Chapter 17: Mechanics of Breathing and Pulmonary Ventilation: Study Notes

Mechanics of Breathing and Pulmonary Ventilation

Respiratory System Functions

The respiratory system is essential for maintaining homeostasis and supporting cellular metabolism. Its functions extend beyond gas exchange to include regulation, protection, and physiological support.

• Exchange of gases: Oxygen is absorbed from the atmosphere into the blood, and carbon dioxide is expelled.

• Acid-base balance: The respiratory system helps regulate blood pH by controlling CO2 levels.

• Vocalization: Air movement through the larynx enables speech.

• Defense: Airways filter pathogens and foreign particles.

• Water and heat loss: The respiratory tract provides a route for loss of water vapor and heat.

• Venous return: The respiratory pump enhances blood return to the heart.

Overview of Respiration

Respiration involves both internal and external processes to supply oxygen and remove carbon dioxide.

• Internal respiration: Cellular processes such as oxidative phosphorylation use oxygen to produce ATP.

• External respiration: Includes four main steps:

  1. Pulmonary ventilation (breathing)

  2. Exchange of gases between lungs and blood

  3. Transport of gases in blood

  4. Exchange of gases between blood and body tissues

The Respiratory Tract

The respiratory tract consists of a series of airways that conduct air from the external environment to the lungs and facilitate gas exchange.

• Conducting zone: Includes the nose, pharynx, larynx, trachea, bronchi, and bronchioles. Functions as an air passageway, warms and humidifies air, and filters particles.

• Respiratory zone: Includes respiratory bronchioles, alveolar ducts, alveoli, and alveolar sacs. Site of gas exchange via diffusion.

Functions of the Conducting Zone

• Air passageway (dead space: ~150 mL)

• Increases air temperature to body temperature

• Humidifies air

• Epithelium contains:

  • Goblet cells: Secrete mucus

  • Ciliated cells: Move particles toward mouth (mucus escalator)

Anatomical Features of the Conducting and Respiratory Zones

Structure	Inner Diameter (mm)	Cilia	Goblet Cells	Cartilage	Smooth Muscle
Larynx	15-20	+	+	++ (C-shaped)	+
Trachea	12-18	+	+	++ (C-shaped)	+
Primary Bronchi	12-16	+	+	++	+
Secondary Bronchi	5-10	+	+	+	++
Tertiary Bronchi	2-5	+	+	+	++
Smaller Bronchi	1-2	+	+	+	++
Bronchioles	<1	+	+	-	+++
Terminal Bronchioles	<0.5	+	-	-	+++
Respiratory Bronchioles	<0.5	-	-	-	+++
Alveolar Sacs	<0.5	-	-	-	-

Epithelium and Mucus Secretion

• Submucosal glands and epithelial cells secrete saline and mucus.

• Cilia move mucus toward the pharynx, removing trapped pathogens and particles.

• Saline secretion involves ion transport:

  • NKCC brings Cl- into epithelial cells from ECF.

  • Apical anion channels (including CFTR) allow Cl- to enter the lumen.

  • Na+ follows Cl- into the lumen, creating a concentration gradient for water to follow.

Function and Structure of the Respiratory Zone

The respiratory zone is specialized for efficient gas exchange between air and blood.

• Structures:

  • Respiratory bronchioles

  • Alveolar ducts

  • Alveoli

  • Alveolar sacs

• Respiratory membrane: Composed of the epithelial cell layer of alveoli and the endothelial cell layer of capillaries.

Alveoli

• Site of gas exchange; ~300 million alveoli provide a large surface area.

• Rich capillary network forms a sheet over alveoli.

• Alveolar pores allow air flow between alveoli.

• Type I alveolar cells: Form the wall of alveoli.

• Type II alveolar cells: Secrete surfactant, reducing surface tension.

• Alveolar macrophages: Ingest foreign particles and pathogens.

Structures of the Thoracic Cavity

The thoracic cavity houses and protects the lungs, and its structure is essential for breathing mechanics.

• Chest wall: Includes rib cage, sternum, thoracic vertebrae, and muscles (internal/external intercostals, diaphragm).

• Pleura: Double-layered membrane lining lungs and chest wall; pleural sac surrounds each lung.

• Intrapleural space: Contains intrapleural fluid (20-25 mL), reducing friction and aiding lung expansion.

Forces for Pulmonary Ventilation

Air movement in and out of the lungs is driven by pressure gradients created by changes in lung volume.

