34.8 Circulation and Respiratory Systems

34.8.2 Describe the basic anatomy (structure) of the respiratory system.

34.8.4 Describe the physiology (function) of the respiratory system.

34.8.10 Describe the role of the lungs in oxygen and carbon dioxide transfer.

The Respiratory System

• Air can be drawn into the body through the nose and mouth.  
• The nose warms, moistens and filters air which is then taken to the lungs via the windpipe (trachea).  
• The two lungs are the organs within which the exchange of gases between the atmosphere and blood takes place.  
• The lungs occupy almost the entire chest cavity and together with the heart are the only major organs in the chest.
• The trachea divides into two bronchi within the chest which supply the left and right lungs.  
• Each bronchus divides many times, like the branches of a tree, to produce millions of tiny passages or bronchioles each of which ends with many grape-like air sacs called alveoli.  
• Each alveolus is covered with fine capillaries which carry blood and, because the walls of the alveolus are very thin and a pressure gradient exists across them, oxygen readily diffuses into the blood and carbon dioxide is removed from the blood.
• Oxygen is taken up by the protein haemoglobin in the red blood cells and transported in the blood throughout the body.  
• Haemoglobin is a highly complex and specialised oxygen transport system that allows far more oxygen to be carried by blood than could be achieved by simple solution.
• The haemoglobin also has the property of remaining bound to oxygen molecules until it enters an area of very low oxygen tension where the oxygen is released to diffuse into the tissues.  
• Carbon dioxide is then diffused into the blood from the tissues of the body, and is carried in solution in the blood as carbonic acid back to the lungs to be exhaled.
• The lungs are protected by the ribcage.  
• A sheet of muscle called the diaphragm is located beneath the lungs.  
• Breathing is performed by movement of the ribcage and diaphragm.  
• To inhale, the muscles between the ribs contract, which pulls the ribcage upwards and outwards - at the same time the diaphragm contracts downwards - thus the volume of the chest is increased and the lungs expand.  
• This produces a negative pressure in the lungs which consequently fill with air.  
• To breathe out the ribcage muscles relax and the diaphragm rises causing the chest volume to reduce and air to be exhaled from the lungs.