How Does The Human Respiratory System Work?

The human respiratory system pulls oxygen from the air into the bloodstream and pushes carbon dioxide out. Air travels through the nose, pharynx, larynx and trachea, then splits into the bronchi and bronchioles, finally reaching about 500 million alveoli in the lungs where gas exchange with the blood happens. The system also warms, filters and humidifies every breath.

Did you know that your right lung is slightly larger than your left lung? The left lung is narrower and has only two lobes, because it has to make space for the heart, which is slightly tilted to the left. The right lung has three lobes and is shorter and broader than the left lung because the liver sits right below it, pushing the diaphragm up on that side. Since the right lung is bigger, it ends up supplying your body with more oxygen than the left one!

Did you also know that at any given time, you’re breathing mostly through one nostril? This is called the nasal cycle, and it’s driven by the autonomic nervous system swelling the lining of one nostril while the other shrinks back, swapping over every couple of hours.

However, these are mere superficial facts about the organs in charge of supplying the body with oxygen. The nose and lungs, along with the pharynx, the larynx, the trachea and the bronchi, make up the respiratory system.

So, how does the respiratory system work?

Functions Of The Respiratory System

Your respiratory system is in charge of supplying the body with oxygen. It would be hopelessly inefficient for each cell of the body to source its own oxygen supply. The respiratory system therefore takes in oxygen from the atmosphere and hands it over to the circulatory system, which transports that oxygen throughout the body.

Besides taking in oxygen, the respiratory system also expels carbon dioxide (CO₂). The cells produce CO₂ as a byproduct of metabolism, and that excess CO₂ must be thrown out. The respiratory system swaps fresh oxygen for old CO₂, which is called gas exchange.

Expelling the CO₂ also maintains the pH of blood at an optimum level of 7.4.

The respiratory system also makes it possible for us to speak and smell.

Organs Of The Respiratory System

These critical functions for life are carried out by the different parts of the respiratory system: the nose or nasal cavity, the pharynx, the larynx, the trachea, the bronchi, and the lungs.

The air first enters through the nose. The epithelial cells in the nose produce mucus, which moistens the air, while blood vessels bring in heat to warm the air. The mucus also traps any dirt and pathogens that might try to invade the lungs.

Also inside the nose are millions of olfactory receptors that help us smell the world around us.

From the nose, the air moves down the pharynx and larynx and into the trachea. The pharynx and larynx connect the nasal cavity to the trachea and also keep our air passages open when breathing. The vocal cords sit inside the larynx, and as air rushes over them, we can produce sound.

The trachea is a long tube held open by C-shaped cartilage rings and smooth muscle. The inside of the trachea is lined with mucus, again, to keep foreign particles out and keep the incoming air nice and moist.

Think of the trachea like a long passageway. As you move down the trachea, it splits into the two main bronchi, which keep branching into ever-finer bronchioles, roughly 23 generations of splits in all. Through these divergent passageways, air travels deep into the lungs.

This keeps happening until the bronchioles end in the alveoli of the lungs.

The alveoli are where the exchange of gases takes place. They are tiny air-filled sacs, sort of like clusters of grapes or microscopic balloons. Each one is wrapped in a fine mesh of capillaries, which bring in oxygen-poor and carbon dioxide-rich blood. There are roughly 480-500 million alveoli in a pair of adult lungs, and together their walls unfold to about 70 m² (around 750 sq ft) of gas-exchange surface, all packed inside your chest.

Inspiration – Breathing In

Everything begins with the brain. The respiratory center in the brainstem (mostly the medulla oblongata, with the pons fine-tuning the rhythm) senses that the body needs more oxygen, or that CO₂ levels in the blood are climbing, and sends signals to the respiratory system to inhale.

In response, the muscles and bones around the lungs leap into action. The diaphragm, a dome-shaped sheet of muscle under the lungs, contracts and flattens downward, while the external intercostal muscles between the ribs pull the rib cage up and out. Together they pull and push the lungs, making them expand and contract.

Breathing relies on an important principle – pressure differences. The difference between the pressures inside the lungs and that of the outer environment dictates whether air will flow into the lungs or out of them. This pressure difference is created by changing the volume of the lungs. Remember, pressure and volume are inversely related, so the lower the volume, the higher the pressure inside.

To breathe in, we expand our lungs. This causes the pressure inside the lungs to be lower than the pressure of the atmosphere. Naturally, air will flow from an area of higher pressure to an area of lower pressure, meaning that the air from outside will make its way into the lungs.

The exchange of gases in the alveoli takes place on the principle of partial pressures. Partial pressure is the pressure that a gas exerts when it’s in a mixture. So, if there is a mixture of 3 gases in a jar, each gas will exert its own pressure in the jar. The total pressure in the jar will be the sum of the partial pressures of the three gases.

In the alveoli, the partial pressure of oxygen is high, whereas in the blood vessel, the partial pressure of oxygen is low. As a result, oxygen moves from the alveoli, where it has high partial pressure, into the blood vessel, where it has low partial pressure.

Similarly, carbon dioxide’s partial pressure is high in the blood vessel and low in the alveoli, so it flows the other way, from the blood vessel into the alveoli to be breathed out.

This silent exchange of gases happens constantly across all those hundreds of millions of alveoli. At rest, a healthy adult breathes about 12-16 times a minute, moving roughly 500 mL (about a pint) of air with each breath, and that adds up to around 6-8 liters of air shuttled in and out every single minute.

From here, the now oxygen-rich blood will make its way to the rest of the body to supply the cells with oxygen.

Expiration – Breathing Out

To expel the carbon dioxide, the diaphragm and intercostal muscles relax. The diaphragm springs back up into its dome shape, the rib cage drops, and the lungs are squeezed inwards by their own elastic recoil. This reduces the volume inside the lungs, which raises the pressure inside them above atmospheric pressure, so the stale, CO₂-rich air rushes out. And just like that, you’ve completed a full cycle of breathing!

A Final Word

Our entire respiratory system is also a front line for our immune system, and the two work hand in glove. The mucus and tiny hairs (called cilia) in the nose and trachea trap dust, pollen and microbes, then sweep them up and out before they can reach the lungs. Deeper down, the alveoli have their own resident bodyguards, immune cells called alveolar macrophages, which engulf any pathogens that slip past. The airway lining also secretes antimicrobial proteins and recruits more immune cells from the blood when an infection takes hold, which is why a respiratory infection can quickly become a whole-body affair.

For the most part, this elaborate act of supplying oxygen happens unconsciously. We don’t have to think about getting enough oxygen, unless you’re doing yoga or have just finished exercising. Breathing is one of the most instinctual things we do as living things, the first and last thing we do.

So, the next time you catch yourself breathing in deeply and enjoying the smell of a warm sunny day outside, don’t forget to thank your hard-working lungs for making all of that possible!