What is the structure and function of the respiratory system and how are respiratory issues managed?

Structure and Function of the Respiratory System

The respiratory system is a complex organ system responsible for gas exchange between the atmosphere and the human body, with components working together to facilitate oxygen intake and carbon dioxide removal.

Anatomical Structure

• The respiratory system consists of complex structures including the lungs, upper and lower rib cage, diaphragm, and abdominal compartments, each with distinct mechanical properties 1
• The conducting airways connect the trachea (generation 0) to the alveolar gas exchange surface through a hierarchical network with sequential, irregular, dichotomous branching pattern 1
• Airways have multilayered walls with mucous membrane, smooth muscle, and cartilage components 1
• Pulmonary arteries follow airways in a similar branching pattern with additional "supernumerary" branches that arise from main vessels to perfuse nearby parenchyma 1
• Pulmonary veins course independently of airways in intermediate positions related to interlobular septa, converging on the left atrium in four main stems 1

Physiological Function

• The primary function of the respiratory system is to exchange oxygen and carbon dioxide between the circulating blood and the external environment 2
• This gas exchange occurs through multiple transport phenomena including oscillatory air flow, heat and water vapor exchange, mucus transport, and air-blood gas exchange 2
• Normal ventilation participates in the maintenance of an acid-base buffer system, allowing for excretion of CO2 produced from metabolic processes 3
• The respiratory microbiota acts as a gatekeeper providing resistance to colonization by respiratory pathogens and contributes to the maturation and maintenance of respiratory immunity 4

Respiratory Mechanics

• With increasing severity of airflow obstruction (as in COPD), expiration becomes flow-limited during tidal breathing, initially during exercise and later at rest 5
• Functional residual capacity (FRC) increases due to both static factors (loss of lung elastic recoil) and dynamic factors at the end of expiration 5
• Intrinsic positive end-expiratory pressure (PEEPi) develops as an inspiratory threshold load that must be countered by the contracting inspiratory muscles 5
• Increased FRC can impair inspiratory muscle function and coordination, although diaphragm contractility when normalized for lung volume may be preserved 5

Ventilation-Perfusion Relationships

• Ventilation-perfusion (V'/Q') inequality is the major mechanism impairing gas exchange and leading to arterial hypoxemia in respiratory disorders 5
• In severe respiratory disease, abnormal V'/Q' distributions may include areas with very high V'/Q' (emphysematous regions) and areas with very low V'/Q' (partially blocked airways) 5
• Most patients with respiratory disorders have mild to moderate increase in dead space ventilation 5
• The absence of shunt suggests that collateral ventilation and hypoxic pulmonary vasoconstriction are efficient mechanisms for maintaining gas exchange 5

Management of Respiratory Disorders

Assessment

• Asynchronous and paradoxic motion of both rib cage and abdomen might predict ventilatory failure 1
• Correlations between routine lung function tests and respiratory blood gases or patterns of V'/Q' distribution are generally poor 5
• Significant hypoxemia or hypercapnia is rare with FEV1 >1.0 L 5
• Respiratory assessment should evaluate dyspnea/fatigue, functional status, and gas exchange abnormalities 5

Therapeutic Approaches

• For respiratory disorders with bronchospasm, beta-adrenergic medications like albuterol are the primary treatment, working by stimulating adenyl cyclase to form cyclic AMP which mediates bronchial smooth muscle relaxation 6
• Albuterol has been shown to have more effect on the respiratory tract with fewer cardiovascular effects than isoproterenol at comparable doses 6
• For adults and children ≥2 years weighing at least 15 kg, the usual dosage is 2.5 mg of albuterol administered 3-4 times daily by nebulization 6
• In critically ill patients, positive pressure ventilation aims to improve arterial blood gases and unload respiratory muscles 1
• Active or passive mobilization and muscle training should be instituted early to prevent weakness in hospitalized patients 1
• Positioning, splinting, passive mobilization, and muscle stretching help preserve joint mobility and skeletal muscle length in immobile patients 1

Management of Acute Respiratory Failure

• Acute respiratory failure is characterized by significant deterioration of arterial blood gas tensions (hypoxemia and hypercapnia) 5
• During exacerbations, V'/Q' abnormalities worsen and contribute to increased PaCO2, enhanced by alveolar hypoventilation 5
• Airway resistance, end-expiratory lung volume, and PEEPi increase substantially during acute respiratory failure 5
• Administration of oxygen corrects hypoxemia but may worsen V'/Q' balance, potentially contributing to increased PaCO2 5
• Mechanical ventilation may be necessary when patients develop significant shunting, suggesting complete airway occlusion possibly by bronchial secretions 5

Special Considerations

• Respiratory disorders are often associated with multiple comorbidities that impact symptoms and outcomes 5
• Common comorbidities include cardiovascular disease, metabolic disturbances, skeletal muscle dysfunction, and psychological conditions 5
• Nutritional interventions are important in respiratory management, with recommendations to achieve ideal body weight while avoiding high-carbohydrate diets that may increase carbon dioxide production 5
• Psychosocial support and patient/family education can improve quality of life by focusing on coping skills, medication management, and general health 5