Respiratory Physiology: Alveoli, Gas Laws, & Respiratory Cycle

Respiratory Physiology

Overview

The respiratory system is essential for gas exchange, enabling the body to obtain oxygen and eliminate carbon dioxide. This process involves both physical and biochemical mechanisms, including the movement of air, diffusion of gases, and cellular metabolism.

Respiratory System Functions

Main Functions

• Obtain O2 for body’s cells: Oxygen is required for cellular metabolism and energy production.

• Eliminate CO2 produced by body’s cells: Carbon dioxide is a waste product of metabolism and must be removed to maintain homeostasis.

Processes Involved

• Cellular Respiration: Metabolic reactions within mitochondria that use oxygen to produce energy and generate carbon dioxide as a byproduct.

• External Respiration: All processes required to move gases between the external environment and the body’s cells, including ventilation and gas exchange.

Cellular Respiration

Intracellular Processes

• Use of O2: Oxygen is utilized by mitochondria to metabolize nutrients and produce ATP (energy).

• Production of CO2: Carbon dioxide is generated as a result of cellular respiration reactions.

• Energy Derived from Nutrients: The breakdown of carbohydrates, fats, and proteins provides energy for cellular activities.

Respiratory Quotient (RQ)

• Definition: The ratio of CO2 produced to O2 consumed during metabolism.

• Formula:

$RQ = \frac{\text{CO}_2\ \text{produced}}{\text{O}_2\ \text{consumed}}$

• Example: If CO2 produced = 200 ml/min and O2 consumed = 250 ml/min, then $RQ = \frac{200}{250} = 0.8$.

External Respiration

Ventilation and Gas Exchange

• Ventilation (Breathing): The process of exchanging external air with the air in the lungs (alveoli).

• Mechanics of Breathing:

  • O2 and CO2 are exchanged between alveoli and blood by diffusion across pulmonary capillaries.

  • O2 and CO2 are transported between lungs and tissues via the bloodstream.

  • O2 and CO2 are exchanged between tissue cells and blood by diffusion across systemic capillaries.

Non-Respiratory Functions of the Respiratory System

Additional Roles

• Water loss and heat elimination: The respiratory tract helps regulate body temperature and fluid balance.

• Maintains acid-base balance: By controlling CO2 levels, the respiratory system helps maintain blood pH.

• Enables vocalizations: Air movement through the larynx allows for speech and other sounds.

• Defends against inhaled foreign matter: Mucus and cilia trap and remove particles and pathogens.

• Removes, modifies, activates, or inactivates materials passing through pulmonary circulation: The lungs can metabolize or filter substances from the blood.

• Sensory organ of smell: The nose contains olfactory receptors for detecting odors.

Physics of Respiration

Pressure Gradients and Gas Laws

• Pressure Gradient: Air moves from regions of higher pressure to lower pressure. Breathing involves alternating and reversing pressure gradients to move air in and out of the lungs.

• Boyle’s Law: The mechanical process of ventilation depends on volume changes in the thoracic cavity, which lead to pressure changes and thus airflow.

$P_1V_1 = P_2V_2$

• As volume increases, pressure decreases.

• As volume decreases, pressure increases.

Alveoli and Gas Exchange

Structure and Function

• Alveoli: Clusters of thin-walled, inflatable air sacs at the end of terminal bronchioles; primary site of gas exchange.

• Surrounded by pulmonary capillaries: Facilitates rapid diffusion of gases.

• Interstitial space: Thin barrier between alveolus and capillary network, optimizing diffusion.

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

• Type II alveolar cells: Secrete pulmonary surfactant.

Alveolar Surfactant and Surface Tension

• Surface tension: Created by water molecules at the surface of the alveolar fluid, which are more strongly attracted to each other than to air.

• Effects: Reduces lung compliance (increases work of breathing), decreases alveolus size (reduces gas exchange), and increases with area.

• Alveolar surfactant: A phospholipoprotein that decreases surface tension and promotes lung stability by counteracting the effects of surface tension.

• Mechanism: Surfactant reduces hydrogen bonding in the liquid film lining each alveolus, decreasing resistance to stretch and preventing alveolar collapse.

Pleural Sac and Pressure Gradients

Pleural Structure

• Pleural sac: Double-walled, closed sac that separates each lung from the thoracic wall and other structures.

• Pleural cavity: The interior of the sac, containing intrapleural fluid.

• Intrapleural fluid: Lubricates the pleural surfaces, allowing them to slide smoothly during respiration.

Transmural Pressure Gradient

• Definition: The difference in pressure across the lung wall (between alveolar pressure and intrapleural pressure).

• Importance: Maintains lung expansion and prevents collapse.

Respiratory Cycle

Phases of Breathing

• Before inspiration: Intra-alveolar pressure equals atmospheric pressure (760 mmHg); no air flow.

• Inspiration: Thoracic cavity enlarges, intra-alveolar pressure drops below atmospheric pressure, air flows into lungs.

• Expiration: Respiratory muscles relax, thoracic cavity volume decreases, intra-alveolar pressure rises above atmospheric pressure, air flows out of lungs.

Key Pressures

• Atmospheric pressure: 760 mmHg

• Intra-alveolar pressure: Varies with breathing phase (e.g., 759 mmHg during inspiration, 761 mmHg during expiration)

• Intrapleural pressure: Slightly less than atmospheric pressure (e.g., 756 mmHg at rest, decreases during inspiration)

Respiratory Muscles

Muscles Involved in Breathing

• Diaphragm: Primary muscle of inspiration; contraction increases thoracic volume.

• External intercostal muscles: Assist in elevating the ribs during inspiration.

• Internal intercostal muscles: Aid in forced expiration by depressing the ribs.

• Innermost intercostal muscles: Provide additional support to the thoracic wall.

Summary Table: Key Pressures in the Respiratory Cycle

Phase	Atmospheric Pressure (mmHg)	Intra-Alveolar Pressure (mmHg)	Intrapleural Pressure (mmHg)	Air Flow Direction
Before Inspiration	760	760	756	No air flow
Inspiration	760	759	754	Into lungs
Expiration	760	761	756	Out of lungs

Example Application

• Alveolar Surfactant: By decreasing surface tension, surfactant prevents alveolar collapse and promotes lung stability, making breathing easier and more efficient.

• Boyle’s Law in Breathing: During inspiration, the diaphragm contracts, increasing thoracic volume and decreasing intra-alveolar pressure, causing air to flow into the lungs.

Additional info: Some context and definitions were expanded for clarity and completeness, including the explanation of pressure gradients, the role of surfactant, and the summary table of pressures during the respiratory cycle.