Reviewing anatomy and physiology of the respiratory tract

The respiratory system distributes air to the alveoli, where gas exchange — the addition of oxygen (02) and the removal of carbon dioxide (C02) from pulmonary capillary blood — takes place. Certain specialized structures within this system play a vital role in preparing air for use by the body. The nose, for example, contains vestibular hair that filter the air and an extensive vascular-network that warms it. The nose also contains a layer of goblet cells and a moist mucosal surface; water vapor enters the airstream from this mucosal surface to fully saturate inspired air as it's warmed in the upper airways. Ciliated mucosa in the posterior portion of the nose and nasopharynx, as well as major portions of the tracheobronchial tree, propels particles deposited by impaction or gravity to the oropharynx, where the particles are swallowed.

External respiration.

The external component of respiration - ventilation or breathing - delivers inspired gas to the lower respiratory tract and alveoli. Contraction and relaxation of the respiratory muscles move air into and out of the lungs. Ventilation begins with the contraction of the inspiratory muscles: the diaphragm (the major muscle of respiration) descends, while external intercostal muscles move the rib cage upward and outward. Air then enters the lungs in response to the pressure gradient between the atmosphere and the lungs. The lungs adhere to the chest wall and diaphragm because of the vacuum created within the pleural space. As the thorax expands, negative pressure is created in the intrapleural space, causing the lungs also to expand and draw in the warm, humidified air. The accessory muscles of inspiration, which include the scalene and sternocleidomastoid muscles, raise the clavicles, upper ribs, and sternum. The accessory muscles are not used in normal inspiration but are used in certain disease states, when diaphragm function is impaired.

Normal expiration is passive; the inspiratory muscles cease to contract, and the elastic recoil of the lungs causes the lungs to contract. These actions raise the pressure within the lungs above atmospheric pressure, moving air from the lungs to the atmosphere. Active expiration causes the pleural pressure to become less negative.

An adult lung contains an estimated 300 million alveoli; each alveolus is supplied by many capillaries. To reach the capillary lumen, 02 must cross the alveolocapillary membrane, which consists of an alveolar epithelial cell, a thin interstitial space, the capillary basement membrane, and the capillary endothelial cell membrane. The 02 tension of air entering the respiratory tract is approximately 160 mm Hg. In the alveoli, inspired air mixes with C02 and water vapor, lowering the oxygen pressure to approximately 100 mm Hg. Because alveolar partial pressure of 02 is higher than that present in mixed venous blood entering the pulmonary capillaries (approximately 40 mm Hg), O2 diffuses across the alveolocapillary membrane into the blood.