The Open Door Web Site
Part III : How Living Organisms Breathe : Index
Breathing in the Air Summary
Model of the thorax, showing the lungs,
The lung fish has rudimentary lungs
INTO THE THORAX
The trachea is a flexible pipe which travels from the throat down into the thorax. The trachea is supported by rings of a material called cartilage. Cartilage is flexible but strong. It stops the trachea collapsing as inspired and expired air move through it. The cartilage rings also allow us to continue breathing when the neck is bent. The inside of the trachea is covered by millions of microscopic hairs called cilia (singular = cilium). The cilia are covered with a layer of liquid called mucus. Very fine dust particles and microbes get trapped in the mucus and cilia. Although the cilia are attached at one end, each one is able to move (beat). As the cilia beat they tend to push the dust-containing mucus toward the nose and the mouth. In this way the lungs are kept free of most of the dust and microbes which we inhale.
In the lungs
The air passes down the trachea into the thorax. The trachea branches into two smaller tubes called bronchi (singular: bronchus). The bronchi direct the air into each lung where they divide and divide again to form a three dimensional network of tubes called bronchioles. This network of bronchioles is known as the bronchial tree. At the end of each of the smallest bronchioles is a microscopic air sac called an alveolus (plural : alveoli). The inspired air reaches the end of its journey when it enters the alveoli.
There are millions of alveoli in each lung. They make the lung tissue feel spongy. Just under the wall of each alveolus is a network of blood capillaries. The large numbers of capillaries give the lungs their pink colour. The blood in the capillaries picks up oxygen from the air in the alveoli and leave behind carbon dioxide and water vapour. This changed air is then expired.
The Oxygen in the Blood
The blood in the capillaries comes from the heart and, once it has lost its carbon dioxide and gained oxygen, the same blood moves back to the heart. It is no accident that the heart is the other organ found in the thorax since the lungs and the heart work very closely together.
The oxygen-rich blood returns from the lungs to the heart. The heart then pumps this blood to all the different parts of the body. The oxygen is used by the living tissues for respiration. Respiration releases energy which keeps our bodies alive. One of the waste products of respiration is carbon dioxide. So this is how the body consumes oxygen and produces carbon dioxide. Respiration also produces water, some of which can be used by the body. The carbon dioxide must be excreted from the body, so it travels in the blood as it returns to the lungs.
Animals that use Lungs
All four groups of vertebrates - mammals, birds, reptiles and amphibians, breathe using lungs. Only one type of fish, the lung fish, have lungs. The structure of the breathing system is not always the same, however, even amongst these vertebrate groups. Reptiles and birds do not have a diaphragm, they only use their ribs to expire and inspire. Amphibians, such as frogs, have very simple lungs, like empty sacs, although they can stretch them a lot. So amphibian’s lungs are not spongy like mammalian lungs.
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