Respiratory System: Structure, Function, and Physiology Study Guide

Respiratory System Overview

General Organization and Function

The respiratory system is responsible for gas exchange, supplying oxygen to the body and removing carbon dioxide. It consists of a series of organs and passageways that conduct air to the lungs and facilitate respiration.

• Major Divisions: Upper respiratory tract (nose, nasal cavity, pharynx, larynx) and lower respiratory tract (trachea, bronchi, bronchioles, alveoli).

• Primary Functions: Air conduction, gas exchange, protection from pathogens, vocalization.

• Respiratory Passages: Air flows from the nasal cavity → pharynx → larynx → trachea → bronchi → bronchioles → alveoli.

Structural Components of the Respiratory System

Conducting Zone vs. Respiratory Zone

The respiratory system is divided into the conducting zone and the respiratory zone, each with distinct functions.

• Conducting Zone: Includes nasal cavity, pharynx, larynx, trachea, bronchi, and bronchioles. Functions to filter, warm, and moisten air and conduct it to the lungs.

• Respiratory Zone: Includes respiratory bronchioles, alveolar ducts, and alveoli. Site of gas exchange between air and blood.

Nasal Cavity and Nasal Conchae

The nasal cavity is lined with mucosa and contains nasal conchae (turbinates) that increase surface area and enhance air filtration and humidification.

• Epithelium Types: Pseudostratified ciliated columnar epithelium with goblet cells.

• Function: Warms, moistens, and filters inspired air; traps particles in mucus.

Pharynx and Larynx

The pharynx is a muscular tube that connects the nasal cavity to the larynx and esophagus. The larynx contains vocal cords and is involved in sound production and airway protection.

• Pharynx Epithelium: Varies by region; stratified squamous epithelium in the oropharynx and laryngopharynx.

• Larynx: Contains cartilage (thyroid, cricoid, arytenoid) and vocal folds (true and false vocal cords).

Trachea and Bronchi

The trachea is a tube supported by C-shaped rings of hyaline cartilage, which prevent collapse. It branches into the right and left primary bronchi, which further divide into secondary and tertiary bronchi.

• Epithelium: Pseudostratified ciliated columnar epithelium with goblet cells.

• Bronchial Tree: Branching system of airways; bronchi → bronchioles → terminal bronchioles → respiratory bronchioles → alveolar ducts → alveoli.

Bronchioles and Alveoli

Bronchioles are small airways lacking cartilage, leading to alveolar ducts and alveoli, the primary sites of gas exchange.

• Epithelium: Simple cuboidal in bronchioles; simple squamous in alveoli.

• Alveoli: Thin-walled sacs surrounded by capillaries; site of oxygen and carbon dioxide exchange.

Respiratory Mucosa and Defense Mechanisms

Respiratory Mucosa

The respiratory mucosa lines most of the respiratory tract and consists of ciliated epithelial cells and mucus-producing goblet cells.

• Function: Traps and removes particles and pathogens; cilia move mucus toward the pharynx for swallowing.

• Olfactory Mucosa: Located in the superior nasal cavity; contains olfactory receptors for smell.

Pulmonary Lobules and Pleura

Pulmonary Lobules

Pulmonary lobules are the smallest subdivisions of the lung visible to the naked eye, each supplied by a bronchiole and surrounded by connective tissue.

• Function: Compartmentalize lung tissue for efficient gas exchange.

Pleura

The pleura are serous membranes that surround each lung and line the thoracic cavity.

• Parietal Pleura: Lines the thoracic wall and diaphragm.

• Visceral Pleura: Covers the surface of the lungs.

• Pleural Cavity: Space between pleurae containing pleural fluid, which reduces friction during breathing.

Mechanics of Breathing

Phases of Pulmonary Ventilation

Pulmonary ventilation consists of two phases: inhalation and exhalation, driven by pressure changes in the thoracic cavity.

• Inhalation: Occurs when alveolar pressure ($P_{alv}$) falls below atmospheric pressure ($P_{atm}$).

• Exhalation: Occurs when alveolar pressure ($P_{alv}$) rises above atmospheric pressure ($P_{atm}$).

Pressures Involved in Breathing

Several pressures are important in the mechanics of breathing:

• Atmospheric Pressure ($P_{atm}$): Pressure of air outside the body.

• Intrapulmonary Pressure ($P_{alv}$): Pressure within the alveoli.

• Intrapleural Pressure ($P_{ip}$): Pressure within the pleural cavity; normally lower than $P_{alv}$.

• Transpulmonary Pressure ($P_{tp}$): Difference between $P_{alv}$ and $P_{ip}$; keeps lungs inflated.

Key Equations:

• $P_{tp} = P_{alv} - P_{ip}$

Muscles of Respiration

Breathing involves several muscles:

• Primary Muscles of Inhalation: Diaphragm and external intercostals.

• Accessory Muscles of Inhalation: Sternocleidomastoid, scalenes, pectoralis minor.

• Muscles of Exhalation: Internal intercostals, abdominal muscles (active exhalation).

Alveolar Structure and Gas Exchange

Alveolar Cells and Surfactant

Alveoli contain two main cell types:

• Type I Alveolar Cells: Simple squamous cells for gas exchange.

• Type II Alveolar Cells: Secrete surfactant, which reduces surface tension and prevents alveolar collapse.

Gas Exchange Mechanism

Gas exchange occurs by diffusion across the respiratory membrane, which consists of alveolar and capillary walls.

• Factors Affecting Gas Exchange: Thickness of membrane, surface area, partial pressure gradients.

Airway Resistance and Bronchoconstriction

Airway Resistance

Airway resistance is affected by the diameter of airways, smooth muscle tone, and presence of mucus or inflammation.

• Bronchodilation: Relaxation of smooth muscle increases airway diameter and decreases resistance.

• Bronchoconstriction: Contraction of smooth muscle decreases airway diameter and increases resistance.

Lung Volumes and Capacities

Definitions and Measurements

Lung volumes and capacities are important for assessing respiratory function.

• Tidal Volume (TV): Volume of air inhaled or exhaled in a normal breath (~500 mL).

• Vital Capacity (VC): Maximum amount of air exhaled after a maximal inhalation.

• Total Lung Capacity (TLC): Total volume of air in the lungs after maximal inspiration.

• Residual Volume (RV): Air remaining in lungs after maximal exhalation.

Key Equation:

• $VC = TV + IRV + ERV$

IRV: Inspiratory Reserve Volume; ERV: Expiratory Reserve Volume

Summary Table: Respiratory System Structures and Functions

Structure	Epithelium Type	Main Function
Nasal cavity	Pseudostratified ciliated columnar	Filters, warms, moistens air
Pharynx	Stratified squamous	Conducts air and food
Larynx	Pseudostratified ciliated columnar	Sound production, airway protection
Trachea	Pseudostratified ciliated columnar	Conducts air, prevents collapse
Bronchi	Pseudostratified ciliated columnar	Conducts air, branches to lungs
Bronchioles	Simple cuboidal	Conducts air, regulates airflow
Alveoli	Simple squamous	Gas exchange

Additional info:

• Some context and definitions were inferred to provide a complete, self-contained study guide.

• Equations and table entries were expanded for clarity and academic completeness.