Lungs Health Article

Respiration

Respiration is the process by which the body takes in oxygen and emits carbon dioxide. The following is a summary of the steps of respiration:

• ventilation
• interchange of CO₂ and O₂ between alveolar air and blood in lung capillaries
• 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 in metabolism

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. Bulk flow may be thought of as occurring between the outside air, the air in most of the lung, and the air in the alveolar sacs. Flow of some gases (especially oxygen and carbon dioxide) also occurs between the alveolar air and the blood. It is important to note that the pressure of individual gases is different in different types of air. For example, air going into the lungs is rich in oxygen and low in carbon dioxide. Air leaving the lungs is rich in carbon dioxide and low in oxygen. The different concentrations (or pressures) of individual gases are known as the partial pressures, and the partial

pressure of each individual gas adds up to the total pressure of the gas.

When air is inspired (taken in), it has a higher partial pressure of oxygen than the air already in the lung, and a lower partial pressure of carbon dioxide. Therefore, inspired air allows oxygen to flow from the area of highest pressure (inspired air) to the alveolar sacs (that have a lower partial pressure of oxygen), and into the bloodstream. The same inspired air has a low partial pressure of carbon dioxide, so carbon dioxide leaves the bloodstream (where it has a high partial pressure), enters the alveolar air (where the pressure is lower), and is passed onto the inspired air (where the partial pressure is even lower). Thus, carbon dioxide gas and oxygen gas both move from areas of highest pressure to lowest pressure in an attempt to reach a pressure (or concentration) equilibrium. This process is called gas exchange. After gas exchange has taken place, the air is expired, or expelled to rid the body of air that has a high concentration (partial pressure) of carbon dioxide gas. Then the process begins again.

Lung expansion and contraction

The concept of bulk flow (explained above) and Boyle's law explain the expansion and contraction of the lung. Boyle's law states that, at constant temperature, an increase in the volume of a container (lung) lowers the pressure of a gas, and a decrease in the container (lung) volume raises the pressure. Thus, when the volume of the lung expands, the pressure inside the lung is lowered, and when the volume of the lung contracts, the pressure inside the lung rises.

Inspiration occurs when the muscles of inspiration increase the volume of the thoracic cavity. The decrease in pressure in the cavity causes the lungs to expand to fill the cavity, which lowers the pressure inside the lung. Since air flows from areas of high pressure to low pressure, air fills the lungs to equalize the air pressure inside the lungs with the outside air, and inspiration occurs. The difference between the internal pressure in the lung and the pressure of the outside air is called the transpulmonary pressure.

During expiration, the muscles of inspiration relax, and the lung contracts. The decreased volume causes increased pressure inside the lungs, which results in air being expired, or expelled. In normal adults, expiration does not require any effort.

Role in human health

The lungs ability to extract oxygen from the atmosphere and supply it to the body's tissues is essential for metabolism and therefore for life. Disease and disorder can interfere with the body's normal function and slow a normally healthy person. Serious interference with the lung's function can cause hypoxia and even death.