Ventilation, breathing, and external respiration all mean the same thing; the movement of air into and out of the lungs. This movement occurs because of a pressure gradient (ventilation block arrow) between the atmospheric air (Patm) and alveolar air (Pa).
The direction (arrow is double-headed) of the pressure gradient alternates; when alveolar pressure is greater than atmospheric pressure exhalation occurs, when alveolar pressure is lower then inhalation occurs. Alveolar pressure (Pa) is controlled by the activity of the diaphragm. When the diaphragm contracts the alveolar pressure is reduced--an inverse relationship represented by the dashed arrow.
The respiratory bronchioles (neck of the flask) contain smooth muscle tissue that narrows the diameter of these air passages as it contracts-- bronchoconstriction. As these muscles relax, bronchodilation (BCL) occurs. Air flows through the air passages with less resistance when these vessels are wider. It is the local concentration of carbon dioxide (CO2) that controls these smooth muscles.
There is a direct relationship (solid arrow) between the concentration of carbon dioxide in the bronchioles and the extent of their bronchodilation (BDL). A high carbon dioxide level causes a high degree of bronchodilation and increases the ease of air flow. Alveoli (round part of flask) that do not receive a good blood flow (i.e., perfusion) do not receive a good supply of carbon dioxide. Respiratory bronchioles connected to such alveoli are narrow. This adaptation of the air passages to the blood flow is called 'ventilation-perfusion coupling.'
The flow of blood through any tissue is called 'perfusion.' Like systemic arterioles, pulmonary arterioles contain smooth muscle tissue (donut) that alters the diameter of these vessels. However, unlike systemic arterioles, there is a direct relationship between local oxygen levels and vasodilation (VDL) as indicated by the solid arrow.
A unique anatomical design insures that alveoli that have a high oxygen concentration will receive the best perfusion. As shown in the map, pulmonary arterioles travel parallel and adjacent to air passages. Capillaries from these arterioles surround alveoli at the end of these passages. When air reaches the thin-walled respiratory bronchioles and alveoli, oxygen diffuses into the surrounding interstitium. The pulmonary arterioles vasodilate and blood flow (flow block arrow) is increased.
Last Updated: 7/18/2005