Breathing and gaseous exchange
Joanna George-Johnson, Contributor

During certain exercises we are often told to inhale and exhale, but do we actually understand the many processes that allow this to take place? Today, we will be focusing on breathing and gaseous exchange. This topic is very important and affects our daily lives. By the end of the topic, you should be able to:

i) Describe and explain the importance of breathing and gaseous exchange in plants and animals.

ii) Name and give characteristics of gaseous exchange surfaces.

Breathing is often confused with respiration. However, there is a great difference! Breathing is simply the process by which air is drawn into and pushed out of the body. Breathing aids in the process of respiration. This relationship can be seen if someone who is prevented from breathing dies, which is the principle behind suffocation.

Additionally, gaseous exchange in the lungs and breathing are often wrongly used interchangeably. Gaseous exchange in the lungs is the means by which we get oxygen from the air into our blood and carbon dioxide out of our blood into the air. Breathing and gaseous exchange are important to ensure that organisms that respire aerobically (with the use of oxygen) have a continual supply of oxygen. They also provide a pathway by which carbon dioxide can be emitted from the body. If carbon dioxide was left in the body and accumulates, it would be poisonous to the body.

The breathing mechanism is carried out by a bunch of cavities and tubes which make up the respiratory system. A diagram of the respiratory system is shown in Diagram 1.

When breathing in (inspiration), the lung is expanded, causing air to flow in. The lung collapses to blow air out (expiration).

Expanding the lungs

In order to expand the lungs, the following activities have to take place:

i) External intercostal muscles contract and internal intercostal muscles relax which causes the ribs to move up and out.

ii) Diaphragm muscles contract and flatten the diaphragm, resulting in an increase in the volume of the thorax and the lungs.

iii) Pressure inside the lungs is therefore decreased and so, air is drawn in.

In the expiration process, the opposite occurs:

I) Internal intercostal muscles contract and external intercostal muscles relax, which causes the ribs to move down and in.

II) Diaphragm muscles relax, causing the diaphragm to have an upward dome shape, resulting in the decrease in the volume of the thorax and the lungs.

III) Pressure inside the lungs is therefore increased and so, air is pushed out.

The process of breathing has now facilitated the presence of air in the lungs, enabling gaseous exchange. Some persons are of the impression that only oxygen is taken into an organism and carbon dioxide released. This is not so, since air is what is taken in and pushed out. The table below shows the main constituents of inhaled and exhaled air.

Gas	% inhaled air	% exhaled air
Oxygen	21	17
Carbon dioxide	0.04	4
Nitrogen	78	78

As was said earlier, gaseous exchange is important for the movement of oxygen into blood and carbon dioxide out of the blood. This process is accomplished via diffusion. Gaseous exchange is therefore important to facilitating respiration. The process occurs at gaseous exchange surfaces. The gaseous exchange surface in amoeba is the cell membrane; in fish, it is the walls of the gill lamellae; in plants the spongy mesophyll cells in the leaf; and in humans, it is all of the alveoli in the lungs.

Diagram showing gaseous exchange surface of fish

Gaseous exchange surfaces are made to suit their role:

i) They are thin and usually consist of one cell. This allows for gases to diffuse rapidly.

ii) They have a large surface area, allowing for more gases to be diffused.

iii) They are moist, which aids in the diffusion process since gases must dissolve for diffusion to take place.

iv) A rich blood supply in organisms that have blood. A blood supply close to respiratory or gaseous exchange will allow gases to diffuse more rapidly.

Questions

Diagram showing gaseous exchange surface of plants

1. Give another place in humans other than the lungs where gaseous exchange occurs.

2. What is the main difference between inhaled and exhaled air in terms of their composition? Why are they different?

3. Name the process by which oxygen gets into the blood and carbon dioxide gets into the lungs from the blood.

Diagram showing gaseous exchange surface of humans

Joanna George-Johnson teaches at Ardenne High School.