Mechanics of Breathing: Structure, Function, and Gas Laws of the Respiratory System

Chapter 17: Mechanics of Breathing

17.1 The Respiratory System

The respiratory system is essential for gas exchange, pH regulation, protection, and vocalization. It consists of specialized structures that facilitate the movement of air and the exchange of gases between the atmosphere and the blood.

• Cellular Respiration: The metabolic process by which cells use oxygen to produce energy from food, generating carbon dioxide as a waste product.

• External Respiration: The exchange of gases between the atmosphere and the body, involving four integrated processes:

  1. Exchange of air between the atmosphere and the lungs (ventilation: inspiration and expiration)

  2. Exchange of O2 and CO2 between the lungs and the blood

  3. Transport of O2 and CO2 by the blood

  4. Exchange of gases between the blood and the cells

Functions of the Respiratory System:

• Exchange of gases (O2 and CO2) between the atmosphere and the blood

• Homeostatic regulation of body pH

• Protection from inhaled pathogens and irritating substances

• Vocalization

Respiratory System Structures

• Conducting System (Airways):

  • Upper respiratory tract: Mouth, nasal cavity, pharynx, larynx

  • Lower respiratory tract: Trachea, primary bronchi, their branches, lungs

• Alveoli: Tiny air sacs (singular: alveolus) that serve as the primary site of gas exchange

• Thoracic Cage: Bones and muscles of the thorax and abdomen, including the spine, rib cage, diaphragm, intercostal muscles, sternocleidomastoids, and scalenes

• Pleural Sacs: Double membranes surrounding each lung, containing pleural fluid to reduce friction and hold the lungs against the thoracic wall

Airways and Alveoli

• Airways: Connect the lungs to the external environment (pharynx → larynx → trachea → primary bronchi → bronchioles)

• Functions of Airways:

  • Warm air to body temperature

  • Add water vapor to humidify air

  • Filter out foreign material

• Alveolar Cells:

  • Type I: Gas exchange

  • Type II: Produce surfactant (reduces surface tension)

• Pulmonary Circulation: High flow, low pressure system; right ventricle → pulmonary trunk → pulmonary arteries → lungs → pulmonary veins → left atrium

Table: Airway Branching in the Lower Respiratory Tract

System Name	Name	Diameter (mm)	How Many?	Cross-Sectional Area (cm2)
Conducting system	Trachea	18-22	1	2.5
Conducting system	Primary bronchi	10-16	2	2.5
Conducting system	Smaller bronchi	1-10	8	8
Conducting system	Bronchioles	0.5-1	103	100
Exchange surface	Alveoli	0.3	8 x 106	104

17.2 Gas Laws

Gas laws describe the physical principles that govern the movement and behavior of gases in the respiratory system. Understanding these laws is essential for explaining how gases are exchanged and transported in the body.

• Atmospheric Pressure: The pressure exerted by the weight of air in the atmosphere; at sea level, it is 760 mm Hg.

• Gases Move Down Pressure Gradients: Gases flow from regions of higher pressure to regions of lower pressure.

• Dalton's Law: The total pressure of a mixture of gases is the sum of the pressures of the individual gases.

  • Partial Pressure: The pressure exerted by an individual gas in a mixture; calculated as:

  $P_{gas} = P_{atm} \times \% \text{ of gas}$

  • Example: For O2 at sea level (21% of air): $P_{O_2} = 760 \text{ mm Hg} \times 0.21 = 160 \text{ mm Hg}$

• Boyle's Law: Describes the inverse relationship between pressure and volume for a gas at constant temperature and number of moles:

  $P_1V_1 = P_2V_2$

  • If volume decreases, pressure increases, and vice versa.

• Ideal Gas Law: Relates pressure, volume, temperature, and number of moles:

  $PV = nRT$

  • Where $P$ = pressure, $V$ = volume, $n$ = moles, $R$ = universal gas constant, $T$ = temperature (Kelvin)

• Effect of Water Vapor: In humid air, subtract water vapor pressure from total pressure before calculating partial pressures.

