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From birth, we carry out a process that is fundamental for our life that we are not even aware of. Involuntarily, our lungs continuously fill up with air in order to capture the necessary oxygen that will enable cellular respiration and the development and functioning of our entire body. Even when we sleep, the respiratory system works non-stop.
Although the gas exchange or hematosis (where the transfer of O2 and CO2 takes place) occurs directly in the pulmonary alveoli, the respiratory ways are the perfect route to conduct the inhaled air towards the lungs.

The journey or air

Although pulmonary ventilation (respiration) is an automatic and constant process, each time we breathe a series of specialized structures spring into action which ease air’s trip. Because respiration not only involves deeply inhaling the air that surrounds us; the true exchange, where we take advantage of the oxygen and discard the carbon dioxide, occurs at  a pulmonary level, specifically in the pulmonary alveoli.
When we breathe, air enters through our nose (it can also do so through the mouth), where it is heated, humidified and cleaned. Then, it passes on to the pharynx where it encounters a true filter, which intercepts and destroys the pathogen organisms: the tonsils.
Once this immunological barrier is sorted, air continues its journey through the larynx and then the trachea.
This last structure is a true elastic tube that, at the end of its route, splits into two bronchi that enter the lungs. Each bronchus branches just like a tree, ending in some elastic sacks (pouches), final destination of the inspired air. These small structures, called pulmonary alveoli, are the ones in charge of performing the gas exchange. With an appearance similar to a bunch of balloons, each time we inhale air, they take advantage of the oxygen and discarding the carbon dioxide.
A continuous, automatic process with different stages, but so fast that many times we are not even aware of it. A real air route.

Gas exchange

We already said respiration is a much more complex process that just breathing in through the nose. Transportation of oxygen to all of the organs and tissues of our body not only involves the work of the respiratory system, but also of the cardiovascular system.

In earlier chapters we saw how blood is the true means of transportation and collector of the gases involved in respiration.

Now we shall get to know the «loading and unloading station» of these molecules, the exchange place of oxygen and carbon dioxide.

Inhaled air ends its trip in the pulmonary alveoli. Each alveolus is adapted to receive and contain it as long as hematosis lasts. Its membrane has a great surface and they are also surrounded by an enormous amount of capillaries that, through the process of diffusion, capture oxygen and eliminate carbon dioxide.

When air enters each pulmonary alveolus it contains, approximately, 20.8% oxygen, 0.04% carbon dioxide, 78.6% nitrogen and 0.56% water vapor. While in the alveolar cavity, oxygen heads towards the capillaries in order to be transported to every area of our body by the red blood cells. However, to be more precise, oxygen is loaded and transported in a molecule housed in the red blood cells: hemoglobin.

It is in charge of capturing the oxygen molecules and taking them to each cell that requires so.From the capillaries, the pulmonary alveoli also receive the main waste product of cellular respiration, carbon dioxide. Through expiration (expulsion of air from the lungs), this gaseous compound is released towards the outside of our body. In this stage, the gaseous composition of the air that entered through the airways has clearly changed. 

The exhaled air contains the same percentage of nitrogen (78.6%), but the carbon dioxide (4%) and water vapor (1.8%) levels increase. Logically, the amount of oxygen expulsed decreases (15.6%).

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