Physiology of How Breathing Works

Central Neural Control

Breathing is a semi-automatic process that is largely directed by parts of the brain known as the cerebral cortex, medulla, and pons. Together, these components comprise the central neural control, also known as the “respiratory drive.”

The medulla and pons work together to regulate normal inspiration and expiration. The cerebral cortex “steps in” when there are conscious changes in breathing (like holding your breath) or unconscious changes (such as breathing while singing).

Sensory Input Systems

Sensory input systems are local “substations” that transfer nerve signals to and from the brain to regulate the muscles of the respiratory muscles. Four groups of receptors make up this system:

• Mechanoreceptors, located throughout the respiratory tract, provide information to the brain regarding the mechanical status of the lungs, including the breathing rate, the air space in the lungs, and problems like inhaled irritants.
• Metaboreceptors are connected to the muscles of the chest cavity and stimulate the expansion of the chest when the breathing rate is increased.
• Central chemoreceptors monitor for changes in blood gases in the brain and spinal cord, increasing or decreasing the breathing rate when needed.
• Peripheral chemoreceptors monitor for abnormal changes in blood gases in arteries, also increasing or decreasing the breathing rate when needed.
  

Respiratory Muscles

Information from the sensory input system is translated by the brain into electrical signals that direct the movement of respiratory muscles.

These muscles include:

• Diaphragm: This is a large, dome-shaped muscle situated beneath the lungs that contracts rhythmically, continually, and, most often, involuntarily.
• Internal intercostal muscles: These muscles, which line the chest wall, pull down the rib cage to push air out of the lungs.
• External intercostal muscles: These muscles, situated outside of the chest wall, lift the rib cage to pull air into the lungs.
• Abdominal muscles: These muscles compress the abdominal organs, pushing them up into the diaphragm to drive air from the lungs.
  

Upper Airways

When you breathe, the journey of air in and out of the body occurs in the upper respiratory tract. This is composed of muscles, soft tissues, and bony or cartilaginous structures that maintain the unobstructed flow of air in and out of the lungs.

The main structures of the upper respiratory tract include:

• Nose and nostrils
• Nasal cavity
• Mouth
• Pharynx (throat)
• Larynx (voice box)
• Trachea (windpipe)

Some of the muscles of the upper respiratory tract, including the trachealis muscle on the back of the trachea, work in tandem with the respiratory muscles to expel irritants like smoke or water from the lungs by inducing coughing.

Other muscles, like the pharyngeal constrictor muscles, contract voluntarily when you have to hold your breath.

Major Airways

At the end of your trachea, the airway splits into two branches called bronchi which serve as the main airways to each lung. Individually, they are called the right and left bronchus.

The bronchi branch off into smaller bronchioles which look similar to the branches of a tree. As the branches get smaller, they transition from stable cartilage to smooth muscles, which can widen (dilate) or narrow (constrict).

The walls of these air passages are lined with goblet cells that produce mucus. The purpose of the mucus is to protect the airways from irritants or infections that can harm them.

However, when mucus is overproduced, such as with diseases like COPD and cystic fibrosis, the airflow may be blocked, causing shortness of breath and a “wet” productive cough.

Lungs

The two lungs—a left and right lung—are situated in the chest cavity and separated by the mediastinum, which contains the heart and other organs. The left lung is slightly smaller than there right and has a notch in it to house the heart.

Each lung is divided into lobes: The left has two lobes and the right has three. Together, the lungs are surrounded by a two-layered membrane called the pleural sac that attaches them to the chest wall and, in turn, the intercostal muscles.

The lungs themselves are soft and spongy, They are mostly made up of air spaces filled with connective tissues and hundreds of millions of tiny air sacs called alveoli, where oxygen and carbon dioxide are exchanged.

Each alveolus is connected to a bronchiole.

What Happens When You Breathe

During inspiration, the drawing down of the diaphragm and lifting of the external intercostal muscles creates a negative pressure—or vacuum—inside the chest cavity that pulls air into the lungs.

During expiration, the contraction of the diaphragm and abdominal muscles, in tandem with the drawing down of the internal intercostal muscles, reduces the space in the chest cavity, pushing air out of the lungs.

In terms of gas exchange, the respiratory tract works with the circulatory system as follows to achieve this:

• During inhalation, oxygen molecules diffuse through the wall of alveoli into tiny blood vessels called capillaries. The oxygen-rich blood is then fed to the pulmonary vein for delivery to the heart.
• During exhalation, carbon dioxide delivered to the lungs via the pulmonary artery diffuses through the walls of the capillaries into the alveoli.

The entire process is repeated 12 to 20 times per minute in a healthy adult. During exercise or strenuous activity where more oxygen is needed, the respiration rate is increased.