Learning Objectives
Upon completion of this chapter, you will be able to:
- Describe the major structures of the respiratory system, including the role of each in respiration.
- Describe the process of respiration, with clinical examples of problems that can arise with alterations in the respiratory membrane.
- Differentiate the common respiratory tract infections.
- Discuss the processes involved in ventilation and gas exchange disorders.
Below are key terms related to the respiratory system:
The respiratory system is essential for survival, bringing oxygen into the body, allowing for the exchange of gases, and leading to the expulsion of carbon dioxide and other waste products. Its normal functioning depends on an intricate balance of the nervous, cardiovascular, and musculoskeletal systems. Numerous conditions can affect the respiratory tract and interfere with the body's ability to ensure adequate oxygenation and gas exchange.
The respiratory system consists of two major components: the conducting airways and the respiratory airways. The conducting airways are composed of the upper respiratory tract (nose, mouth, pharynx, larynx, trachea) and bronchi that move air in and out of the lung tissues. The respiratory airways are responsible for gas exchange. The lobules of the lungs consist of a bronchiole and arteriole, pulmonary capillaries, and veins. Alveolar sacs are small pouches at the ends of the bronchioles where gas is exchanged between air and blood. The bronchi and lobules of the lungs can be categorized as the lower respiratory tract.
The conducting airways are primarily involved in ventilation, the movement of air in and out of the body. These airways also change the quality of the atmospheric air by warming, filtering, and moistening it prior to reaching the alveoli. Air usually moves into the body through the nose and into the nasal cavity. The nasal hairs catch and filter foreign substances. The air is warmed and humidified as it passes by blood vessels close to the surface of the epithelial lining in the nasal cavity. Oxygen moves more efficiently when in warm and humid air, making respiration easier.
The epithelial lining contains goblet cells that produce mucus. This mucus traps dust, microorganisms, pollen, and any other foreign substances. The epithelial cells also contain cilia, microscopic, hairlike projections of the cell membrane, which are constantly moving and directing the mucus and any trapped substances down toward the throat. The action of the goblet cells and cilia is commonly called the mucociliary escalator or blanket.
Pairs of sinuses (air-filled passages through the skull) open into the nasal cavity. Because the epithelial lining of the nasal passage is continuous with the lining of the sinuses, the mucus produced in the sinuses drains into the nasal cavity and then into the throat.
Air moves from the nasal cavity into the pharynx and larynx. The larynx contains the vocal chords and the epiglottis, which closes during swallowing to protect the lower respiratory tract from any foreign particles. From the larynx, air proceeds to the trachea, the main conducting airway into the lungs. The trachea bifurcates, or divides, into two main bronchi, which further divide into smaller and smaller branches. All of these tubes contain mucous-producing goblet cells and cilia to entrap any particles that may have escaped the upper protective mechanisms. The cilia in these tubes move the mucus up the trachea and into the throat, where again it can be swallowed or coughed into the oral cavity.
The walls of the trachea and conducting bronchi are highly sensitive to irritation. When receptors in the walls are stimulated, a central nervous system (CNS) reflex is initiated and a cough results. The cough causes air to be pushed through the bronchial tree under tremendous pressure, cleaning out any foreign irritant. This reflex, along with the similar sneeze reflex (which is initiated by receptors in the nasal cavity), forces foreign materials directly out of the system, opening it for more efficient flow of gas.
Throughout the airways, many macrophage scavengers move freely about the epithelium and destroy invaders. Mast cells are present in abundance and release histamine, serotonin, adenosine triphosphate, and other chemicals to ensure a rapid and intense inflammatory reaction to any cell injury. The end result of these various defense mechanisms is that the lower respiratory tract is kept as free as possible from pathogens that can cause respiratory infection, which could interfere with essential gas exchange.
Question
Beyond cilia and mucus, what other protective reflexes and cellular defenses are present in the conducting airways?
Answer the question above the continue reading. iTELL evaluation is based on AI and may not always be accurate.
The lower respiratory tract, also known as the respiratory airways, is composed of the lobules and alveoli. The lobules include the smallest bronchioles, an arteriole, pulmonary capillaries, and veins. The bronchial tubes are composed of three layers: cartilage, smooth muscle, and epithelial cells. The cartilage keeps the tube open, but it becomes progressively less abundant as the bronchi divide and get smaller. The smooth muscles are able to contract and narrow the airways.
The muscles in the bronchi become smaller and less abundant as the bronchi divide, with only a few muscle fibers remaining in the terminal bronchi and alveoli. The epithelial cells are similar in structure and function to the epithelial cells in the nasal passage. The alveoli, or respiratory sacs, at the end of the bronchioles form the respiratory membrane. The alveoli are the smallest units of the lung and are the functional units where gas exchange occurs.
The lungs are two spongy organs that fill the chest cavity, separated by the mediastinum, which contains the heart, esophagus, thymus gland, and various blood vessels and nerves. The lungs are made up of the bronchial tree, the alveoli, the blood supply to the lungs, and elastic tissue, which allows for expansion and recoil. The left lung has two lobes, and the right lung has three lobes. The lung tissue receives its blood supply from the bronchial artery, which branches directly off the aorta. The alveoli receive unoxygenated blood from the right ventricle via the pulmonary artery, a process referred to as pulmonary perfusion.
