Lower Respiratory Tract Anatomy and Function

Lower Respiratory Tract Overview

• The lower respiratory tract includes vital organs and structures such as:

  • Larynx

  • Trachea

  • Lungs

  • In birds, additionally includes air sacs and a unique vocal organ called the syrinx.

Larynx

• Structure & Function:

  • A short, tube-shaped, bilaterally symmetrical musculocartilaginous organ.

  • Composed of muscles and several cartilages lined with a nervous membrane.

  • Connected to the hyoid bone and trachea via ligaments and muscles.

  • Main functions:

  • Acts as a protective mechanism by opening or closing before anything enters the lower respiratory tract.

  • Facilitates vocalization in domestic animals (birds have a different structure for vocalization).

  • Assists in swallowing (epiglottis helps close off the larynx).

Cartilages in the Larynx

• Four key cartilages include:

  1. Epiglottic Cartilage: Elastic, prevents food from entering the larynx during swallowing.

  2. Thyroid Cartilage: Has a body and lamina (varied size across species).

  3. Arytenoid Cartilage: Paired, creates the glottis, and aids in vocalization.

  4. Cricoid Cartilage: Hyaline cartilage shaped like a signet ring.

• Species-specific differences:

  • Carnivores & Horses: Sharp, pointed free apex of epiglottis.

  • Ruminants & Pigs: Rounded free apex of epiglottis.

  • Thyroid notch is present in horses but absent in other species.

Trachea

• Extends from the cricoid cartilage to its bifurcation into bronchus.

• Characteristics:

  • Composed of C-shaped hyaline cartilage rings connected by ligaments.

• Comparative anatomy across species:

  • Length:

  • Small ruminants: ~25 cm

  • Pigs: 15-20 cm

  • Dogs: ~10 cm

  • Tracheal Rings: Vary in number and shape, with differences in muscle placement (inside vs. outside the rings) based on species.

Key Differences by Species:

• Dogs: 42-46 cartilaginous rings, slender U-shaped, no dorsal opposition.

• Pigs: Slightly more overlapping than dogs, with muscle inside.

• Horses: 55-60 rings, no dorsal opposing in cervical and thoracic regions.

• Ruminants: Have bronchus before primary bronchi, known as tracheal bronchus.

Lungs and Bronchi

• Lungs differ in structure, size, and number of lobes among species:

  • Carnivores: Two lobes on the left (cranial and caudal), three on the right (plus middle and accessory lobes).

  • Pigs: Similar to carnivores but vary in formation and division of lobes.

  • Horses: Simplest structure; left lung divided into two lobes, and right lung has cranial, caudal, and accessory lobes but no middle lobe.

Air Sac Functionality in Birds

• Structure: Nine air sacs, thin-walled, contribute to mechanical ventilation, not gas exchange.

• Function:

  • Facilitate continuous unidirectional airflow during respiration.

  • Communicate with lungs, reducing metabolic heat during flight.

  • Their role extends into the anatomy of pneumatized bones.

Unique Characteristics of Bird Respiratory Systems

• Dynamics: Unlike mammals, bird lungs do not change significantly in volume.

• Ossified Cartilages: Birds have tightly stacked cartilages in trachea that may often ossify except in pigeons.

• Syrinx: Junction of trachea and bronchi that is responsible for sound production, formed by various cartilaginous components and specialized muscles.

• Parabronchi: Site of gas exchange instead of alveoli, allowing for efficient respiratory mechanics during flight.