the air sacs are embedded in a tight network of capillary vessel. these thin vessels are located in the walls between alveoli. Every day ,7000 to 8000 liters of co2 rich blood, low in o2 ,from the systemic circulation flow through the lungs in the capillary vessels
capillary areas in the upper sections of the lungs are collapsed in a state of rest, and are only supplied with blood when placed under stress. after oxygenation of the blood, the blood is collected via four pulmonary veins the flow into the left atrium of the heart.
Under physiological conditions,2-5% of the blood flow bypasses the alveoli via arteriovenous anastomoses (shunts) and is not oxygenated