Gas exchange occurs in the alveoli. In this process, carbon dioxide diffuses from the blood and oxygen is transferred to the blood. The exchange of gases at the alveolar level is called respiration. The alveolar sac holds the gas, allowing needed oxygen to diffuse across the respiratory membrane into the capillary while carbon dioxide, which is more abundant in the capillary blood, diffuses across the membrane and enters the alveolar sac to be expired. Type I alveolar cells are extremely thin cells that occupy most of the surface area of the alveoli.
The respiratory membrane is where gas exchange occurs. It is made up of the capillary endothelium, the capillary basement membrane, the interstitial space, the alveolar basement membrane, the alveolar epithelium, and the surfactant layer. The sac is able to stay open because the surface tension of the cells is decreased by the lipoprotein surfactant. Absence of surfactant leads to alveolar collapse. Surfactant is produced by the type II cells in the alveoli. These cells have other metabolic functions, including the conversion of angiotensin I to angiotensin II by angiotensin-converting enzyme, the degradation of serotonin, and possibly the metabolism of various hormones.
The oxygenated blood is returned to the left atrium via the pulmonary veins; from there, it is pumped throughout the body to deliver oxygen to the cells and to pick up waste products. The carbon dioxide that diffuses into the alveoli is moved back out into the atmosphere in the process of ventilation.
Ventilation, or the act of breathing, is controlled by the CNS. The inspiratory muscles—diaphragm, external intercostals, and abdominal muscles—are stimulated to contract by the respiratory center in the brain, which includes neurons in the pons and the medulla. The respiratory center receives input from chemoreceptors (neuroreceptors sensitive to carbon dioxide, oxygen, and acid levels) and lung receptors. The rate and/or depth of breathing can be increased quickly if the chemoreceptors note higher acid levels or carbon dioxide levels. Low oxygen levels will also trigger breathing, but this is a secondary or back-up system in people who do not have chronically high carbon dioxide levels.
The lung receptors consist of stretch receptors and irritant receptors. If the stretch receptors are stimulated, expiration will be triggered. Irritant receptors can protect airways from noxious stimuli (e.g., dust, cigarette smoke, cold air) by stimulating airway constriction. People can also have voluntary control of breathing, for example, when talking or singing.
The vagus nerve, a predominantly parasympathetic nerve, plays a key role in stimulating diaphragm contraction and inspiration. Vagal stimulation also leads to bronchoconstriction or tightening. The sympathetic system also innervates the respiratory system, leading to increased rate and depth of respiration and dilation of the bronchi to allow freer flow of air through the system.
Question
How is the process of ventilation regulated, and what roles do the central nervous system, chemoreceptors, and lung receptors play?
Answer the question above the continue reading. iTELL evaluation is based on AI and may not always be accurate.
Key Points
- The respiratory system has two parts: the conducting airways, which moves air in and out of the lungs, and the respiratory airways, where gas exchange occurs.
- Nasal hairs, mucous-producing goblet cells, cilia, the superficial blood supply of the upper respiratory tract, and the cough and sneeze reflexes all work to keep foreign substances from entering the lower respiratory tract.
- Gas exchange occurs across the respiratory membrane in the alveolar sac. The type II cells of the alveoli produce surfactant, which reduces surface tension to keep the alveoli open for gas exchange.
- The CNS controls ventilation, which depends on a functioning muscular system and a balance between the sympathetic and parasympathetic systems.
Several conditions or disorders of the conducting and respiratory airways can interfere with the functioning of the respiratory system. These problems can range from generalized discomfort to life-threatening changes in gas exchange. Having a basic understanding of the processes at work will facilitate the understanding of the drugs that are used to treat these disorders.
Respiratory tract infections are among the most common conditions affecting the conducting airways. They primarily involve an inflammatory response of the mucosal layer, often triggered by pathogens that also stimulate immune cells. These infections lead to a cascade of events that can impair airflow and gas exchange.
A number of viruses cause the common cold. These viruses invade the tissues of the upper respiratory tract, initiating the release of histamine and prostaglandins and causing an inflammatory response. As a result, the mucous membranes become engorged with blood, tissues swell, and goblet cells increase mucus production. These effects cause symptoms like sinus pain, nasal congestion, runny nose, sneezing, watery eyes, scratchy throat, and headache. Swelling can also block the eustachian tube, potentially leading to an ear infection (otitis media).
Seasonal rhinitis, commonly known as hay fever, is an inflammation of the nasal cavity caused by an allergic reaction to specific antigens (e.g., pollen, mold, dust), resulting in a vigorous inflammatory response, nasal congestion, sneezing, stuffiness, and watery eyes.
Sinusitis occurs when the epithelial lining of the sinus cavities becomes inflamed, often causing severe pain due to pressure against the bone. If untreated, microorganisms can travel up the sinus passages and into brain tissue or affect eyesight.
