The Respiratory System – Anatomy & Physiology

Updated: May 23

Before we dig into the anatomy and physiology we have to understand that The respiratory system has three primary functions:

  1. Supplying oxygen to the body

  2. Eliminating carbon dioxide from the body

  3. Maintaining the acid-base balance

Let’s start by having an overview of the respiratory system.

The whole process starts when we breathe in (inhale) oxygen from the air. Oxygen travels through our nose, into our windpipes to our lungs.  From our lungs, the oxygen travels through a capillary system which leads to our body tissues.

At the same time, carbon dioxide oppositely leaves our bodies (exhale). So, from our body tissues into our lungs, and all the way out of our nose.

An explanation of the anatomy of the human respiratory system. Human Respiratory anatomy labelled

Respiratory System Anatomy and Physiology – Part 1

Now that we understood the basic function of the respiratory system, we can start by looking into more detail of the anatomy and physiology.

Breathing in encourages our diaphragm to contract and pull itself down, at the same time, our chest muscles contract to open up the chest space, allowing air to be sucked in like a vacuum.

On the other hand, when we breathe out, our diaphragm and chest muscles relax, this allows our lungs to spring back to their standard size, and push air out.

Our nasal cavities act like bouncers at a club; they have several ways to keep any harmful bacteria or particles out of our body. Firstly, they are lined by cells that produce a sticky mucus containing lysozymes. This mucus helps in killing off bacteria. Along with that, the nasal cavities also have nose hairs coated in this mucus. These hairs trap large particles of dust or pollen making them into tiny clumps of boogers which can then be removed by “blowing your nose”.

The air which has been filtered through your nose will then move into the paranasal sinuses. The paranasal sinuses are four hollow spaces within the bone structure of the face: frontal, ethmoid, sphenoid and maxillary. Think of them as quality control officers; they evaluate the air and add moisture and warmth to it to make it optimal for the body. Besides that, your voice is amplified thanks to your sinuses. In fact, that is why you sound so different when they get blocked during a cold.

Illustration of the human paranasal sinuses

Once the air is nice and warm, it moves into the pharynx. The pharynx is divided into three parts:

  1. Nasopharynx: Upper part, connects to the nasal cavity

  2. Oropharynx: Middle part, connects to the oral cavity

  3. Laryngopharynx: Lower part, connects to the larynx (aka. Voicebox)

Respiratory System Anatomy and Physiology – Part 2

Do you remember that time when you made someone laugh while drinking and water came out of their nose? That was because the Uvula (bell in the back of the mouth) and the soft palate (roof of the mouth) didn’t work correctly. When they do function properly, they keep food or drinks from travelling up to the nasopharynx.

Up until the larynx, food and air travel through the same ‘pipe’, however, the body has gates installed at the top of the larynx that determines which way food and air will go. This gate is known as the epiglottis, and it seals the airway shut while you are eating. This method prevents food from entering the ‘windpipe’ aka. trachea, and directs it into the oesophagus.

The body has a cough reflex that kicks in if anything other than air goes into the larynx

Respiratory System Anatomy and Physiology – Part 3

Ok, are you on board so far?

Great. So, the next part of the respiratory system anatomy and physiology is the trachea, the windpipe. As it goes further down, the trachea splits into two mainstem bronchi, one leading to the left lung and the other leading to the right lung. The point where the trachea divides is known as the carina.

As we already know, our bodies are symmetrical so what happens on one side repeats itself on the other side. However, there is an exception when it comes to the lungs because they have slightly differently shaped structures.

  1. The right lung has three lobes, and a broader and more vertical mainstem bronchus.

  2. The left lung has two lobes, and a tighter and more horizontal mainstem bronchus.

an illustration of human lungs, trachea, cricoid cartilage, bronchi and bronchioles

The main stem bronchi branch out to several smaller bronchi, which all divide into even smaller ones and so on. As a support system, the trachea and the first three levels of bronchi all have cartilage rings to keep a rigid shape.

