The Respiratory System

The Respiratory System

The respiratory system is the system involved with breathing and supplying the cells in the body with oxygen.

Respiration, on the other hand, is a process in which energy is produced in the cells.


When you breathe you inhale and exhale. Inhalation goes as follows:

Air enters the body through the nasal passage or mouth. Here it’s warmed, moistened and filtered.


It then moves into trachea (or windpipe).


From here it then passes down one of the two bronchi: the left one takes the air to the left lung and the right one takes the air to the right lung.


The bronchi then split up into bronchioles in the lungs.


The bronchioles lead to alveoli which are small sacs in which gaseous exchange takes place. Here oxygen is taken from the air and enters the blood while carbon dioxide passes out of the blood and into the alveoli

Inhaling and exhaling are also a physical process. The main muscles involved are:

  • the intercostal muscles where are situated between the ribs
  • the diaphragm which can be found between the thorax and abdomen

When you inhale:

  • The intercostal muscles contract which makes the ribcage expand.
  • The diaphragm contracts as well and pulls downwards which makes it flatter. This causes the volume in the chest to increase.
  • When the pressure in the thorax decreases, air is pulled into the lungs.

When you exhale:

  • The intercostal muscles relax which makes the ribcage drop in and down.
  • The diaphragm also relaxes and moves upwards. This causes the chest volume to decrease.
  • The increase in pressure in the thorax squeezes the air out of the lungs.


Respiration is the process by which cells release energy. When you exercise your muscles require a lot more energy than when you’re resting.

The body is able to respire in two ways:

  • aerobically
  • anaerobically

Aerobic respiration requires oxygen. You respire aerobically when you’re resting and when you start to exercise. In order to get enough oxygen to your muscles you need to breath deeper and your heart rate has to increase so that blood can be pumped more quickly around the body in order to get oxygen to the muscles and take away carbon dioxide.

Aerobic respiration is represented by:

glucose + oxygen ? energy + water + carbon dioxide

As you begin to exercise harder a point is reached by which the heart simply can’t beat fast enough and so not enough oxygen is able to reach the muscles. This means that they need to start respiring anaerobically. In anaerobic respiration no oxygen is used but not as much energy is produced because the glucose can’t be broken down completely. As a result, the waste product lactic acid is produced.

Anaerobic respiration is represented by:

glucose ? energy + lactic acid

Inhaled and exhaled air

The air you inhale and the air you exhale are different.

Exhaled air contains less oxygen then inhaled air because oxygen is used by the body for respiration and it contains more carbon dioxide because this is expelled by the body as a waste product.

Gas Approximate % found in inhaled air Approximate % found in exhaled air
OxygenCarbon dioxideNitrogen 210.0479 16479

Due to the fact that the concentrations of these gases are different it’s possible for gaseous exchange to take place in the alveoli.