Respiratory Physiology and Spirometry

Respiratory Physiology and Spirometry

Introduction

• Course: BIO 2061: Human Anatomy & Physiology II Lab

• Instructor: Dr. Eddie Hernandez

Pulmonary Ventilation

• Definition: Pulmonary ventilation consists of two phases:

  • Inspiration: Gases flow into the lungs.

  • Expiration: Gases exit the lungs.

• Nature of the Process:

  • Mechanical process dependent on volume changes in the thoracic cavity.

  • Volume changes lead to pressure changes, which subsequently result in the flow of gases to equalize pressure.

Boyle’s Law

• Definition: Boyle's Law describes the relationship between pressure and volume of gases.

  • Mathematical expression: If volume increases, pressure decreases and vice versa, or:
    $P imes V = k$ (where k is a constant).

  • Inverse relationship implies:

  • If volume increases ($V<em>{increase}$), then pressure decreases ($P</em>{decrease}$).

  • If volume decreases ($V<em>{decrease}$), then pressure increases ($P</em>{increase}$).

Mechanics of Respiration: Lung Pressures

1. Atmospheric Pressure (Patm):

   • Standard pressure exerted by air surrounding the body at sea level, measured as 760 mmHg.

2. Intrapulmonary Pressure (Ppul):

   • Pressure within the alveoli.

3. Intrapleural Pressure (Pip):

   • Pressure within the pleural cavity, typically about 4 mmHg less than intrapulmonary pressure.

4. Transpulmonary Pressure (Ptp):

   • The pressure difference between intrapleural and intrapulmonary pressures, calculated as:
     $P<em>{tp} = P</em>{pul} - P_{ip} = 4 ext{ mmHg}$.

Mechanics of Inspiration

• During inspiration:

  • The diaphragm and intercostal muscles contract.

  • Thoracic cavity volume increases.

  • Resulting decrease in thoracic pressure causes air to flow into the lungs.

Mechanics of Expiration

• During expiration:

  • The diaphragm and intercostal muscles relax.

  • Thoracic cavity volume decreases.

  • Resulting increase in thoracic pressure causes air to flow out of the lungs.

Clinical Assessment of Ventilation

• Several respiratory volumes can be utilized to assess respiratory status:

  • Respiratory volumes can be combined to calculate respiratory capacities, providing insight into a person’s respiratory status.

  • Spirometer: A clinical tool used to measure patient’s respiratory volumes.

Spirometry: Lung Volumes

1. Tidal Volume (TV):

   • The amount of air moved into and out of the lungs during quiet respiration (approximately 500 ml).

2. Inspiratory Reserve Volume (IRV):

   • The amount of air that can be forcibly inhaled beyond the tidal volume, ranging between 2100–3200 ml.

3. Expiratory Reserve Volume (ERV):

   • The amount of air that can be forcibly exhaled from the lungs, typically measured at 1000–1200 ml.

4. Residual Volume (RV):

   • The amount of air that always remains in the lungs (~1200 ml), crucial for keeping alveoli inflated.

Spirometry: Lung Capacities

• Lung capacities are combinations of two or more respiratory volumes:

1. Inspiratory Capacity (IC):

   • Calculated as: $IC = TV + IRV$.

2. Functional Residual Capacity (FRC):

   • Calculated as: $FRC = RV + ERV$.

3. Vital Capacity (VC):

   • Calculated as: $VC = TV + IRV + ERV$.

4. Total Lung Capacity (TLC):

   • Calculated as: $TLC = TV + IRV + ERV + RV$ (sum of all lung volumes).

Clinical Assessment: Pulmonary Function Tests

• Spirometry can help distinguish between:

1. Obstructive Pulmonary Disease:

   • Characterized by increased airway resistance (e.g., bronchitis).

   • In conditions of hyperinflation, TLC, FRC, and RV may increase.

2. Restrictive Pulmonary Disease:

   • Characterized by reduced TLC due to diseases such as tuberculosis or exposure to environmental agents like fibrosis.

   • In these cases, VC, TLC, FRC, and RV decline because lung expansion is compromised.

Gas Exchange

• Definition: Involves the exchange of O2 and CO2 across respiratory membranes.

• Influencing Factors:

  • Partial pressure gradients and gas solubilities.

  • Thickness and surface area of the respiratory membrane.

  • Ventilation-perfusion coupling: The matching of alveolar ventilation with pulmonary blood perfusion.

Partial Pressure Gradients

• A steep partial pressure gradient for O2 exists between blood and lungs:

  • Venous blood PO2 = 40 mm Hg.

  • Alveolar PO2 = 104 mm Hg.

• The gradient for CO2 is less steep:

  • Venous blood PCO2 = 45 mm Hg.

  • Alveolar PCO2 = 40 mm Hg.

Ventilation-Perfusion Coupling

• Perfusion: Refers to the blood flow reaching alveoli;

  • Controlled by PO2 that adjusts arteriolar diameter.

• Ventilation: Amount of gas reaching alveoli;

  • Controlled by PCO2 that adjusts bronchiolar diameter.

Oxygen Transport

• Transport Methods:

  1. 2% of molecular O2 is dissolved in plasma.

  2. 98% is bound to hemoglobin (Hb) in red blood cells (RBCs):

  • Each Hb molecule comprises four polypeptide chains, each with an iron-containing heme group.

  • Each Hb can transport up to four oxygen molecules.

Carbon Dioxide Transport

• Transport Forms:

  1. 7 to 10% of CO2 is dissolved in plasma as PCO2.

  2. 20% of CO2 is bound to the globin part of hemoglobin, known as carbaminohemoglobin.

  3. 70% is transported as bicarbonate ions (HCO3–) in plasma:

  • The formation of bicarbonate involves CO2 combining with water to form carbonic acid (H2CO3), which quickly dissociates into bicarbonate and H+.

  • This reaction is catalyzed by the enzyme carbonic anhydrase.