The Respiratory System: Structure, Function, and Gas Laws

The Respiratory System

Overview of Respiratory System Function

The respiratory system is responsible for the exchange of gases between the atmosphere and the body, supporting cellular respiration and maintaining homeostasis. It involves a series of anatomical structures and physiological processes that ensure efficient oxygen uptake and carbon dioxide removal.

• Cellular respiration: The metabolic process by which cells use oxygen to produce ATP, releasing carbon dioxide as a waste product.

• External respiration: The exchange of gases between the atmosphere and the blood, and between the blood and body cells.

• Ventilation: The mechanical movement of air into (inspiration) and out of (expiration) the lungs.

• Gas transport: The movement of oxygen and carbon dioxide in the blood between the lungs and tissues.

• Internal respiration: The exchange of gases between the blood and the body’s cells.

Bulk Flow in the Respiratory System

• Air flows from regions of higher pressure to lower pressure.

• A muscular pump (primarily the diaphragm) creates pressure gradients necessary for ventilation.

• Resistance to airflow is mainly determined by the diameter of the airways.

Respiratory System Structures

Conducting System (Airways)

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

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

Alveoli

• Alveoli (singular: alveolus): Tiny air sacs where gas exchange occurs.

• Type I alveolar cells: Specialized for gas exchange.

• Type II alveolar cells: Produce surfactant to reduce surface tension.

• Alveoli are closely associated with capillaries for efficient gas exchange.

Thoracic Cage and Associated Muscles

• Composed of the spine, rib cage, and associated muscles (diaphragm, intercostal muscles, sternocleidomastoids, scalenes).

• Pleural sacs surround each lung, containing pleural fluid to reduce friction and hold the lungs against the thoracic wall.

Airway Branching

System Name	Name	Divisions	Diameter (mm)	How Many?	Cross-Sectional Area (cm2)
Conducting system	Trachea	1	18	1	2.5
Conducting system	Primary bronchi	2	12	2	2.5
Conducting system	Bronchioles	~8	1	~65,000	~1,400
Exchange surface	Alveoli	~24	0.3	~8 x 106	~1 x 104

Additional info: Table values are representative; actual numbers may vary by source.

Airways and Their Functions

• Air passes through the pharynx, larynx (containing vocal cords), trachea, primary bronchi, and bronchioles.

• Airways warm, humidify, and filter inspired air:

  • Warming air to body temperature

  • Adding water vapor

  • Filtering out foreign material

Gas Laws Relevant to Respiration

Atmospheric Pressure and Gas Movement

• Atmospheric pressure is the force exerted by the weight of air in the atmosphere.

• Gases move down pressure gradients (from high to low pressure).

Dalton’s Law of Partial Pressures

• Air is a mixture of gases; each gas exerts its own partial pressure.

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

• Formula: $P_{gas} = P_{atm} \times \text{fraction of gas}$

Boyle’s Law

• Describes the inverse relationship between pressure and volume for a gas at constant temperature.

• Formula: $P_1V_1 = P_2V_2$

Partial Pressures in Humid Air

Gas	pgas in dry 25°C air	pgas in 25°C air, 100% humidity	pgas in 37°C air, 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

To calculate partial pressure of a gas in humid air: $P_{O_2,\ inhumid\ air} = (P_{atm} - P_{H_2O}) \times \%\ of\ gas$

Example: $P_{O_2} = (760 - 24) \times 0.21 = 155\ mm\ Hg$

Ventilation

Respiratory Cycle and Lung Volumes

• One respiratory cycle = 1 inspiration + 1 expiration.

• Lung volumes are measured using a spirometer.

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

• Inspiratory reserve volume (IRV): Additional volume inspired above tidal volume.

• Expiratory reserve volume (ERV): Volume forcefully exhaled 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): $TLC = VC + RV$

• Other capacities include inspiratory capacity and functional residual capacity.

Pulmonary Function Tests

• Used to assess lung volumes and capacities.

• Commonly measured with a spirometer.

Mechanics of Breathing

• Air flows due to pressure gradients: $\text{Flow} \propto \Delta P / R$

• Inspiration occurs when alveolar pressure decreases below atmospheric pressure.

• Expiration occurs when alveolar pressure increases above atmospheric pressure.

• Expiration can be passive (relaxation of muscles) or active (muscle contraction).

Pressure Changes During Breathing

• During quiet breathing, pressure changes in the alveoli and pleural cavity drive airflow.

• Subatmospheric pressure in the pleural cavity keeps the lungs inflated.

Lung Compliance and Elastance

• Compliance: Ability of the lung to stretch.

  • High compliance: Stretches easily.

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

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

Surfactant and the Law of LaPlace

• Law of LaPlace: $P = 2T/r$ (P = pressure, T = surface tension, r = radius)

• Surfactant reduces surface tension, making it easier to inflate smaller alveoli.

• Premature infants may lack adequate surfactant, leading to newborn respiratory distress syndrome (NRDS).

Factors Affecting Airway Resistance

Factor	Affected by	Mediated by
Length of system	Constant, not a factor	—
Viscosity of air	Humidity, altitude	—
Diameter of airway	Physical obstruction, smooth muscle tone	Mucus, parasympathetic/sympathetic input
Bronchoconstriction	Parasympathetic input	Muscarinic receptors
Bronchodilation	Sympathetic input	β2-receptors, epinephrine

Airway Diameter and Resistance

• Wider airways have less resistance: $R \propto L / r^4$

• Bronchoconstriction (parasympathetic) increases resistance.

• Bronchodilation (sympathetic, β2 receptors) decreases resistance.

Ventilation Efficiency

• Total pulmonary ventilation: Volume of air moved in and out of lungs per minute.

• Formula: $\text{Total pulmonary ventilation} = \text{ventilation rate} \times V_T$

• Anatomic dead space: Volume of airways not involved in gas exchange.

• Alveolar ventilation: $\text{Alveolar ventilation} = \text{ventilation rate} \times (V_T - V_D)$

• Maximum voluntary ventilation: Maximum amount of air that can be moved in and out of the lungs per minute.

Normal Ventilation Values

Parameter	Normal Value
Total pulmonary ventilation	6 L/min
Total alveolar ventilation	4.2 L/min
Maximum voluntary ventilation	125–170 L/min
Respiration rate	12–20 breaths/min

Alveolar Gas Composition

• Oxygen entering alveoli ≈ oxygen entering the blood.

• Fresh air entering the lungs is about 10% of total lung volume at the end of inspiration.

• Ventilation and alveolar blood flow are matched for efficient gas exchange.

Local Control of Ventilation and Perfusion

• Local mechanisms adjust bronchiole and arteriole diameter to match airflow (ventilation) and blood flow (perfusion).

• CO2 and O2 levels regulate constriction and dilation of airways and blood vessels.

Gas Composition	Bronchioles	Pulmonary Arteries	Systemic Arteries
PCO2 increases	Dilate	Constrict	Dilate
PCO2 decreases	Constrict	Dilate	Constrict
PO2 increases	—	Dilate	Constrict
PO2 decreases	—	Constrict	Dilate

Key Terms

• Cellular respiration

• External respiration

• Ventilation, inspiration, expiration

• Conducting system, airways, alveolus

• Upper/lower respiratory tract

• Diaphragm, intercostal muscles, pleural sacs

• Dalton's law, Boyle's law

• Tidal volume (VT), inspiratory reserve volume (IRV), expiratory reserve volume (ERV), residual volume (RV)

• Minute volume, anatomic dead space, alveolar ventilation