Gas Exchange Health Article

Definition

Gas exchange is the process by which oxygen is transferred from the atmosphere to bodily tissues for use in metabolism; and the gas produced by metabolism, carbon dioxide, is transferred from tissues to the atmosphere.

Overview of gas exchange

The process of gas exchange has several steps. The following is a summary of the steps:

• ventilation (breathing)
• interchange of CO₂ and O₂ between air in the lungs' alveoli and blood in lung capillaries by diffusion
• transport of CO₂ and O₂ through the bloodstream
• interchange of CO₂ and O₂ between blood in lung capillaries and alveolar air by diffusion
• use of O₂ and production of CO₂ by cells through metabolism

Ventilation

The transfer of oxygen from the atmosphere to the tissues starts with the inspiration of air into the lungs. The lungs consist mainly of tiny air-containing alveolar sacs. The alveoli are small hollow sacs connecting to the larger terminal bronchioles of the airways. The air adjacent to the surfaces of the alveolar wall are lined by a single cell layer of flat epithelial cells called type I alveolar cells. In between these type I cells are thicker and more rounded type II alveolar cells, which produce a detergent-like fluid. In the alveolar walls, the fluid and connective tissue fills the interstitial space and is interspersed with capillaries. In some places the interstitial space is nonexistent, and the epithelial cell membranes are in direct contact with the capillaries. The blood in the capillaries is separated from the air molecules by a single layer of flat epithelial cells. The surface area in a single alveolus, because of the undulating terrain of type I and II epithelial cells, is roughly the size of a medium-sized room. There are around 300 million alveoli in the adult male. Therefore, there is a large amount of surface area placing air and the blood stream in close proximity. This trait is needed for gas exchange to easily occur. The respiratory system also needs a continual supply of fresh air. This air is supplied to the lungs through the nose and mouth, trachea, and bronchi. Ventilation is the interchange of air between the atmosphere and the alveoli by bulk flow. Bulk flow is the movement of air from a region of high pressure to one of low pressure.

The physics of gas exchange

In order to understand why oxygen and carbon dioxide are able to diffuse from their respective areas of high concentration, Dalton's Law must first be presented. It states that in a mixture of gases, the pressure exerted by each gas is independent of the pressure exerted by the others. It is why carbon dioxide can move out of the bloodstream while oxygen is diffusing into the blood stream. The concentration of oxygen (O₂) will not affect the activity of carbon dioxide (CO₂).

Henry's law explains why CO₂ can move from the blood stream into the airspace of the lung, and O₂ can move from that airspace into the bloodstream. It states that the amount of gas dissolved will be directly proportional to the partial pressure of the gas with which the liquid is in equilibrium. At equilibrium, the partial pressures of the gas molecules in liquid and gaseous phases must be identical. Elemental gas can move from air into or out of a liquid where there is a pressure difference.

Interchange

During inspiration, the partial pressure of oxygen (PO₂) in the lung (105 mm Hg) is higher than that in the arteries of the alveoli (40 mmHg). This pressure difference allows O₂ to transfer into the blood stream. The partial pressure of carbon dioxide (PCO₂) in the lung (40 mmHg) is less than the arterial partial pressure of the alveoli (46 mmHg). This pressure difference allows carbon dioxide to diffuse into the lung and eventually into the atmosphere. The ventilation of the lungs allows for the continual renewal of imbalance and need for breathing and metabolism to continue.

Transport

The circulatory system continually supplies blood in need of oxygenation and the ventilation of CO₂ to the lungs. It arrives in the lungs with a PO₂ of 40 mmHg and a CO₂ of 46 mmHg and leaves the lungs with a PO₂ of 100 mmHg and a CO₂ of 40 mmHg. From the lungs, the oxygenated blood travels through the pulmonary veins to the left side of the heart and into the systemic arteries. The blood eventually flows to the tissue capillaries where another pressure difference occurs.