Respiratory System Physiology: Structure, Function, and Gas Laws

Chapter 17: Mechanics of Breathing

Overview of Respiratory System Functions

The respiratory system is essential for gas exchange, regulation of blood pH, protection from inhaled substances, and vocalization. Its structure and function are closely integrated with the cardiovascular system.

• Exchange of Gases: O2 and CO2 are exchanged between the atmosphere and blood.

• Homeostatic Regulation: The system regulates body pH by selectively retaining or excreting CO2.

• Protection: Airways protect from inhaled pathogens and irritating substances.

• Vocalization: Air movement across vocal cords enables speech and singing.

Bulk Flow of Air vs. Blood

Both air and blood are fluids, and their movement follows similar physical principles, though with key differences.

• Flow Direction: Movement occurs from regions of high pressure to low pressure.

• Pump Mechanism: Muscular pumps (heart for blood, diaphragm for air) create pressure gradients.

• Resistance: Fluid resistance is influenced by the diameter of tubes (airways or blood vessels).

• Difference: Air is a compressible gas mixture; blood is an incompressible liquid.

17.1 The Respiratory System

Integrated Processes of External Respiration

External respiration involves three exchanges and one transport process:

1. Exchange of air between atmosphere and lungs (ventilation)

2. Exchange of O2 and CO2 between lungs and blood

3. Transport of O2 and CO2 via blood

4. Exchange of gases between blood and cells

Major Components of the Respiratory System

• Conducting System: Passages/airways leading from external environment to exchange surface of lungs

• Exchange Surface: Alveoli and associated pulmonary capillaries

• Musculoskeletal Support: Bones and muscles of thorax and abdomen

Structures of Upper and Lower Respiratory Tract

• Upper Respiratory Tract: Mouth, nasal cavity, pharynx, larynx

• Lower Respiratory Tract: Trachea, primary bronchi, bronchioles, lungs

• Thoracic Cage: Ribs, spine, muscles (diaphragm, intercostals, sternocleidomastoids, scalenes)

Pleural Sacs

Pleural sacs enclose the lungs, providing lubrication and cohesion for lung movement.

• Pleural Membranes: Two layers of elastic connective tissue and capillaries, separated by thin fluid layer (~25-30 mL)

• Functions: Reduce friction, hold lungs tight against thoracic wall

Airways and Gas Exchange

Path of Oxygen Molecule

Oxygen travels from the mouth/nose through the pharynx, larynx, trachea, bronchi, and bronchioles to the alveoli, where gas exchange occurs.

• Airway Division: Airways branch into progressively smaller bronchi and bronchioles, increasing cross-sectional area and decreasing air velocity.

Airways Warm, Humidify, and Filter Inspired Air

• Warming: Air is warmed to body temperature (37°C) to protect alveoli.

• Humidifying: Moisture is added to prevent drying of exchange surfaces.

• Filtering: Cilia and mucus trap foreign particles; CFTR channels regulate mucus hydration.

Alveoli: Site of Gas Exchange

• Structure: Alveoli are clusters at the ends of terminal bronchioles, composed of a single layer of epithelial cells (type 1 and type 2).

• Type 1 Cells: Thin, cover 95% of alveolar surface, site of gas exchange.

• Type 2 Cells: Secrete surfactant, reduce surface tension, aid lung expansion.

• Blood Vessels: Fill 80-90% of space between alveoli, facilitating rapid gas exchange.

Pulmonary Circulation

• Low Oxygen Blood: From right ventricle to pulmonary arteries, then lungs.

• Oxygenated Blood: Returns to left atrium via pulmonary veins.

• Low Pressure System: Pulmonary pressure is low; right ventricle does not need to pump forcefully.

17.2 Gas Laws

Properties of Gases

• Blood: Incompressible liquid

• Air: Compressible mixture of gases

Dalton's Law

States that total pressure exerted by a mixture of gases is the sum of pressures exerted by individual gas components.

• Partial Pressure: $P_{gas} = (P_{atm} - P_{H2O}) \times \% \text{ of gas}$

Boyle's Law

Describes the inverse relationship between pressure and volume for a set amount of gas molecules.

• Equation: $P_1V_1 = P_2V_2$

• Application: As chest volume increases, alveolar pressure decreases, and air flows into lungs.

