Chapter 21: The Respiratory System – Study Notes

The Respiratory System

Overview of the Respiratory System

The respiratory system is essential for gas exchange, supplying oxygen to the body and removing carbon dioxide. It is divided into anatomical and functional regions, each with specialized roles in respiration.

• Upper vs. Lower Respiratory Tracts: The upper respiratory tract includes the nose, nasal cavity, pharynx, and larynx. The lower respiratory tract consists of the trachea, bronchi, bronchioles, and lungs.

• Conducting vs. Respiratory Zones: The conducting zone transports air (nose to terminal bronchioles), while the respiratory zone (respiratory bronchioles, alveolar ducts, alveoli) is where gas exchange occurs.

• Major Functions: Includes gas exchange, regulation of blood pH, voice production, olfaction, and protection from pathogens.

• Four Respiratory Processes:

  1. Pulmonary ventilation: Movement of air in and out of the lungs.

  2. Pulmonary gas exchange: Exchange of gases between alveoli and blood.

  3. Gas transport: Movement of gases in the blood.

  4. Tissue gas exchange: Exchange of gases between blood and tissues.

Anatomy of the Respiratory System

The respiratory system's structure is adapted for efficient air conduction and gas exchange.

• Airway Pathway: Air enters through the nose/mouth → pharynx → larynx → trachea → bronchi → bronchioles → alveoli.

• Gross Anatomy: Each region (nasal cavity, pharynx, larynx, trachea, bronchi, lungs) has unique features and functions.

• Pleural and Thoracic Cavities: The lungs are housed in the thoracic cavity, surrounded by pleural membranes that reduce friction during breathing.

• Pulmonary Blood Vessels and Nerves: Pulmonary arteries carry deoxygenated blood to the lungs; veins return oxygenated blood to the heart.

• Histology: The respiratory tract is lined with various epithelial types (e.g., pseudostratified ciliated columnar in trachea, simple squamous in alveoli). Alveoli contain type I (gas exchange) and type II (surfactant-secreting) cells.

• Respiratory Membrane: Thin barrier (alveolar epithelium, capillary endothelium, and basement membrane) for efficient gas diffusion.

• Functional Adaptations: Changes in epithelial and connective tissue support airway function and protection.

Pulmonary Ventilation

Pulmonary ventilation is the process of moving air into and out of the lungs, driven by pressure and volume changes.

• Pressure-Volume Relationship: According to Boyle’s Law, pressure and volume are inversely related: $P_1V_1 = P_2V_2$.

• Muscles of Breathing: The diaphragm and external intercostals are primary inspiratory muscles; accessory muscles assist during forced breathing.

• Pressure Changes: Atmospheric pressure (external), intrapulmonary pressure (within alveoli), and intrapleural pressure (within pleural cavity) fluctuate during breathing.

• Factors Affecting Ventilation:

  • Airway resistance: Increased resistance (e.g., asthma) impedes airflow.

  • Pulmonary compliance: Ease of lung expansion; decreased in fibrosis.

  • Alveolar surface tension: Surfactant reduces tension, preventing alveolar collapse.

• Respiratory Volumes and Capacities:

  • Tidal volume (TV): Air moved per breath (~500 mL).

  • Inspiratory reserve volume (IRV): Extra air inhaled after normal inspiration.

  • Expiratory reserve volume (ERV): Extra air exhaled after normal expiration.

  • Residual volume (RV): Air remaining after maximal exhalation.

  • Capacities: Combinations of volumes (e.g., vital capacity = TV + IRV + ERV).

Gas Transport through the Blood

Oxygen and carbon dioxide are transported in the blood via distinct mechanisms, crucial for maintaining homeostasis.

• Oxygen Transport: Most oxygen binds reversibly to hemoglobin: $\mathrm{Hb} + \mathrm{O}_2 \rightleftharpoons \mathrm{HbO}_2$.

• Carbon Dioxide Transport: CO2 is transported as dissolved gas, bound to hemoglobin, or as bicarbonate (HCO3-): $\mathrm{CO}_2 + \mathrm{H}_2\mathrm{O} \rightleftharpoons \mathrm{H}_2\mathrm{CO}_3 \rightleftharpoons \mathrm{H}^+ + \mathrm{HCO}_3^-$.

• pH Regulation: Increased CO2 lowers plasma pH (acidosis); decreased CO2 raises pH (alkalosis).

• Ventilation Disorders:

  • Hyperventilation: Excessive ventilation decreases CO2, causing alkalosis.

  • Hypoventilation: Inadequate ventilation increases CO2, causing acidosis.

Putting It All Together: The Big Picture of Respiration

Respiration involves integrated processes: ventilation, gas exchange, transport, and tissue utilization, all regulated to meet metabolic demands.

• Integration: Each step is interdependent; disruption in one affects overall oxygen delivery and CO2 removal.

Neural Control of Ventilation

Breathing is regulated by neural centers in the brainstem, responding to chemical and neural stimuli.

• Brainstem Centers: Medullary respiratory centers (dorsal and ventral groups) and pontine centers coordinate rhythm and depth of breathing.

• Chemical and Neural Stimuli: CO2, O2, and pH levels are detected by chemoreceptors; stretch and irritant receptors provide feedback.

• Central vs. Peripheral Chemoreceptors:

  • Central: Located in the medulla, sensitive to CO2 and pH in cerebrospinal fluid.

  • Peripheral: Located in carotid and aortic bodies, sensitive to blood O2, CO2, and pH.

Diseases of the Respiratory System

Respiratory diseases can be classified by their effect on lung function and underlying pathophysiology.

• Restrictive Diseases: Reduce lung expansion (e.g., pulmonary fibrosis).

• Obstructive Diseases: Increase airway resistance (e.g., asthma, COPD).

• Pathophysiology: Each disease alters normal respiratory mechanics and gas exchange.