2-3
2-3. STRUCTURE AND FUNCTION
a. Nose. Inhaled air is warmed, moistened, and filtered in the nasal cavities. Filtering is done by the cilia (tiny hair-like projections) of the mucous membrane lining the nasal passages.
b. Pharynx. The pharynx, or throat, connects the nose and mouth with the lower air passages and the esophagus. It is divided into three sections, the nasopharynx, the oropharynx, and the laryngopharynx. Both air and food pass through the pharynx. Air passes from the nose and mouth to the larynx, while food passes from the mouth to the esophagus. The walls of the pharynx contain masses of lymphoid tissue called the adenoids and tonsils.
c. Larynx. The larynx, or voice box, connects the pharynx with the trachea. Two membranous bands in the wall of the larynx are called vocal cords. Vibrations of the vocal cords produce sound.
d. Trachea. The trachea, or windpipe, is a tube that carries air from the larynx to the bronchi. It is held open by cartilaginous rings and is lined with cilia and mucous glands to keep dust and dirt out of the lungs.
e. Bronchi. The trachea divides to form the two main bronchi. One bronchus enters each lung and divides into many smaller air passages called bronchioles. The bronchioles terminate in the final air spaces, called the alveoli.
f. Lungs.
(1) The lungs are the organs of respiration. They are elastic structures contained within the thoracic cavity. The upper, pointed border of each lung, called the apex, extends above the clavicle. The lower border, or base, rests upon the diaphragm. Each lung is divided into sections called lobes. The right lung has three lobes, termed upper, middle, and lower. The left
lung has only two lobes, referred to as upper and lower. Inside each lung, millions of tiny air sacs, called alveoli, are interlaced in a network of capillaries. Certain cells in the alveolar walls secrete a lipid-rich material called surfactant. Surfactant helps to maintain the elastic quality of the alveolar membrane and assists with the transfer of gases.
(2) Oxygen-poor blood is pumped from the heart's right ventricle, through the right and left pulmonary arteries, and into the lungs. Paralleling the branching of the respiratory tree, the arteries divide and subdivide within the lungs. Arteries divide into arterioles, and arterioles divide into the capillaries that surround the alveoli.
(3) Each lung is enclosed by a membranous sac formed of two layers of serous membrane called pleura. One layer covers the lungs and is called the visceral pleura. The other lines the chest cavity and is called the parietal pleura. The space between the two layers, the pleural cavity, contains a small amount of fluid that lubricates the surfaces. Between the two lungs is the mediastinum, the central thoracic cavity containing the heart, the great vessels, the esophagus, and the lower trachea.
