Respiratory System: Lower Respiratory System + Avian Respiratory Anatomy

Lower Respiratory System Overview

• The respiratory system is crucial for gas exchange in animals;
  • This section focuses on lower respiratory anatomy in mammals and birds.

Trachea

• Definition: A firm tube connecting the larynx to the lungs.
• Structure:
  • Composed of cartilaginous rings:
  • In most species: C-shaped rings that are incomplete dorsally.
  • In birds: Complete tracheal rings.
  • Inner layer: Made of pseudostratified ciliary epithelium.
  • Produces mucus that serves as a barrier.
  • Cilia: Hair-like projections that help move foreign material and mucus upwards to the larynx for swallowing.

Bronchi

• Bifurcation: The trachea bifurcates into two main bronchi around the 4th to 6th intercostal space.
  • Larger bronchi are similar to the trachea in structure: cartilaginous rings and inner mucosal layer.
  • Branch into smaller bronchi, eventually ending in bronchioles.

Bronchioles

• Definition: Smaller branches of bronchi that lead to the alveoli.
• Structure:
  • Lack cartilage, replaced by glandular cells, muscle, and epithelium.
  • Narrow, with a diameter of less than 1mm.
  • Terminal bronchioles lose mucus-producing cells; replace them with surfactant-producing cells.
• Function: Surfactant reduces surface tension, enabling lung expansion during inflation and preventing alveoli from collapsing.

Alveoli

• Definition: Sac-like structures where gas exchange occurs.
• Structure: Surrounded by thin epithelium, closely associated with pulmonary capillaries for O2 and CO2 exchange.

Lungs

• Structure:
  • Composed of left and right sides, attached at roots (main bronchi, pulmonary vessels, lymphatics, nerves).
  • Expand during air inhalation; typically spongy, light pink.
  • Right lung is generally larger than the left.

Parts of the Lung

• Apex: Cranial direction.
• Base: Caudal direction (toward the diaphragm).
• Costal surface: Concave surface facing the ribs.
• Medial surface: Uneven surface facing heart, esophagus, trachea.
• Dorsal surface: Closest to the spine.

Lung Lobes

• Defined by bronchial branching:
  • Cranial lobe (left and right): Has cranial and caudal parts.
  • Middle lobe (right only): Absent in horses.
  • Caudal lobe (left and right).
  • Accessory lobe (right only): Present in pigs and ruminants.

Pleura

• Serous membranes surrounding thoracic structures (heart, lungs).
• Types:
  • Visceral pleura: Directly lines lungs.
  • Mediastinal pleura: Lines heart and midline structures.
  • Parietal pleura: Lines ribs and diaphragm.

Pleural Cavity

• Normally a potential space with a few milliliters of fluid to facilitate lung movement.
• Negative pressure allows lung expansion; if breached, the lung may collapse due to air influx.

Avian Respiratory System

• Very different from mammalian respiratory systems.
• Critical to understand due to the susceptibility of poultry to respiratory diseases.

Nasal Cavity

• Components:
  • Nostrils (nares) at the beak's base, leading to the nasal cavity.
  • Septum separates nasal cavity into two halves.
  • Contains conchae for olfaction, pollutant filtering, thermoregulation.
  • Nasal Gland: Secretes salt in marine birds to allow seawater consumption.

Larynx and Trachea in Birds

• Larynx: Comprised of only arytenoid and cricoid cartilage, lacking an epiglottis to close the glottis.
• Trachea: Features tightly stacked complete cartilaginous rings, longer than in many species, located close to the esophagus.

Syrinx

• No vocal folds; vocalization occurs in the syrinx located at the terminal part of the trachea.
• Unique structure in birds:
  • Musculature for sound production varies among species; songbirds have more muscle complexity.
  • Common site for foreign body obstruction.

Avian Bronchi and Lungs

• Lungs: Small, non-lobed, loosely attached to body wall in the cranio-dorsal part of the cavity.
  • Primary bronchi split into 40-50 secondary bronchi.

Parabronchi

• Secondary bronchi develop into 400-500 parabronchi.
• Two regions:
  • Paleopulmonic region: Tightly arranged parabronchi loops.
  • Neopulmonic region: Looser arrangement of loops.