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3.1. Exchange Surfaces and breathing (3.1.2 Mammalian gaseous exchange…
3.1. Exchange Surfaces and breathing
3.1.1 Exchange surfaces
Three factors to affect the need for an exchange system
size
If there was a single layer of cells, diffusion distance is short so that gas exchange is efficient
In multicellular organisms, diffusion is too slow to enable a sufficient supply to the innermost cells due to the long diffusion pathway
Level of activity
Cells of an active organism needs a good supply of nutrients and oxygen. Therefore needs an efficient gaseous exchange system
Metabolic activity uses energy from food and requires oxygen to release the energy in aerobic respiration
Surface area to volume ratio
Small organisms have a small surface area and a small volume. Therefore they are referred to having a large surface area to volume ratio, so their surface area is large enough to supply all cells with sufficient oxygen
Large organisms have a large surface area and a large volume. As the size increases, the volume rises more quickly than the surface area. Their surface area is relatively small compared with their volume so therefore have a small surface area to volume ratio
Features of a good exchange surface
A large surface area to provide more space for molecules to pass through. This is often achieved by folding the walls and membranes involved
A thin barrier to reduce the diffusion distance and that barrier must be permeable to the substances being exchanged
A good blood supply. This is important to maintain a steep concentration gradient so that diffusion can occur rapidly
3.1.2 Mammalian gaseous exchange system
In mammals, the gaseous exchange consists of the lungs and airways
Air can pass into the lungs through the nose and along the trachea, bronchi and bronchioles then reaching the air-filled sacs called alveoli
The lungs are protected by the ribcage. The ribs are held together by the intercostal muscles
Gases pass by diffusion through the thin walls of the alveoli. Oxygen passes from the air in the alveoli to the blood in the capillaries. Carbon dioxide passes from the blood to their air in the alveoli. The lungs must maintain a steep concentration gradient in each direction in order to ensure that diffusion can continue
Alveoli are very small but because there are so many of them - they produce a large surface area
Alveoli are lined by a thin layer of moisture, which evaporates and is lost as we breathe out. The lungs must produce a surfactant that coats the internal surface of the alveoli to rudece the cohesive forces between the water molecules, as these forces tend to make the alveoli collapses
Thin barrier to reduce the diffusion distance
The alveolus wall is one cell thick
The capillary wall is one cell thick
Both walls consist of squamous cells - this means flattened or very thin
The capillaries are in close contact with the alveolus walls
The capillaries are so narrow that the red blood cells are squeezed against the capillary wall - making them closer to the air in the alveoli and reducing their rate of flow
A good blood supply
The blood system transports carbon dioxide from the tissues to the lungs. This ensures that the concentration of carbon dioxide in the blood is higher than that in the air of the alveoli. Therefore carbon dioxide diffuses into the alveoli
The blood also transports oxygen away from the lungs. This ensures that the concetration of oxygen in the blood is kept lower than that in the alveoli - so that oxygen diffuses into the blood
Inspiration (inhaling)
The diaphragm contracts to move down and become flatter - this displaces the digestive organs downwards
The external intercostal muscles contract to raise the ribs
The volume of the chest cavity is increased
The pressure in the chest cavity drops below the atmospheric pressure
Air is moved into the lungs
Expiration (exhaling)
The diaphragm relaxes and is pushed up by the displaced organs underneath
The external intercostal muscles relax and the ribs fall; the internal intercostal muscles can contract to help push air out more forcefully
The volume of the chest cavity is decreased
The pressure in the lungs increases and rises above the pressure in the surrounding atmosphere
Air is moved out of the lungs