P: Respiratory System 1

Respiratory System Overview

Introduction

The respiratory system is essential for gas exchange, supplying oxygen to the body and removing carbon dioxide. It is divided into functional zones and involves several anatomical structures and physiological processes.

Respiration in Three Steps

Overview of Respiratory Processes

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

• Gas Exchange: Occurs at two levels: external respiration (between alveoli and blood) and internal respiration (between blood and tissues).

• Cellular Respiration: Utilization of oxygen by cells and elimination of carbon dioxide as a metabolic waste product.

Key Equation:

$C_6H_{12}O_6 + 6O_2 \rightarrow 6CO_2 + 6H_2O + ATP$

• Acidosis & Toxicity: Excess CO2 can lead to acidosis and toxicity in the body.

Functional Zones of the Respiratory System

Conducting Zone

The conducting zone consists of anatomical structures that air passes through before reaching the respiratory zone. Its main functions are humidification, warming, filtration, and cleaning of inspired air.

• Structures: Nasal cavity, pharynx (nasopharynx, oropharynx, laryngopharynx), larynx, trachea, bronchi, and terminal bronchioles.

• Functions:

  • Humidifies and warms incoming air.

  • Filters out particles and pathogens.

  • Contains olfactory/scent receptors for smell.

  • Tonsils are part of the immune system and help protect against pathogens.

• Goblet Cells: Specialized cells in the conducting zone secrete mucin, forming mucus that traps small particles. Mucociliary clearance removes trapped particles from the airways.

Respiratory Zone

The respiratory zone is where gas exchange occurs. It includes structures such as respiratory bronchioles and alveolar sacs, which provide a large surface area for efficient gas exchange.

• Structures: Respiratory bronchioles, alveolar ducts, alveolar sacs.

• Cells:

  • Type I alveolar cells: Thin cells covering most of the alveolar surface, facilitating gas exchange.

  • Type II alveolar cells: Produce surfactant, which reduces surface tension and prevents alveolar collapse.

• Alveolar-Capillary Membrane: Extremely thin, allowing rapid diffusion of gases between alveolar air and blood.

Structure of the Respiratory System: Conducting Zone Details

Anatomical Features

• Nasal Cavity: Entry point for air, lined with mucosa for filtration and humidification.

• Pharynx: Divided into nasopharynx, oropharynx, and laryngopharynx; directs air and food appropriately.

• Larynx: Contains vocal cords; involved in sound production and airway protection.

• Trachea: Main airway leading to bronchi; lined with ciliated epithelium and goblet cells.

• Bronchi & Bronchioles: Branching tubes that conduct air to the respiratory zone.

Protective Mechanisms

Mucociliary Clearance

• Mucus: Traps dust, pathogens, and other particles.

• Cilia: Move mucus upward toward the pharynx for removal.

Special Procedures and Airway Management

Tracheostomy

• Definition: Surgical procedure creating an opening in the trachea to bypass the upper airway.

• Indications: Airway obstruction, need for prolonged ventilation.

• Complications: Inflated cuff secures airway but may prevent speech and increase risk of aspiration.

Respiratory Zone: Gas Exchange

Alveolar Structure and Function

• Alveoli: Tiny air sacs where gas exchange occurs; grouped in alveolar sacs.

• Surface Area: Large surface area maximizes gas exchange efficiency.

• Surfactant: Produced by Type II cells, prevents alveolar collapse.

Pleural Membranes and Lung Inflation

Pleural Structure

• Visceral Pleura: Covers the lungs.

• Parietal Pleura: Lines the thoracic wall.

• Pleural Cavity: Space between pleurae filled with fluid to reduce friction.

• Transpulmonary Pressure: Normally negative pressure prevents lung collapse.

Mechanics of Breathing

Boyle's Law and Pressure Changes

Breathing is driven by changes in pressure and volume within the thoracic cavity, described by Boyle's Law.

$P_1V_1 = P_2V_2$

• Inspiration: Diaphragm contracts downward, external intercostals contract, increasing thoracic volume and decreasing pressure, drawing air in.

• Forced Inspiration: Accessory muscles (pectoralis minor, sternocleidomastoid) assist in increasing thoracic volume.

• Expiration: Normally passive; muscles relax, thoracic volume decreases, pressure increases, air is expelled.

• Forced Expiration: Internal intercostals and abdominal muscles contract to further increase pressure and expel air.

Lung Volumes and Capacities

Definitions and Calculations

• Tidal Volume (TV): Amount of air inhaled or exhaled during quiet breathing.

• Inspiratory Reserve Volume (IRV): Extra air inhaled after a normal inspiration.

• Expiratory Reserve Volume (ERV): Extra air exhaled after a normal expiration.

• Residual Volume (RV): Air remaining in lungs after forced expiration.

• Dead Space: Air in conducting zone not involved in gas exchange (anatomical dead space ~150 mL).

Lung Capacities

• Inspiratory Capacity (IC): $IC = TV + IRV$

• Functional Residual Capacity (FRC): $FRC = ERV + RV$

• Vital Capacity (VC): $VC = IRV + TV + ERV$

• Total Lung Capacity (TLC): $TLC = VC + RV$

Fresh Air Reaching Alveoli

Percentage of fresh air reaching alveoli can be calculated as:

$\text{Fresh Air Percentage} = \frac{\text{Tidal Volume} - \text{Dead Space}}{\text{Tidal Volume}} \times 100\%$

• Example: If tidal volume is 500 mL and dead space is 150 mL, fresh air percentage is $\frac{500 - 150}{500} \times 100\% = 70\%$.

• At higher altitudes, tidal volume may increase (e.g., 700 mL), so $\frac{700 - 150}{700} \times 100\% = 78\%$.

Summary Table: Lung Volumes and Capacities

Volume/Capacity	Definition	Formula
Tidal Volume (TV)	Air in/out during quiet breathing	-
Inspiratory Reserve Volume (IRV)	Extra air inhaled after normal inspiration	-
Expiratory Reserve Volume (ERV)	Extra air exhaled after normal expiration	-
Residual Volume (RV)	Air remaining after forced expiration	-
Inspiratory Capacity (IC)	Max air inhaled after normal expiration	$IC = TV + IRV$
Functional Residual Capacity (FRC)	Air remaining after normal expiration	$FRC = ERV + RV$
Vital Capacity (VC)	Max air exhaled after max inhalation	$VC = IRV + TV + ERV$
Total Lung Capacity (TLC)	Total air in lungs after max inspiration	$TLC = VC + RV$

Additional info: Some details about the pharynx, tonsils, and tracheostomy were inferred for completeness. The equations and table were expanded for academic clarity.