3.1.1 Exchange Surfaces

Question 1

Why do multicellular organisms require

specialised gas exchange surfaces?

Answer 1

Their smaller surface area to volume ratio
means the distance that needs to be crossed
is larger and substances cannot easily enter
the cells as in a single-celled organism.

Question 2

How is surface area to volume ratio

calculated?

Answer 2

Ratio = Surface area
—————–
Volume

Question 3

Name three features of an efficient gas

exchange surface.

Answer 3

1. Large surface area, e.g. root hair cells.
2. Thin/short distance, e.g. alveoli.
3. Steep concentration gradient, maintained
   by blood supply or ventilation, e.g. gills.

Question 4

Describe the trachea and its function in
the mammalian gaseous exchange
system.

Answer 4

● Wide tube supported by C-shaped cartilage to keep
the air passage open during pressure changes.
● Lined by ciliated epithelium cells which move
mucus, produced by goblet cells, towards the throat
to be swallowed, preventing lung infections.
● Carries air to the bronchi.

Question 5

Describe the bronchi and their function in
the mammalian gaseous exchange
system.

Answer 5

● Like the trachea they are supported by rings of
cartilage and are lined by ciliated epithelium cells
and goblet cells.
● However they are narrower and there are two of
them, one for each lung.
● Allow passage of air into the bronchioles.

Question 6

Describe the bronchioles and their
function in the mammalian gaseous
exchange system.

Answer 6

● Narrower than the bronchi.
● Do not need to be kept open by cartilage, therefore
mostly have only smooth muscle and elastic fibres
so that they can contract and relax easily during
ventilation.
● Allow passage of air into the alveoli.

Question 7

Describe the alveoli and their function in
the mammalian gaseous exchange
system.

Answer 7

● Mini air sacs, lined with epithelium cells, site of
gas exchange.
● Walls only one cell thick, covered with a
network of capillaries, 300 million in each lung,
all of which facilitates gas diffusion.

Question 8

Explain the process of inspiration and
the changes that occur throughout the
thorax.

Answer 8

● External intercostal muscles contract (while internal
relax), pulling the ribs up and out.
● Diaphragm contracts and flattens.
● Volume of the thorax increases.
● Air pressure outside the lungs is therefore higher than
the air pressure inside, so air moves in to rebalance.

Question 9

Explain the process of expiration and the
changes that occur throughout the
thorax.

Answer 9

● External intercostal muscles relax (while internal
contract), bringing the ribs down and in.
● Diaphragm relaxes and domes upwards.
● Volume of the thorax decreases.
● Air pressure inside the lungs is therefore higher than the
air pressure outside, so air moves out to rebalance.

Question 10

Explain how a spirometer works.

Answer 10

Used to measure lung volume. A person
breathes into an airtight chamber which
leaves a trace on a graph which shows
the volume of the breaths.

Question 11

Define vital capacity.

Answer 11

The maximum volume of air that can be
taken in or expelled from the lungs in
one breath. Can be calculated from the
spirometer graph by finding the
maximum amplitude.

Question 12

Define tidal volume.

Answer 12

The volume of air we breathe in and out
during each breath at rest. Can be
calculated from the spirometer graph by
finding the amplitude at rest.

Question 13

Define breathing rate.

Answer 13

The number of breaths we take per minute.
Can be calculated from the spirometer
graph by counting the number of peaks in
one minute.

Question 14

Name and describe the two main

features of a fish’s gas transport system.

Answer 14

Gills = located within the body, supported by arches, along
which are multiple projections of gill filaments, which are
stacked up in piles.
Lamellae = at right angles to the gill filaments, give an
increased surface area. Blood and water flow across them
in opposite directions (countercurrent exchange system).

Question 15

Explain the process of gas exchange in

fish.

Answer 15

● Buccal cavity volume increased to enable water to
flow in, reduced to increase pressure.
● Water is pumped over the lamellae by the
operculum, oxygen diffuses into the bloodstream.
● Waste carbon dioxide diffuses into the water and
flows back out of the gills.

Question 16

How does the countercurrent exchange
system maximise oxygen absorbed by
the fish?

Answer 16

Maintains a steep concentration gradient, as
water is always next to blood of a lower
oxygen concentration. Keeps rate of
diffusion constant and enables 80% of
available oxygen to be absorbed.

Question 17

Name and describe the three main
features of an insect’s gas transport
system.

Answer 17

● Spiracles = holes on the body’s surface which may be
opened or closed by a valve for gas or water exchange.
● Tracheae = large tubes extending through all body
tissues, supported by rings to prevent collapse.
● Tracheoles = smaller branches dividing off the tracheae.

Question 18

Explain the process of gas exchange in

insects.

Answer 18

● Gases move in and out of the tracheae through
the spiracles.
● A diffusion gradient allows oxygen to diffuse into
the body tissue while waste CO2
diffuses out.
● Contraction of muscles in the tracheae allows
mass movement of air in and out.