Gaseous exchange surfaces
“Happiness is when what you think, what you say and what you do are in harmony.”
L– Mahatma Ghandi AST WEEK, we looked at gaseous exchange in animals and this week we will be looking at the surfaces where this exchange takes place. What we discussed was actually only a part of the story because we spoke only about animals (humans) living on land. You remember, don’t you, that a vast majority of our animals are aquatic, living in freshwater or marine environments. For these animals, the lung would not be an appropriate organ for the exchange of gases because it cannot function in water. In the other aquatic animals, the lung is replaced by the gills. Aquatic mammals such as whales and dolphins do not have gills; they have lungs, so they cannot remain underwater for very long periods. They have to come to the surface to breathe. Each gill has dark red string-like structures known as the gill lamellae. This is where gaseous exchange takes place. The gill rakers serve to prevent debris from damaging the filaments.
The oxygen that enters is found dissolved in the water which passes over the gills when the fish opens its mouth. After the oxygen is extracted, the water passes out through the opening formed by the bony gill cover, known as the operculum. The following diagram shows the movement of the water.
In plants, gaseous exchange takes place mostly in the leaf. Do you remember the characteristics of the leaf that you learnt when you were doing photosynthesis? Remember the spongy mesophyll layer with the intercellular air spaces which served as accumulation areas for the gases? The factors which made the leaf suitable for supplying carbon dioxide for photosynthesis are the same factors which make it suitable for providing oxygen for respiration.
In all of these organisms, the sites where the gases are exchanged have certain features in common.
CHARACTERISTICS OF GASEOUS EXCHANGE SURFACES
1. They must have a large surface area to facilitate the diffusion of gases
a. Man – The lungs have many air sacs/alveoli. If these are laid out side by side, they would measure over 100m2
b. Fish – In the fish, the respiratory surface where gaseous exchange takes place is the gill. Each gill has these dark red string-like structures which are known as the gill lamellae.
c. In the plant, the irregular-shaped cells of the spongy mesophyll provide a large surface area for the exchange of gases. 2. They must be thin. a. Gases move by diffusion, and diffusion can only take place across short distances. 3. They must be moist. a. Gases must dissolve in moisture in order to be able to diffuse. 4. They need a constantly moving transport medium. a. The medium must be constantly moving in order to maintain the concentration gradient so that diffusion can continue to take place.
There are many habits that human beings pursue which can harm their bodies. One of these is cigarette and, or marijuana smoking. This practice harms the lung, the organ which, as we have seen, provides our bodies with the oxygen that we need to keep us alive. We are now going to take a look at the effects that the smoking of cigarettes have on the lungs.
EFFECTS OF CIGARETTE SMOKING
It produces lung cancer. 25% of people who smoke die from lung cancer.
It causes lung cancer in non-smokers through second-hand smoke.
It promotes heart disease.
CIGARETTE SMOKE CONTAINS TAR
Nicotine
Tar Carbon monoxide
NICOTINE
It is the presence of nicotine in the smoke that makes the cigarette addictive.
1. Reduces airflow into and out of the lungs
2. Paralyses cilia, preventing them from removing dust and germs from the air going into the lungs.
3. Raises blood pressure.
4. Raises heart rate.
5. Increases the risk of osteoporosis.
1. Sticks to the cells in the lungs.
2. Promotes cancer.
3. Damages lung tissue.
4. Breaks down alveoli, causing a reduction in the number of alveoli and, hence, the size of the respiratory surface.
5. Causes bronchitis, affecting breathing.
CARBON MONOXIDE
1. Puts haemoglobin out of action by combining with it, hence less oxygen is transported.
2. Causes breathlessness.
3. Reduces the birth weight of babies born to mothers who smoke.
In our next lesson we will begin a new topic!