Functionality lungs are elastic bags devoid of muscles that allow them to expand by themselves. The lungs react rather passively to pressure changes with the thoracic cavity. Contraction of the diaphragm causes it to flatten and enlarge the thoracic cavity from top to bottom. Contraction of the rib muscles upward, and outward, enlarges the thoracic cavity from front to back and from side to side. This sudden increase in volume forces air into the lungs. Conversely, contraction of the abdominal muscles and other rib muscles reduces the volume of the thoracic cavity and forces air out of the lungs.
The distance between the air in the alveoli and the blood in the capillaries, is only about 0.1µ. This facilitates easy exchange of gases at lung surface between the air and blood by diffusion. It is observed that oxygen partial pressure in the lungs is 100 mm Hg pO2 while the blood coming to the lungs from the heart is 40 mm Hg. Therefore, O2 diffuses into the blood until its concentration has reached that of the alveolar air. The partial pressure of CO2 in the blood reaching the lung is 46 mm Hg pCO2 while that in the lung is 40 mm Hg. Thus blood gives off CO2 in the lung until its tension equals that in the lung.
The distance between the air in the alveoli and the blood in the capillaries, is only about 0.1µ. This facilitates easy exchange of gases at lung surface between the air and blood by diffusion. It is observed that oxygen partial pressure in the lungs is 100 mm Hg pO2 while the blood coming to the lungs from the heart is 40 mm Hg. Therefore, O2 diffuses into the blood until its concentration has reached that of the alveolar air. The partial pressure of CO2 in the blood reaching the lung is 46 mm Hg pCO2 while that in the lung is 40 mm Hg. Thus blood gives off CO2 in the lung until its tension equals that in the lung.
