Post-Departure Reflections

Anatomy & Physiology – The Respiratory System


The primary function of the respiratory system is to keep the blood oxygenated by inhaling oxygen and exhaling carbon dioxide. The system is divided into two parts.

The upper respiratory tract is comprised of the following:

  • Nose. The nose is the entrance to the respiratory tract. Hairs within the nostrils help filter out foreign particles from entering the respiratory system.
  • Pharynx. Situated behind the mouth, the pharynx is the passage to the stomach and the lungs.
  • Larynx. Located at the top of trachea, the larynx contains vocal cords; it is also called the voice box.
  • Trachea. The trachea is a tube-like structure that helps move air from the larynx to the bronchi; it is also called the windpipe. It is about 10-12 centimeters in length and 2.5 centimeters wide.


The lower respiratory tract is comprised of the following:

  • Bronchi. The trachea divides into tubes called bronchi. Bronchi are composed of hyaline cartilage and further branch out until they become bronchioles. They contain mucous membranes and cilia which further filter out foreign matter not yet removed from incoming air.
  • Bronchioles. These are the branches of the bronchi that conduct air into the lungs. They are very small and are made up of smooth muscle cells.
  • Alveoli. The alveoli are air sacs in the lungs where gas exchange occurs. Alveoli walls are thin, moist, and surrounded by capillaries.
  • Lungs. These make up the body’s primary respiration organ.


Respiratory system

The Lungs

The lungs are the main organs of our respiratory system. They are located in the upper part of our chest on either side of the heart. The lungs are covered in a membrane called the pleura that enables the lungs to move smoothly when we breathe.

The lungs are separated from each other by the mediastinum, an area that contains the heart and its vessels, the trachea, esophagus, and thymus.

The lungs are not the same in either size or shape. The left lung has two lobes called the left upper lobe and left lower lobe. The right lung has three lobes called the right upper lobe, right middle lobe and right lower lobe. The left is smaller than the right lung because it shares space with the heart.



The act of breathing consists of two phases, inspiration and expiration.



Inspiration (when you inhale) is the process where oxygen enters the respiratory system through the mouth and the nose. The nasal passages contain mucous that helps filter out contaminants like dust or pollen that can cause irritation. These contaminants are forced out of the respiratory tract by coughing or sneezing.


The nasal passages contain mucus membranes that warm and humidify the breathed in air in order to prevent the trachea from becoming dry.The air passes from the nose and mouth through the larynx, which prevents food or liquid entering the respiratory tract. The air then enters the trachea.

In the chest, the trachea splits into two smaller tubes called the bronchi. Each bronchus then divides again forming the bronchial tubes, which lead directly into the lungs where they divide into many smaller tubes that connect to tiny, spongy air sacs called alveoli.

The average adult’s lungs contain about 600 million alveoli, which are surrounded by capillaries. The inhaled oxygen passes into the alveoli and then diffuses through the capillaries into the arterial blood.

Below the lungs is a muscle called the diaphragm that plays an important role in respiration. During inhalation, the diaphragm contracts, creating a vacuum that helps pull air into the lungs.



At the same time oxygen is passing into the alveoli, blood from the veins release carbon dioxide into the alveoli during expiration (when you exhale). The carbon dioxide exits the lungs via the bronchial tubes, travels back up the trachea and larynx, and then is breathed out of the mouth or nose.

During exhalation, the diaphragm relaxes which helps in forcing the air out of the lungs.


Central Control

Breathing, also called ventilation, is an involuntary process controlled by a region in the medulla called the respiratory center, which transmits nerve impulses to the diaphragm, causing inhalation. Stretch receptors in the alveoli and bronchioles detect inhalation and send inhibitory signals to the respiratory center, causing exhalation.

Ventilation can also be a voluntary action, controlled by the cortex, such as when we hold our breath or blow out candles.



Another function of the respiratory system is vocalization. The process is how humans are able to make sounds and speak as air passes through the larynx and makes the vocal cords vibrate. 


The respiratory system keeps the blood oxygenated in order to maintain life. In general, humans can only live around three minutes without air before starting to suffer organ failure or brain damage. In addition to air, people need nourishment. The next lesson explains how the digestive system extracts nutrients from the food we eat to keep the cells fueled.

Respiratory System, Part 1: Crash Course A&P

Supplemental Material

Respiratory System, Part 1: Crash Course

Respiratory System – An Overview

Respiratory Anatomy

Course Discussion