What Gas Do We Breathe Out

Have you ever stopped to think about what it really means when we breathe? To get enough of the right kind of air for our bodies to function properly, we have to take in the right amount of oxygen and expel all that nasty stuff known as carbon dioxide. It’s not just about having clean air — it’s also about getting rid of the bad stuff.
In this article, we’ll look at how gas exchange works and why we need to do it every minute of every day.
Breathing has been going on since the beginning of life itself. When you were still inside your mother’s womb, you breathed through amniotic fluid, which contained only nitrogen and water vapor. And while you were growing up outside your home, you took in plenty of fresh air with each breath, but you didn’t know any better than to keep breathing it in until you got someplace where there was pollution or smog. Once you started walking around outside, you began taking in the polluted air, or smog, because you needed more oxygen to fuel your body. As soon as you learned how to walk, you had to learn how to use your muscles, which required oxygen. So now you’re no longer just absorbing oxygen passively; you’ve got to make sure you have enough of it by moving around.
The first thing you did once you left the safety of the warm, cozy confines of the womb was to begin making loud sounds with your mouth. You opened your mouth wide so that you could inhale enough air to fill your stomach. After you swallowed, the air then traveled down into your chest cavity and filled your lungs. Your diaphragm, the muscle that separates your chest from your abdomen, relaxed to allow the expansion of the lungs and to help them push the air ahead of them. The air then moved into your windpipe, where it was further expanded, and finally pushed into your bloodstream. All of these actions are performed by the respiratory system, which consists of the nose, pharynx, larynx, trachea, bronchi, lungs, diaphragm, vocal cords and finally, the alveoli. Air travels through the respiratory tract one way, and the gases you inhale travel another way.
As you can see, gas exchange occurs when you inhale air and when you exhale air. In order to understand exactly how this happens, let’s start by looking at what goes on inside your body during gas exchange.
Gas Exchange With Oxygen
Your cells contain proteins called hemoglobin that carry oxygen throughout your body. Hemoglobin binds to oxygen molecules, forming oxyhemoglobin. Oxygen-rich blood transports these complexes through tiny capillaries under the skin near your fingers or toes. There, the hemoglobin releases its cargo of oxygen atoms and unbinds to form deoxyhemoglobin. These freed oxygen atoms enter tissues to supply energy to muscles and other parts of the body. Without this mechanism, your red blood cells would be unable to release their oxygen payload anywhere except back into the venous circulation.
So far, we’ve looked at how oxygen gets absorbed into your bloodstream and delivered to the rest of your body. Now let’s take a quick peek at what’s happening on the opposite end of this equation.
Carbon Dioxide and Other Gases
Carbon dioxide, along with nitrogen and water vapor, makes up most of the atmosphere above us. However, when you exhale, you don’t simply push these three substances out of your lungs. Instead, carbon dioxide is released into the air by special membrane-bound protein structures within your lung tissue called mitochondria.
Mitochondria convert food nutrients like carbohydrates and fats into usable forms of ATP, which is used to power muscles and other body functions. They do this by using enzymes to break apart glucose to pyruvic acid. Pyruvic acid is then converted into acetyl CoA, which is transported across the inner membrane of the mitochondria. From here, the citric acid cycle takes place, in which acetyl CoA is broken down into succinic acid, fumarate and malate, which produce energy. During this conversion, carbon dioxide is produced as a byproduct.
Once the energy-producing reactions pass beyond the mitochondrial membranes, they transport hydrogen ions across the outer membrane into the intermembrane space. Here, the electrons of the hydrogen ions combine with oxygen, which produces water as an electron acceptor. Then, the resulting negatively charged electrons move through the series of respiratory chain proteins embedded in the inner membrane. Finally, they reach molecular iron centers in the structure, where they reduce the iron centers to generate protons, which cause the chains to swing outward and away from the inner side. A proton gradient develops between the inner and outer sides of the mitochondrial membrane, causing H+ ions to flow across the membrane. Carbon dioxide exits the cell via diffusion, driven by the concentration difference between the inside and outside of the cell wall.
Now you know how the respiratory system exchanges gases. But how does this amazing system work? Find out next.
The Respiratory System
The respiratory system includes several different types of organs, including the nose, throat, sinuses, voice box, lungs, trachea, bronchi and bronchioles. Each organ plays a crucial role in gas exchange. Let’s review each one and see how it contributes to the overall process.
Nose and Sinuses:
The nasal passage leads from the nostrils directly to the nasopharynx. The nose filters incoming air for smell, temperature regulation and protection against pathogens. Nasal secretions include mucus, which helps trap bacteria and viruses before they can reach the olfactory region and the sensory receptors. Bacteria and viruses also may be removed by cilia lining the nasal passages.
The sinuses lie behind the nasal septae and drain into the facial vein. The paranasal sinuses hold air under pressure and provide a cushioning effect for the face. The ethmoid sinuses receive drainage from the eyes, ears and nose. The frontal sinuses, located just above the eyebrows, drain into the sphenopalatine artery. The maxillary antra drain into the infraorbital veins, and the anterior cranial fossa drains into the superior sagittal sinus. The last of the major sinuses, the petrous sinuses, drain into the carotid arteries.
Pharynx:
The pharynx begins in the nasal cavity. Its main function is to propel food past the tongue and teeth into the digestive system. The epiglottis prevents regurgitation of food into the tracheobronchial tree. The soft palate seals off the roof of the oral cavity from the nasal cavity.
Larynx:
The larynx is the uppermost part of the trachea. It sits just below the glottis, the opening formed by the vocal folds. Two cartilage rings suspend the vocal folds over the trachea. The vocal folds are made of elastic connective tissue covered with mucosa, which allows the folds to vibrate freely. The larynx contains two small openings, the false vocal cords at the front and the true vocal cords at the rear. The false vocal cords open widely forward to become the vocal folds.
Trachea:
The trachea is the central tube joining the larynx to the bronchii at the base of the lungs. The smooth muscle walls of the trachea constrict to control airflow. The trachealis muscle surrounds the trachea and restricts its movement toward the neck. The cricothyroid ligament attaches the thyroid gland to the lower border of the trachea.
Bronchi:
Airflow through the bronchi is controlled by muscular action. Smooth muscles surround the bronchi and divide them into lobules. Muscles attached to the walls of the bronchi contract and expand the airway diameter. Lymphatic vessels collect lymph fluid from the smaller airways and send it to larger nodes for cleaning. The large airways terminate in clusters of terminal bronchioles. Here, branching tubes lead to microscopic hairlike projections called Clara cells, which line the airways to absorb moisture.
Alveoli:
After passing through the bronchioles, the free ends of the airways join together to create a network of thin-walled sacs lined with type I epithelial cells. Alveolar walls contain many small pores that prevent liquid from entering but permit gas exchange. Surrounded by capillaries and macrophages, these alveoli stretch into hundreds of millions of individual units. Within the alveolus, surfactant lines the interior surface of the alveolar sacs. Type II pneumocytes secrete surfactant into the alveolar spaces.
To recapitulate, when we breathe, our lungs draw oxygenated air in through the nose and mouth into the trachea, which carries it down to the bronchi. From there, the air passes into the bronchioles and eventually into the alveoli. Inside the alveoli, oxygen diffuses across the thin semipermeable films of lipids secreted by type I epithelial cells.

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