• Bulk flow: Air moves from regions of high to low pressure.

• Inspiration: Pressure in lungs is less than atmospheric pressure.

• Expiration: Pressure in lungs is greater than atmospheric pressure.

Pulmonary Pressures

• Atmospheric pressure (Patm): 760 mm Hg at sea level; used as reference (set Patm = 0 mm Hg).

• Intra-alveolar pressure (Palv): Pressure of air in alveoli; varies with respiration phase.

• Intrapleural pressure (Pip): Pressure inside pleural sac; always negative under normal conditions, less than Palv.

Equation for Air Flow

Air flow is determined by the pressure gradient and airway resistance:

$\text{Flow} = \frac{P_{atm} - P_{alv}}{R}$

Boyle's Law

Boyle's Law describes the inverse relationship between pressure and volume for gases:

$P_1 V_1 = P_2 V_2$

• As lung volume increases, alveolar pressure decreases, allowing air to flow in.

• As lung volume decreases, alveolar pressure increases, driving air out.

Determinants of Intra-Alveolar Pressure

• Lung expansion: Increases alveolar volume, decreases Palv, air flows in.

• Lung recoil: Decreases alveolar volume, increases Palv, air flows out.

Factors Affecting Pulmonary Ventilation

• Lung compliance: Ease with which lungs can be stretched.

• Airway resistance: Resistance to airflow, influenced by airway diameter and mucus.

Lung Compliance

• High compliance: Easier to inspire.

• Low compliance: Requires more force from inspiratory muscles.

• Elastance: Elastic recoil; low elastance means high compliance.

• Surface tension: Thin fluid layer in alveoli creates tension; surfactant reduces surface tension, increasing compliance.

Airway Resistance

• Smaller airways increase in number, keeping overall resistance low.

• Increased resistance makes breathing harder; caused by smooth muscle contraction and mucus secretion.

• Bronchoconstriction: Narrowing of airways.

• Bronchodilation: Widening of airways.

• Extrinsic control:

  • Parasympathetic nervous system: Bronchoconstriction.

  • Epinephrine (hormonal): Bronchodilation.

• Intrinsic control:

  • CO2: Bronchodilation.

  • Histamine: Bronchoconstriction and increased mucus (asthma, allergies).

Respiratory Volumes and Air Flows

• Lung volumes and capacities: Measured by spirometry.

• Pulmonary function tests: Assess lung health and diagnose diseases.

• Alveolar ventilation: Volume of fresh air reaching gas exchange areas per minute.

Pulmonary Function Tests

• Obstructive pulmonary diseases: Increased airway resistance, increased residual volume, decreased vital capacity (e.g., COPD, asthma).

• Restrictive pulmonary diseases: Difficulty expanding lungs, decreased compliance, decreased total lung and vital capacity (e.g., pulmonary fibrosis, asbestos exposure).

Ventilation Equations

• Minute ventilation: $\text{Minute ventilation} = V_T \times RR$ Where $V_T$ = tidal volume, $RR$ = respiration rate.

• Normal values: $V_T = 500$ mL, $RR = 12$ breaths/min, $\text{Minute ventilation} = 6000$ mL/min.

• Anatomical dead space: Conducting zone air (~150 mL) does not participate in gas exchange.

• Alveolar ventilation: $\text{Alveolar ventilation} = (V_T \times RR) - (DSV \times RR)$ Where DSV = dead space volume. Example: $(500 \text{ mL/breath} \times 12 \text{ breaths/min}) - (150 \text{ mL/breath} \times 12 \text{ breaths/min}) = 4200 \text{ mL/min}$

Summary Table: Pulmonary Volumes and Capacities

Volume/Capacity	Male (mL)	Female (mL)
Tidal Volume (TV)	500	500
Inspiratory Reserve Volume (IRV)	3000	1900
Expiratory Reserve Volume (ERV)	1000	700
Residual Volume (RV)	1200	1100
Vital Capacity (VC)	4600	3100
Total Lung Capacity (TLC)	5800	4200

Example: Spirometry

• Spirometry: Measures lung volumes and capacities by recording air movement during breathing cycles.

• Used to diagnose obstructive and restrictive lung diseases.

Additional info:

• Surfactant is critical for preventing alveolar collapse, especially in premature infants.

• Pneumothorax occurs when air enters the pleural cavity, causing lung collapse.