External respiration

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External respiration refers to the exchange of gases between the atmosphere and the pulmonary loop of circulation including the lungs. Oxygen is drawn in through the respiratory tract, (nasal passages, into the pharynx, to the trachea, the bronchial tubes to the alveoli sacs) and is then delivered to the blood which transports oxygen throughout the body. From the lungs, oxygen is transported across the thin membranes of the alveoli and the border of the capillary and attracted to the hemoglobin molecule within the red blood cell.

External respiration is a passive process in that energy (ATP) is not expended by cells of the body as oxygen passes from the air outside the nostrils to the red blood cell. Diffusion is the process utilized which means that the partial pressure of oxygen in the atmosphere helps provide the energy needed for diffusion to occur. External respiration also transports carbon dioxide from the blood in the capillaries of the respiratory membrane through the capillary wall, through the alveoli wall, to the respiratory tract, and out into the surrounding air.

• Introductory Anatomy: Respiratory System, University of Leeds