Respiratory System Overview

Respiratory System Overview

• Respiration: The process of gas exchange involved in supplying oxygen (O2) to and removing carbon dioxide (CO2) from cells.
  • Internal Respiration: Metabolic processes in mitochondria where O2 and nutrients produce ATP, generating CO2.
  • External Respiration: Exchange of O2 and CO2 between the environment and cells through ventilation (breathing).
  • Gas exchange occurs in the pulmonary capillaries via diffusion.
  • O2 leaves and CO2 enters blood at systemic capillaries by diffusion.

Alveoli

• Structure: Thin-walled, surrounded by pulmonary capillaries.
• Gas Exchange:
  • Occurs through diffusion over short distances.
  • Types of Alveolar Cells:
  • Type I Cells: Single layer of flattened cells facilitating gas exchange.
  • Type II Cells: Secrete surfactant, reducing surface tension.
  • Alveolar Macrophages: Immune cells that help clear pathogens and debris.

Respiratory Pressure Gradients

• Air Movement: Air moves from areas of higher pressure to areas of lower pressure.
  • Pressure Definitions:
  • Atmospheric Pressure: 760 mm Hg at sea level.
  • Intra-alveolar Pressure: Rapidly equilibrates with atmospheric pressure; important for airflow.
  • Intrapleural Pressure: Averages 4 mm Hg less than atmospheric pressure, vital for lung inflation.

Respiratory Mechanics

• Lungs: Normally partially stretched due to the cohesiveness of intrapleural fluid and transmural pressure, pushing the lungs outward and thoracic wall inward.
• Pneumothorax: Occurs when pleural cavity is punctured, leading to lung collapse due to pressure equalization (760 mm Hg in intrapleural space).

Air Flow and Boyles Law

• Air Movement: Described by pressure gradients; air enters the lungs when intra-alveolar pressure is less than atmospheric pressure and vice versa.
• Boyle's Law: Pressure inversely related to volume, critical for understanding lung mechanics.

Respiratory Muscles

• Inspiration: Active process involving contraction of respiratory muscles (e.g., diaphragm accounts for 75% of volume increase).
• Expiration: Typically a passive process due to muscle relaxation; forced expiration involves abdominal and internal intercostal muscles.

Changes in Intra-Alveolar Pressure

• Inspiration: Involves contraction of muscles, decreasing intrapleural pressure from 756 to 754 mm Hg, and intra-alveolar pressure from 760 to 759 mm Hg.
• Expiration: Generally passive; forced expiration is active.

Air Flow Variables

• Air flow follows similar principles to blood flow, described by the equation $F = \frac{\Delta P}{R}$, where R is resistance determined by airway radius.
• Nervous System Influence: Autonomic responses regulate bronchiole resistance through bronchoconstriction and bronchodilation, impacting conditions like COPD.

Pulmonary Elasticity

• Elastic Recoil: Refers to the lungs' ability to return to resting state after stretching.
• Compliance: Represents how easily the lungs stretch; more compliant lungs require less effort to inflate.
• Factors Influencing Elasticity and Compliance:
  • Elastin in connective tissue, alveolar surface tension due to fluid lining.

Forces Preventing Collapse of Alveoli

• Forces preventing collapse:
  • Transmural pressure gradient;
  • Pulmonary surfactant;
  • Alveolar interdependence.

Surfactant Role

• Pulmonary Surfactant: Mixture secreted by Type II cells, lowering surface tension, promoting lung compliance, and preventing collapse.
• Newborn Respiratory Distress Syndrome: Occurs due to lack of surfactant.

Measuring Lung Volumes

• Key Lung Volumes:
  • Tidal Volume (TV): 500 mL - amount per breath.
  • Inspiratory Reserve Volume (IRV): 3000 mL - extra air that can be inhaled.
  • Expiratory Reserve Volume (ERV): 1000 mL - extra air that can be exhaled.
  • Residual Volume (RV): 1200 mL - air remaining after max expiration.
  • Vital Capacity (VC): 4500 mL - total air exhaled after max inhalation.
  • Total Lung Capacity (TLC): 5700 mL - includes VC and RV.

Ventilation Concepts

• Pulmonary Ventilation: Calculated as $TV \times \text{respiratory rate}$; shows total air movement per minute.
• Alveolar Ventilation: More significant for assessing gas exchange, calculated as $(TV - \text{dead space}) \times \text{respiration rate}$.

Regulation of Air Flow

• Local controls affect smooth muscles to optimize air and blood flow efficiency, influenced by CO2 and O2 levels.
  • Increased CO2 results in bronchodilation; decreased O2 results in vasoconstriction.

Gas Exchange Principles

• Gas exchange at capillaries driven by partial pressure gradients; O2 diffuses from alveoli to capillaries, and CO2 from capillaries to alveoli.
• Diffusion Factors: Must consider surface area, distance, and molecular properties (e.g., CO2 diffuses faster due to its higher solubility).

Oxygen Transport and Hemoglobin

• O2 transport through the circulatory system occurs in two forms: dissolved (1.5%) and chemically bound to hemoglobin (98.5%).
• Hemoglobin's O2 binding is influenced heavily by partial pressure; % saturation of hemoglobin changes with varying PO2 levels.

Carbon Dioxide Transport

• CO2 is transported in three forms: dissolved in plasma (10%), bound to hemoglobin (30%), and as bicarbonate ions in plasma (60%).
• Haldane effect: facilitates O2 unloading and CO2 loading at tissue levels.

Regulation of Respiration

• Controlled by the brain’s medullary respiratory center, which responds to chemical signals, primarily PCO2 but also O2 and H+ ion concentrations.