The Respiratory System

Overview of the Respiratory System in Veterinary Practice

The respiratory system delivers oxygen to the body's tissues and removes carbon dioxide, a waste product of cellular metabolism. As a veterinary assistant or veterinary technician, you will monitor breathing, assist with oxygen supplementation, help position patients for intubation, and recognize signs of respiratory distress. A solid understanding of anatomy is the foundation for all of these tasks.

Upper Respiratory Tract

The upper respiratory tract includes all structures from the external openings down to, but not including, the lungs.

Nares (nostrils) are the external openings through which air enters. In dogs and cats, the nares should be open and moist. In brachycephalic breeds such as English Bulldogs and Persian cats, the nares may be stenotic — abnormally narrowed — forcing the animal to work harder to breathe.

Nasal cavity is divided by the nasal septum into left and right passages. Scroll-like bones called turbinates (nasal conchae) line the nasal cavity and warm, humidify, and filter incoming air. The nasal cavity opens caudally into the nasopharynx.

Pharynx is the common passageway shared by the digestive and respiratory systems. The soft palate separates the nasopharynx (above) from the oropharynx (below). In brachycephalic animals, an elongated soft palate extends too far caudally and partially blocks the glottis, contributing to airway obstruction.

Larynx is the cartilaginous structure sometimes called the voice box. Its primary function in veterinary medicine is protecting the airway from aspiration during swallowing. Key cartilages include the epiglottis, thyroid cartilage, cricoid cartilage, and the paired arytenoid cartilages. The opening between the arytenoids is the glottis, through which an endotracheal tube passes during intubation. In cats, laryngospasm — a sudden, protective closure of the glottis — is common during intubation attempts and can be a life-threatening emergency; this is why lidocaine is often applied to the feline larynx before intubation.

Trachea is a semi-rigid tube composed of C-shaped cartilaginous rings connected by trachealis muscle on the dorsal surface. This open dorsal surface allows the esophagus to expand during swallowing. In dogs and horses, the rings are nearly complete C-shapes. In cats, the rings overlap slightly. In cattle, some rings are complete O-shapes. The trachea bifurcates at the carina into left and right principal (mainstem) bronchi.

Lower Respiratory Tract

The lower respiratory tract begins at the carina and includes the bronchial tree and lungs.

Bronchi and bronchioles branch progressively from principal bronchi into lobar bronchi, segmental bronchi, and finally terminal bronchioles. Terminal bronchioles lead to respiratory bronchioles and then to alveolar ducts. No gas exchange occurs until the respiratory bronchioles and beyond; everything proximal is considered anatomical dead space.

Alveoli are microscopic air sacs with extremely thin walls where gas exchange occurs by diffusion across the respiratory membrane. Type II pneumocytes produce surfactant, a phospholipid substance that reduces surface tension and prevents alveolar collapse at end-expiration. Neonatal animals born prematurely may lack adequate surfactant, causing respiratory distress.

Lungs are divided into lobes. The dog has seven lobes: left cranial (with cranial and caudal parts), left caudal, right cranial, right middle, right caudal, and right accessory. The cat has a similar pattern but with minor differences in lobe subdivision. Cattle have well-developed accessory lobes and more distinct lobation due to thicker interlobular septa — an important consideration because bovine pneumonia can be confined to certain lobules more readily.

Species Comparison Highlights

| Feature | Dog | Cat | Horse | Cow |

|---|---|---|---|---|

| Tracheal rings | C-shaped | Overlapping | C-shaped | Some complete |

| Lung lobes | 7 | 7 (similar) | 7 | Distinctly lobulated |

| Laryngospasm risk | Low | High | Low | Low |

Brachycephalic Airway Syndrome and Anesthetic Risk

Brachycephalic breeds (English Bulldog, French Bulldog, Pug, Boston Terrier, Persian cat) have been selectively bred for a shortened skull. This creates a mismatch between the skull length and the volume of soft tissue, resulting in brachycephalic obstructive airway syndrome (BOAS). The anatomical features of BOAS that directly increase anesthetic risk include:

1. Stenotic nares — reduced airflow even at rest; worsened under sedation.

2. Elongated soft palate — occludes the glottis, making visualization for intubation difficult and increasing post-extubation obstruction risk.

3. Narrowed trachea (tracheal hypoplasia) — limits the size of endotracheal tube that can be placed, reducing ventilation.

4. Everted laryngeal saccules — mucosal tissue that folds into the airway lumen, further narrowing the glottis.

5. Hypoplastic trachea — present in some bulldogs; requires a smaller endotracheal tube than expected for body weight.

As a veterinary assistant, your monitoring responsibilities with brachycephalic patients include watching for increased respiratory effort, audible stridor, prolonged recovery time, oxygen saturation readings, and being prepared to assist with re-intubation. These patients should never be left unattended during recovery and should be extubated as late as safely possible to maintain a patent airway.

Safety Points

• Always verify that endotracheal tube size and cuff inflation are confirmed by the veterinarian or credentialed technician before you assume the airway is secure.
• Brachycephalic patients in lateral or dorsal recumbency are at elevated risk; monitor continuously.
• Report any change in respiratory rate, rhythm, or effort immediately — do not wait.
• Oxygen is a drug; administration rate and duration are directed by the veterinarian.