Breathing is the term given to the process of taking air into and out of the lungs.
Two important structures for breathing are the diaphragm and intercostal muscles.
The diaphragm is a sheet of muscle that separates the chest (or thoracic) cavity from the rest of the body.
The intercostal muscles are found between the ribs and they control rib movement.
The diaphragm contracts and moves downwards. The intercostal muscles contract and move the ribs upwards and outwards. This increases the size of the chest and decreases the air pressure inside it which sucks air into the lungs.
When exercise begins, inspiration can be assisted by the pectoral muscles and the sternocleidomastoid which help to lift the ribs up and out even further.
The diaphragm relaxes and moves back to its domed shape. The intercostal muscles relax so the ribs move inwards and downwards under their own weight. This decreases the size of the chest and increases the air pressure in the chest so air is forced out of the lungs.
During exercise, this passive process of relaxation becomes active as the abdominal muscles pull the ribs downwards and inwards even further.
Gaseous exchange occurs at the alveoli in the lungs and takes place by diffusion. The alveoli are surrounded by capillaries so oxygen and carbon dioxide diffuse between the air in the alveoli and the blood in the capillaries.
Diffusion is the movement of gas from an area of high concentration to an area of low concentration.
There is a high concentration of oxygen in the alveoli and a low concentration of oxygen in the blood, so oxygen diffuses from the alveoli into the blood.
There is a high concentration of carbon dioxide in the blood and a low concentration in the alveoli, so carbon dioxide diffuses from the blood into the alveoli.
Both oxygen and carbon dioxide are capable of combining with an iron-rich protein in the blood called haemoglobin. Haemoglobin carries oxygen to be exchanged at the working muscle and carbon dioxide to be exchanged at the lung.
Capillaries surround the alveoli in the lungs. Both the capillaries and alveoli walls are very thin - just one cell thick. They are made of semi-permeable membranes which allow oxygen and carbon dioxide to pass through them.
Describe the process of gaseous exchange at the muscles.
In the muscle there is a high concentration of carbon dioxide and in the bloodstream there is a high concentration of oxygen.
Oxygen diffuses from the bloodstream into the muscles and carbon dioxide diffuses into the blood from the muscles.