Table: Partial Pressures of Atmospheric Gases at 760 mm Hg

Gas	pgas in dry 25°C air	pgas in 25°C, 100% humidity	pgas in 37°C, 100% humidity
O2 (21%)	160 mm Hg	155 mm Hg	150 mm Hg
CO2 (0.03%)	0.25 mm Hg	0.24 mm Hg	0.23 mm Hg
Water vapor	0 mm Hg	24 mm Hg	47 mm Hg

17.3 Ventilation

Ventilation refers to the mechanical process of moving air into and out of the lungs. It is driven by pressure gradients created by changes in lung volume.

• Respiratory Cycle: One inspiration followed by one expiration.

• Lung Volumes:

  • Tidal Volume (VT): Volume of air moved during a normal respiratory cycle.

  • Inspiratory Reserve Volume (IRV): Additional volume that can be inspired above tidal volume.

  • Expiratory Reserve Volume (ERV): Additional volume that can be expired after normal expiration.

  • Residual Volume (RV): Volume of air remaining in the lungs after maximal exhalation.

• Lung Capacities:

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

  • Total Lung Capacity (TLC): $TLC = IRV + ERV + V_T + RV$

  • Inspiratory Capacity: $IC = IRV + V_T$

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

• Pulmonary Function Tests: Use a spirometer to measure lung volumes and capacities.

Mechanics of Breathing

• Air Flow: Proportional to the pressure gradient ($\Delta P$) and inversely proportional to resistance ($R$):

  $\text{Flow} \propto \frac{\Delta P}{R}$

• Inspiration: Occurs when alveolar pressure decreases below atmospheric pressure.

• Expiration: Occurs when alveolar pressure increases above atmospheric pressure.

• Passive vs. Active Expiration: Quiet breathing is passive; forced expiration is active.

Intrapleural Pressure

• Normally negative (subatmospheric, about -3 mm Hg)

• Prevents lung collapse by keeping lungs inflated

• Pneumothorax: Air in the pleural cavity causes lung collapse

Lung Compliance and Elastance

• Compliance: Ability of the lung to stretch

  • High compliance: Stretches easily

  • Low compliance: Requires more force (e.g., fibrosis, inadequate surfactant)

• Elastance: Ability to return to resting volume when stretching force is released

Surfactant and the Law of LaPlace

• Law of LaPlace: Describes the relationship between pressure, surface tension, and radius in alveoli:

  $P = \frac{2T}{r}$

  • $P$ = pressure, $T$ = surface tension, $r$ = radius

  • Smaller alveoli have higher pressure unless surfactant reduces surface tension

• Surfactant: Surface-active agent (mixture of proteins and phospholipids) that disrupts the cohesive force of water, reducing surface tension and the work of breathing

• Clinical Note: Premature infants may develop Newborn Respiratory Distress Syndrome (NRDS) due to inadequate surfactant production

Summary Table: Key Gas Laws

Law	Equation	Description
Dalton's Law	$P_{total} = \sum P_{gas}$	Total pressure is the sum of partial pressures of individual gases
Boyle's Law	$P_1V_1 = P_2V_2$	Pressure and volume are inversely related at constant temperature
Ideal Gas Law	$PV = nRT$	Relates pressure, volume, temperature, and moles of gas
Law of LaPlace	$P = \frac{2T}{r}$	Pressure in a bubble (alveolus) is proportional to surface tension and inversely proportional to radius

Example: If the radius of an alveolus is halved and surface tension remains constant, the pressure required to keep it open doubles. Surfactant reduces this pressure, especially in smaller alveoli, preventing collapse.

Additional info: The notes above integrate textbook content with academic context, including definitions, equations, and clinical relevance for a comprehensive understanding of respiratory mechanics.