Blood Gases Health Article

Definition

Blood gases are defined as the mixture of gases, including oxygen (O₂), carbon dioxide (CO₂), and nitrogen (N₂), dissolved in the fluid fraction of blood.

Description

Oxygen from the air is transported from the lungs to all tissues of the body, where it is needed for metabolism; and carbon dioxide, a by-product of metabolism, is taken from the tissues to the lungs to be eliminated.

Respiration

The overall process of delivering oxygen to the tissues and carrying carbon dioxide away is called respiration. When the blood reaches the tissues, oxygen diffuses into the cells, and carbon dioxide diffuses from the cells into the blood. In the lungs, the air enters a branching complex of multiple air sacs, called alveoli, where tiny capillaries separate the air from the red blood cells by only a very thin membrane, about 0.3 [.mu]m thick. During respiration the inspired air is filtered and moistened by the nose and tracheal linings and is completely saturated with water vapor by the time it enters the alveolar sac.

In the alveolus, oxygen diffuses into the blood, and carbon dioxide diffuses out of the blood to mix with the alveolar air. Diffusion is a very rapid process, and the gases do not have time to totally equilibrate across the alveolar membrane. A small pressure difference for each gas develops. About 2% of the blood flow through the lungs bypasses the pulmonary capillaries and does not become oxygenated; thus, the partial pressure of oxygen is somewhat higher in the alveolus than in the blood. This pressure difference, calculated for the lung as a whole, is called the arterial-alveolar (A–a) gradient.

Composition of gases in air

Dry air is made up of 20.98% oxygen, 0.04% carbon dioxide, 78.06% nitrogen, and 0.92% other gases (mostly argon). In blood-gas analysis, the content of the gases oxygen and carbon dioxide are reported in terms of their partial pressures, with normal values for oxygen (PO₂) of 75 mm to 100 mm of mercury (mm Hg) and for carbon dioxide (PCO₂) of 35 to 45 mm Hg. The partial pressure of water vapor in the lung, where the air is completely water-saturated, at body temperature (98.6°F, or 37°C) is 47 milliliters of mercury (mm Hg).