The inner part of the trachea is covered in smooth muscle, and this plays an essential role because the smooth muscles have nerves of the autonomic nervous system in them.

The autonomic system has control over how your body reacts in certain situations, like when you are chasing your dog, or when you are tanning on the beach. Your body will respond differently in the two scenarios.

In the first one, you are under stress so your ‘fight or flight’ reflexes kick in. This is where your sympathetic nerves start to work; they stimulate the beta 2 adrenergic receptors and widen your airways to allow more air to get in.

In the second scenario, you are relaxed, so your body goes into ‘rest and digest’ mode. Here your parasympathetic nerves are activated, they signal the muscarinic receptors and reduce the diameter of your airways because you do not require such high flow of oxygen.

Ok so to recap:

Stress = sympathetic nerves = beta 2 adrenergic receptors = wide airways

Relaxed = parasympathetic nerves = muscarinic receptors = narrow airways

Respiratory System Anatomy and Physiology – Part 4

If you thought that’s all there is to know about airways, you’re mistaken. Because your body loves you and wants to protect you in the best possible way, it has ciliated columnar cells and a few goblet cells that secrete mucus in your large airways. Together, the mucus and the ciliated columnar cells trap particles and push them towards the pharynx. Here particles can either be swallowed or spat out.  This process is called the mucociliary escalator.

After the first three levels of bronchi, the airways progress into bronchioles that are even smaller and do not require cartilage to stay open. Bronchioles go on for approximately 17 levels and are often referred to as conducting bronchioles. The bronchial arteries supply oxygenated blood to the conducting bronchioles.

Just like your larger airways, the walls of conducting bronchioles are lined with ciliated columnar cells and goblet cells, but they have also been upgraded to contain club cells. Club cells produce glycosaminoglycans which act as a protector to the bronchial epithelium.

Clubs cells are as dope as they sound to be because they can transform themselves into ciliated columnar cells and help in replacing old ciliated columnar epithelial cells.

Respiratory System Anatomy and Physiology – Part 5

The conducting bronchioles branch further into the terminal bronchioles, which ultimately lead to what are known as alveoli. Alveoli are small pouches that fill up with air and make up the last part of the respiratory pathway. It is estimated that our lungs have around 500 million alveoli, all of which are lined by fine epithelial cells known as pneumocytes.

illustration of human lungs with emphasis on the alveoli

There are two types of pneumocytes:

  1. Pneumocytes Type I: make up about 95% of alveolar cells, involved in gas exchange between alveoli and blood

  2. Pneumocytes Type II: produce surfactant to reduce the surface tension of alveoli and maintain the structure open. They can also transform into Type I to reproduce new cells.

In the rare cases, where a tiny particle makes it all the way to the alveoli, the lungs have alveolar macrophages that can engulf it and push it up to the conducting bronchioles to get on the mucociliary escalator.

The pneumocytes are attached to endothelial cells of blood vessels by a protein layer known as the basement membrane. These three layers of pneumocytes, basement membrane and endothelial cells are the blood-gas barrier and it is what separates air from the blood. The blood that reaches these endothelial cells is deoxygenated as it is coming from pulmonary arteries.

At this point, oxygen freely diffuses into the blood, and the now oxygenated blood will circulate the heart and the rest of the body. At the same time, carbon dioxide from the deoxygenated blood diffuses into the alveoli and finds its way out through all the passages mentioned.

computer generate image of the alveolus gas exchange

And that is it! You just made it through the whole Respiratory System Anatomy and Physiology !!

Let Us Recap:

The respiratory system is what makes gas exchange possible

  1. Oxygen inhaled from the air makes the following journey: mouth or nose > pharynx > larynx > trachea > mainstem bronchi > conducting bronchioles > terminal bronchioles > alveoli > capillaries > body tissues.

  2. Carbon Dioxide in the body takes the same pathway as oxygen but in reverse and finds it’s way into the world.

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