Ventilation of the Lungs:  the drawing of air in and out of the conducting pathways from the nose or mouth to the alveoli

Perfusion of the Lungs:  the transfer of blood to and from the lung

Ventilation:Perfusion Equality: Only when ventilation and perfusion are both functional, and functionally coordinated (optimized) can gas exchange between the blood and the alveolar air be efficient (more on this below).  You may have noticed that, at rest, the minute respiratory volume (sometimes called pulmonary ventilation rate) and cardiac output are approximately equal... that is, about 6 liters per minute each.  Young athletes may be able to drive their cardiac output as high as 30 liters per minute, and their minute respiratory volume as high as 150 liters per minute; while this huge increase in ventilation is achieved through an increase in rate and depth of respiration, the greatest contribution to this increase occurs naturally (reflexly) through depth of respiration.  Deeper breaths allow the dead space to account for a smaller portion of each breath... and the effect of anatomic dead-space would not be as effectively countered by simply increasing rate of respiration.  Often, small-medium increases in ventilation are associated with increased depth, but slightly decreased rates of respiration.

The Respiratory Zone: After the terminal bronchi (TBL = the end of the conducting pathway) we get to the parts of the lung which are important to physiologic gas exchange.

RBL = respiratory bronchiole - limited gas exchange occurs here

AD = alveolar duct - gas exchange occurs here, but less than in alveoli

AS = alveolar sacs - gas exchange occurs readily because of the very small distances between the alveolar space and the capillary lumen; in addition, an overwhelming amount of the surface area available for gas exchange is found within the alveolar sacs

Pulmonary Circulation: The surface area of the respiratory zone in the human may be as great as 85 m².  This surface area is often likened to the surface area of one of these! 

 Essentially, gas exchange occurs in the alveoli, although a small amount of gas exchange (1%) occurs in the respiratory bronchioles.  There may be several hundred million alveoli, each about 1/3 mm in diameter.  The alveolar blood supply originates from the pulmonary artery leaving the right ventricle, such that deoxygenated blood can be delivered to the lungs for oxygenation and to reduce CO₂ and H⁺ concentrations.  The pulmonary artery and arterioles tend to follow the bronchioles and then break up to form the capillary network that travels over the surface of the alveoli.  The capillaries collect into veins and then the pulmonary veins return to the left side of the heart, such that freshly oxygenated blood can be delivered to the systemic circulation.

Diagram B

The alveoli sit in grape-like arrangements within the lungs.  The walls of the alveoli are richly perfused with blood; you may want to think of the pulmonary capillary network as a sheet of blood perfusing the lung parenchyma.  Blood cells travel across only 2 alveoli before they are fully oxygenated and returned to venous circulation and then carried back to the left atrium.  The system is designed so that O₂ travels the smallest distance possible for gas exchange. The total amount of interstitial tissue under the alveolar epithelium is ordinarily very small in a healthy lung; this means the interstitial tissue occupies very little space such that the vast bulk of the lung may be occupied by parenchyma which is dedicated to gas exchange.  The capillary endothelium outlines the capillary wall.  O₂ in the lung only has to diffuse across the alveolar epithelium, then across a very tiny interstitial space, then across the capillary basement membrane and across the capillary endothelium to the RBC's.  This sounds like a long distance but it is actually a very small distance.  The sketch, below, depicts the very small distance O₂ must travel to get to the RBC's!  Note that the distance is so small that the term, "alveolar-capillary" membrane is used.

Diagram C

Blood vessels to the lung (the pulmonary arterioles) have very thin walls with very little smooth muscle.  The pulmonary circulation is a low pressure/low resistance circuit and the pulmonary vessels do not have to constrict to the extent that the systemic vessels do.  Pulmonary vessels do not have to generate much resistance.  Pulmonary blood vessels and capillaries are virtually surrounded by alveoli and are not anchored in any other way.  The arrangement of the lung, then, with i) very little vascular smooth muscle, ii) major blood and lymph vessels aligned with the air conducting tubules, and iii) very little connective tissue, optimizes the lung for gas exchange!

There is a bronchiolar artery as well as a pulmonary artery.  In my experience, many students will not have addressed the bronchiolar artery in anatomy class (there may be just too much anatomy).  The bronchiolar artery is actually part of the systemic circulation, delivering oxygenated blood from the left ventricle to supply the conducting pathways of the lung (bronchi, bronchioles) with O₂ and nutrients.  This blood is not brought to the lung to be oxygenated but is carried back to the heart by the pulmonary veins; note that this means that some deoxygenated blood from the airways is dumped into the pulmonary veins on the way back to the left atrium (an anatomic systemic-pulmonary shunt that results in minute dilution of oxygen in the blood arriving at the left ventricle)!  Thus, oxygenated blood leaving the lungs has a pO₂ of about 100 mmHg while oxygenated blood leaving the left ventricle has an oxygen concentration of only 95 mmHg.  Unlike coronary arteries, bronchiolar arteries are not essential for life; that is, bronchiolar arteries do not need to be anastomosed during a lung transplant.

Most of you will have covered the Thebesian veins (venae cordis minimae) in anatomy class.  Recall that the Thebesian veins collect as much as 30% of the blood from the heart and drain through the foramina venarum minimarum directly into all of the heart chambers, although the majority of the blood dumps into the chambers on the right side... my anatomy instructors used to love make reference to the venae cordis minimae dextrad... exercising their Latin).  Nonetheless, the smaller portion of blood dumping from the Thebesian veins into the left side of the heart (venae cordis minimae sinistrad) will also contribute to the reduction in pO₂ of blood leaving the left ventricle.

NOTE: the lungs are a significant storage site for extra blood.  If you are lying down, blood tends to accumulate in the lungs due to hydrostatic pressure.

Lymphatics in the lungs: Lymphatics occur along the conducting airways and around the vessels along the conducting pathways and in the visceral and parietal pleura.  Lymphatics do not occur among the alveoli as this would interfere with gas exchange by increasing the distance gases would have to diffuse in the exchange process.  The lymphatics get larger as they travel along the larger air conducting pathways.  Notice in diagram B that the lymphatics do not follow the arterioles and venules into the alveolar arrangements.