Respiration is the system whereby oxygen is taken into the body by way of the lungs and passed into the bloodstream; this is called inspiration. The carbon dioxide, one of the waste products of energy, is carried by the bloodstream back to the lungs and expelled from the body; this is called expiration. The exchange between oxygen and carbon dioxide in the blood takes place in the tissues of the lungs.

The respiratory system is made up of:

  • Trachea
    • Bronchi
    • Lungs
    • Bronchioles
    • Alveolar sacs
    • Alveoli

And these are shown in the diagrams below


The trachea is a tough tube of ringed cartilage. The inside is lined with small, hair-like projections called cilia. Their function is to act as filters of the air drawn down into the lungs, and remove the particles of dust and foreign matter. These are then swept up towards the mouth, mixed with mucus, and as a result by the cilia moving in a continuous wave-like sweeping motion. The trachea fits next to the oesophagus where both food and air are drawn in at the mouth. The entrance to the trachea has to be closed when food is being swallowed to prevent food particles being carried into the lungs. This entrance is closed by a flap called the epiglottis. If you look into the back of someone’s mouth you can see the epiglottis, the small flap hanging down at the back of the throat.

At its bottom end, the trachea divides into two smaller tubes called bronchi.


The bronchi, which are also made of tough cartilage rings, lead into the lungs, one to each lung. Once inside the lungs, the bronchi divide into smaller and smaller branches called bronchioles, and these lead into small air sacs called alveoli which look like very small balloons. Around each alveolus there is a network of capillaries which connect the pulmonary artery vessels to those of the pulmonary vein. Each air sac, consisting of many alveoli, is called an alveolar sac.

Figure 2: The Trachea

Figure 3: Alveoli Surrounded by Capillaries


Each lung is a cone-shaped, soft, spongy mass of tissue consisting of tubes and air sacs. Lungs are elastic and can expand and contract as air is breathed in and out, and they are well supplied with blood vessels which divide into many thread-like capillaries. The lungs lie in a special cavity in the body called the pleural cavity; and the outside of each lung is lined with a special, slippery membrane called the pleura. The pleura allow the lungs to slide easily during breathing.

Figure 4: The Lungs


The act of breathing is done by the lungs expanding and contracting in order to draw air in and then push it out of the lungs. The chest cavity, which contains the heart and lungs, is bounded on the sides by the ribs, and at the bottom by the diaphragm.

This is a muscular plate which stretches from one side of the body to the other, and divides the chest cavity from the abdominal cavity which contains the stomach and intestines. Inside the chest cavity there is a slight vacuum, or negative pressure. By expanding the ribs outwards and pulling the diaphragm downwards, the size of the chest cavity is increased, the vacuum is increased, and the lungs expand, drawing air in. This process is known as inspiration. Try it for yourself. Take a deep breath and you will feel your ribs moving outwards. The whole movement, both of the ribs and the diaphragm, is muscular. When more air is needed, as in running, the muscles between the ribs work harder expanding the ribs further.

When both lungs have been filled with air, the diaphragm and ribs relax and push inwards towards the lungs. This decreases the vacuum inside the chest cavity by making it smaller. The lungs contract pushing the air out in the process known as expiration.

Respiration is normally an involuntary movement which goes on without any conscious thought. However, a person or animal does have some voluntary control when taking deep breaths, holding the breath and so on, although this can be done only for short periods. The rate and depth of breathing is controlled by the respiratory centre situated in the brain. This centre is stimulated by the levels of carbon dioxide in the blood. When the carbon dioxide level rises, the speed and depth of breathing is increased to expel it and increase the amount of oxygen.

Figure 5: Ribs


Blood which is low in oxygen and high in carbon dioxide is brought to the lungs by the pulmonary artery, which branches out and ends in capillaries surrounding the alveoli. The air in the alveoli which has just been breathed in is rich in oxygen, and a very rapid exchange takes place with oxygen passing through the thin walls of the alveoli into the capillaries, and carbon dioxide passing in the opposite direction, into the alveoli. The blood, now rich in oxygen leaves the lungs through the pulmonary vein, which carries it to the heart from where it is pumped around the body and used up in the production of energy.

Figure 6: Gaseous Exchange

    Conscious: aware of and responding to one’s surroundings.   Voluntary: done, given, or acting of one’s own free will.

Figure 7: Gaseous Exchange

Table 1: Normal Body Temperatures and Respiration Rates of Some Farm Animals

 Body Temp Normal Respiration Rate Per Min
Horse37.5 – 38.59 – 10
Foal39.3 – 38.514 – 15
Cow / Bull37.5 – 39.512 – 16
Calf38.5 – 40.027 – 30
Sheep38.5 – 40.012 – 15
Lamb38.5 – 40.515 – 18
Goat38.5 – 41.012 – 15
Kid38.5 – 40.512 – 20
Fowl± 40.715 – 30
Dog37.5 – 39.014 – 16
Cat38.0 – 39.520 – 30
Pig38.0 – 40.010 – 20