The Respiratory System: Structure, Function, and Regulation

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 zones, each with distinct 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 pulmonary ventilation, gas exchange, sound production, olfaction, and protection from pathogens.

• Four Respiratory Processes:

  1. Pulmonary Ventilation: Movement of air into 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 between lungs and tissues.

  4. Tissue Gas Exchange: Exchange of gases between blood and tissues.

Anatomy of the Respiratory System

The respiratory system's structure supports its function in air conduction and gas exchange. Each region has specialized features and histology.

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

• Gross Anatomy: The pleural and thoracic cavities house the lungs, while pulmonary blood vessels facilitate gas exchange.

• Histology: The respiratory tract transitions from pseudostratified ciliated columnar epithelium (upper) to simple squamous epithelium (alveoli). Alveoli contain type I (gas exchange) and type II (surfactant-secreting) cells.

• Respiratory Membrane: Thin barrier for efficient gas exchange, composed of alveolar and capillary walls plus their fused basement membranes.

• Functional Adaptations: Epithelial and connective tissue changes along the tract optimize air conduction, filtration, and exchange.

Pulmonary Ventilation

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

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

• Muscles of Ventilation: Inspiratory muscles (diaphragm, external intercostals) increase thoracic volume; accessory muscles assist during forced breathing.

• Pressure Changes: Atmospheric pressure (outside air), 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: The lung's ability to stretch; decreased in fibrosis.

  • Alveolar Surface Tension: Surfactant reduces tension, preventing alveolar collapse.

Respiratory Volumes and Capacities

• Tidal Volume (TV): Air moved in/out during normal 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, each involving reversible chemical reactions.

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

• Carbon Dioxide Transport: CO2 is transported dissolved in plasma, bound to hemoglobin, or as bicarbonate ions. The reversible reaction: $\mathrm{CO}_2 + \mathrm{H}_2\mathrm{O} \rightleftharpoons \mathrm{H}_2\mathrm{CO}_3 \rightleftharpoons \mathrm{H}^+ + \mathrm{HCO}_3^-$

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

• Hyperventilation: Excessive ventilation decreases CO2, causing alkalosis.

• Hypoventilation: Insufficient ventilation increases CO2, causing acidosis.

Putting It All Together: The Big Picture of Respiration

Respiration involves coordinated processes: ventilation, gas exchange, and transport, regulated to meet metabolic demands.

• Integration: All steps must function efficiently for effective 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.

• Chemical Stimuli: CO2, O2, and pH levels are detected by chemoreceptors.

• Central vs. Peripheral Chemoreceptors: Central (medulla) respond to CO2/pH in cerebrospinal fluid; peripheral (carotid/aortic bodies) respond to blood O2, CO2, and pH.

Diseases of the Respiratory System

Respiratory diseases are classified by their effects on airflow and lung mechanics.

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

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

• Basic Pathophysiology: Disease mechanisms disrupt normal ventilation, gas exchange, or both.