Anatomy Of Respiratory System

The Human Respiratory System

The respiratory system is a complex network of organs and structures responsible for the vital process of respiration, where oxygen is exchanged with carbon dioxide in the body. It plays a crucial role in ensuring that oxygen reaches the bloodstream and is distributed to all the cells while removing carbon dioxide, a waste product of metabolism.

Components of the Respiratory System

The respiratory system is divided into two main parts based on structure and function:

1. Upper Respiratory System:

   • Nose
   • Nasal Cavity
   • Pharynx (Throat)
   • Associated Structures
      

2. Lower Respiratory System:

   • Larynx (Voice Box)
   • Trachea (Windpipe)
   • Bronchi
   • Lungs

Functional Zones

The respiratory system can also be divided based on function into two zones:

1. Conducting Zone:

   • Components: Nose, nasal cavity, pharynx, larynx, trachea, bronchi, bronchioles, terminal bronchioles.
   • Function: The conducting zone includes a series of interconnected cavities and tubes that transport air into the lungs. The primary functions here are to filter, warm, and moisten the incoming air to make it suitable for gas exchange in the lungs.

2. Respiratory Zone:

   • Components: Respiratory bronchioles, alveolar ducts, alveolar sacs, alveoli.
   • Function: This is the primary site for gas exchange. Oxygen from the inhaled air passes through the walls of the alveoli into the blood, and carbon dioxide from the blood passes into the alveoli to be exhaled.

Respiratory Mucosa

• Structure: The respiratory mucosa is a layer of pseudostratified ciliated columnar epithelial cells that secrete mucus.
• Location: It is found in the nose, sinuses, pharynx, larynx, and trachea.
• Function: The mucus traps contaminants like dust and microorganisms, while the cilia move the mucus upwards towards the mouth where it can be expelled or swallowed.

Nose and Nasal Cavity

• Nose:

  • Function: The nose serves as the entrance for air into the respiratory system. It filters large particles through hairs in the nostrils.
  • Structure: The nose has two portions:
    • External Nares (Choanae): Openings to the exterior.
    • Internal Nares: Openings that lead to the pharynx.
  • Support: The external portion is supported by bone and cartilage and lined with mucous membranes.
     

• Nasal Cavity:

  • Description: The nasal cavity is the internal part of the nose, which is divided into two portions. It is lined with a mucous membrane and contains cilia that trap and move particles.
  • Nasal Vestibule: The anterior portion just inside the nostrils, surrounded by cartilage.
  • Nasal Septum: A vertical partition that divides the nasal cavity into right and left sides. The anterior portion is primarily made up of hyaline cartilage.

Paranasal Sinuses

• Description: The paranasal sinuses are air-filled spaces located within the bones of the skull. These include the frontal, ethmoidal, sphenoidal, and maxillary sinuses.
• Function: They help add resonance to the voice and are lined with pseudostratified ciliated columnar epithelium. The sinuses communicate with the nasal cavity through ducts.

The Pharynx

The pharynx is an essential part of the human respiratory and digestive systems. It is a funnel-shaped muscular tube, about 13 cm long, that plays a crucial role in both breathing and swallowing.

Structure and Location

• The pharynx starts at the internal nares and extends to the level of the cricoid cartilage, which is the most inferior cartilage of the larynx.
• It is located posterior to the nasal and oral cavities, superior to the larynx, and anterior to the cervical vertebrae.

Function

• The pharynx serves as a common passageway for both air and food.
• It is composed of skeletal muscles and lined with a mucous membrane.
• The muscles of the pharynx are arranged in two layers: an outer circular layer and an inner longitudinal layer.
• The relaxed skeletal muscles help keep the pharynx open (patent), while the contraction of these muscles aids in deglutition (swallowing).

Divisions of the Pharynx

The pharynx is divided into three regions:

1. Nasopharynx (Uppermost Portion)

   • Location: Posterior to the nasal cavity and superior to the soft palate.
   • Structure: Lined with pseudostratified ciliated columnar epithelium. The lateral walls of the nasopharynx connect to the auditory (eustachian) tubes, which open into the middle ear.
   • Function: Primarily a passage for air. The uvula of the soft palate elevates during swallowing to prevent food from entering the nasopharynx.
   • Special Features: Contains the pharyngeal tonsil (adenoids) and is involved in the equalization of air pressure between the pharynx and the middle ear.
      

