Respiratory System

The human respiratory system is responsible for taking oxygen into the respiring cells and carbon dioxide out of the respiring cells. All the body processes, for example movement, growth and reproduction, require energy. Unlike plants which can obtain energy directly from the sun, animals (humans included) obtain energy only from the food they eat. This energy is extracted from the chemical energy in the food through respiration.

There are two types of respiration:

  • Aerobic respiration – the breakdown of carbohydrates in the presence of in the presence of oxygen to release energy.
  • Anaerobic respiration – the breakdown of carbohydrates in the absence of oxygen to produce less energy.

In humans, the oxygen needed for respiration is obtained from the air through the the lungs.

Structure of the respiratory system

Respiratory system

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The lungs are enclosed in the thorax ( the chest region). The lungs are connected to the back of the mouth by the windpipe (trachea). The trachea then splits into two smaller tubes, called bronchi (singular bronchus). Each bronchus enter each lung and branches into even smaller tubes called bronchioles. At the end of the bronchioles are thin-walled, pouch-like air sacs (alveoli).

The lungs are enclosed by the ribs which form a cage called a ribcage. The ribcage has the following functions:

  • to protect the lungs
  • to protect the heart
  • to ventilate the lungs



During inhaling the intercoastal muscles of the ribcage contract, lifting the ribs. The volume of the chest cavity increases, reducing the pressure inside the cavity. Atmospheric pressure forces air into the lungs.

The air enters through the nasal passage. The functions of the nasal passage are:

  • to warm the air
  • to moisten the air
  • to filter dust

The air then moves down through the larynx (voice box) into the trachea. From there the air moves into the lungs. The larynx is the organ that generates voice when you speak.



During exhaling, the intercoastal muscles relax, allowing the ribs to drop back. The volume of the chest cavity decreases, increasing pressure inside the cavity. The air is pushed out of the lungs.

Gaseous exchange

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The movement of air into and out of the lungs, brings in the oxygen into the lungs and expels the carbon dioxide from the lungs. Gaseous exchange refers to the exchange of oxygen and carbon dioxide, between the air and the blood vessels of the alveoli in the lungs.

The alveoli have thin elastic walls surrounded by a dense network of capillaries with deoxygenated. The alveoli also have a large moist surface area to make it possible to take in oxygen and give out carbon dioxide efficiently.

The concentration of oxygen in the alveoli is higher than in the blood. The oxygen then diffuses into the blood along the concentration gradient. In the blood the oxygen combines with the haemoglobin in the red blood cells to form oxyhaemoglobin. The oxygen is then carried to all parts of the body by the blood.

The concentration of the carbon dioxide in the blood is higher than in alveoli, so the carbon dioxide moves from the blood into the alveoli. The carbon dioxide release is then expelled when exhaling.

Changes in the composition of breathed air

carbon dioxide0.044
water vapourvariable but lowsaturated