The Pulmonary System

The pulmonary system brings oxygen into the body and expels metabolic wastes in the form of gases. Physician specialists who treat conditions of the pulmonary system are internists who have subspecialty certifications in pulmonary medicine (pulmonologists). Physician specialists who operate on the LUNGS and related structures are thoracic surgeons. This section, “The Pulmonary System,” presents an overview of the structures and functions of the pulmonary system, a discussion of pulmonary health and disorders, and entries about the health conditions that can affect the lungs and related structures.

Functions of the Pulmonary System

The LUNGS bring life-giving oxygen into the body and remove toxic gaseous wastes from it. An asymmetrical pair, these spongy structures rhythmically expand and compress about 15 to 20 times a minute. Expansion, or inhalation, draws air and oxygen into the lungs; compression, or exhalation, expels carbon dioxide and other gases that are metabolic waste byproducts of cellular activity. The structures of the nasal cavity and the upper airway (THROAT) bring air, containing about 21 percent oxygen, into the body. The NOSE and SINUSES warm and moisturize the air.

Carrying that air to the lungs are the TRACHEA, bronchi, and bronchioles—a branching structure of progressively smaller airways. The air’s destination is the alveoli, tiny membranous sacs that cluster grapelike at the ends of the bronchioles. A webbing of capillaries (tiny blood vessels) surrounds each ALVEOLUS, carrying erythrocytes (red BLOOD cells) waiting to receive oxygen molecules and release carbon dioxide molecules. This process, the OXYGEN–CARBON DIOXIDE EXCHANGE, gives the body life.

The lungs: asymmetry in synchronization

The lungs fill the thoracic cavity from neck to DIAPHRAGM and sternum to spine. Though paired, the lungs differ somewhat in structure. The right lung is larger than the left lung, containing three lobes to the left lung’s two. The left lung must accommodate the HEART, which nestles into an indentation called the cardiac notch. Some anatomists consider the small tail of tissue in the left lung that drops behind the heart, called the lingula, as a structure separate from either lobe of the left lung though most designate it a segment of the left upper lobe. Each lobe of the lung further contains structural divisions called bronchopulmonary segments, 10 among the three lobes of the right lung and 8 among the two lobes of the left lung. A bronchial cluster—which includes bronchi, bronchioles, alveoli, blood vessels, LYMPH VESSELS, and nerves—branches into each segment.

The cells of the respiratory tract are primarily epithelial cells, the same kind of cells that make up the SKIN. The epithelial cells lining the trachea and bronchi contain cilia, tiny hairlike projections that sweep debris, such as dust and pollen, and excess mucus from the respiratory tract outward to the throat for coughs to expel them from the body. A tissue-thin membrane, the PLEURA, covers the outer surface of the lungs. The pleura secretes serous fluid to lubricate the lungs in their perpetual movement, protecting them from friction and irritation. Lung tissue is highly porous, with the substance of the lungs being about 15 percent solid tissue and 85 percent air and blood.

The heart pumps the body’s full volume of blood—about five liters—through the lungs once each minute to pick up oxygen and release carbon dioxide. The blood, which flows through the lungs in a closed circuit from the heart via the PULMONARY ARTERIES and back to the heart via the PULMONARY VEINS, pulses through a dense, meshlike capillary network surrounding the alveoli. Pulmonary and cardiovascular mechanisms maintain an intricate balance between the flow of blood and the flow of air, with the blood flow constantly adjusting to flood into CAPILLARY BEDS surrounding alveoli that have strong air flow and recede from those with diminished air flow. This balance provides the greatest efficiency for getting oxygen into the bloodstream.

The bronchial tree: trachea, bronchi, bronchioles, and alveoli

Like a hollow trunk, the trachea supports the treelike bronchial structure that brings air into the lungs. The trachea extends from the back of the throat about four and a half inches down to the midchest, where it branches into the right BRONCHUS and left bronchus. The spine in the back and the sternum in the front protect the trachea’s path. The trachea’s most vulnerable exposure is at the front of the neck, where it passes in front of the ESOPHAGUS before dropping behind the sternal notch. C-shaped bands of CARTILAGE help protect the trachea as well as give it the rigidity necessary to maintain an open passageway against ever-changing air pressures. Long fibers of smooth MUSCLE complete the posterior wall of the trachea. The trachea terminates with the branching of the right main bronchus, going to the right lung, and left main bronchus, going to the left lung. Like the trachea, the bronchi contain smooth muscle with rings of cartilage for support and STRENGTH. The smooth muscle fibers of the trachea and the bronchi contract and expand in response to the air pressure changes of inhalation and exhalation. Each bronchus quickly divides to branch to the lung’s lobes (lobular bronchi), and further subdivides into segmental bronchi, branching bronchi, and ultimately the very tiny and cartilage-free bronchioles. The alveoli cluster at the ends of the bronchioles.

