3.1.1: Exchange surfaces Flashcards
Why do multi-cellular organisms require exchange surfaces?
- Have a low SA to V ratio –> diffusion alone, directly into cells, would take too long/be too slow
- May have higher metabolic demands
- Multi-cellular so longer diffusion pathway –> distance would be too great hence transport system connected to exchange surface needed
Features of an efficient exchange surface
- Good blood supply to maintain concentration gradient
- High total surface area
- Thin walls/short diffusion distance to allow rapid diffusion
Function of cartilage
- Prevents closing of trachea when air pressure is below atmospheric
- Gap at the back of the C-shaped ring allows a bolus of food to be swallowed
Function of smooth muscle in the lungs
• Able to constrict airways to reduce intake if noxious substances are in the surrounding air
Function of elastic fibres in the lungs
- Facilitates and encourage relaxation of smooth muscle
* Become distorted when the smooth muscle contracts, recoil when it relaxes, thus dilating the airway
Function of goblet cells in the lungs
Secrete mucus to trap particles such as dust and bacteria, reducing the risk of irritation/infection in the lungs
Function of the ciliated epithelial cells
Cilia beat rhythmically to waft mucus out of the lungs and up the trachea so that it is swallowed.
Squamous epithelium
• Layer 1 cell thick
• Thin cells walls
⟶ Allows rapid diffusion
Endothelial cells
Line capillary walls
Why the surface of the alveolus is moist
Increases the humidity of incoming air, reducing evaporation from the exchange surface.
Purpose of lung surfactant
Prevents alveoli collapsing when air is exhaled
Mechanism of inspiration (mammals)
Requires energy
1) Diaphragm contracts, flattening
2) External intercostal muscles contract moving ribs upwards and outwards
3) Thorax volume increases, so pressure in the thorax is decreased
4) Atmospheric air pressure is now higher than pressure in the thorax, so air is drawn into the lungs, equalising the pressure inside and outside the thorax
Mechanism of passive expiration (mammals)
Passive process
1) Diaphragm relaxes into resting dome shape
2) External intercostal muscles relax, so ribs move down and inwards under gravity
3) Elastic fibres of alveoli return to normal length
4) Volume of thorax decreases, so pressure inside the thorax increases
5) Pressure inside the thorax is now greater than atmospheric air pressure, so air moves out of the lungs to equalise the pressure.
Mechanism of active expiration (mammals)
Active process
1) Abdominal muscles contract, forcing diaphragm up
2) Internal intercostal muscles pull ribs down and in
⟶ This decreases volume
Tidal volume
The volume of air moved into or out of the lungs during a single respiratory cycle under resting conditions.
Vital capacity
Maximum volume of air you can move into or out of your lungs in a single respiratory cycle.
Inspiratory reserve volume
The volume of air that you can breathe in over and above the tidal volume.
Expiratory reserve volume
The volume of air that you can voluntarily expel after completion of a normal, quiet respiratory cycle.
Process of gaseous exchange in insects
Air enters and leaves through spiracles
- -> passes along tracheae
- -> most gas exchange takes place in tracheoles
- -> oxygen diffuses directly into cells, not via a transport medium
Spiracles
Where air enters and leaves the insect (water is also lost); many can be opened and closed by sphincters in response to oxygen demands
Tracheae
- Lined with spirals of chitin, which keeps them open if the insect’s body is compressed
- Relatively impearmeable to gases so little gaseous exchange takes place in tracheae
Tracheoles
- 0.6-0.8 µm in diameter
- No chitin lining so freely permeable to gases
- Spread between tissues in insect, in between individual cells
- Where most gaseous exchange takes place
Insects with high energy demands - methods to supply extra oxygen needed
- Mechanical ventilation of the tracheal system
* Collapsible enlarged trachea (air sacs)
Collapsible enlarged trachea (air sacs)
Act as air reservoirs, can be inflated and deflated by ventilating movements of thorax and abdomen.
Mechanical ventilation
Muscular pumping movements of thorax and abdomen pump air into the system –> changes the volume and thereby pressure of tracheae and tracheoles so air drawn in
Why is gas exchange in water difficult?
- Water has lower oxygen content than air
* Water more viscous than air; mechanical ventilation would require too much energy
Adaptations of the gills
- Large surface area
- Good blood supply
- Gill layers are thin
- Tips of gill filaments overlap
- Water and blood flow in opposite directions (countercurrent flow)
- Covered by operculum
Why large SA important for gills
Allows maximum area for diffusion.
Why good blood supply important for gills
Maintains concentration gradient therefore enables rapid diffusion
Why gill layers being thin is important for the gills
Short diffusion distance allows fast diffusion
Why tips of adjacent gill filaments overlapping is important for the gills
Provides resistance to the flow of water, increasing time in which gas exchange can occur; exposes larger SA.
Why countercurrent flow is important for gills
Maintains concentration gradient between oxygen in blood and oxygen in water; enables more gas exchange to take place.
Why operculum covering is important for the gills
Operculum opens and closes to maintain flow over gills
Mechanism of ventilation (bony fish)
1) Mouth opens (operculum is closed)
2) The buccal cavity floor is lowered
3) This increases the volume and decreases the pressure of the buccal cavity compared to the water outside
4) Water rushes into the mouth down a pressure gradient
5) Opercular cavity expands
6) The buccal cavity floor is raised and the muscles surrounding it contract
7) The pressure inside the buccal cavity is now higher than the pressure in the opercular cavity
8) Water moves from the buccal cavity over the gills into the opercular cavity
9) The mouth is now closed and the operculum opens
10) The sides of the opercular cavity move inwards, increasing the pressure
11) Water rushes out of the fish through the operculum
Explain countercurrent flow
• Countercurrent exchange maintains a concentration gradient through the whole capillary; equilibrium is never reached
∵ The conc of O₂ in the water is always higher than that of the blood parallel to it
• If blood and water flowed in same direction, equilibrium would be reached part way along the capillary
⟶ This would limit the oxygen that could diffuse into the blood
