You need to know about three things:

  1. Tissue Respiration getting energy out of glucose.
  2. Ventilation of the Lungs getting air in and out of your lungs.
  3. Gaseous Exchange swapping Oxygen and Carbon Dioxide.

Try to avoid using simple words like “breathing” if this does not make it absolutely clear what you mean. Tissue Respiration, Ventilating the Lungs and Gaseous Exchange are terms with precise meanings and you must know what these are.

Tissue Respiration.

Tissue respiration is the release of energy, usually from glucose, in the tissues of all animals, green plants, fungi and bacteria. All these living things require energy for other processes such as growth, movement, sensitivity, and reproduction.

The most efficient form of respiration is aerobic respiration: this requires oxygen. When oxygen is not available, some organisms can respire anaerobically i.e. without air or oxygen. Yeast can respire in both ways. Yeast gets more energy from aerobic respiration, but when it runs out of oxygen it does not die. It can continue to respire anaerobically, but it does not get so much energy from the sugar. Yeast produces ethanol (alcohol) when it respires anaerobically and ultimately the ethanol will kill the yeast.

We can respire in both ways too. Normally we use oxygen, but when we are running in a race, we may not get enough oxygen into our blood, so our muscles start to respire anaerobically. Unlike yeast we produce lactic acid. Of course if we produced alcohol in our muscles it would make us drunk! Fine thing if you are running away from a predator and you end up drunk! Making lactic acid is not much better. Lactic acid causes cramp.

Glucose + Oxygen = Carbon Dioxide + Water + Energy

This word equation means: “sugar and oxygen are turned into carbon dioxide and water releasing energy”. You must memorise the word equation (and the balanced chemical equation if you want a grade A, B or C). Get help memorising the equations

Glucose = Carbon Dioxide + Ethanol + Energy

This word equation means: “glucose is turned into carbon dioxide and ethanol releasing energy”. You must memorise this word equation.

Ventilating the Lung.

This is the proper term for what many people call breathing. If you open a window, you can ventilate a room and get rid of all the nasty smells of BO, cigarette smoke, and cooking; but if you open your mouth it is not enough to ventilate your lungs. You must actually suck air into your lungs. We do this by contracting the diaphragm when we are sitting down and “breathing” gently, and by raising and expanding our rib cage when we are “breathing” deeply. Sucking air into the lungs is called inhaling. Inhaled air is mixed with the stale air already in our lungs; so although the air we inhale contains 20% Oxygen, the air in our alveoli (alveolar air) only contains 14% Oxygen.

Don’t expect all the stale used air to come out just by opening your mouth, it must be pushed out. This happens when the diaphragm relaxes and the muscles of the abdomen (tummy) push the lungs up. The rib cage can also be pulled down and in. So this is how you exhale. Although alveolar air only contains 14% Oxygen, it gets mixed with rather fresher air in your trachea. This means that exhaled air may contain 16% Oxygen.

When I inhale and exhale as deeply as I possibly can, about 5.5 Litres of air comes in and out.

Gaseous Exchange.

All forms of respiration require some form of gaseous exchange. In aerobic respiration, Oxygen must enter our blood and Carbon Dioxide must leave the blood through our lungs. Gaseous exchange is the exchange of Oxygen and Carbon Dioxide across a respiratory surface. Many animals which live in water or very wet places use gills for gaseous exchange. Animals which live on dry land use lungs. Our lungs have an enormous surface area so that Oxygen can get into the blood quickly enough and Carbon Dioxide can get out of our blood quickly enough. Our lungs contain billions of very tiny sacs called alveoli. Each alveolus is microscopic; but if we took all the alveoli in someone’s lungs and laid them flat side by side we would end up with a sheet the size of a tennis court. As well as having a very, very, very large surface area, the walls of out alveoli are incredibly thin, so the distance between the air in our lungs and the blood in our capillaries is very, very, very small.

So in your exam remember that respiratory surfaces:

  1. have a very large surface area and
  2. are very thin.

These two things allow the respiratory gases (Oxygen and Carbon Dioxide) to get in or out of the blood fast enough. If you don’t believe this, find someone who has been smoking cigarettes for fifty years. They might have a disease called emphysema. What happens is that instead of having billions of very tiny alveoli, they have millions of larger ones; this means that the surface area of their lungs is not the size of a tennis court but just the size of a dining room table. People with emphysema get out of breath very quickly. Even getting out of the chair to change channels on the TV makes them puff and pant as though they had run the marathon. Perhaps that is why we need remote controls for our TVs and radios.

Gaseous exchange is also necessary for photosynthesis. Green plants do respire: at night time they exchange gases just as we do, Oxygen in and Carbon Dioxide out. In the daytime they do just the opposite. Carbon Dioxide enters a plant because it is needed for photosynthesis, and Oxygen leaves. This is still called gaseous exchange.