2. Oropharynx (Middle Portion)

   • Location: Posterior to the oral cavity, extending from the soft palate to the hyoid bone.
   • Structure: Lined with nonkeratinized stratified squamous epithelium, which is well-suited to handle the wear and tear from food passing through.
   • Function: Acts as a common pathway for both air and food. It opens into the oral cavity via an archway called the fauces.
   • Special Features: Contains the palatine tonsils and lingual tonsils, which are part of the body’s first line of defense against ingested or inhaled foreign materials.
      

3. Laryngopharynx (Lowermost Portion)

   • Location: Inferior part of the pharynx, extending from the hyoid bone to the level where it splits into the esophagus and larynx.
   • Structure: Lined with nonkeratinized stratified squamous epithelium.
   • Function: Permits the passage of both food and air. The laryngopharynx opens into the esophagus (posteriorly) and the larynx (anteriorly).

The Lower Respiratory Tract and Larynx

The lower respiratory tract includes essential structures that facilitate the passage and exchange of air in the lungs. It is composed of the trachea, bronchial tree, lungs, alveolus, and alveoli. The lower respiratory tract is divided into the conducting airway and the respiratory portion, each serving specific roles in respiratory function.

1. Conducting Airways

These structures serve as passageways for air to travel to and from the lungs. They include:

• Pharynx: A muscular tube that connects the nasal cavity and mouth to the larynx and esophagus. It serves as a pathway for both air and food.
• Trachea (Windpipe): A rigid tube that connects the larynx to the bronchi, allowing air to flow into the lungs.
• Bronchi: The two main branches from the trachea that lead into each lung, further dividing into smaller bronchioles.
• Terminal Bronchioles: The smallest branches of the bronchi that lead to the respiratory portion of the lungs.

2. Respiratory Portion

This part of the respiratory system is where the actual gas exchange occurs, and it includes:

• Respiratory Bronchioles: The smallest airways leading from the terminal bronchioles, these mark the beginning of the respiratory portion.
• Alveolar Ducts and Alveoli: Tiny sac-like structures where oxygen and carbon dioxide are exchanged between the lungs and the bloodstream.

The Larynx (Voice Box)

The larynx is a vital structure located in the neck that serves as both an air passageway and the site of sound production. It is a short, cylindrical airway that begins at the laryngopharynx and ends at the trachea, measuring about 5 cm in length.

Anatomical Boundaries and Position

• Superiorly: Attached to the hyoid bone and opens into the laryngopharynx.
• Inferiorly: Continuous with the trachea.
• Posteriorly: Located in front of the esophagus.
• Position: Lies in the midline of the neck, anterior to the esophagus, and opposite the C4 to C6 cervical vertebrae.

Functions

• Conducting Air: The larynx channels air into the lower respiratory tract.
• Sound Production: It contains the vocal cords, which vibrate to produce sound.

Structure and Cartilages

The larynx is composed of nine cartilages (three unpaired and three paired) that form its framework:

• Unpaired Cartilages:
  • Thyroid Cartilage: Also known as the Adam's apple, this V-shaped cartilage forms the anterior wall of the larynx.
  • Cricoid Cartilage: A ring-shaped cartilage forming the inferior boundary of the larynx.
  • Epiglottis: A leaf-shaped, elastic cartilage that acts as a lid over the glottis, preventing food from entering the trachea during swallowing.
     
• Paired Cartilages:
  • Arytenoid Cartilages: Triangular cartilages located at the posterior, superior border of the cricoid cartilage, involved in vocal cord movement.
  • Cuneiform Cartilages: Club-shaped cartilages that support the vocal cords and the lateral epiglottis.
  • Corniculate Cartilages: Horn-shaped cartilages on top of the arytenoid cartilages, contributing to vocal cord function.

