Reference Article: Respiratory system facts, function and diseases.

The human respiratory system is a series of organs responsible for taking in oxygen and expelling carbon dioxide. The primary organs of the respiratory system are the lungs, which carry out this exchange of gases as we breathe.

The lungs work with the circulatory system to pump oxygen-rich blood to all cells in the body. The blood then collects carbon dioxide and other waste products and transports them back to the lungs, where they’re pumped out of the body when we exhale, according to the American Lung Association.

The human body needs oxygen to sustain itself. After only about five minutes without oxygen, brain cells begin dying, according to the National Institute of Neurological Disorders and Stroke, which can lead to brain damage and ultimately death. [Gasp! 11 Surprising Facts About the Respiratory System]

In humans, the average breathing, or respiratory rate, mostly depends on age. A newborn’s normal breathing rate is about 40 to 60 times each minute and may slow to 30 to 40 times per minute when the baby is sleeping, according to Stanford Children’s Health. The average resting respiratory rate for adults is 12 to 16 breaths per minute, and up to 40 to 60 breaths per minute during exercise, according to the European Respiratory Society.

Parts of the respiratory system

As we breathe, oxygen enters the nose or mouth and passes the sinuses, which are hollow spaces in the skull that help regulate the temperature and humidity of the air we breathe.

From the sinus, air passes through the trachea, also called the windpipe, and into the bronchial tubes, which are the two tubes that carry air into each lung (each one is called a bronchus). The bronchial tubes are lined with tiny hairs called cilia that move back and forth, carrying mucus up and out. Mucus is a sticky fluid that collects dust, germs and other matter that has invaded the lungs and is what we expel when we sneeze and cough.

The bronchial tubes split up again to carry air into the lobes of each lung. The right lung has three lobes while the left lung has only two, to accommodate room for the heart, according to the American Lung Association. The lobes are filled with small, spongy sacs called alveoli, which is where the exchange of oxygen and carbon dioxide occurs.

The alveolar walls are extremely thin (about 0.2 micrometers) and are composed of a single layer of tissues called epithelial cells and tiny blood vessels called pulmonary capillaries. Blood in the capillaries picks up oxygen and drops off carbon dioxide. The oxygenated blood then makes its way to the pulmonary vein. This vein carries oxygen-rich blood to the left side of the heart, where it is pumped to all parts of the body. The carbon dioxide the blood left behind moves into the alveoli and gets expelled in our exhaled breath.

The diaphragm, a dome-shaped muscle at the bottom of the lungs, controls breathing and separates the chest cavity from the abdominal cavity. When air gets taken in, the diaphragm tightens and moves downward, making more space for the lungs to fill with air and expand. During exhalation, the diaphragm expands and compresses the lungs, forcing air out.