Science 8th grade
UNIT 4: Respiration 4.2 Gas exchange
Science 8th grade
UNIT 4: Respiration 4.2 Gas exchange
Air sacs in the lungs
The photograph on the right shows what your lungs look like inside, magnified about 300 times. You can see that they are full of spaces. These spaces are called air sacs or alveoli, and they are full of air.
You cannot see them in the photograph, but there are lots of very tiny blood capillaries in the living tissue between the air sacs (which looks brown in the photograph).
How gas exchange takes place
The diagram on the right shows one of these air sacs and a nearby blood capillary.
The blood inside the capillary has come from the heart. Before that, it came from the organs of the body. The cells in these organs used up oxygen and produced carbon dioxide. So this blood contains only a little oxygen, and a lot of carbon dioxide.
The air inside the air sac has come from outside the body. It contains a lot of oxygen and very little carbon dioxide.
Oxygen therefore diffuses from the air sac into the blood capillary. It diffuses into the red blood cells. (You can read about diffusion on page 70).
Carbon dioxide diffuses from the blood air inside inside the capillary into the air sac.
Questions
1) Name the red pigment inside red blood cells that helps to carry oxygen.
2) Name the large blood vessel that carries blood from the heart to the lungs.
$A+I$ 3) Using what you know about particle theory, explain how oxygen diffuses from an air sac into the blood.
$A+I$ 4) The walls of the air sacs and the capillaries are both very thin. Suggest how this helps gas exchange to take place quickly.
Activity 4.2 (Why are air sacs so small?)
SE: Your teacher will give you two Petri dishes filled with agar jelly.
1) Use a cork borer with a diameter of 10 mm to make 8 holes in the jelly in one dish. Space the holes evenly over the dish.
2) Use a cork borer with a diameter of 5mm to make 32 small holes in the jelly in the second dish. Try to space the holes evenly over the dish.
3) Using a dropper pipette, carefully fill each hole in both dishes with a solution of a coloured dye. Record what happens after five minutes and after half an hour.
Questions
${A_1}$: Describe your observations in each dish.
${A_2}$: The holes you made in the jelly represent the air sacs in the lungs.
The coloured dye represents oxygen in the air sacs.
Explain how your observations help to show what happens to oxygen in the lungs.
${A_3}$: The total volume of the liquid in the 8 large holes is the same as the total volume in the 32 small holes. Use your results to explain why it is better to have a lot of very tiny air sacs in your lungs, rather than a few big ones.