Porth's Essentials of Pathophysiology, 4e

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Respiratory Function

U N I T 6

Innervation The lung is innervated by both the sympathetic and parasympathetic divisions of the autonomic nervous system. The parasympathetic fibers, which are derived from vagal nerves, and the sympathetic fibers, which originate in the upper thoracic and cervical ganglia, form the pulmonary plexuses that enter the lung in the region of the hilus. Fibers from the plexus follow the major bronchi and blood vessels into the lung to inner- vate bronchial smooth muscle cells, blood vessels, and epithelial cells (including the goblet and submucosal glands). There is no voluntary motor innervation of the lung, nor are there any pain fibers. Pain fibers are found only in the pleura. The parasympathetic (cholinergic) fibers are excit- atory neurons that respond to acetylcholine. Stimulation of the parasympathetic nervous system is responsible for airway constriction, blood vessel dilation, and increased glandular secretion. The sympathetic nervous system, which responds to the catecholamines norepi- nephrine and epinephrine, produces bronchodilation, blood vessel constriction, and inhibition of glandular secretion. ■■ The primary function of the respiratory system is gas exchange, with oxygen from the air being transferred to the blood and carbon dioxide from the blood being eliminated into the atmosphere. ■■ Functionally, the respiratory system can be divided into two parts: the conducting airways (nasopharynx, oropharynx, larynx, trachea, and bronchi and bronchioles), through which air moves as it passes between the atmosphere and the lungs, and the respiratory airways of the lungs (terminal bronchioles and alveoli), where gas exchange takes place. ■■ There are two types of alveolar cells: type I and type II.Type I alveolar cells provide the surface area for the gas exchange function of the lung. Type II alveolar cells secrete surface-active surfactants that serve to decrease alveolar surface tension (surfactants B and C) and mediate the immune destruction of pathogens that have entered the lung (surfactants A and D). ■■ The lungs are provided with a dual blood supply: the pulmonary circulation, which provides for the gas exchange function of the lungs; and the bronchial circulation, which supplies blood to the conducting airways and supporting structures of the lung. SUMMARY CONCEPTS

■■ Innervation of the lungs occurs by way of the sympathetic and parasympathetic divisions of the autonomic nervous system. Parasympathetic innervation produces airway constriction and an increase in respiratory secretions; whereas sympathetic innervation produces bronchodilation and a decrease in respiratory tract secretions.

Exchange of Gases Between the Atmosphere and the Lungs The exchange of gases between the atmosphere and the lungs occurs along a pressure gradient, moving from an area of higher pressure to one of lower pressure. Basic Properties of Gases The air we breathe is made up of a mixture of gases, mainly nitrogen and oxygen. These gases exert a com- bined pressure called the atmospheric pressure. The pressure at sea level is defined as 1 atmosphere, which is equal to 760 millimeters of mercury (mm Hg) or 14.7 pounds per square inch (PSI). Respiratory pressures— the pressures within the alveoli and other respiratory structures—are always expressed relative to atmo- spheric pressure, which is assigned a value of 0 mm Hg. This means that a respiratory pressure of +15 mm Hg is 15 mm Hg above atmospheric pressure, and a respiratory pressure of −15 mm Hg is 15 mm Hg less than atmospheric pressure. Respiratory pressures often are expressed in centimeters of water (cm H 2 O) because of the small pressures involved (1 mm Hg = 1.35 cm H 2 O pressure). The pressure exerted by a single gas in a mixture is called the partial pressure. The capital letter “P” fol- lowed by the chemical symbol of the gas (e.g., PO 2 ) is used to denote its partial pressure. The law of partial pressures states that the total pressure of a mixture of gases, as in the atmosphere, is equal to the sum of the partial pressures of the different gases in the mix- ture. If the concentration of oxygen at 760 mm Hg (1 atmosphere) is 21%, its partial pressure is 160 mmHg (760 × 0.21). Water vapor is different from other types of gases; its partial pressure is affected by temperature but not atmospheric pressure. The relative humidity refers to the percentage of moisture in the air compared with the amount that the air can hold without causing conden- sation (100% saturation). Warm air holds more mois- ture than cold air. This is the reason that precipitation in the form of rain or snow commonly occurs when the relative humidity is high and there is a sudden drop in atmospheric temperature. The air in the alveoli, which remains 100% saturated at normal body temperature,