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Respiratory System

Functional Anatomy of the Respiratory System

1. The nasal cavity, the chamber within the nose, is divided medially by a nasal septum and separated from the oral cavity by the palate(Figure 13.1). The nasal cavity is lined with a mucosa, which warms, filters, and moistens incoming air. The mucosa also contains receptors for sense of smell. Paranasal sinuses and nasolacrimal ducts drain into the nasal cavity.

2. The pharynx (throat) is a mucosa-lined, muscular tube with three regions nasopharynx, oropharynx, and laryngopharynx. The nasopharynx functions in respiration only; the others serve both respiratory and digestive functions. The pharynx contains tonsils, which act as part of the body's defense system.

3. The larynx (voice box) is a cartilage structure; most prominent is the thyroid cartilage (Adam's apple). The larynx connects the pharynx with the trachea below. The laryngeal opening (glottis) is hooded by the epiglottis, which prevents entry of food or drink into respiratory passages when swallowing. The larynx contains the true vocal cords, which produce sounds used in speech.

4. The trachea (windpipe) extends from larynx to primary bronchi. The trachea is a smooth muscle tube lined with a ciliated mucosa and reinforced with C-shaped cartilage rings, which keep the trachea open.

5. Right and left primary bronchi result from subdivision of the trachea. Each plunges into the hilus of the lung on its side.

6. The lungs are paired organs flanking the mediastinum in the thoracic cavity. The lungs are covered with visceral pleura; the thorax wall is lined with parietal pleura. Pleural secretions decrease friction during breathing. The lungs are primarily elastic tissue, plus passageways of the respiratory tree. The smallest passageways end in clusters of alveoli.

7. The conducting zone includes all respiratory passages from the nasal cavity to the terminal bronchioles; they conduct air to and from the lungs. Respiratory bronchioles, alveolar ducts and sacs, and alveoli which have thin walls through which gas exchanges are made with pulmonary capillary blood are respiratory zone structures (Figure 13.2).

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Respiratory Physiology

1. Mechanics of breathing: Gas travels from high-pressure to low-pressure areas. Pressure outside the body is atmospheric pressure; pressure inside the lungs is intrapulmonary pressure; pressure in the intrapleural space is intrapleural pressure (which is always negative). Movement of air into and out of the lungs is called pulmonary ventilation, or breathing (Figure 13.3). When inspiratory muscles contract, intrapulmonary volume increases, its pressure decreases, and air rushes in (inspiration). When inspiratory muscles relax, the lungs recoil and air rushes out (expiration). Expansion of the lungs is helped by cohesion between pleurae and by the presence of surfactant in alveoli.

2. Nonrespiratory air movements: Nonrespiratory air movements are voluntary or reflex activities that move air into or out of the lungs. These include coughing, sneezing, laughing, crying, hiccuping, yawning.

3. Respiratory volumes and capacities: Air volumes exchanged during breathing are tidal volume, inspiratory reserve volume, expiratory reserve volume, and vital capacity (Figure 13.4). Residual volume is non-exchangeable respiratory volume and allows gas exchange to go on continually.

4. Respiratory sounds: Bronchial sounds are sounds of air passing through large respiratory passageways. Vesicular breathing sounds occur as air fills alveoli.

5. External respiration, gas transport, and internal respiration: Gases move according to laws of diffusion (Figure 13.5). Oxygen moves from alveolar air into pulmonary blood. Most oxygen is transported bound to hemoglobin inside RBCs. Carbon dioxide moves from pulmonary blood into alveolar air. Most carbon dioxide is transported as bicarbonate ion in plasma. At body tissues, oxygen moves from blood to the tissues, whereas carbon dioxide moves from the tissues to blood.

6. Control of respiration

   1. Nervous control: Neural centers for control of respiratory rhythm are in the medulla and pons. Reflex arcs initiated by stretch receptors in the lungs also play a role in respiration by notifying neural centers of excessive overinflation.

   2. Physical factors: Increased body temperature, exercise, speech, singing, and nonrespiratory air movements modify both rate and depth of breathing.

   3. Volition: To a degree, breathing may be consciously controlled if it does not interfere with homeostasis.

   4. Emotional factors: Some emotional stimuli can modify breathing. Examples are fear, anger, and excitement.

   5. Chemical factors: Changes in blood levels of carbon dioxide are the most important stimuli affecting respiratory rhythm and depth. Carbon dioxide acts directly on the medulla via its effect on reducing blood pH. Rising levels of carbon dioxide in blood result in faster, deeper breathing; falling levels lead to shallow, slow breathing. Hyperventilation may result in apnea and dizziness, due to alkalosis. Oxygen is less important as a respiratory stimulus in normal, healthy people, but it is the stimulus for those whose systems have become accustomed to high levels of carbon dioxide.

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Respiratory Disorders

1. The major respiratory disorders are emphysema, chronic bronchitis, and lung cancer. A significant cause is cigarette smoking.

2. Emphysema is characterized by permanent enlargement and destruction of alveoli. The lungs lose their elasticity, and expiration becomes an active process.

3. Chronic bronchitis is characterized by excessive mucus production and its pooling in lower respiratory passageways, which severely impairs ventilation and gas exchange. Patients may become cyanotic as a result of chronic hypoxia.

4. Lung cancer is extremely aggressive and metastasizes rapidly. The three most common lung cancers are squamous cell carcinoma, adenocarcinoma, and small cell carcinoma.

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Developmental Aspects of the Respiratory System

1. Premature infants have problems keeping their lungs inflated due to lack of surfactant in their alveoli. (Surfactant is formed late in pregnancy.)

2. The most important birth defects of the respiratory system are cleft palate and cystic fibrosis.

3. The lungs continue to mature until young adulthood.

4. During youth and middle age, most respiratory system problems are a result of external factors, such as infections and substances that physically block respiratory passageways.

5. In old age, the thorax becomes more rigid and lungs become less elastic, leading to decreased vital capacity. Protective mechanisms of the respiratory system decrease in effectiveness in elderly persons, predisposing them to more respiratory tract infections.