Veterinary Internal Medicine Nursing

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62 | How to understand the respiratory system quickly and easily as a vet nurse

Laura Jones

We know that respiratory patients are often very challenging.

They tend to be really stressed, and teetering on a knife-edge, balancing their increased demand for oxygen with a disease impacting their oxygenation.

Careful handling, supportive care and approaching these patients confidently, the right way, are essential to give them the best care and avoid complications.

In today’s episode, we’ll be laying the foundations for the rest of the series, taking a brief journey back to A&P and all of the need-to-know information about the respiratory system - because when we understand how it’s supposed to work, we can better understand what to do when it goes wrong.

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So, let’s waste no more time and dive straight into the structure and function of the respiratory system.

The respiratory system begins at the nose and ends at the distal alveoli. Its primary function is to deliver oxygen to the lungs, where it is exchanged with carbon dioxide. This oxygen is then transported to the body’s cells and tissues via the bloodstream, where it enters the cells and is used to generate energy.

The respiratory system can be divided into several major components, including the upper airways, lower airways, pulmonary parenchyma (tissue) and the pleural space.

Each area plays an essential role in respiratory function, and it’s important we understand them - because disorders in each region have different clinical signs and nursing considerations.

Let’s start by looking at the upper airways.

The upper airways include the nose, the sinuses, the larynx and the pharynx.

The nose and sinuses

These provide olfaction and are also responsible for temperature regulation in hyperthermic patients.

The nasal passageways are split into the dorsal, ventral and middle meati. Each passageway contains small scroll-like turbinate bones covered with a thin mucosal layer - these turbinate bones are responsible for warming and humidifying inhaled air and trapping foreign material and particles.

The sinuses are linked to the nasal cavities. They are open spaces—air-filled cavities within the skull. The frontal sinuses are located at the back of the nose and extend to the top of the head. They can also be affected in patients with nasal disease - for example, due to infection or tumours spreading from the nose.

We see many nasal disorders in our patients, many of which we’ll look at in dedicated episodes. Rhinitis, inhaled foreign bodies, fungal infections, cat ‘flu, and many other diseases all cause upper respiratory signs in our patients, such as sneezing, nasal discharge, congestion and anorexia.

The pharynx

The pharynx is an open space that acts as a passageway for the respiratory and digestive systems. It’s split into the nasopharynx (the back of the nose - so when you place a nasal feeding tube, the tube passes through the nasopharynx to be swallowed and enter the digestive tract) and the oropharynx (the mouth).

The larynx

The larynx is at the end of the oropharynx. It is the end of the upper airway and protects the tracheal entrance from aspiration. It also serves as an air passageway and permits vocalisation since it contains the vocal folds.

The larynx functions as a valve, opening and closing to permit and restrict entry to the trachea as needed.

It is made up of several cartilages connected by muscles. The most important of these are the arytenoid cartilages and the epiglottis - the epiglottis is the ‘flap’ that sits over the trachea, and the arytenoid cartilages are the ones affected in laryngeal paralysis patients. 

And then we’ve got the lower airways

The lower airways include the trachea, bronchi, bronchioles, and alveoli. I’m going to talk about the alveoli separately, just as they are complex and deserve their own moment - they’re often referred to as the pulmonary parenchyma, aka the lung ‘tissue’ - but they technically are also part of the lower airways, since they’re attached to the bronchi.

The trachea

The trachea carries air from the larynx to the bronchi and ultimately to the alveoli. It’s divided into two sections - the extrathoracic trachea (ie. the cervical trachea) and the intrathoracic trachea. 

The trachea comprises C-shaped cartilage rings connected by smooth muscle and a ligament called the dorsal ligament. The cartilage rings give the trachea its shape and keeps it open, and the muscle allows the diameter to change as needed during coughing, sneezing and swallowing.

Inside the trachea, the muscles and cartilage are lined with mucosa made up of ciliated epithelium and goblet cells. The goblet cells secret mucous which traps inhaled particles, which the cilia sweep cranially for it to be coughed and swallowed.

The bronchi and bronchioles

The bronchi and bronchioles are essentially smaller versions of the trachea. They have the same anatomical structure - cartilage rings and smooth muscle - and that’s important for us to note because it means diseases like tracheal collapse can affect the bronchi, too.

The trachea ends at the carina, where it bifurcates into the left and right mainstem bronchi. These supply each lung with air, and divide into lobar bronchi (which each supply a lung lobe) and then progressively smaller and smaller bronchioles.

These bronchioles continue to divide until they end in a terminal bronchus, containing an alveolar air sac.

Basically, think of the airways as an upside-down tree. The trachea is the trunk; the two main branches are the mainstem bronchi, and then the bronchioles branch off of them, getting smaller and smaller until they end in a leaf - our alveoli.

