Click images to enlarge.
Inhaled air contains 21% oxygen. Exhaled air contains 16% oxygen at rest. Inhaled air contains 0.04% carbon dioxide. Exhaled air contains 4% carbon dioxide at rest.
The human body is made up of millions of cells – all need a constant supply of oxygen to allow energy to be released and to remove waste products. The respiratory system takes in oxygen from air that is breathed and transfers it to the blood in the lungs. Oxygen is then transported to cells in the body via the circulatory system, which transfers it in exchange for carbon dioxide. Carbon dioxide is then transported to the lungs and removed from the blood to the air outside.
The lungs are 2 thin walled elastic sacs that are found within the thoracic cavity (chest). This is an airtight area which is protected by the ribs at the back and front, and below by a strong elastic sheath called the diaphragm. The diaphragm separates the thoracic cavity from the rest of the body and is extremely important for breathing.
Other muscles that are very important for breathing are the intercostal muscles which are found between the ribs and control their movement. The lungs are surrounded by the pleural membrane which is a slippery double skin that keeps the lungs moist. These membranes slide against each other as the lungs expand and contract reducing friction with the surrounding ribs and the diaphragm.