Interactive Primary Newsletter 30 Late developers, classification & model lungs

And the heart must pause for breath The primary function of the respiratory system is to supply the blood with oxygen. When we breathe in, we inhale air containing oxygen and carbon dioxide. Breathing out we exhale air with a lower concentration of oxygen and a higher concentration of carbon dioxide than inhaled air.   Lung model This exchange of gases is the respiratory system’s means of getting oxygen into the blood whilst removing waste carbon dioxide. The oxygenated (red) blood then delivers oxygen to all parts of the body. Oxygen in the air we breathe enters the respiratory system through our mouth and/or nose. The air then passes through the larynx (voice box) and the trachea (windpipe), which is a tube that enters the chest cavity. Here the trachea splits into two smaller tubes called the bronchi (right) Fig.4 Inhale - diaphragm pulled down Each bronchus then divides again forming the bronchial tubes which lead directly into the lungs where they divide into many smaller tubes, each of which connects to tiny sacs (alveoli). Each alveolus is in close contact with a tiny blood vessel called a capillary. The blood in these capillaries has been around the body so has a low concentration of oxygen and a relatively high concentration of carbon dioxide. Oxygen diffuses from the air in the alveolar sacs across the alveolar and capillary membranes into the blood. Carbon dioxide diffuses from the blood across the alveolar and capillary membranes into the alveolar sac and is breathed out. So how do we make the bits move to allow us to breathe in and out? A large, dome-shaped muscle (diaphragm) lies across the bottom of the chest cavity. As it contracts and relaxes, breathing takes place. When it contracts, ‘fresh’ air rushes into the lungs (Fig.4). When the diaphragm relaxes, air containing waste carbon dioxide is released from the lungs as the pressure inside is greater than that outside (Fig.5). Fig.5 Exhale - diaphragm & chest muscles relax Both the diaphragm and the muscles between the ribs (intercostal muscles) contribute to changes in the size of the chest. Hence the pressure within the chest (thoracic) cavity results in changes in the size of the lungs. As the diaphragm and intercostal muscles contract, the volume of the chest cavity increases and the pressure within, reduces. This allows air from outside to flow into the lungs Lung model A simple model showing the mechanism of the diaphragm is easily made. All you need is a plastic drinks bottle, 2 drinking straws, 2 balloons and a plastic bag. Two holes are drilled in the cap of the bottle to take the drinking straws. The straws are then pushed through the cap and a small balloon is taped to each straw.                          Fig.6 Straws fed through bottle top The bottom of the plastic bottle can easily be cut off with scissors. A plastic sandwich bag (from the supermarket) is then taped over the bottom of the bottle and hey presto you have a model of the working of the diaphragm. As the plastic is pulled down the balloons (lungs) inflate, as the plastic is pushed up the lungs deflate. One limitation of this model is that it can’t represent movement of the rib cage. The photos (Figs. 6 and 7) show the finished model.                                   Fig.7 Lung models