Aerobic Respiration in Animals

You are here

Aerobic Respiration in Animals

After eating, our food is digested and the small molecules are absorbed into the blood. The blood carries the molecules to each cell where they are used to build new molecules or are used in respiration to release energy to 'power' the cells. So animals need to breathe to get the oxygen for respiration.

We make two sets of breathing movements:

In called inspiration also called inhalation
Out called expiration or exhalation

The human breathing system:

breathing system

We breathe in by using 2 lots of muscles. The diaphragm is a muscle sheet below the lungs, which flattens and pulls down as it contracts. Then between the ribs are intercostal muscles which act to pull the ribcage up and outwards.

breathing system

We feel as if we suck air into our lungs but actually it is pushed in. Our muscles make the pressure in our chest lower than atmospheric pressure outside so air is forced in.

Now have a go at putting the sequence of events into the correct order. Enter the sequence from the first (1) up to the last (6):

Install Flash

Try again at working out the correct sequence, this time for the events of expiration. Remember to add the numbers from the first (1) up to the last (6):

Install Flash

Expiration, breathing out, is a bit simpler. Usually all we have to do is to let the two sets of muscles relax, so that the diaphragm is moved back up to its original position, and our ribcage falls back into its normal place. This pushes the air out of the lungs.

Breathing isn't just about making movements. It is about moving gas molecules.

The air that we breathe out has more carbon dioxide in it (4%) than we breathe in (0.04%). It has less oxygen too, the air we breathe in has about 21% oxygen whereas that which we breathe out has 16%.

So there are changes going on. But where do they occur? In the lungs!

The lungs aren't just bags holding gas. You need to see them as a long series of tubes that finally reach dead-ends.

The air enters the lungs down the trachea which branches into the right and left bronchi. Each bronchus then divides further into bronchioles. After about 20 branchings you reach the air sacs, the alveoli.

Each alveolus has a thin layer of epithelial cells separating the air from blood capillaries - a bit like the villi in the digestive system.

The alveoli have a similar job too, they are involved in moving molecules about.

Oxygen molecules diffuse from the alveoli into the blood stream, where there is a lower concentration of oxygen. The carbon dioxide diffuses the otherway, from the high concentration in the blood to the alveoli.