Gaseous exchange in animals
“Never give up on a dream because of the time it will take to accomplish it. The time will pass anyway.” – Earl Nightingale
HELLO THERE! How have you been doing? I hope everything has been going well for you and that you are keeping up with the workload. Remember, the work becomes easier when you approach it in an organised manner; so if you have not been doing so, you need to begin to do so right now! Time passes so quickly when you have a lot to do!
This week we continue our discussion on respiration, and we will be doing this by looking at the method of gaseous exchange in animals and plants.
First, we will look at gaseous exchange in animals. We referred to this in the first lesson; do you remember the name given to the process? Yes, that’s right, it is called breathing!
Aerobic respiration requires a continuous supply of oxygen and there is a lot of it in the air. Unfortunately, animals cannot use it while it is in the air; they have to get it inside their bodies. This happens by a process known as breathing. Animals breathe in air to get the oxygen that is used in respiration and they breathe out the carbon dioxide that is being constantly produced by respiration (revise the equations). This exchange of the gases takes place in the lungs.
The lungs are found in the thorax, surrounded and protected by the ribs which form a cage around them. Below the ribs is a sheet of muscle which is known as the diaphragm. The diaphragm separates the thorax from the abdomen below. Breathing comes about when the ribs and the diaphragm work together to get air into and out of the lungs.
Look at Figure 1. Air enters through the nose and mouth and passes down through the trachea (windpipe). The air that enters might be cold, dry and dirty, especially in countries with high pollution indices, and air of this quality can cause damage to the lungs. So, as the air passes through the nose and down the through the trachea, it is: Warmed
Moistened
Filtered and cleaned
Dust and germs are trapped in the mucus that is secreted by the cells lining the nasal cavity and the trachea. The trapped dirt is passed up back to the throat and nose by the tiny hairs found on the cells. These hairs are known as cilia. When we blow our noses and cough, we get rid of what is brought up.
Look at the two diagrams in Figure 2. Put your hands on your chest as you breathe in (inhale) and breathe out (exhale). Do you feel any difference in your ribs as you carry out these actions? Let us see what is happening?
BREATHING IN
Look at figure 1, identify the intercostal muscles. These muscles contract to raise your ribs upwards and outwards.
Identify the diaphragm, the muscles which make up the diaphragm contract, causing the diaphragm to flatten.
Both movements cause the volume of the thorax to increase. The increase in the volume of the thorax causes the pressure in the thorax to decrease.
The decrease in the pressure in the thorax makes the pressure there lower than the external air pressure, and this causes air to enter into the lungs.
BREATHING OUT
The intercostal muscles relax. What do you think this does? Did you say, ‘It causes the ribs to move downwards and inwards?’ You did! If you did, you are correct!
The muscles of the diaphragm relax, causing the diaphragm to bulge upwards.
Both movements cause the volume of the thorax to decrease. The decrease in the volume of the thorax causes the pressure in the thorax to increase.
The increase in the pressure makes the pressure in the lung greater than the pressure of the external air, and air is forced out of the lungs.
Now that the air is in your lungs, what happens next?
The site of gaseous exchange is actually deeper in the lungs in a bunch of air sacs known as the alveoli (singular – alveolus). How is the gas exchanged? If your answer is by diffusion, then your answer is correct!
The alveoli have the following characteristics:
They have thin walls.
They are moist.
They are connected to a transport system because each alveolus is surrounded by blood capillaries. When the air with the oxygen reaches the alveoli, the oxygen dissolves in the water lining the alveoli (remember they are moist) and diffuses into the blood. The carbon dioxide travels in the opposite direction.
Guess what, students? We have come to the end of the lesson and we will need to look at the characteristics of gaseous exchange surfaces before we begin to look at gaseous exchange in plants! See you next week!