Notes on the Respiratory System from Dr. Jesse Winters

Respiratory System Overview

• Dr. Jesse Winters, Bio 109, Fall 2025

General Characteristics of the Respiratory System

• Functions:

  • The respiratory system comprises passages that filter, moisten, and warm incoming air, transporting it into the body and the lungs.

  • Respiration: The entire process of exchanging gases between the atmosphere and body cells.

  • Ventilation (Breathing): The act of moving air in and out of the lungs.

  • External Respiration: Gaseous exchange between the air in the lungs and the blood.

  • Transport of Gases by Blood: Movement of oxygen and carbon dioxide within the blood.

  • Internal Respiration: Gaseous exchange between blood and body cells.

    • Oxygen use and carbon dioxide production occur during cellular respiration within cells.

Organs of the Respiratory System

• Divided into Two Groups:

  • Upper Respiratory Tract:

  • Components include the nose, nasal cavity, sinuses, pharynx, and larynx.

  • Lower Respiratory Tract:

  • Components consist of the trachea, bronchial tree, and lungs.

Nose and Nasal Cavity

• Nostrils (External Nares): Openings for air entry and exit in the nasal cavity.

• Internal Hairs: Function to prevent large particles from entering the nasal cavity.

• Nasal Cavity: Hollow space behind the nose, divided by the nasal septum, which directs air to the nasopharynx.

• Nasal Conchae (Turbinate Bones): Curl out from the lateral walls of the nasal cavity forming passageways known as:

  • Superior Meatus

  • Middle Meatus

  • Inferior Meatus

  • Function: Support the mucous membrane lining and increase surface area for air filtration.

• Mucous Membrane: Comprised of pseudostratified ciliated epithelium abundantly containing goblet cells for mucus secretion.

  • Cilia push mucus and trapped particles towards the pharynx for swallowing, preventing lower airway entry.

Sinuses and Pharynx

• Sinuses (Paranasal Sinuses): Air-filled cavities located in skull bones (frontal, sphenoidal, ethmoid, maxillary) that open into the nasal cavity and are lined with mucous membranes.

• Pharynx: Space located posterior to nasal and oral cavities and larynx, serving as a passageway for food and air. It also aids speech sound production.

  • Regions of Pharynx:

  • Nasopharynx: Superior to the soft palate; passageway for breathing; auditory tubes open within it.

  • Oropharynx: Located posterior to the mouth; serves as a passage for food and air.

  • Laryngopharynx: Inferior to oropharynx; extends to the esophagus and larynx.

Larynx

• Description: Enlarged airway above the trachea; facilitates air passage and prevents foreign object inhalation; houses the vocal cords.

• Composition: Framework of muscles and cartilage supported by elastic tissue.

• Epiglottis: Flap-like structure that closes the trachea during swallowing to prevent food entry.

• Vocal Folds: Two pairs of horizontal folds:

  • False Vocal Cords: Upper folds; do not produce sound but assist in airway closure during swallowing.

  • True Vocal Cords: Lower folds; responsible for sound production when air passes between them.

• Glottis: Space between the true vocal cords, which closes during swallowing.

Trachea

• Also Known As: Windpipe.

• Structure: Flexible cylindrical tube extending downward, located anterior to the esophagus, bifurcating into right and left main bronchi.

• Inner Lining: Comprised of ciliated mucus membrane with goblet cells, trapping particles that are moved upward into the pharynx for swallowing.

• Cartilage: Incomplete hyaline cartilage rings present to prevent collapse of the trachea.

Bronchial Tree

• Structure:

  • Main Bronchi: Right and left main bronchi branch from the trachea.

  • Lobar Bronchi: Three on the right, two on the left; they branch from the main bronchi.

  • Segmental Bronchi: Supply lung portions known as bronchopulmonary segments.

  • Intralobular Bronchioles: Smaller branches of the segmental bronchi that enter lung lobules.

  • Terminal Bronchioles: Branch from intralobular bronchioles, ~50-80 in each lobule.

  • Respiratory Bronchioles: Short branches originating from terminal bronchioles.

  • Alveolar Ducts: Branch from each respiratory bronchiole.

  • Alveolar Sacs: Thin-walled outpouchings.

  • Alveoli: Microscopic air sacs; primary sites of gas exchange.

Lungs

• Structure: Soft, spongy organs encased by the diaphragm and thoracic cage.

• Hilum: Region on the medial surface of each lung where bronchi and large blood vessels enter.

• Pleura Layers:

  • Visceral Pleura: Adhered to the lung surface.

