3.1.1: Exchange surfaces

Question 1

Why do multi-cellular organisms require exchange surfaces?

Answer 1

• 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

Question 2

Features of an efficient exchange surface

Answer 2

• Good blood supply to maintain concentration gradient
• High total surface area
• Thin walls/short diffusion distance to allow rapid diffusion

Question 3

Function of cartilage

Answer 3

• 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

Question 4

Function of smooth muscle in the lungs

Answer 4

• Able to constrict airways to reduce intake if noxious substances are in the surrounding air

Question 5

Function of elastic fibres in the lungs

Answer 5

• Facilitates and encourage relaxation of smooth muscle

* Become distorted when the smooth muscle contracts, recoil when it relaxes, thus dilating the airway

Question 6

Function of goblet cells in the lungs

Answer 6

Secrete mucus to trap particles such as dust and bacteria, reducing the risk of irritation/infection in the lungs

Question 7

Function of the ciliated epithelial cells

Answer 7

Cilia beat rhythmically to waft mucus out of the lungs and up the trachea so that it is swallowed.

Question 8

Squamous epithelium

Answer 8

• Layer 1 cell thick
• Thin cells walls
⟶ Allows rapid diffusion

Question 9

Endothelial cells

Answer 9

Line capillary walls

Question 10

Why the surface of the alveolus is moist

Answer 10

Increases the humidity of incoming air, reducing evaporation from the exchange surface.

Question 11

Purpose of lung surfactant

Answer 11

Prevents alveoli collapsing when air is exhaled

Question 12

Mechanism of inspiration (mammals)

Answer 12

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

Question 13

Mechanism of passive expiration (mammals)

Answer 13

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.

Question 14

Mechanism of active expiration (mammals)

Answer 14

Active process
1) Abdominal muscles contract, forcing diaphragm up
2) Internal intercostal muscles pull ribs down and in
⟶ This decreases volume

Question 15

Tidal volume

Answer 15

The volume of air moved into or out of the lungs during a single respiratory cycle under resting conditions.

Question 16

Vital capacity

Answer 16

Maximum volume of air you can move into or out of your lungs in a single respiratory cycle.

Question 17

Inspiratory reserve volume

Answer 17

The volume of air that you can breathe in over and above the tidal volume.

Question 18

Expiratory reserve volume

Answer 18

The volume of air that you can voluntarily expel after completion of a normal, quiet respiratory cycle.

Question 19

Process of gaseous exchange in insects

Answer 19

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

Question 20

Spiracles

Answer 20

Where air enters and leaves the insect (water is also lost); many can be opened and closed by sphincters in response to oxygen demands

Question 21

Tracheae

Answer 21

• 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

Question 22

Tracheoles

Answer 22

• 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

Question 23

Insects with high energy demands - methods to supply extra oxygen needed

Answer 23

• Mechanical ventilation of the tracheal system

* Collapsible enlarged trachea (air sacs)

Question 24

Collapsible enlarged trachea (air sacs)

Answer 24

Act as air reservoirs, can be inflated and deflated by ventilating movements of thorax and abdomen.

Question 25

Mechanical ventilation

Answer 25

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

Question 26

Why is gas exchange in water difficult?

Answer 26

• Water has lower oxygen content than air

* Water more viscous than air; mechanical ventilation would require too much energy

Question 27

Adaptations of the gills

Answer 27

• 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

Question 28

Why large SA important for gills

Answer 28

Allows maximum area for diffusion.

Question 29

Why good blood supply important for gills

Answer 29

Maintains concentration gradient therefore enables rapid diffusion

Question 30

Why gill layers being thin is important for the gills

Answer 30

Short diffusion distance allows fast diffusion

Question 31

Why tips of adjacent gill filaments overlapping is important for the gills

Answer 31

Provides resistance to the flow of water, increasing time in which gas exchange can occur; exposes larger SA.

Question 32

Why countercurrent flow is important for gills

Answer 32

Maintains concentration gradient between oxygen in blood and oxygen in water; enables more gas exchange to take place.

Question 33

Why operculum covering is important for the gills

Answer 33

Operculum opens and closes to maintain flow over gills

Question 34

Mechanism of ventilation (bony fish)

Answer 34

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

Question 35

Explain countercurrent flow

Answer 35

• 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