Respiratory System

To live, we need oxygen. Our respiratory system has adapted to allow gaseous exchange of atmospheric air and the by products of respiration to travel in and out of the body. The respiratory system works together with the cardiovascular system to provide oxygen and remove carbon dioxide (by product of respiration). Fitness instructors and personal trainers learn about the respiratory system during their level 2 gym instructor modules and level 3 personal trainer modules. It is important to understand the structures and functions, this allows us to plan safe and effective training and build an awareness of the lifestyle factors that can affect the functions of the respiratory system.

Respiratory Pathway

Air passes into and out of the body through the following respiratory pathway:

  1. Nose / Mouth
  2. Pharynx
  3. Larynx
  4. Trachea
  5. Bronchus
  6. Bronchioles
  7. Alveoli

When we breathe in, air passes from the top of the pathway into the bottom of the pathway. When we breathe out, air passes from the bottom of the pathway to the top of the pathway. Oxygen and carbon dioxide move from high concentrations to low concentrations.

Functions of the Respiratory System

The function of the respiratory system is simple;

  • Take in oxygen and;
  • Remove carbon dioxide


  • Inspiration/inhalation: – drawing air into the lungs.
  • Expiration/exhalation: expelling air out of the lungs.
  • External respiration: exchange of gases between the lungs and blood
  • Internal respiration: exchange of gases between the blood and all cells

Mechanics of Breathing

Air conditioning units in buildings constantly draw external air into buildings and draw out the air from within the building, expelling it into the external environment. Our respiratory system does a similar job except the air is either rich with oxygen and low in carbon dioxide or low in oxygen and high in carbon dioxide. To understand the mechanics of ventilation we must understand the mechanics of breathing in and out.

When you breath in or Inhale:

The internal intercostal muscles relax and the external intercostals contract, pulling the rib cage up and out. The diaphragm contracts and pulls down. The lung volume increases, decreasing the air pressure inside the lungs, drawing air into the lungs.

When you Breath out or Exhale:

The external intercostals relax and the internal intercostals contract, pulling the rib cage down and in. The diaphragm relaxes, drawing up Lung volume decreases and the air pressure inside the lungs increases, forcing air out of the lungs.

Breathing is triggered by rising amounts of carbon dioxide within the blood and stretch receptors within the respiratory muscles.

Gaseous Exchange

Gas exchange takes place in the alveoli within the lungs. Alveoli are small sac like structures, their features include:

  • Thin walls (1 cell thick)
  • Large surface area
  • Moist surface
  • Numerous capillaries

Oxygen and carbon dioxide flow from high concentrations to low concentrations and are aided by negative pressure, produced by the mechanics of breathing.


Diffusion is the movement of gas from a high concentration to a low concentration. When we breathe in oxygen is pulled into the lungs and travels down the trachea, to the bronchioles and into the Alveoli. Oxygen is transferred from the Alveoli to the blood via “diffusion” and carbon dioxide is transported from the blood to the alveoli. This process continually maintains the constant flow of oxygen and carbon dioxide.

Oxygen and carbon dioxide are carried to and from cells by binding with the iron rich protein haemoglobin. This transport protein carries oxygen to be exchanged at working muscle cells and carbon dioxide from working cells to be exchanged at the alveoli.

Inhaled & Exhaled Air

Atmopheric air and exhaled air contain the exact same gases but they contain different amounts of the same gases. Specifically exhaled air contains:

  • Less oxygen
  • More carbon dioxide
  • Slightly more nitrogen

Lung Volumes

Tidal Volume: The amount of air breathed in during one normal breathe. Average tidal volume is 0.5l (500ml).

Inspiratory reserve volume: is the maximum amount of air that can be taken in to the lungs after one normal breathe.

Expiratory reserve volume: is the maximum amount of air that can be forced out of the lungs after one normal breath out.

Residual reserve volume: is the amount of air in the lungs after one maximum breathe out. This air keeps the lungs partially inflated to protect the internal structures of the lungs from damage.

Effects of Exercise on Breathing

During exercise there is an increased demand for oxygen and the removal of carbon dioxide. This means that cells of the body “respire” more during exercise than they do when resting. The cardio-respiratory system is the respiratory system and the cardiovascular system working together to remove waste products from working cells and deliver oxygen to working cells.

During exercise we know that, heart rate increases to send more deoxygenated blood to the capillaries of the alveoli within the lungs to pick up oxygen and drop off carbon dioxide. To accommodate this need by the cardiovascular system, the respiratory system responds. The respiratory system increases breathing rate (frequency of breaths) and the depth of breathing (tidal volume) increases. With an increased breathing rate and depth, tidal volume increases to support the delivery of oxygen and the removal of increased levels of carbon dioxide.

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