Speaking of those alveoli…

The alveoli are the functional units of our respiratory system and the tissues responsible for gaseous exchange.

Each terminal bronchus ends in an alveolus. This is an empty sac lined with a membrane one-cell-layer-thick. Surfactant, a substance secreted by the alveoli, helps to keep the sac open and facilitate gaseous exchange.

So how does gaseous exchange occur?

Inhaled oxygen-rich air passes through the lower airways and reaches the alveoli. From here, oxygen diffuses across the thin alveolar membranes and into the surrounding alveolar capillaries down a concentration gradient.

At the same time, waste gases such as carbon dioxide diffuse out of the blood and into the alveoli for elimination. As the alveolar air is exhaled, that CO2 is removed.

Ok, so the lungs are responsible for gaseous exchange. But what else?

Whilst oxygenation is the most important function of our respiratory system, it isn’t all it does. The lungs play an essential role in managing acid base balance, by controlling how much carbon dioxide they eliminate.

Carbon dioxide acts as an acid inside the body, so by eliminating additional CO2 (by hyperventilating), our bodies can increase our pH, and by retaining CO2 (through hypoventilating), they can decrease our pH.

Most of the time, our patients will do this deliberately to compensate for something else in the body that is causing acid-base imbalances. For example, if I have a DKA patient with metabolic acidosis, they’ll likely hyperventilate to reduce their CO2 and try to return their pH to normal.

However, primary respiratory diseases can also impact CO2 levels - for example, during respiratory obstruction or in patients with severe hypoxia causing respiratory fatigue.

What problems do we see with the alveoli?

Many disorders affect the alveoli, including:

  • Pulmonary oedema

  • Pneumonia

  • Pulmonary fibrosis

  • Pulmonary contusions

  • Smoke inhalation

  • Pulmonary haemorrhage

Regardless of the underlying cause, pulmonary diseases affect gaseous exchange, causing varying degrees of hypoxia. If this hypoxia progresses, patients work harder and harder to breathe, causing respiratory muscle fatigue and hypoventilation.

Then we have the pleural space.

The pleural space is the area between the lungs and the thoracic wall. Though it isn’t directly responsible for ventilation, pleural space disorders will impact our patients’ ventilation and oxygenation.

The pleura are membranes that line the outside of the lungs and the inside of the thoracic wall, allowing them to move against each other during breathing without friction.

Between these membranes is a small theoretical space and a tiny volume of fluid (so small you can’t detect it with imaging - less than 10-20ml in humans, so even less in our patients!), which acts as a lubricant.

This is absolutely fine - until something enters that pleural space that shouldn’t be there.

Pleural space disease occurs when air, fluid or tissue enters that space, increasing its size, and reducing the space available for the lungs to expand. The more content within the pleural space, the more the lungs are compressed, and the less our patient can ventilate.

Usually, the pleural space fills with:

  • Air - causing a pneumothorax

  • Low-protein fluid - causing a pleural effusion

  • Exudate (pus) - causing a pyothorax

  • Lymphatic fluid from digestion (chyle) - causing a chylothorax

  • Blood - causing a haemothorax

  • Tissue - e.g. due to neoplasia or a diaphragmatic hernia

We’ll chat more about pleural space disorders later in this series, but understanding which your patient has is vital - because each of them requiresvery different management, and different nursing care!

And then, lastly, we have the mediastinum.

The mediastinum isn’t part of the respiratory system itself, but like the pleural space, mediastinal disease will impact respiration.

The mediastinum is the central part of the thoracic cavity. It’s essentially the bit in between the lungs, containing the thymus, lymph nodes, thoracic duct, and organs like the heart, trachea and oesophagus.

Disorders like mediastinal neoplasia - such as thymoma and lymphoma - are relatively common. When these occur, these structures enlarge and can compress the lungs, affecting ventilation and oxygenation. They can also cause pleural effusion in some cases.


So there you have it - the need-to-knows of the respiratory system and how it impacts our patients. 

Understanding where in the respiratory tract the abnormality is and how that disease causes the clinical signs we see is the first step in managing respiratory patients - so that we can plan and deliver care that really helps them. 

We’re diving into all of the common respiratory diseases in the rest of this series to help you stabilise and care for these patients confidently. So be sure to head back to the podcast each Friday and tune in!

Did you enjoy this episode? If so, I’d love to hear what you think. Take a screenshot and tag me on Instagram (@vetinternalmedicinenursing) so I can give you a shout-out and share it with a colleague who’d find it helpful!

Thanks for learning with me this week, and I’ll see you next time!

References and Further Reading