Pharyngitis, laryngitis, and bronchitis are inflammations of the pharynx, larynx, and bronchi, respectively. They are commonly caused by viral or bacterial infections, frequently seen with influenza. Acute bronchitis is often viral, while chronic bronchitis is commonly caused by noxious stimuli like cigarette smoke and recurrent infections.
Pneumonia is an inflammation of the lungs caused by bacterial or viral invasion or aspiration of foreign substances. The rapid inflammatory response leads to localized swelling, engorgement, and exudation, affecting the respiratory membrane and decreasing gas exchange. Symptoms include difficulty breathing, fatigue, fever, noisy breath sounds, and poor oxygenation.
Tuberculosis is caused by Mycobacterium tuberculosis, which replicates slowly but is resistant to destruction. It primarily infects lung tissue and spreads via respiratory droplets. Symptoms include cough (often bloody tinged), fatigue, shortness of breath, fever, chills, weight loss, and night sweats.
A number of disorders affect the lower respiratory tract, including atelectasis, bronchiectasis, asthma, chronic obstructive pulmonary disease (COPD), cystic fibrosis (CF), and respiratory distress syndrome (RDS). All of these disorders involve an alteration in the ability to move gases in and out of the lungs to some degree, and they all have the potential to hinder gas exchange.
Atelectasis, the incomplete expansion of alveoli, can result from outside pressure (e.g., pulmonary tumor, pneumothorax, pleural effusion) or airway blockage (e.g., mucous plug, edema, secretions). It commonly occurs after surgery due to decreased tidal volume and accumulation of secretions. Insufficient surfactant can also cause atelectasis. Patients may present with crackles, dyspnea, fever, cough, hypoxia, and changes in chest wall movement. Treatment involves clearing airways, oxygen delivery, and assisting ventilation, with a chest tube for pneumothorax.
Bronchiectasis is a chronic disease involving the bronchi and bronchioles, characterized by dilation of the bronchial tree and chronic infection and inflammation. With chronic inflammation, bronchial epithelial cells are replaced by fibrous scar tissue, leading to loss of protective mucus and ciliary movement. This results in chronic infections in the lower lung tissue. Patients often have an underlying medical condition making them susceptible to infections, presenting with fever, malaise, myalgia, arthralgia, and a purulent, productive cough.
Question
What are some common causes and clinical manifestations of atelectasis and bronchiectasis?
Answer the question above the continue reading. iTELL evaluation is based on AI and may not always be accurate.
Asthma is characterized by reversible bronchospasm, inflammation, and hyperactive airways. It is triggered by allergens, nonallergic inhaled irritants, infections, exercise, and emotions, which cause inflammation mediated by eosinophils, lymphocytes, and mast cells. Cytokine-mediated inflammation, mucous production, and edema contribute to obstruction. Symptoms include wheezing, shortness of breath, chest tightness, and/or cough. Treatment involves preventing episodes, antiinflammatory medications, and bronchodilators. The extreme case is status asthmaticus, a life-threatening bronchospasm unresponsive to usual treatment.
Chronic obstructive pulmonary disease (COPD) is a progressive, not completely reversible, chronic obstruction of airways, often related to cigarette smoking or other noxious stimuli. It includes two related disorders: emphysema and chronic bronchitis. Both result in airflow obstruction on expiration, increased lung inflation, and poor gas exchange. Emphysema involves loss of elastic tissue and destruction of alveolar walls, leading to alveolar hyperinflation and collapse on expiration. Chronic bronchitis is diagnosed by long-term airway inflammation with mucous secretion, edema, and other inflammatory signs. COPD is diagnosed with spirometry (breathing test), showing a low peak flow rate of expired air, shortness of breath, and/or productive cough.
Cystic fibrosis (CF) is a hereditary disease involving the epithelial lining of the respiratory, gastrointestinal, and reproductive tracts. It results from a defective gene on chromosome 7 that makes the epithelial membrane less permeable to chloride. This alteration in chloride transport decreases sodium and water excretion, leading to the accumulation of copious amounts of thick secretions in the lungs. These thick secretions obstruct the airways, causing recurrent infections and potential destruction of lung tissue. Treatment includes preventing and treating infections, chest physical therapy, and nutritional therapy.
Respiratory distress syndrome (RDS) causes obstruction at the alveolar level and is frequently seen in premature infants whose lungs have not fully developed and whose surfactant levels are low. Surfactant is necessary for lowering surface tension in the alveoli to keep them open. Low surfactant levels lead to atelectasis, decreased gas exchange, and low oxygen levels. Treatment aims to instill surfactant to prevent atelectasis and expand the lungs.
Acute respiratory distress syndrome (ARDS) is characterized by progressive loss of lung compliance and increasing hypoxia, typically resulting from severe insults like cardiovascular collapse, major burns, severe trauma, or rapid depressurization. It involves extensive epithelial cell damage and increased permeability in the alveolar-capillary membrane, allowing fluid, proteins, and blood cells into the lung tissue and alveoli. Treatment involves reversing the underlying cause and supportive care, including mechanical ventilation.
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