Epithelium of the Larynx

• Above the Vocal Folds: Lined with nonkeratinized stratified squamous epithelium, which provides protection from abrasion.
• Below the Vocal Folds: Lined with pseudostratified ciliated columnar epithelium, containing cilia and goblet cells. The mucus produced traps dust and other particles, which are then moved upwards towards the pharynx by ciliary action.

Vocal Folds (Vocal Cords)

• Ventricular Folds: Also known as false vocal cords, these do not play a direct role in sound production.
• Vocal Folds: Also known as true vocal cords, these elastic structures vibrate to produce sound when air is forced through them.
• Glottis: The space between the vocal folds, which can be closed during swallowing to prevent food from entering the airway.

Blood and Nerve Supply

• Blood Supply: The larynx receives blood from the superior and inferior laryngeal arteries and is drained by the thyroid veins, which connect to the internal jugular vein.
• Nerve Supply: The larynx receives parasympathetic innervation from the superior and recurrent laryngeal nerves, branches of the vagus nerve, and sympathetic innervation from the superior cervical ganglia.

Epiglottis

• The epiglottis acts as a protective flap that closes over the glottis during swallowing, ensuring that food and liquids do not enter the airway.

Parts of the Larynx

1. Vocal Cords: Essential for sound production.
2. Glottis: The opening between the vocal cords.
3. Epiglottis: Protects the airway during swallowing.
4. Subglottis: The area below the vocal cords.
5. Supraglottis: The area above the vocal cords, including the epiglottis and arytenoids.

The Trachea

Structure and Location

• Definition: The trachea, commonly known as the windpipe, is a flexible, cylindrical tube that serves as a vital airway in the respiratory system.
• Size: Approximately 10-12 centimeters (4-5 inches) in length and about 2.5 centimeters (1 inch) in diameter.
• Location: Extends from the larynx (voice box) down into the thorax, passing through the mediastinum. It lies anterior (in front) to the esophagus and inferior (below) to the larynx.

Anatomy of the Trachea

• Cartilaginous Support:

  • The tracheal wall is supported by 15 to 20 C-shaped rings of hyaline cartilage.
  • Function of Cartilage Rings:
    • Provide rigidity and maintain an open airway, preventing collapse during inhalation.
    • The open part of the "C" faces posteriorly towards the esophagus, allowing the trachea to expand slightly during swallowing.
       

• Trachealis Muscle:

  • The posterior part of the trachea (where the cartilage rings are open) is spanned by the trachealis muscle, a band of smooth muscle.
  • Function:
    • Allows flexibility for the esophagus to expand anteriorly as food passes down.
    • Can contract to reduce the diameter of the trachea, aiding in expelling air forcefully (e.g., coughing).

Layers of the Tracheal Wall

1. Mucosa (Innermost Layer):

   • Lined with pseudostratified ciliated columnar epithelium containing goblet cells.
     • Function:
       • The goblet cells secrete mucus that traps inhaled particles.
       • The cilia move the mucus laden with dust and microorganisms upward toward the pharynx to be swallowed or expelled.
          

2. Submucosa (Outer Layer):

   • Composed of loose connective tissue and contains seromucous glands that produce additional mucus.
     • Function:
       • Provides support and elasticity to the trachea.
       • The glands aid in keeping the airway moist.

Bifurcation of the Trachea

• At the level of the sternal angle (approximately at the fifth thoracic vertebra), the trachea divides into two smaller tubes:
  • Right Primary (Main) Bronchus
  • Left Primary (Main) Bronchus
• The division point is marked by an internal ridge called the carina.
  • Carina:
    • A sensitive area that triggers coughing when irritated by foreign particles.

The Bronchial Tree

Overview

• The bronchial tree is a highly branched system of air passages that conduct air from the trachea into the lungs.
• Function: Distributes air to the lungs' interior and facilitates gas exchange at the alveolar level.

Anatomy of the Bronchial Tree

1. Primary (Main) Bronchi:

   • Right Primary Bronchus:
     • Shorter, wider, and more vertical than the left.
     • Aspiration of foreign objects is more likely to occur here.
   • Left Primary Bronchus:
     • Longer, narrower, and more horizontal.
   • Support: Both have incomplete rings of hyaline cartilage for support.
      

