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Pathophysiology
Clinical meaning
The respiratory system is responsible for gas exchange, the continuous process of delivering oxygen from the atmosphere to the blood and removing carbon dioxide from the blood to the atmosphere. Understanding respiratory anatomy, physiology, and pathophysiology at the cellular level is essential for practical nursing practice because respiratory assessment findings directly reflect underlying cellular processes, and nursing interventions are designed to optimize cellular oxygenation.
Anatomy of the Respiratory System:
The respiratory system is divided into the upper and lower respiratory tracts. The upper respiratory tract includes the nose, pharynx (nasopharynx, oropharynx, laryngopharynx), and larynx. These structures warm, humidify, and filter inspired air. The nasal cavity contains turbinates (conchae), shelf-like structures covered with highly vascular mucous membrane that increase the surface area for warming and humidifying air. The mucous membrane contains goblet cells that secrete mucus, trapping particles larger than 10 micrometers in diameter. The nasopharynx contains adenoid tissue (pharyngeal tonsils) that provides immune surveillance. The larynx contains the vocal cords and the epiglottis, a cartilaginous flap that closes over the laryngeal inlet during swallowing to prevent aspiration.
The lower respiratory tract begins at the trachea, a rigid tube maintained open by C-shaped cartilage rings with a posterior membranous wall shared with the esophagus. The trachea is lined with pseudostratified ciliated columnar epithelium with goblet cells, forming the mucociliary escalator. The trachea bifurcates at the carina into the right and left main bronchi. The right main bronchus is shorter, wider, and more vertical than the left, which is why aspirated foreign bodies and aspiration pneumonia most commonly affect the right lung, particularly the right lower lobe.
The bronchi undergo progressive branching (approximately 23 generations of branching from the trachea to the alveoli), with each generation becoming smaller in diameter but greater in total cross-sectional area. As the airways become smaller, the structure changes: cartilage gradually disappears (completely absent in bronchioles less than 1 mm in diameter), smooth muscle becomes the predominant structural component, and the epithelium transitions from ciliated columnar to cuboidal to squamous. Terminal bronchioles are the smallest conducting airways and mark the boundary between the conducting zone (anatomical dead space, where no gas exchange occurs) and the respiratory zone.
The Alveolar-Capillary Unit:
The alveoli are the functional units of gas exchange, numbering approximately 300 million in adult lungs and providing a total surface area of approximately 70 to 100 square meters. Each alveolus is a thin-walled sac surrounded by an extensive network of pulmonary capillaries. The alveolar wall is composed of two types of epithelial cells. Type I alveolar cells (pneumocytes) are thin, flat squamous cells that cover approximately 95 percent of the alveolar surface area. Their extreme thinness (as little as 0.1 micrometers) minimizes the diffusion distance for gas exchange. Type II alveolar cells are cuboidal cells that produce pulmonary surfactant, a phospholipoprotein mixture (primarily dipalmitoylphosphatidylcholine, or DPPC) that reduces surface tension in the alveoli. Without surfactant, the high surface tension at the air-liquid interface would cause the alveoli to collapse, particularly the smaller alveoli (according to LaPlace's law, smaller spheres with higher surface tension require greater pressure to remain inflated). Type II cells also serve as progenitor cells that can proliferate and differentiate into type I cells after lung injury, playing a critical role in alveolar repair.
The alveolar-capillary membrane, across which gas exchange occurs, consists of four layers: the surfactant layer, the alveolar epithelium (type I cell), the fused basement membranes of the alveolar epithelium and capillary endothelium, and the capillary endothelium. This membrane is remarkably thin (0.5 to 1.0 micrometers total thickness) to facilitate rapid gas diffusion. Gas exchange occurs by passive diffusion according to Fick's law: the rate of diffusion is proportional to the surface area and the partial pressure gradient across the membrane, and inversely proportional to the membrane thickness.
Physiology of Gas Exchange:
Oxygen and carbon dioxide move between alveolar air and pulmonary capillary blood by passive diffusion driven by partial pressure gradients. Inspired atmospheric air at sea level has a PO2 of approximately 159 mmHg (21 percent of 760 mmHg total atmospheric pressure). By the time air reaches the alveoli, it has been humidified (water vapor pressure of 47 mmHg reduces available gas pressure) and mixed with residual gas, producing an alveolar PAO2 of approximately 100 mmHg. Mixed venous blood arriving at the pulmonary capillaries has a PvO2 of approximately 40 mmHg. This 60 mmHg pressure gradient drives oxygen diffusion from the alveolus into the blood. Oxygen diffusion is normally complete within the first third of the capillary transit time, providing a substantial reserve.
