The Respiratory System
The respiratory system consists of the nasal cavity (the cavity inside the nose), the pharynx (an opening for both food and air), the larynx (the opening connecting the pharynx to the rest of the respiratory system), the trachea (this is the windpipe leading to the lungs), the bronchi (two smaller tubes branching off the trachea), and the lungs (the main organ of respiration and is where oxygen uptake occurs).
During low level activities we primarily breathe air through the nasal cavity where dust particles are filtered out by a layer of tiny hairs. However, during higher levels of physical exertion we breathe air through the mouth as well as the nose. After travelling through the nose and mouth the air then passes through the pharynx, then through the larynx and along the trachea. The air then passes through the bronchi before entering the lungs.
Air enters the lungs through the bronchi. The bronchi divide into the secondary bronchi – two in the left and three in the right lung – and then divide again into the tertiary bronchi. The tertiary bronchi then subdivide into the bronchioles, which are less than 1mm in diameter, and then subdivide again to the terminal bronchioles. The diameter of the bronchi and bronchioles is controlled by smooth muscle. During exercise the smooth muscles will relax allowing a greater passage of air into the lungs. However, this is opposite of what happens during an asthma attack. In this case the smooth muscles contract and greatly reduce the air flow to the bronchioles.
The bronchioles extend into the alveoli which are small air filled sacs. It is in these small air filled sacs that gas exchange takes place. Each alveoli is like a tiny bubble that is covered by a network of tiny blood vessels (capillaries). The walls of both the alveoli and the capillaries are just one cell thick. This allows oxygen to easily penetrate through the alveoli walls, through the capillary walls, and then into the bloodstream. When in the blood stream the oxygen combines with haemoglobin within the red blood cells. From here it travels to the heart and is then pumped around the body where it will be taken up and used by various organs. Haemoglobin has a high affinity for oxygen, this means that it will take up oxygen readily but will not release it until there are low levels of oxygen in the surrounding tissue. Therefore, haemoglobin will only release the oxygen where it is needed.
Carbon dioxide, a waste product produced during energy metabolism, passes through the capillary walls, through the alveoli walls, into the alveoli and diffuses into the lungs where it is exhaled into the surrounding air.
When we breathe air from our surrounding atmosphere it contains approximately 79% nitrogen, 21% oxygen, 0.03% carbon dioxide and 1% of other gases. But when we breathe out the air contains approximately 79% nitrogen, 17% oxygen, 4% carbon dioxide, and 1% other gases. Therefore our bodies use a small amount of the oxygen that we breathe in and during metabolism this is converted to carbon dioxide as a waste product.
Air is sucked into the lungs through the process of inhalation. In this process the diaphragm (muscle that sits below the lungs) contracts and flattens, this pushes the rib cage out. The intercostals muscles (muscles between the rib cage) then contract which lifts the rib cage upwards and draws air into the lungs. The process of breathing out is called exhalation. During this process the diaphragm relaxes, and then the intercostals muscles relax pulling the rib cage downwards and force air out of the lungs.