Porth's Essentials of Pathophysiology, 4e

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Respiratory Function

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For example, the removal of one lung reduces the diffus- ing capacity by one half. The thickness of the alveolar– capillary membrane and the distance for diffusion are increased in persons with pulmonary edema or pneu- monia. The characteristics of the gas and its molecular weight and solubility determine how rapidly the gas dif- fuses through the respiratory membranes. Carbon diox- ide, for example, diffuses 20 times more rapidly than oxygen because of its greater solubility in the respira- tory membranes.

Dissolved oxygen and carbon dioxide exert a par- tial pressure that is designated in the same manner as the partial pressure in the gas state. In the clinical set- ting, blood gas measurements are used to determine the partial pressure of oxygen (PO 2 ) and carbon dioxide (PCO 2 ) in the blood. Arterial blood commonly is used for measuring blood gases. Venous blood is not used because venous levels reflect the metabolic demands of the tissues rather than the gas exchange function of the lungs. The PO 2 of arterial blood normally is above 80 mm Hg, and the PCO 2 is in the range of 35 to 45 mm Hg. Normally, the arterial blood gases are the same or nearly the same as the partial pressure of the gases in the alveoli. The arterial PO 2 often is written PaO 2 , and the alveolar PO 2 as PAO 2 , with the same types of designations being used for PCO 2 . This text uses PO 2 and PCO 2 to designate both arterial and alveo- lar levels of the gases. OxygenTransport Oxygen is transported both in the dissolved state and in chemical combination with hemoglobin. Hemoglobin carries about 97% of oxygen in the blood and is the main transporter of oxygen. The remaining 3% of the oxygen is carried in plasma in the dissolved state. Only the dissolved form of oxygen (i.e., PO 2 ) passes through the cell membranes and makes itself available for use in tissue metabolism. The oxygen content of the blood (measured as mL of O 2 per deciliter [dL] or 100 mL of blood) includes both the oxygen carried by hemoglobin and in the dissolved state. HemoglobinTransport Hemoglobin is a highly efficient carrier of oxygen. Hemoglobin with bound oxygen is called oxyhe- moglobin , and when oxygen is removed, it is called deoxygenated or reduced hemoglobin. Each gram of hemoglobin carries approximately 1.34 mL of oxygen when it is fully saturated. This means that a person with a hemoglobin level of 14 g/dL carries 18.8 mL of oxygen per dL (i.e., 1.34 × 14 g/dL hemoglobin) of blood. In the lungs, oxygen moves across the alveolar-cap- illary membrane, through the plasma, and into the red blood cell, where it forms a loose and reversible bond with the hemoglobin molecule. In normal lungs, this process is rapid, so that even with a fast heart rate, the hemoglobin is almost completely saturated with oxy- gen during the short time it spends in the pulmonary capillaries. As the oxygen moves out of the capillaries in response to the needs of the tissues, the hemoglobin saturation, which usually is approximately 95% to 97% as the blood leaves the left side of the heart, drops to approximately 75% as the mixed venous blood returns to the right side of the heart. The efficiency of the hemoglobin transport sys- tem depends on the ability of the hemoglobin mol- ecule to bind oxygen in the lungs and release it as it is needed in the tissues. Oxygen that remains bound to hemoglobin cannot participate in tissue metabolism.

SUMMARY CONCEPTS

Oxygen and Carbon Dioxide Transport Although the lungs are responsible for the exchange of gases with the external environment, it is the blood that transports these gases between the lungs and body tis- sues. The blood carries oxygen and carbon dioxide in the physically dissolved state and in combination with hemoglobin. Carbon dioxide also is converted to bicar- bonate and transported in that form. ■■ The primary function of the lungs, which is gas exchange, requires matching of ventilation and perfusion so that equal amounts of air and blood enter the respiratory portion of the lungs. ■■ Dead air space refers to areas of the lungs that are ventilated but not perfused.The anatomic dead air space represents the volume of air that moves through the conducting airways, but does not participate in air exchange.The physiologic dead space is the total volume of dead air space, including the anatomic dead space and dead space associated with ventilated but unperfused alveoli. ■■ Shunt refers to blood that moves from the left to the right side of the circulation without being oxygenated. In an anatomic shunt, blood moves directly from the venous to the arterial side of the circulation without moving through the lungs. In a physiologic shunt, there is an absence of ventilation in a perfused portion of the lung. ■■ Diffusion or movement of gases across the alveolar–capillary membranes of the lung is influenced by the: (1) difference in the partial pressures of the gas on either side of the membrane; (2) surface area available for diffusion; (3) diffusion characteristics of the gas; and (4) thickness of the alveolar-capillary membrane.