Carbon dioxide diffuses approximately 20 times more rapidly than oxygen because of its much higher solubility in the alveolar-capillary membrane (despite having a smaller partial pressure gradient: PACO2 is approximately 40 mmHg, and PvCO2 is approximately 46 mmHg). This is why CO2 retention (hypercapnia) is a late finding in respiratory disease: significant gas exchange impairment must occur before CO2 elimination is affected.
Oxygen Transport in the Blood:
Once oxygen diffuses into the pulmonary capillary blood, it is transported in two forms. Approximately 1.5 percent is dissolved in plasma (measured as PaO2), and approximately 98.5 percent is bound to hemoglobin (measured as SaO2 or SpO2 by pulse oximetry). Each hemoglobin molecule can bind four oxygen molecules. The relationship between PaO2 and hemoglobin saturation is described by the oxyhemoglobin dissociation curve, which has a characteristic S-shaped (sigmoidal) form. This S-shape has critical clinical significance. On the upper flat portion of the curve (PaO2 60 to 100 mmHg), large changes in PaO2 produce relatively small changes in oxygen saturation, providing a safety buffer. On the steep middle portion (PaO2 20 to 60 mmHg), small decreases in PaO2 cause dramatic drops in saturation and oxygen delivery. This explains why a patient with SpO2 of 90 percent (PaO2 approximately 60 mmHg) is at a critical tipping point: any further decrease in PaO2 will cause a precipitous fall in oxygen saturation.
Factors that shift the oxyhemoglobin dissociation curve to the right (decreased hemoglobin affinity for oxygen, facilitating oxygen release to tissues) include increased temperature, increased PCO2, decreased pH (acidosis), and increased 2,3-diphosphoglycerate (2,3-DPG). These conditions are found in metabolically active or stressed tissues, where enhanced oxygen release is beneficial. A right shift means higher PaO2 is needed to achieve the same oxygen saturation. Factors that shift the curve to the left (increased affinity, hemoglobin holds onto oxygen more tightly) include decreased temperature, decreased PCO2, increased pH (alkalosis), decreased 2,3-DPG, carbon monoxide, and fetal hemoglobin. A left shift means oxygen is loaded more efficiently in the lungs but released less readily to the tissues.
Ventilation Regulation:
Breathing is controlled by the respiratory center in the medulla oblongata and pons. The medullary respiratory center contains the dorsal respiratory group (primarily inspiratory neurons) and the ventral respiratory group (containing both inspiratory and expiratory neurons, active during forced breathing). The pneumotaxic center in the pons modulates the depth and rate of breathing by limiting the duration of inspiration.
Chemical regulation of breathing involves two systems. Central chemoreceptors on the ventral surface of the medulla respond primarily to changes in cerebrospinal fluid pH, which reflects arterial PCO2. Carbon dioxide freely crosses the blood-brain barrier and is converted to carbonic acid by carbonic anhydrase, releasing hydrogen ions that stimulate the central chemoreceptors. This is the primary driver of normal breathing: even small increases in PaCO2 (2 to 3 mmHg above normal 35-45 mmHg) significantly increase ventilatory drive. Peripheral chemoreceptors in the carotid bodies (innervated by the glossopharyngeal nerve, CN IX) and aortic bodies (innervated by the vagus nerve, CN X) respond primarily to decreased PaO2. However, they are not significantly stimulated until PaO2 drops below approximately 60 mmHg. This peripheral hypoxic drive becomes the primary respiratory stimulus in patients with chronic CO2 retention (some COPD patients), where chronically elevated PaCO2 has blunted the central chemoreceptor response. This is the physiological basis for the clinical guideline of maintaining SpO2 at 88 to 92 percent in COPD patients: excessive supplemental oxygen can suppress the hypoxic drive in these patients, leading to hypoventilation and CO2 narcosis.
