LO2 of Unit 1 – Respiratory System (2021)

The Structure and Function of
the Respiratory System

Learning outcomes
By the end of this section, you will be able to:

2.1 Describe the structure and function of the respiratory system

2.2 Identify the primary muscles involved in breathing

2.3 Describe the route of air through the respiratory system

2.4 Describe the process of gaseous exchange (internal and external respiration)

The structure and function of the respiratory system

The lungs are located on either side of the chest and are the respiratory system’s major organs. The primary function of the respiratory system is breathing. Breathing involves inhaling air into the lungs and exhalation of carbon dioxide as a waste product.

The respiratory system has three primary functions:

  1. Provide a large surface area for gas exchange between air and circulating blood
  2. Inhale and exhale air to and from exchange surfaces
  3. Protect respiratory surfaces from environmental variations (cold/heat) and clean, moisten and warm incoming air

Structure of the lungs/labelled.

The primary muscles involved in breathing

Intercostals and diaphragm

Inhalation and exhalation


Breathing is the process of taking air into and expelling it from the lungs. Muscles contract and relax around the lungs, which changes air pressure inside the lungs. Air pressure outside the lungs is greater than the air pressure inside the lungs, and air rushes in. In contrast, if air pressure is greater inside the lungs, the air is expelled into the atmosphere. Breathing is mainly an involuntary (unconscious) process but can be controlled voluntarily (conscious breathing).


Breathing in or ‘inhalation’ is the act of taking air from the atmosphere into the lungs. It occurs due to a change in pressure between the atmosphere and the lungs. The diaphragm is a primary breathing muscle under the rib cage, like an upside-down hammock separating the thoracic and abdominal cavities. When the diaphragm contracts, the muscle drops and flattens out. Simultaneously, the external intercostal muscles (attached between and onto the costal bones) contract pulling the costal (ribs) bones up, out and to the sides. The lungs then expand, increasing their volume, but lowering the pressure inside. The result is that atmospheric pressure is higher than the lungs’ pressure, so air rushes in, filling the lungs.


Breathing out or ‘exhalation’ is the process of letting air out from the lungs into the atmosphere. It occurs when the intercostal muscles and diaphragm relax, causing the rib cage to return and the diaphragm to lift to its resting position. This action decreases the lungs volume, causing pressure inside the lungs to rise above atmospheric pressure, resulting in the air rushing out.

However, during vigorous aerobic exercise, breathing is increased to meet the needs of oxygen intake. The intercostal muscles and diaphragm, along with the abdominal muscles, actively contract to force exhalation, thereby allowing faster breathing.

The route of air through the respiratory system

Air is made up of many gases and oxygen that humans need to survive.

During breathing, oxygen enters the lungs’ alveoli via external respiration.

Oxygens journey moves past the following structures.

Pie or table listing gas %

Diagramme of lung structures

Nose and mouth

Air must first enter the nose of mouth. Under resting conditions, conditioning air enters the nose and into the nasal cavities. The benefit of nose breathing is that air can be warmed, filtered and humidified before entering other areas. 

During exercise, the body requires more oxygen and much faster than can be taken in via the nose. The mouth opens and breathing rate increases allowing greater air volumes to enter the lungs.


The pharynx is the throat area. It receives air from the nasal cavity and the oral cavity.


The larynx is positioned at the top of the trachea. It’s a vital structure directing air into the lungs. It also has the critical function of creating sound for speech.

The trachea (windpipe)

The trachea is a 4-inch long tube beginning just under the larynx (voice box) dividing into two smaller tubes called bronchi. The trachea is composed of about 20 cartilage rings which can widen and lengthens slightly with each breath. 


Bronchi is plural for bronchus. Two tubes following on from the trachea taking oxygen into the lungs’ right and left lobes. The two main bronchi branch into secondary bronchi and then branch into tertiary bronchi.


Bronchioles are small tubes made up of smooth muscle tissue branching from the right and left bronchi and connecting to the alveoli.


The alveoli are the terminal destination for oxygen and are tiny grape-like air sacs arranged in clusters throughout the lungs. Each alveolus has a large surface area. Only one cell thick allows the relatively easy passage of oxygen and carbon dioxide (CO2) between the alveoli and blood vessels called capillaries.

The process of gaseous exchange

We need oxygen to live. Once oxygen is in our body, it combines with foodstuff, resulting in chemical energy (ATP) synthesis to power life’s activities. At the same time, waste products such as carbon dioxide and water are discarded. The entire process of taking in oxygen and discarding the waste is called cellular respiration. Cellular respiration is divided into the following:


  1. Internal respiration
  2. External respiration

What is internal respiration?

Internal respiration is the gas exchange between the blood and the cells and from the cells to the blood. Tissues like skeletal muscles require oxygen to carry out cellular respiration by which the cells produce energy in the form of ATP, by metabolising food such as glucose. 

Cellular respiration occurs in the mitochondria of every cell. Through diffusion, oxygen passes from the blood into the tissues. Diffusion is a natural phenomenon whereby materials flow along with a concentration gradient such as from a high concentration area to one of a lower concentration. It is a passive process requiring no energy.  

Oxygen diffuses into cells because the blood is rich in oxygen being higher in concentration than oxygen in cells. Likewise, carbon dioxide as waste diffuses into the blood from cells due to there being a higher concentration from ATP synthesis. Carbon dioxide is expelled from the body.

Concentration gradient

What is external respiration?

External respiration is the exchange of gases with the external environment and occurs in the lungs’ alveoli. Oxygen-depleted blood transported from metabolising tissues, flows through the pulmonary capillaries where oxygen diffuses from the alveolar air into the blood. Carbon dioxide diffuses out of the blood into the alveolar air. 

Both oxygen and carbon dioxide exchanges occur until each gas’s equilibrium is established. Thus, the blood which leaves the lungs is called oxygen-rich blood. This oxygen-rich blood flows to the metabolising tissues, responding to their high oxygen demands. Both oxygen and carbon dioxide are transported through the blood by binding with haemoglobin, found in red blood cells. Some of the carbon dioxides are transported by dissolving in the plasma.

Alveoli/capillaries and Capillaries/cells