Day 3 OBJECTIVES Describe the structure and function of the lungs and the pleural coverings.

Main Bronchi The right and left main (primary) bronchi are formed by the division of the trachea. Each main bronchus runs obliquely before it plunges into the medial depression(hilum) of the lung on its own side. The right main bronchus is wider, shorter, and straighter than the left. Consequently, it is the more common site for an inhaled foreign object to become lodged. By the time incoming air reaches the bronchi, it is warm, cleansed of most impurities, and well humidified. The smaller subdivisions of the main bronchi within the lungs are direct routes to the air sacs. After entering the lungs, the main bronchi subdivide into smaller and smaller branches (secondary and tertiary bronchi, and so on), finally ending in the smallest of the conducting passageways, the bronchioles. Because of this branching and rebranching of the respiratory passageways within the lungs, the network formed is often referred to as the bronchial, or respiratory, tree. All but the smallest branches have reinforcing cartilage in their walls.    

Main Bronchi (con’t) The terminal bronchioles lead into respiratory zone structures, even smaller conduits that eventually terminate in alveoli, or air sacs. The respiratory zone, which includes the respiratory bronchioles, alveolar ducts, alveolar sacs, and alveoli, is the only site of gas exchange. There are millions of the clustered alveoli, which resemble bunches of grapes, and they make up the bulk of the lungs. All other respiratory passages are conducting zone structures that serve as conduits to and from the respiratory zone. Consequently, the lungs are mostly air spaces. The balance of the lung tissue, its stroma, is mainly elastic connective tissue that allows the lungs to recoil passively as we exhale. Thus, in spite of their relatively large size, the lungs weigh only about 2 ½ pounds, and they are soft and spongy.

Lungs The lungs are fairly large organs. They occupy the entire thoracic cavity except for the most central area, the mediastinum, which houses the heart (in its inferior pericardial cavity region), the great blood vessels, bronchi, esophagus, and other organs. The narrow superior portion of each lung, the apex, is just deep to the clavicle. The broad lung area resting on the diaphragm is the base. Each lung is divided into lobes by fissures; the left lung has two lobes, and the right lung has three. The surface of each lung is covered with a visceral serosa called the pulmonary, or visceral, pleura, and the walls of the thoracic cavity are lined by the parietal pleura.  The pleural membranes produce pleural fluid, a slippery serous secretion which allows the lungs to glide easily over the thorax wall during breathing movements. The lungs are held tightly to the thorax wall, and the pleural space is more of a potential space than an actual one. This is essential for normal breathing. the lungs are mostly air spaces. The balance of the lung tissue, its stroma, is mainly elastic connective tissue that allows the lungs to recoil passively as we exhale. In spite of their relatively large size, the lungs weigh only about 2 ½ pounds, and they are soft and spongy.

HOMEOSTATIC IMBALANCE Pleurisy inflammation of the pleura, can be caused by decreased secretion of pleural fluid. The pleural surfaces become dry and rough, resulting in friction and stabbing pain with each breath. Conversely, the pleurae may produce excessive amounts of fluid, which exerts pressure on the lungs. This type of pleurisy hinders breathing movements, but it is much less painful than the dry rubbing type.

The Respiratory Membrane The walls of the alveoli are composed largely of a single, thin layer of squamous epithelial cells. A sheet of tissue paper is much thicker.  Alveolar pores connect neighboring air sacs and provide alternative routes for air to reach alveoli whose feeder bronchioles have been clogged by mucus or otherwise blocked. The external surfaces of the alveoli are covered with a “cobweb” of pulmonary capillaries. Together, the alveolar and capillary walls, their fused basement membranes, and occasional elastic fibers construct the respiratory membrane (air-blood barrier), which has gas (air) flowing past on one side and blood flowing past on the other. The gas exchanges occur by simple diffusion through the respiratory membrane—oxygen passing from the alveolar air into the capillary blood and carbon dioxide leaving the blood to enter the gas-filled alveoli. It has been estimated that the total gas exchange surface provided by the alveolar walls of a healthy man is 50 to 70 square meters, or approximately 40 times greater than the surface area of his skin. The final line of defense for the respiratory system is in the alveoli. Remarkably efficient alveolar macrophages, sometimes called “dust cells,” wander in and out of the alveoli picking up bacteria, carbon particles, and other debris. The cuboidal cells produce a lipid (fat) molecule called surfactant, which coats the gas-exposed alveolar surfaces and is very important in lung function.

The respiratory membrane is composed of squamous epithelial cells of the alveoli, the capillary endothelium, and the scant basement membranes between. Surfactant-secreting cells are also shown. Oxygen diffuses from the alveolar air into the pulmonary capillary blood; carbon dioxide diffuses from the pulmonary blood into the alveolus. Neighboring alveoli are connected by small pores.

REVIEW What is the order of the following parts of the human respiratory system from the site where air enters the nostrils to the site where air reaches the end passages of the lungs— bronchi, larynx, nasal cavity, alveoli, trachea, pharynx, bronchioles? Nasal cavity, pharynx, larynx, trachea, bronchi, bronchioles, alveoli. Which main bronchus is the most likely site for an inhaled object to become lodged? Why? The right because it’s wider and straighter. The lungs are mostly passageways and elastic tissue. What is the role of the passageways? Of the elastic tissue? The passageways conduct air. The elastic tissue allows lungs to recoil passively when exhaling, saving energy. Name the four structures that make up the respiratory zone. Respiratory bronchiole, alveolar duct, alveolar sac, and individual alveoli.

WORK ON VOCABULARY WORDS UNTIL 11:15 1. Clearly explain the difference between external and internal respiration. 2. Trace the route of air from the nares to an alveolus. 3. Why is it important that the trachea be reinforced with cartilaginous rings? What is the advantage of the fact that the rings are incomplete posteriorly? 4. Where in the respiratory tract is the air filtered, warmed, and moistened? 5. The trachea has goblet cells that produce mucus. What is the specific protective function of the mucus? 6. In terms of general health, what is the importance of the fact that the pharyngotympanic . tubes and the sinuses drain into the nasal cavities and nasopharynx? 7. What is it about the structure of the alveoli that makes them an ideal site for gas exchange? WORK ON VOCABULARY WORDS UNTIL 11:15