The alveoli are the work stations of the lungs, and each lung contains about 150 million of them. Each microscopic alveolus looks like a small sac; an alveolar cluster contains dozens of alveoli that bubble from the end of a bronchiole. The alveolar membrane, the thickness of a single cell, forms the interface between the pulmonary system and the cardiovascular system, allowing the oxygen and carbon dioxide molecules to cross from the air within the lungs and the blood within the capillaries. Their bunched arrangement vastly extends the surface area available for oxygen exchange within the confined space of the chest. The total surface area of the alveoli, if spread out flat, is about the size of a tennis court.

Breathing: mechanics, physics, and molecular exchange

The balance between oxygen and carbon dioxide in the blood regulates pulmonary respiration (BREATHING). As carbon dioxide from cellular METABOLISM accumulates in the blood, it reaches a threshold that triggers a sequence of biochemical signals to the brainstem. The brainstem then initiates a RESPIRATORY CYCLE, sending NERVE signals that trigger the diaphragm (the flat, broad muscle that forms the base of the thoracic cavity) and the intercostal muscles (the muscles between the ribs) to contract. In response the diaphragm flattens, pulling the floor of the thoracic cavity downward. The intercostal muscles simultaneously contract to pull the ribs outward and upward. The synchronized movements enlarge the thoracic cavity, drawing air into the lungs. When the diaphragm and the intercostal muscles relax, the thoracic cavity returns to its resting size and the lungs expel air.

With each breath the EPIGLOTTIS, a flap of cartilaginous tissue at the back of the throat, opens and closes like a valve to allow air to pass in and out of the trachea and to keep other substances (such as saliva, food, and drink) from entering the trachea and lungs. With the changing of air pressure that occurs between inhalation and exhalation, oxygen molecules migrate through the micrometers-thin membrane walls of the alveoli into the blood circulating through the capillaries that surround the alveoli. Carbon dioxide molecules cross back in exchange. The respiratory cycle repeats about 20,000 times every 24 hours, varying in rate according to the body’s oxygen needs.

ALTITUDE AND OXYGEN

Though the percentage of oxygen in the air remains constant regardless of altitude, the concentration of oxygen molecules decreases as altitude increases. The concentration of oxygen molecules in the air is greater at sea level than in the mountains. At a higher altitude the LUNGS must work harder to extract enough oxygen to meet the body’s needs. In the short term the body compensates by raising the RESPIRATION RATE—breathing faster. After about 72 hours at a higher altitude, the decreased concentration of oxygen in the air stimulates the BONE MARROW to produce more erythrocytes (red BLOOD cells), which increases the blood’s capacity to carry oxygen.

Health and Disorders of the Pulmonary System

The lungs rhythmically pull air into and release air from the body in a choreography the BRAIN coordinates with the HEART RATE and BLOOD PRESSURE to maintain the vital supply of oxygen to tissues throughout the body. Aerobic conditioning through regular physical exercise, not smoking, and maintaining healthy weight are among the key factors that keep the lungs functioning efficiently across the span of life.

Lung capacity peaks during the 20s and gradually declines with increasing age. People who remain aerobically fit continue to have strong pulmonary function despite this decline, as their lungs still have enough reserve to meet the body’s needs. People whose lifestyles are sedentary are more likely to experience a decline in lung capacity, apparent in shortness of breath when climbing stairs, when walking distances, or with sudden bursts of physical exertion.

The lungs are subject to few BIRTH DEFECTS. Congenital disorders of the lungs include hereditary disorders, such as CYSTIC FIBROSIS, and conditions resulting from prematurity or inadequate development of the lungs before birth, such as chronic lung disease of infancy. Cardiovascular anomalies, notably structural malformations such as transposition of the great arteries (TPA) or hypoplastic left heart syndrome (HLHS), affect the integration of function between the heart and the lungs.

Cigarette smoking, CARDIOVASCULAR DISEASE (CVD), and long-term exposure to irritants such as industrial chemicals are the leading causes of acquired pulmonary disease. Because the lungs face continual exposure to external pathogens, they are vulnerable to viral, bacterial, and fungal invaders. Immune cells—notably lymphocytes, macro-phages, and neutrophils—reside within the walls of the bronchi, detecting and eliminating most pathogens before they can cause localized INFECTION. However, those that slip through the body’s defense mechanisms can cause serious diseases such as PNEUMONIA (viral or bacterial).