The Respiratory System

Overview and Major Functions

The respiratory system is essential for supplying oxygen (O2) to the body and removing carbon dioxide (CO2). This is achieved through four key processes:

• Pulmonary Ventilation: The act of breathing, moving air into and out of the lungs.

• External Respiration: Gas exchange between the alveoli and the blood.

• Transport of Respiratory Gases: The movement of O2 and CO2 in the blood throughout the body.

• Internal Respiration: Gas exchange between blood and tissue cells, supporting cellular respiration.

Note: Detailed anatomy is covered in laboratory sessions. Homeostatic imbalances are important for clinical understanding.

Mechanics of Breathing

Pressure Relationships in the Thoracic Cavity

Breathing is driven by pressure differences between the atmosphere and the thoracic cavity:

• Atmospheric Pressure (Patm): The pressure exerted by air surrounding the body, standard value is 760 mm Hg.

• Intrapulmonary Pressure (Ppul): Pressure within the alveoli; fluctuates with breathing but equalizes with atmospheric pressure.

• Intrapleural Pressure (Pip): Pressure within the pleural cavity; always about 4 mm Hg less than intrapulmonary pressure, creating a partial vacuum.

Factors maintaining lung inflation:

• Surface tension of pleural fluid adheres parietal and visceral pleura.

• Intrapulmonary pressure exceeds intrapleural pressure, pushing lungs outward.

• Atmospheric pressure pushes the thoracic cavity against the lungs.

Factors promoting lung collapse:

• Elastic recoil of lung tissue.

• Surface tension of alveolar fluid (minimized by surfactant).

Atelectasis (collapsed lung) occurs if intrapleural and intrapulmonary pressures equalize, often due to chest wounds or pleural tears. Air in the pleural space is called pneumothorax.

Pulmonary Ventilation and Boyle's Law

• As lung volume increases, pressure decreases, and vice versa (Boyle's Law).

• Inspiration: Diaphragm and external intercostals contract, increasing thoracic volume and decreasing intrapulmonary pressure, causing air to flow in.

• Expiration: Usually passive; muscles relax, thoracic volume decreases, intrapulmonary pressure rises, and air flows out. Forced expiration is active.

Boyle's Law equation:

Physical Factors Affecting Ventilation

• Airway Resistance: Friction in airways impedes airflow. Increased resistance (e.g., bronchoconstriction) reduces airflow.

• Lung Compliance: The ease with which lungs expand. Decreased by loss of elasticity, airway blockage, increased alveolar surface tension, or thoracic cage rigidity.

• Lung Elasticity: Necessary for passive expiration; loss impairs exhalation.

• Alveolar Surface Tension: Water in alveoli creates tension, but surfactant (from type II cells) reduces this, preventing alveolar collapse.

Gas Flow equation:

Respiratory Volumes and Capacities

• Tidal Volume (TV): Air moved in/out during normal breathing.

• 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.

Dead Space: Volume of air in conducting passages not involved in gas exchange.

Pulmonary Function Tests

• Minute Respiratory Volume (MRV): Total gas flow per minute (normal ~6 L/min).

• Forced Vital Capacity (FVC): Gas expelled after deep breath and forceful exhalation.

• Forced Expiratory Volume (FEV): Gas expelled during intervals of FVC (e.g., FEV1 = first second, normally 80%).

• Obstructive disorders: Decreased FEV; Restrictive disorders: Decreased FVC.

Alveolar Ventilation Rate (AVR) measures effective gas exchange:

Normal AVR ≈ 4200 mL/min. Increasing tidal volume (not rate) is more effective for increasing AVR.

Gas Exchange

Gas Laws Relevant to Respiration

• Dalton's Law of Partial Pressures: Each gas in a mixture exerts its own pressure; total pressure is the sum of partial pressures.

• Henry's Law: The amount of gas dissolved in a liquid is proportional to its partial pressure and solubility. CO2 is highly soluble, O2 is less so, N2 is nearly insoluble.

