The Respiratory System: Structure and Function

The Respiratory System

Primary Functions of the Respiratory System

The respiratory system is essential for gas exchange, regulation of blood pH, vocalization, olfaction, and protection of delicate tissues in the lungs.

• Gas exchange: Brings oxygen (O2) into the body and removes carbon dioxide (CO2).

• Regulation of blood pH: Helps control the acidity of blood by managing CO2 levels.

• Voice production: Air passing through the vocal cords allows speech and other sounds.

• Olfaction (smelling): Air carries odor molecules to the olfactory receptors in the nose.

• Protection: Filters, warms, and humidifies incoming air to protect delicate tissues in the lungs.

Organization of the Respiratory Tract

The respiratory system is divided into the upper and lower respiratory tracts, each with distinct structures and functions.

• Upper respiratory tract:

  • Nose

  • Nasal cavity

  • Paranasal sinuses

  • Pharynx (throat)

  These structures filter, warm, and moisten the air before it enters the lower respiratory tract.

• Lower respiratory tract:

  • Larynx (voice box)

  • Trachea (windpipe)

  • Bronchi and bronchioles

  • Lungs (contain alveoli where gas exchange occurs)

Respiratory Epithelium

The respiratory epithelium is primarily pseudostratified columnar epithelial tissue, which helps trap and move particles out of the airways.

• Contains cilia and mucus, which trap particles (like dust and microbes) and move them toward the pharynx for swallowing or expulsion.

• This process helps keep the airways clean and free of debris.

Larynx and Epiglottis

The larynx contains the epiglottis and glottis, which play a role in protecting the airway during swallowing.

• Epiglottis: Covers the opening of the larynx during swallowing, preventing food and liquids from entering the trachea.

Trachea and Bronchi

The trachea (windpipe) leads to the main/primary bronchi and contains C-shaped cartilages for structural support.

• C-shaped cartilages: Prevent collapse of the trachea and maintain an open airway.

Ventilation and Gas Exchange

Ventilation is the process of moving air into and out of the lungs, also known as breathing.

• Gas exchange occurs at the alveoli in the lungs.

Smooth Muscle in Airways

Smooth muscle lines the airways and regulates airflow by constricting or dilating the airways.

• Parasympathetic activation: Constricts the airways by making the smooth muscle contract.

• Sympathetic activation: Dilates the airways by making the smooth muscle relax.

Alveolar Ducts and Elastic Fibers

Alveolar ducts lead to alveolar sacs, which are surrounded by blood capillaries and elastic fibers.

• Blood capillaries are essential for gas exchange.

• Elastic fibers stretch and recoil:

  • Stretch most during inhalation.

  • Recoil during exhalation.

Alveolar Epithelium

The alveolar epithelium is simple squamous epithelium, which allows for efficient gas exchange.

• Thin epithelium allows O2 and CO2 to diffuse easily across the alveolar and capillary walls.

• Two main cell types:

  • Type I alveolar cells: Form the thin walls of the alveoli and are responsible for gas exchange.

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

Respiratory Membrane

The respiratory membrane must be thin for efficient gas exchange. It consists of three layers:

1. Alveolar epithelium (Type I alveolar cells)

2. Capillary endothelium (lining of blood capillaries)

3. Fused basement membrane between alveolar and capillary walls

These layers allow for efficient gas exchange between air in the alveoli and blood in the capillaries.

Pleural Membranes

The pleural membranes line the pleural cavity and surround the lungs.

• Visceral pleural membrane: Covers the surface of the lungs.

• Parietal pleural membrane: Lines the inner surface of the thoracic cavity, including the chest wall and diaphragm.

Primary Inspiratory Muscles

There are two sets of primary inspiratory muscles:

• Diaphragm

• External intercostal muscles

Diaphragm Movement During Breathing

The diaphragm contracts and moves downward during inspiration, and relaxes and moves upward during expiration.

• During inspiration: Diaphragm contracts and moves downward.

• During expiration: Diaphragm relaxes and moves upward.

Eupnea and Hypernea

Eupnea and hypernea describe normal and forced breathing, respectively.

• Eupnea: Normal, quiet breathing at rest, using only the diaphragm and external intercostal muscles.

• Hypernea: Forced breathing, which uses additional muscles such as the internal intercostals and abdominal muscles to move more air in and out of the lungs.

Key Equations

• Ventilation: $\text{Ventilation} = \text{Movement of air into and out of the lungs}$

• Gas Exchange: $\text{O}_2 \text{ (in)} \leftrightarrow \text{CO}_2 \text{ (out)}$

Summary Table: Structures of the Respiratory Tract

Region	Main Structures	Function
Upper Respiratory Tract	Nose, Nasal cavity, Paranasal sinuses, Pharynx	Filter, warm, moisten air
Lower Respiratory Tract	Larynx, Trachea, Bronchi, Lungs (Alveoli)	Conduct air, gas exchange

Summary Table: Alveolar Cell Types

Cell Type	Function
Type I alveolar cells	Form thin walls for gas exchange
Type II alveolar cells	Produce surfactant to reduce surface tension