Breathing
“And breathe, just breathe, oh breathe, just breathe.”
That’s what Anna Nalick tells us in her song titled (funnily enough) Breathe.
But what actually is breathing? According to reference.com, breathing is “the process of inhaling and exhaling air from the lungs.” That’s fairly straight-forward. Let’s look at what actually goes on when we inhale and exhale.
To inhale (breathe in), the diaphragm contracts. The diaphragm is a muscle that lies at the base of the thorax (chest), and separates the thoracic cavity (lungs and heart) from the abdominal cavity (stomach, intestines, etc.). It is dome-shaped, and when it contracts it moves down. As this is happening, the intercostal muscles around the ribs contract, causing the ribs to expand outwards and the sternum to rise.
The contraction of the diaphragm and the intercostal muscles both increase the volume of the thoracic cavity, resulting in a decrease in intrathoracic (within the thorax) pressure to a value below atmospheric pressure. Air moves from a region of higher pressure to a region of lower pressure, and so passes through the nose/mouth and into our body. The air moves through the pharynx (upper part of the throat, i.e. behind the nasal and oral cavities), larynx (where the vocal cords are found), trachea (windpipe) and bronchi (the two tubes leading into the lungs), into the lungs. The lungs inflate with the extra air, and we have just breathed in.
The trachea is a 9-12 cm long tube. It branches into two bronchi (called the left and right bronchi) which lead to the left and right lungs. The bronchi enter the lungs and subdivide into smaller and smaller bronchioles which terminate in air sacs called alveoli. The network of branching bronchi and bronchioles looks like an upside down tree (with smaller and smaller branches coming off), and in fact it is known as the bronchial tree.
In the lungs, the alveoli walls are surrounded by tiny blood vessels called capillaries, which allow oxygen from the air to enter the bloodstream. They also allow carbon dioxide accumulated in the blood to pass into the alveoli, ready to be breathed out.
To exhale (breathe out), the diaphragm and intercostal muscles simply relax. This results in the diaphragm curving upwards, and the ribs and sternum returning to their original position. This decreases the volume of the thoracic cavity, increasing the intrathoracic pressure to above atmospheric pressure and essentially squeezing the air out of the lungs. As the muscles simply relax to do this, the process is known as passive expiration.
Only about 70 % of the air that is inhaled actually reaches the alveoli, with the rest remaining in the air spaces of the nose, throat, larynx, trachea and bronchi.
But why did we even want to get air to the alveoli in the first place? For gas exchange. The air we breathe in is about 21 % oxygen, yet the air we breathe out is only about 16 % oxygen. Some of the oxygen passes into our bloodstream to be delivered to all parts of our body for cellular respiration. On the other hand, carbon dioxide is a waste product of respiration and needs to be removed. Carbon dioxide accounts for about 0.04 % of the air we breathe in, but the air we breathe out contains about 3.6 %.
So what’s the deal with forced deep breathing? Some people do that, right? Yep, it’s called hyperventilation, and some people (e.g. the pearl divers of Asia) may voluntarily do this before going underwater. They take a series of deep inhalations and exhalations before diving. This is not to get more oxygen into their system, but rather to reduce the level of carbon dioxide. By doing this, the urge to breathe is greatly reduced, allowing them to stay underwater for longer. The danger with this is that our sensory receptors are not very sensitive to oxygen levels in the blood, and the brain can become deprived of oxygen before the urge to breathe is felt. The diver may then pass out or faint. Breathing begins again once voluntary (conscious) control is lost, however, this is not such a good thing when you’re underwater!
So that’s breathing in a nutshell. Now remember, as Anna Nalick says, just breathe...