Alveolar Gas Composition

• Alveolar air has higher CO2 and water vapor, and lower O2 than atmospheric air.

• Differences arise from gas exchange, humidification, and mixing of new and old air.

Factors Affecting Gas Exchange

• Gases diffuse from high to low partial pressure.

• O2 has a steeper pressure gradient but lower solubility than CO2.

• Other factors:

  • Respiratory Membrane Thickness: Thicker membranes (e.g., fibrosis, edema) impede exchange.

  • Alveolar Surface Area: Greater area increases exchange; emphysema reduces surface area.

Ventilation–Perfusion Coupling

• Efficient gas exchange requires matching alveolar ventilation with blood flow (perfusion).

• Local mechanisms (O2, CO2, pH) regulate arteriolar diameter to optimize exchange.

Gas Transport

Oxygen Transport

• 98.5% of O2 is carried by hemoglobin (Hb); 1.5% is dissolved in plasma.

• Each Hb molecule has 4 heme groups, each binding one O2 (max 4 O2 per Hb).

• O2 binding changes Hb's shape, increasing affinity for more O2 (cooperative binding).

• O2 unloading is facilitated by low pO2, high pCO2, low pH, high temperature, and increased BPG.

O2-Hb Dissociation Curve: Shows % saturation of Hb at different pO2 values. Steep at low pO2, flat at high pO2. Venous reserve allows O2 delivery during increased demand.

• Bohr Effect: Lower pH (higher H+) weakens Hb-O2 bond, enhancing O2 release.

• BPG (2,3-bisphosphoglycerate): Produced by RBCs during glycolysis, decreases Hb's O2 affinity, promoting unloading.

• Hypoxia: Inadequate O2 delivery due to various causes (see textbook for types).

Carbon Dioxide Transport

• CO2 is transported in three forms:

  • Dissolved in plasma (7–10%)

  • Bound to Hb as carbaminohemoglobin (HbCO2) (20–30%)

  • As bicarbonate ion (HCO3-) in plasma (60–70%)

CO2 transport reactions:

• Carbonic anhydrase in RBCs catalyzes the reaction.

• H+ binds to Hb (buffering), and HCO3- diffuses into plasma (chloride shift maintains charge balance).

• In the lungs, the process reverses, releasing CO2 for exhalation.

Neural Control of Respiration

Respiratory Centers in the Brain

• Medulla Oblongata:

  • Dorsal Respiratory Group (DRG): Sets basic rhythm by stimulating diaphragm and external intercostals.

  • Ventral Respiratory Group (VRG): Active during forced expiration.

• Pons:

  • Pneumotaxic Center: Inhibits inspiration, prevents over-inflation.

  • Apneustic Center: Stimulates inspiration, prolongs inhalation.

Factors Influencing Breathing Rate and Depth

• Reflexes: Irritants trigger airway constriction, coughing, sneezing. Hering-Breuer reflex prevents over-inflation.

• Higher Brain Centers: Hypothalamus (emotions, pain) and cortex (voluntary control) can modify breathing.

• Chemical Factors:

  • Increased pCO2 (hypercapnia) stimulates central chemoreceptors, increasing rate and depth (hyperventilation).

  • Decreased pCO2 (hypocapnia) depresses respiration (hypoventilation or apnea).

  • Low pO2 sensed by peripheral chemoreceptors (carotid, aortic bodies) increases breathing rate if severe.

  • Arterial pH changes (usually from CO2 retention) can also influence breathing.

Example: The Bohr Effect in Exercise

During intense exercise, muscle metabolism increases CO2 and H+ production, lowering blood pH. This enhances O2 unloading from hemoglobin, ensuring active tissues receive more oxygen.

Additional info: For clinical relevance, study homeostatic imbalances such as hypoxia, hypercapnia, and respiratory disorders (obstructive vs. restrictive).