Let’s begin with a definition. Breathing results from the coordination of three intricate interactions: a sensory system, respiratory control, and an effector system. The effector system sends commands to the muscles and organs that regulate breathing.
To function properly, the body’s cells must get their energy from combining oxygen and other nutrients; the waste they produce is carbon dioxide. This is a continuous process: oxygen in, carbon dioxide out. Breathing is what causes this cycle (also known as respiration or ventilation). All three terms are used by physiologists when they are talking about the inhaling and exhaling of air.
The central nervous system initiates respiration spontaneously. The air our bodies takes in is composed of 21% oxygen, which enters the lungs by way of the nose and mouth. This component gas travels past the throat and larynx (voice box), though the trachea (windpipe). The air then goes through two main passageways, into either the right or left lung.
After the air enters the lungs, it then goes into the bronchi (smaller branch-like structures), and then into the bronchioles or alveolar ducts (even smaller branch-like structures). The brochi and bronchioles have small sacs at the ends of each branch called aveoli. The aveoli in the human lungs number around one hundred and fifty million. Through their very thin walls, the aveoli release oxygen into the blood. In exchange, the aveoli take in the bloods carbon monoxide. The lungs exhale this carbon monoxide, in the reverse process of taking oxygen in; that is, through the trachea, past the larynx and throat, and expelled through the mouth and nose.
The brain is ultimately responsible for the automatic process of breathing. The commands originate in the brain stem, which is connected to the spinal cord and the spinal cords’ nerves. Breathing is a reflex, not something that humans have to actively control. We breathe without thinking consciously about it. It is difficult to override this system for any sustained time.
The brain stem uses sensors to regulate breathing. These sensors, called “chemoreceptors,” are aided by nerves and muscles. Chemoreceptors have a very specific job: detecting chemical substances. When chemicals are discovered in the body, chemoreceptors pass whatever information they have to nerves in the central nervous system. The central nervous system then sends that data to the respiratory control center. Working together, the body is constantly monitoring and adjusting pH and oxygen in the arterial blood. Arterial blood comes from the heart and is has the highest levels of oxygen, which is distributed throughout the body.
Breathing is regulated by “homeostatic control”; that is, it strives to maintain stability within the body’s internal environment by way of negative feedback. High levels of carbon dioxide, for example, triggers faster and deeper breathing. This increase breathing decreases the amount of carbon dioxide in the body by increasing the amount of oxygen taken in.
The center for breathing is the respiration control center. The respiration control center gets information from other body parts and then coordinates automatic responses, which sets off other organs and muscles involved in respiration. The respiration control center is in the medulla, which itself is in the lower part of the brain stem. Within the medulla are neurones, which are specialized breathing cells. There are two types of neurones: inspiratory an expiratory. Inspiratory neurones are active during inhaling. Expiratory neurones are active when exhaling. The two work together, maintaining a constant, automatic cycle. Depending on the information received by the neurones, from a many origins, the automatic cycle can be altered or even briefly stopped.
Source: "Breathing, Regulation of." Encyclopedia of Nursing & Allied Health. Ed. Kristine Krapp. Vol. 1. Gale Cengage, 2002.