17.3 Ventilation

Ventilation and Respiratory Cycle

• Ventilation: Bulk flow exchange of air between atmosphere and alveoli.

• Respiratory Cycle: One inspiration followed by one expiration.

Lung Volumes and Capacities

• Tidal Volume (TV): Volume of air during a single inspiration or expiration (avg ~500 mL).

• Inspiratory Reserve Volume (IRV): Additional air inspired after normal inspiration (avg ~3000 mL).

• Expiratory Reserve Volume (ERV): Additional air expired after normal expiration (avg ~1100 mL).

• Residual Volume (RV): Air remaining after maximal exhalation (avg ~1200 mL).

Lung Capacities

• Vital Capacity (VC): $VC = IRV + ERV + TV$

• Total Lung Capacity (TLC): $TLC = VC + RV$

• Inspiratory Capacity (IC): $IC = TV + IRV$

• Functional Residual Capacity (FRC): $FRC = ERV + RV$

Muscles of Ventilation

• Quiet Breathing: Diaphragm, external intercostals, scalenes

• Forced Breathing: Internal intercostals, abdominal muscles

Pressure, Flow, and Resistance

• Relationship: $\text{Flow} \propto \frac{\Delta P}{R}$

• Flow increases with greater pressure gradient; decreases with higher resistance.

Inspiration and Expiration

• Inspiration: Thoracic cavity volume increases, pressure decreases, air flows in.

• Expiration: Thoracic volume decreases, pressure increases, air flows out.

• Passive Expiration: Elastic recoil of lungs and thoracic cage.

• Active Expiration: Internal intercostals and abdominal muscles contract.

Intrapleural Pressure

• Definition: Pressure in fluid-filled space between visceral and parietal pleura; normally subatmospheric.

• Function: Keeps lungs inflated by opposing elastic recoil.

Lung Compliance and Elastance

• Compliance: Ability of lungs to stretch and expand ($\Delta V / \Delta P$).

• Elastance: Ability to resist deformation and return to original shape.

• High Compliance: Lungs stretch easily; low elastance.

• Low Compliance: Lungs resist stretching; high elastance.

Surfactant and Work of Breathing

• Surfactant: Secreted by type 2 alveolar cells; reduces surface tension, aids lung expansion.

• Function: Disrupts cohesion between water molecules, decreasing work needed to expand alveoli.

• Clinical Note: Premature infants may lack surfactant, leading to respiratory distress syndrome (NRDS).

Ventilation Efficiency

Total Pulmonary and Alveolar Ventilation

• Total Pulmonary Ventilation: $\text{Ventilation Rate} \times \text{Tidal Volume}$ Avg = $12 \text{breaths/min} \times 500 \text{mL} = 6 \text{L/min}$

• Anatomic Dead Space: Volume of air in conducting airways not available for gas exchange (avg ~150 mL).

• Alveolar Ventilation: $\text{Ventilation Rate} \times (\text{Tidal Volume} - \text{Dead Space Volume})$ Avg = $4.2 \text{L/min}$

Assessment of Pulmonary Function

Auscultation and Spirometry

• Auscultation: Listening to breath sounds to diagnose pathologies (e.g., diminished sounds, wheezes, crackles, friction rub).

• Spirometry: Measures lung volumes and capacities to assess pulmonary function.

Restrictive Lung Diseases

• Definition: Conditions in which compliance is reduced, making lungs stiff and harder to expand (e.g., pulmonary fibrosis).

Summary Table: Lung Volumes and Capacities

Parameter	Abbreviation	Definition	Average Value
Tidal Volume	TV	Volume of air per breath	500 mL
Inspiratory Reserve Volume	IRV	Additional air inspired after normal inspiration	3000 mL
Expiratory Reserve Volume	ERV	Additional air expired after normal expiration	1100 mL
Residual Volume	RV	Air remaining after maximal exhalation	1200 mL
Vital Capacity	VC	IRV + ERV + TV	Varies
Total Lung Capacity	TLC	VC + RV	Varies

Additional info: These notes expand on the original questions by providing definitions, equations, and clinical context for key concepts in respiratory physiology, suitable for college-level Anatomy & Physiology students.