Adaptations for gas exchange in animals

Question 1

Gas exchange

Answer 1

How much oxygen an organism needs depends on its volume.
The rate that oxygen is absorbed at depends on the surface area available for gas exchange.
Therefore, the surface area to volume ratio of an organism affects:
•the surface adapted for use for gas exchange
•the level of activity of the organism.
As organisms increase in size, their surface area to volume ratio decreases and so specialised
respiratory surfaces are needed.

Question 2

Gas exchange in insects

Answer 2

Insects cannot use their external surface for gas
exchange as they are covered in an impermeable
cuticle to reduce water loss by evaporation.
1. Pairs of spiracles on segments of the thorax
and abdomen.
2. These holes lead to tubes called tracheae
leading to tracheoles.
3. Tracheoles enter muscle cells directly.
They have fluid at the end for dissolving and
diffusion of oxygen.
4. During flight, when oxygen requirements
increase, fluid in tracheoles decreases to
shorten diffusion path and whole-body
contractions ventilate the tracheal system by
speeding up air flow through spiracles.

Question 3

Gas exchange in fish

Answer 3

Fish require a specialised gas exchange surface as:
•they have a smaller surface area to volume ratio
•they are relatively active and so have high metabolic rates making oxygen requirements high
•they require a ventilation mechanism to maintain concentration gradients for gas
exchange.

Question 4

Ventilation in fish

Answer 4

Fish require a ventilation mechanism to push water, a dense medium with low oxygen content, over the high surface area gill filaments. Removal from water causes these gill filaments to collapse, stick together and the gas exchange surface becomes too small for survival.
1. Mouth opens, floor of buccal cavity lowers so volume increases, pressure decreases and
water rushes in.
2. Mouth closes, floor of buccal cavity raises, increasing pressure pushing water over the gills.
3. Pressure in gill cavity increases and water forces operculum open and leaves through it.

Question 5

Gills of fish

Answer 5

The gills have gill filaments made of gill
plates/lamellae (the gas exchange surface
across which the water flows).
Gill rakers prevent large particulates
entering and blocking the gills.

Question 6

Gas exchange surfaces must:

Answer 6

be moist in terrestrial animals
•be thin (short diffusion pathway)
•have a large surface area
•be permeable to gases
•have a good blood supply to maintain
concentration gradients (larger
organisms only).

Question 7

Continuous flow

Answer 7

If water and blood flow in the same direction,
equilibrium is reached and oxygen diffusion
reaches no net movement halfway across the
gill plate.
CARTILAGINOUS FISH

Question 8

Counter current flow

Answer 8

If water and blood flow in opposite directions
across the gill plate, the concentration
gradient is maintained and oxygen diffuses
into the blood across the entire gill plate
BONY FISH

Question 9

Amoeba adapted for gas exchange

Answer 9

Single cell
*Large surface area to volume ratio
*Rate of oxygen diffusion through external surface meets demand.
A low metabolic rate means oxygen demand is low.
*There is a short diffusion distance to the middle of the cell.

Question 10

Flatworm adapt to gas exchange

Answer 10

Multicellular
*Smaller surface area to volume ratio
*Flattened body to reduce diffusion distance so rate of oxygen diffusion through body surface meets demand

Question 11

Earthworm adapt to gas exchange

Answer 11

Multicellular
*Even smaller surface area to volume ratio
*Body surface still used formgas exchange but circulatory system needed to distribute oxygen. Blood vessels are close to skin surface and blood has haemoglobin with a high affinity for oxygen.
*Mucus secreted to moisten surface and slow moving to reduce oxygen demand

Question 12

Ventilation in humans – inspiration

Answer 12

1. External intercostal muscles contract and pull the rib cage up and out.
2. Outer pleural membrane is pulled out. This reduces pressure in the pleural cavity and
   the inner pleural membrane is pulled outward.
3. This pulls on the surface of the lungs and causes an increase in the volume of the alveoli.
4. Alveolar pressure decreases to below atmospheric pressure and air is drawn into the lungs.

Question 13

Gas exchange in humans

Answer 13

Bronchiole
Larynx leading to trachea
Alveoli (The gas exchange surface, lined with surfactant that reduces surface tension and prevents collapse on exhalation.)
Chest cavity
Rib

Intercostal muscles
Pleural membranes with pleural cavity between
Diaphgram

Question 14

Gas exchange in amphibia

Answer 14

Amphibia have aquatic tadpoles that have feathery gills. They don’t
ventilate like fish but movement of the gills through water maintains a
concentration gradient. Tadpoles develop into adults. Adult amphibia live on land and in water but must return to the water to mate and lay eggs. Amphibia have soft, moist skin and exchange gases over their surface at rest. Oxygen and carbon dioxide circulate through a closed circulation system containing haemoglobin. When active, movements of the buccal cavity ventilate lungs, which are simple with few alveoli.