Common Respiratory Assessment Findings and Their Cellular Basis:
Crackles (rales) are discontinuous adventitious lung sounds produced by the sudden opening of collapsed or fluid-filled small airways and alveoli during inspiration. Fine crackles sound like hair being rubbed between fingers near the ear and indicate fluid in the alveoli (pulmonary edema, pneumonia) or opening of atelectatic alveoli. Coarse crackles are louder, lower-pitched sounds produced by air moving through mucus in larger airways (bronchitis, resolving pneumonia). Wheezes are continuous, high-pitched musical sounds produced by air moving through narrowed airways. Narrowing can result from bronchospasm (asthma), mucosal edema, mucus plugging, or external compression. Inspiratory wheezes suggest upper airway narrowing (stridor), while expiratory wheezes are characteristic of lower airway obstruction. Diminished or absent breath sounds over an area may indicate pneumothorax (air in the pleural space absorbing sound), large pleural effusion, severe bronchospasm with minimal air movement, or atelectasis.
Oxygen delivery systems and their selection require understanding the physics of gas flow and the patient' ventilatory pattern. Low-flow systems (nasal cannula, simple face mask) deliver oxygen at flow rates below the patient' inspiratory demand, meaning the patient entrains room air to supplement the delivered oxygen, resulting in a variable FiO2 that depends on the patient' tidal volume, respiratory rate, and inspiratory flow rate. A nasal cannula at 1-6 L/min provides approximately 24-44 percent FiO2 under normal breathing conditions, with each additional liter per minute increasing FiO2 by roughly 4 percent. However, this relationship breaks down with increased respiratory rate or mouth breathing, which dilute the inspired oxygen concentration with more room air. High-flow systems deliver oxygen at flow rates that meet or exceed the patient' total inspiratory demand, providing a fixed, precise FiO2. The Venturi mask uses the Bernoulli principle: oxygen flows through a narrow jet orifice at high velocity, creating a negative pressure gradient that entrains a precise volume of room air through side ports. Different colored adaptors have different orifice sizes and entrainment ratios, providing precise FiO2 from 24-50 percent. High-flow nasal cannula (HFNC) delivers heated, humidified oxygen at flow rates up to 60 L/min, which exceeds normal adult peak inspiratory flow (typically 30-40 L/min), providing a predictable FiO2 up to 100 percent. HFNC also provides several additional physiological benefits: continuous positive airway pressure effect (approximately 1 cmH2O per 10 L/min flow) that recruits collapsed alveoli, dead space washout of the nasopharyngeal reservoir that improves alveolar ventilation efficiency, and mucosal humidification that preserves mucociliary clearance function.
Respiratory isolation procedures for airborne pathogens require understanding the physics of aerosol transmission. Airborne precautions are required for pathogens that remain suspended in the air as droplet nuclei (particles less than 5 micrometers in diameter) for extended periods. These particles are small enough to remain airborne on normal air currents, travel beyond 6 feet, and penetrate deep into the lower respiratory tract to the level of the terminal bronchioles and alveoli. Tuberculosis, measles, varicella, and COVID-19 (during aerosol-generating procedures) require airborne precautions including an airborne infection isolation room (AIIR) with negative pressure ventilation that maintains at least 12 air changes per hour and exhausts air directly outside or through HEPA filtration. The negative pressure differential (minimum 2.5 Pascals) ensures air flows from the corridor into the room, preventing escape of contaminated air when the door is opened. Healthcare workers must wear N95 respirators that filter at least 95 percent of airborne particles 0.3 micrometers or larger, which is the most penetrating particle size (MPPS). Particles both larger and smaller than 0.3 micrometers are actually filtered more efficiently through inertial impaction and diffusion mechanisms respectively. Annual fit testing ensures the N95 creates a proper seal against the face, as even small gaps can reduce filtration efficiency by 50 percent or more.
Incentive spirometry represents a key nursing intervention for preventing postoperative atelectasis, and the nurse must understand the physiological basis for its effectiveness. During normal spontaneous breathing, periodic sighs (deep breaths to approximately three times normal tidal volume) occur every 5-10 minutes, generating transpulmonary pressures sufficient to inflate collapsed alveoli and redistribute surfactant across the alveolar surface. Postoperative patients, particularly after abdominal or thoracic surgery, suppress these sighs due to pain-induced splinting, which reduces tidal volumes and eliminates the periodic deep breaths necessary for alveolar recruitment. Within 24 hours, this shallow breathing pattern leads to absorption atelectasis in dependent lung zones as trapped gas is gradually absorbed through the alveolar-capillary membrane without being replenished by fresh inspired air.
