he primary function of the lungs is gas exchange, which consists of movement of oxygen (O2) into the body and removal of carbon dioxide (CO2). This chapter provides an overview of lung anatomical structure/ function relationships (i.e., upper and lower airways, the two circulatory systems, innervation, and muscles), lung development (at the embryo stage and throughout life), and lung repair. This chapter is designed to provide a broad conceptual understanding of structure/function interactions and is not intended to afford a comprehensive understanding of individual lung structures and anatomy.
The lungs are contained in a space with a volume of approximately 4 L, but they have a surface area for gas exchange that is the size of a tennis court (≈85 m2). This large surface area is composed of myriads of independently functioning respiratory units. Unlike the heart, but like the kidneys, the lungs demonstrate functional unity; that is, each unit is structurally identical and functions just like every other unit. Because the divisions of the lung and the sites of disease are designated by their anatomical locations (e.g., right upper lobe, left lower lobe), it is essential to understand pulmonary anatomy in order to comprehend respiratory physiology and pathophysiological alterations in respiratory diseases.
Upper Airways: Nose, Sinuses, and Pharynx
The respiratory system begins at the nose and ends in the most distal alveolus. Thus the nasal cavity, the posterior pharynx, the glottis and vocal cords, the trachea, and all divisions of the tracheobronchial tree are included in the respiratory system. The upper airway consists of all structures from the nose to the vocal cords, including sinuses and the larynx, whereas the lower airway consists of the trachea, airways, and alveoli. The upper airways “condition” inspired air so that by the time air reaches the trachea, inspired air is at body temperature and fully humidified. The nose also functions to filter, entrap, and clear particles larger than 10 µm in size. The interior of the nose is lined by respiratory epithelial cells interspersed with surface secretory cells. These secretory cells produce important immunoglobulins, inflammatory mediators, and interferons, which are the first line of host defense.
The paranasal sinuses (frontal, maxillary, sphenoid, and ethmoid) are lined by ciliated epithelial cells and surround the nasal passages (
Fig. 20.1A).
The cilia facilitate the movement of mucus from the upper airways and clear the main nasal passages approximately every 15 minutes. The functions of the sinuses are (1) to lessen the weight of the skull, which makes upright posture easier; (2) to offer resonance to the voice; and (3) to protect the brain from frontal trauma. The fluid covering their surfaces is continually being propelled into the nose. In some sinuses (e.g., the maxillary sinus), the opening (ostium) is at the upper edge, which makes them particularly susceptible to retention of mucus. The ostia are readily obstructed by nasal edema (swelling), and retention of secretions and secondary infection (sinusitis) can result. The volume of the nose in an adult is approximately 20 mL, but its surface area is greatly increased by the nasal turbinates, which are a series of three continuous ribbons of tissue that protrude into the nasal cavity (see
Fig. 20.1B ). The nose enables the sense of smell. Neuronal endings in the roof of the nose above the superior turbinate carry impulses through the cribriform plate to the olfactory bulb.