2. Secondary (Lobar) Bronchi:

   • Each primary bronchus divides into secondary bronchi.
   • Right Lung: Three secondary bronchi (one for each lobe).
   • Left Lung: Two secondary bronchi (one for each lobe).
      

3. Tertiary (Segmental) Bronchi:

   • Secondary bronchi further divide into tertiary bronchi.
   • Right Lung: Typically 10 tertiary bronchi.
   • Left Lung: Typically 8 tertiary bronchi.
   • Each supplies a bronchopulmonary segment—a distinct region of the lung.
      

4. Bronchioles:

   • Tertiary bronchi branch into smaller bronchioles.
   • Intralobular Bronchioles:
     • Supply air to the lobules within each bronchopulmonary segment.
   • Terminal Bronchioles:
     • The smallest conducting branches; mark the end of the conducting zone.
        

5. Respiratory Bronchioles:

   • Terminal bronchioles divide into respiratory bronchioles.
   • Function: Begin the respiratory zone where gas exchange can occur.
      

6. Alveolar Ducts and Alveolar Sacs:

   • Respiratory bronchioles lead into alveolar ducts.
   • Alveolar ducts end in clusters called alveolar sacs, which contain multiple alveoli.

Structural Changes Along the Bronchial Tree

• Cartilage:

  • Amount decreases as the airways become smaller.
  • Bronchioles lack cartilage and are supported by smooth muscle.
     

• Smooth Muscle:

  • Amount increases as cartilage decreases.
  • Function:
    • Allows regulation of airway diameter.
    • Bronchodilation: Relaxation of smooth muscle increases airway diameter (sympathetic stimulation).
    • Bronchoconstriction: Contraction decreases airway diameter (parasympathetic stimulation).
       

• Epithelium:

  • Changes from ciliated pseudostratified columnar epithelium in the bronchi to simple cuboidal epithelium in the terminal bronchioles.
  • Cilia and mucus-secreting cells decrease, reducing mucus clearance in smaller airways.

Alveoli

Overview

• Definition: Alveoli are tiny, saccular air sacs where gas exchange occurs between the air and the blood.
• Number: Lungs contain approximately 300 million alveoli, providing a vast surface area (~70 square meters) for gas exchange.
• Appearance: Their clustered arrangement gives the lungs a spongy texture.

Structure of Alveoli

• Alveolar Wall Composition:
  • Extremely thin to facilitate rapid gas diffusion.
  • Surrounded by a network of pulmonary capillaries.

Cells in the Alveolus

1. Type I Alveolar Cells (Pneumocytes Type I):

   • Structure: Simple squamous epithelial cells.
   • Function:
     • Form the majority of the alveolar surface area.
     • Facilitate gas exchange between the alveoli and the pulmonary capillaries.
        

2. Type II Alveolar Cells (Septal Cells or Pneumocytes Type II):

   • Structure: Cuboidal epithelial cells with microvilli on their free surface.
   • Function:
     • Secretion of Surfactant:
       • Surfactant is a lipid-protein substance that reduces surface tension within the alveoli.
       • Importance:
         • Prevents alveolar collapse during exhalation.
         • Has hydrophilic and hydrophobic regions that disrupt water molecule cohesion.
     • Alveolar Fluid Production:
       • Keeps the alveolar surface moist for efficient gas exchange.
          

3. Alveolar Macrophages (Dust Cells):

   • Function:
     • Wander within the alveolar spaces and interalveolar septa.
     • Phagocytose dust particles, debris, and pathogens.
     • Help keep the alveolar surfaces sterile.

Respiratory Membrane

• Composition:
  • Alveolar Epithelium (Type I Cells)
  • Fused Basement Membranes of Alveolar and Capillary Endothelium
  • Capillary Endothelium
• Thickness: Approximately 0.5 micrometers.
• Function: Acts as a barrier across which gas exchange occurs by diffusion.

Gas Exchange Process

• Oxygen Transport:
  • Oxygen from inhaled air diffuses across the respiratory membrane into the blood within the pulmonary capillaries.
• Carbon Dioxide Removal:
  • Carbon dioxide diffuses from the blood into the alveolar space to be exhaled.