  • Parietal Pleura: Lines the inner thoracic cavity and diaphragm top.

  • Pleural Cavity: Contains serous fluid, providing lubrication during lung movement.

• Functional Unit: Lobule, which includes terminal bronchioles, alveolar ducts, alveolar sacs, alveoli, and associated nerves.

Breathing Mechanism

• Cycles of Breathing: An inspiration followed by an expiration constitutes a respiratory cycle.

Major Events in Inspiration

1. Impulses from the phrenic nerves to diaphragm muscles cause contraction.

2. Downward movement of the dome-shaped diaphragm expands the thoracic cavity.

3. Contraction of external intercostal muscles may further elevate the ribs.

4. Intra-alveolar pressure decreases.

5. Atmospheric pressure, being greater than intra-alveolar pressure, pushes air into respiratory tract.

6. Lungs fill with air.

Major Events in Expiration

1. Diaphragm and external intercostal muscles relax.

2. Elastic lung tissues, stretched during inspiration, recoil and increase surface tension, pulling alveolar walls inward.

3. Recoil increases intra-alveolar pressure.

4. Air is expelled from the lungs.

Respiratory Volumes and Capacities

• Lung Volume Measurement: Measured in milliliters (mL); different volumes have specific definitions and implications.

Respiratory Volumes Table

• Tidal Volume (TV):

  • Volume: 500 mL

  • Description: Volume of air displaced during one respiratory cycle.

• Inspiratory Reserve Volume (IRV):

  • Volume: 3,000 mL

  • Description: Maximum volume that can be inhaled post-resting inspiration.

• Expiratory Reserve Volume (ERV):

  • Volume: 1,100 mL

  • Description: Maximum volume exhaled after resting expiration.

• Residual Volume (RV):

  • Volume: 1,200 mL

  • Description: Air volume remaining post-maximum exhalation.

• Vital Capacity (VC):

  • Volume: 4,600 mL

  • Description: Maximum volume exhaled after a deep inhalation; calculated as VC = TV + IRV + ERV.

• Inspiratory Capacity (IC):

  • Volume: 3,500 mL

  • Description: Maximum inhaled air after exhalation of resting tidal volume; calculated as IC = TV + IRV.

• Functional Residual Capacity (FRC):

  • Volume: 2,300 mL

  • Description: Air remaining post-resting tidal volume exhalation; calculated as FRC = ERV + RV.

• Total Lung Capacity (TLC):

  • Volume: 5,800 mL

  • Description: Total air capacity of the lungs; calculated as TLC = VC + RV.

Nonrespiratory Air Movements

Control of Breathing

• Neural Control: Breathing is regulated by neuron groups located in the brainstem, referred to as respiratory areas.

  • Components of Respiratory Areas (In pons and medulla):

  • Pontine Respiratory Group

  • Medullary Respiratory Center: Comprising:

    • Ventral Respiratory Group

    • Dorsal Respiratory Group

  • Functions: Control basic breathing rhythms and adjust rate and depth of breathing as needed.

Alveolar Gas Exchange

• Alveoli Structure: Small spaces with thin walls separating them from each other. Some provide openings (alveolar pores) connecting to adjacent alveoli.

• Wall Composition: Composed of simple squamous epithelium, surrounded by a dense capillary network also composed of simple squamous epithelium; together they compose the respiratory membrane.

• Gas Diffusion Principle: Gases diffuse from areas of higher partial pressure to areas of lower partial pressure.

Gaseous Exchange Example

• Diffusion of CO₂:

  • Alveolar wall PCO₂ = 40 mm Hg

  • Alveolus PO₂ = 104 mm Hg

• Diffusion of O₂:

  • Blood returning to left atrium PCO₂ = 45 mm Hg

  • Blood leaving right ventricle PO₂ = 40 mm Hg

  • Atmospheric oxygen entering capillaries PO₂ = 104 mm Hg

Oxygen and Carbon Dioxide Transport

• Oxygen Transport:

  • 98-99% of oxygen entering alveolar capillaries binds to hemoglobin in red blood cells.

  • Hemoglobin consists of globin protein and heme groups with iron atoms, allowing oxygen binding.

  • Oxyhemoglobin: Hemoglobin with bound oxygen.

• Carbon Dioxide Transport:

  • CO₂ produced during cellular metabolism diffuses into systemic capillaries.

  • Can form bicarbonate ions (releasing H+) or bind to hemoglobin.

  • CO₂ can loosely bind to hemoglobin, forming Carbaminohemoglobin.

  • Deoxyhemoglobin serves as an excellent buffer by binding H+.

Gas Entry Reactions Table