles.

 

PERFUSION: Perfusion refers to pulmonary blood flow and equals the heart rate multiplied by the right ventricular stroke volume.  Pulmonary circulation, which brings and removes gases to and from the lung, begins with the main pulmonary artery (only artery to carry deoxygenated blood) which carries mixed venous blood from the right ventricle to the lungs.   The pulmonary artery branches in parallel with the bronchopulmonary tree.  At the level of the terminal bronchioles, the arteries further divide to form an extensive capillary bed that surrounds the alveoli.  The dense network formed by the pulmonary capillary bed is extremely effective for achieving gas exchange.  By the time blood traverses the capillary network, it is oxygenated and carried to the left atrium by the pulmonary veins. 

 

Pulmonary circulation is quite different from systemic circulation: 

  • The mean pressure of the main pulmonary artery is only 15 mm Hg (25 mm Hg systolic and 8 mm Hg diastolic).  In contrast, the mean pressure in the aorta is 100 mm Hg (120 systolic and 80 mm Hg diastolic). 
  • The pulmonary artery walls are extremely thin, containing relatively little smooth muscle.
  • Pulmonary capillaries are unusual in that they are collapsible.   If the pressure of blood flowing through the capillaries falls below a certain point, the capillaries close off – diverting blood to other pulmonary capillary beds with higher pressure.
  • The arteriole response to changes in oxygen (PAO2) and carbon dioxide (PACO2) are opposite of that seen in systemic arterioles.  Whereas systemic arterioles dilate when carbon dioxide levels increase and/or oxygen levels decrease (to bring more oxygen to metabolically active or ischemic tissue), pulmonary arterioles (and bronchioles) constrict (to divert blood to better oxygenated/ventilated alveoli).  Whereas systemic arterioles constrict when carbon dioxide levels decreases and oxygen levels increases, pulmonary arterioles (and bronchioles) dilate.

 

DIFFUSION: All gases move across the alveolar wall according to the principle of simple diffusion: gas moves from areas of higher concentration to lower concentration.  According to FICK’S LAW, the amount of gas that moves across a sheet of tissue is proportional to the area of the sheet but inversely proportional to its thickness. 

 

The rate of diffusion across the membrane is directly proportional to the partial pressure gradient (the primary factor influencing gas exchange) and most rapid over short distances. Normal alveolar PO2 is 100 mm Hg, whereas PO2 of systemic venous blood is 40 mm Hg.  Oxygen, therefore, moves from the alveoli to the capillaries.  The opposite is true for carbon dioxide. Many pathologic states can alter this system.   In addition to pressure gradient, the amount of oxygen or carbon dioxide that dissolves in the plasma is dependent on the solubility (the diffusion of CO2 through tissue is about 20 times faster than O2 because of the much higher solubility of CO2) of the gas.

 

The blood-gas interface facilitates diffusion quite effectively, with a large surface area for exchange (50-100 meters squared) and an extremely thin barrier for the gases to traverse (as thin as 0.3 micrometers in some areas).  There are over 300 million alveoli in the human lung, each one covered in an extensive network of capillaries.