2.1. GAS EXHANGE AND DIFFUSION
(some of these notes are applied to a specific disease called cystic fibrosis)
The role of mucus in the lungs
• The external and internal intercostal muscles work as antagonistic pairs (meaning they work in different directions to each other)
Inhalation and exhalation (recap)
- Inhalation - Exhalation
I. the external set of intercostal muscles contract I. the external set of intercostal muscles relax
II. Ribs up and out: II. ribs drop down and in
III. Increases the volume of the chest cavity (thorax) III. Decreases the volume of the chest cavity (thorax)
IV. Decreasing air pressure IV. increasing air pressure
V. Drawing air in V. Forcing air out
Cystic fibrosis
• Everyone (usually) has a thin coating of mucus in the tubes of the gas exchange system
- produced continually by GOBLET CELLS in the walls of the airways
→ use of the mucus: any dust, debris or microorganisms entering the airways will be trapped in it
• How is this mucus removed? By the wave-like beating of the cilia that cover the epithelial
cells lining the tubes of the gas exchange surfaces.
HOWEVER, ...
People with CF → have mucus containing LESS WATER (resulting in a sticky mucus which the cilia
find difficult to remove)
What are epithelial cells?
Epithelial cells → form the outer surface of many animals including mammals
→ they line the cavities and tubes within the animal
→ cover internal organs
• They work together as a TISSUE (epithelium -pl. epithelia)
What does the epithelium consist of?
→ 1 or more layers of cells sitting on a BASEMENT MEMBRANE
STRUCTURE:
• There are several types of epithelia
• In the small intestine
→ epithelial cells extend on from the basement membrane
→ the surface facing the intestine lumen is normally covered in microvilli
• In the trachea, bronchi and bronchioles
→ ciliated epithelial cells with cilia on the apical surface (where the cells are)
CF problems
How sticky mucus increases the chance of lung infections
• Microorganisms get trapped in the mucus in the lungs (some are pathogens)
→ USUALLY:
1. the mucus is moved by the cilia into the back of the mouth cavity
2. it is either moved or swallowed
3. the stomach acid kills the microorganisms that are swallowed
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,LUCIA ROMERO
→ BUT FOR PEOPLE WITH CF:
1. The mucus layer is so sticky that the cilia cannot move it
2. Mucus production continues
3. The layers of thickened mucus build up in the airways
4. As there are low levels of oxygen in the mucus... WHY? – Because oxygen diffuses slowly through it
-- The epithelial cells use up more oxygen in people with CF
5. Bacteria thrive in anaerobic conditions
6. White blood cells fight the infections within the mucus
7. As they die, they break down and release DNA that makes the mucus even stickier
8. Repeated lung infections weaken the body’s ability to fight pathogens & cause damage to the structures of the gas exchange system
How sticky mucus reduces gas exchange
1. Gases (e.g., oxygen) cross the walls of the alveoli INTO the blood system by diffusion
2. To supply enough to all the bodies respiring cells gas exchange must be RAPID
The effect of increase in size on surface area
• The LARGER the organism – the more exchange has to happen to meet metabolic needs
• As organisms get larger – the surface area per unit of volume gets less
Specialized gas exchange surfaces
What is a specialized gas exchange surface?
A surface that is adapted for the movement of substances
E.g.,
1. Alveoli – lungs
2. Villi – small intestine (ileum)
3. Spongy mesophyll – leaf
Why do multicellular organisms require these surfaces?
Because their SA to volume ratio is too small to meet the needs of the organism (substances would have too far to travel)
E.g.,
An elephant = small SA:V ratio
An amoeba = big SA:V ratio
What do all features of gas exchange have in common?
1. Large surface area
- rate of diffusion is directly proportional to the difference to surface area
2. Steep concentration gradient
- the steeper the slope the faster the rate of diffusion
- if you stop breathing, diffusion will still take place but slower
3. Thin walls: short diffusion pathway
- rate of diffusion is inversely proportional to the thickness of the gas exchange surface
- (the thicker the surface the lower the diffusion rate)
Fick’s law of diffusion
𝑠𝑢𝑟𝑓𝑎𝑐𝑒 𝑎𝑟𝑒𝑎 × 𝑑𝑖𝑓𝑓𝑒𝑟𝑒𝑛𝑐𝑒 𝑖𝑛 𝑐𝑜𝑛𝑐𝑒𝑛𝑡𝑟𝑎𝑡𝑖𝑜𝑛
𝑟𝑎𝑡𝑒 𝑜𝑓 𝑑𝑖𝑓𝑓𝑢𝑠𝑖𝑜𝑛 ∝
𝑡ℎ𝑖𝑐𝑘𝑛𝑒𝑠𝑠 𝑜𝑓 𝑔𝑎𝑠 𝑒𝑥ℎ𝑎𝑛𝑔𝑒 𝑠𝑢𝑟𝑓𝑎𝑐𝑒
Applying the law to adaptations of mammalian lung surfaces
1. Rate of diffusion – proportional to SA (alveoli have large surface area)
2. -- -- proportional to difference in conc (breathing maintains a difference in gas concentrations)
(blood flow maintains a difference in gas concentrations)
3. -- -- inversely proportional to diffusion distance (walls of alveoli and capillaries = 1 cell thick)
4. -- -- proportional to diffusion constant (cell membranes are relatively permeable to non-polar gas molecules)
5. Diffusion distance reduced due to FLATTENED CELLS (forming alveoli and capillary walls)
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, LUCIA ROMERO
How sticky mucus affects gas exchange
• The sticky mucus in the airways of a person with CF prevent the movement of air into the alveoli beyond the blockage
→ these blockages often allow air to pass when the person BREATHES IN, not when they BREATHE OUT
: resulting in over-inflation of the lung tissue beyond the blockage (damages the elasticity of the lungs)
• People with CF become short of breath when they perform exercise
Adaptations of the mammalian lung
1. Large SA
- alveoli are covered by capillaries
- due to alveoli
2. Thin diffusion pathway
- both capillary and alveoli are one cell thick
- the alveoli are covered by capillaries
3. Steep concentration gradient
- constant ventilation keeps conc O2 in alveoli high
- good blood supply (blood is continually being carried away in the flow) : which means
that in the capillaries, there is a low conc gradient always.
2.2. CELL MEMBRANES
Even though the most widely known membrane is the cell membrane, there are also membranes WITHIN cells that have various functions:
1. Membranes from vesicles: allow certain substances to travel around the cell
2. Keep the DNA inside the nucleus
Cell membrane structure
Glycolipid → carbohydrate + lipid
Glycoprotein → carbohydrate + protein
Membranes exist at the cell surface, what is their function?
- Separate the environments inside and outside, semi, or partially permeable
What is CF caused by → faulty transport protein in the surface membranes of epithelial
cells
Membrane components
Phospholipids
Hydrophilic head → it is polar (one end is slightly positive and the rest is slightly negative)
- this means: the head can attract other polar molecules (e.g., water)
- therefore, it Is hydroPHILIC (attracts water)
Hydrophobic tails → it is non-polar (due to the fatty acids which form them)
- therefore, it is hydroPHOBIC (repels water)
- When added to water: phospholipids become arranged with no contact between the hydrophobic
tails and the water.
Why are the phospholipids arranged like this?
- Because they create a semi-permeable membrane as the cytoplasm and tissue fluid are both solutions of water
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