Movement of Oxygen and Carbon Dioxide

lveoli for efficient gas exchange.

TERMS

oxyhaemoglobin
the form of hemoglobin, loosely combined with oxygen, present in arterial and capillary blood

hemoglobin
iron-containing substance in red blood cells that transports oxygen from the lungs to the rest of the body; it consists of a protein (globulin) and heme (a porphyrin ring with iron at its center)

partial pressure
the pressure one component of a mixture of gases would contribute to the total pressure

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Basic Principles of Gas Exchange

The structure and organization of the lung is meant to maximize its surface area to increase gas diffusion. Due to the enormous number of alveoli (approximately 300 million in each human lung), the surface area of the lung is very large (75 m2). Having such a large surface area increases the amount of gas that can diffuse into and out of the lungs.

Gas exchange during respiration occurs primarily through diffusion: a process in which transport is driven by a concentration gradient and molecules will move from a region of high concentration to a region of low concentration. Gas exchange between the air within the alveoli and the pulmonary capillaries occurs by diffusion . The oxygen must first dissolve before passing through the respiratory epithelium. Gas moves from a region of high partial pressure to a region of low partial pressure, down a partial pressure gradient. Partial pressure (P) is a measure of the concentration of the individual components in a mixture of gases. The total pressure exerted by the mixture is the sum of the partial pressures of the components in the mixture. The rate of diffusion of a gas is proportional to its partial pressure within the total gas mixture.  The distance between the blood and the air within the alveoli is approximately 0.7 micrometers. This distance is decreased during inhalation as the lung distends, allowing extremely fast and efficient diffusion across this tiny distance. The various factors that affect gas exchange include oxygen, carbon dioxide, and the ventilation/perfusion (V/Q) ratio.

Gas exchange
Gas exchange
This schematic demonstrates how gas is exchanged in humans between a capillary and an alveolus.
Oxygen
The partial pressure of oxygen (PO2) is always lower in the alveoli compared to the external environment due to the continuous diffusion of oxygen across the alveolar wall along with the 'diluting' effect of CO2 entering the alveoli as it is travels in the opposite direction to the O2. The PO2 in the alveoli is still higher than that in the capillaries, so oxygen diffuses into the blood. Once through the alveolar and capillary walls, the oxygen combines with hemoglobin to form oxyhaemoglobin that is transported within the bloodstream.

Carbon Dioxide
Carbon dioxide enters the red blood cell as a waste product from cells, which reacts with water to form carbonic acid (CA). CA dissociates to bicarbonate ions and hydrogen ions. These diffuse into plasma, where H+ are buffered by hemoglobin. Approximately 5% of the total body CO2 dissolves in the plasma, 5% is carried as carboxyhaemoglobin on proteins, and 90% is carried as bicarbonate ions in the plasma. The partial pressure of carbon dioxide (PCO2) in the capillaries is higher than that in the alveoli, thus CO2 diffuses into the alveoli where it is exhaled.

V/Q Ratio
The adequacy of pulmonary gas exchange relies on the V/Q ratio (ventilation/perfusion ratio). The alveoli should receive the ideal amounts of blood and gas for gas exchange. In disease situations, the amount of air delivered may be reduced, the alveolar wall may be thickened, or the alveolar surface area may be reduced with the result that less gas is able to diffuse out of the alveolus. Alternatively, blood supply may be impaired so that, despite sufficient ventilation, insufficient exchange occurs to support the body.