1. Objective:
1. Diagram 1 shows diffusion of gas Y from an alveolus to a blood capillary.
Diagram 1
Which of the following is true about the difference in the concentration of gas Y
of a healthy person and a cigarette smoker?
Healthy individual Cigarette smoker
A Higher Higher
B Lower Higher
C Lower Lower
D Higher Lower
2. Which artery carries blood with the lowest oxygen concentration?
A. The pulmonary artery
B. The mesenteric artery
C. The hepatic artery
D. The renal artery
3. Diagram 2 shows part of the systemic circulatory system.
Diagram 2
Which of the following is the type of blood and correct blood vessel for blood
that flows from organ X to the heart?
Type of blood Blood vessel
A Deoxygenated Pulmonary artery
B Deoxygenated Vena cava
C Oxygenated Pulmonary vein
D Oxygenated Aorta

2. 4. The following statements are the characteristics of blood transported by a blood
vessel in the human body.
 High concentration of oxygen
 Low concentration of carbon dioxide
 High blood pressure
What is the blood vessel?
A. Aorta
B. Vena cava
C. Pulmonary artery
D. Pulmonary vein
5. Which of the following processes occurs during gaseous exchange between the
alveolus and the blood capillary?
A. Active transport
B. Simple diffusion
C. Osmosis
D. Facilitated diffusion
Mastery level:
Marks 0-2 3 4-5
Mastery level More improvements Intermediate. Work Excellent!
needed. harder.

3. Structure:
1. Diagram 1 shows the structure of respiratory system in human.
Blood CO2
flow O2
Blood
flow
Diagram 1
(a) On Diagram 1, draw labeled arrow ( ) to show the direction of
 Blood flow
 Oxygen diffusion
 Carbon dioxide diffusion [ 3 marks ]
(b) Explain why the diffusion of oxygen occur at the alveolus.
Diffusion of oxygen occurs because the partial pressure of oxygen in
alveoli is higher than that in blood capillary. Hence, due to concentration
gradient, oxygen diffused from alveoli to blood capillary.
[ 2 marks ]

4. 2. Diagram 2 shows the exchange of respiratory gases X and Y between the alveolus,
blood capillary and the body cells and the transport of the gaseous.
Diagram 2
(a) (i) Name the gas X and Y.
Gas X: Oxygen
Gas Y: Carbon dioxide
[ 2 marks ]
(ii) Explain how the alveolus is structured to increase the efficiency of
gaseous exchange.
The wall of alveolus is only one cell thick which reduces the
diffusion distance. Besides, it has a film of moisture between
alveolus and blood capillary to allow diffusion of gases to occur.
Other than that, there are many alveoli present in the lungs and
all the alveoli are surrounded by nets of blood capillaries, both
which will increase the surface area for gas diffusion. [choose any
2 answers]
[ 2 marks ]
(b) Explain the difference between the concentration of gas X and Y in blood
vessel Q.
The concentration of gas Y is higher than that of gas X in blood vessel
Q. This is because most of gas X had diffused into body cells for
cellular respiration. Gas Y, which is the product of cellular
respiration then diffuse from body cells into blood vessel Q to be
transported to the heart. [ 2 marks ]

5. (c) The concentration of gas X transported in blood vessel P of a cigarette
smoker is usually lower than the one in the healthy individual. Explain
why does this occur?
Tobacco and tar from cigarette will surround the wall of alveoli,
reducing the amount of gas X diffused into blood vessel P from alveoli.
Besides that, carbon monoxide from cigarette will reduce the affinity
of haemoglobin towards gas X, causing gas X unable to be
transported to body cells via blood.
[ 2 marks ]
(d) Describe how gaseous exchange occurs between alveolus and blood
capillaries.
It is due to difference in partial pressure of oxygen and carbon dioxide
on both sides. Partial pressure of oxygen in alveolus is higher than
that in blood vessel and vice versa for partial pressure of carbon
dioxide. Due to concentration gradient, oxygen diffuses from alveolus
to blood vessel and vice versa for carbon dioxide.
[ 3 marks ]
3. Diagram 3 shows the gaseous exchange of oxygen and carbon dioxide between
the lungs and surrounding blood capillaries.
W W
V W
W X
Y
Z
Diagram 3
Name V, W, X, Y and Z shown in diagram 3. [ 5 marks ]
V : Oxygen gas
W: Carbon dioxide gas
X : Alveolus

6. Y : Red blood cells
Z : Blood capillary
Mastery level:
Marks 0-9 10 - 16 17 - 21
Mastery level More improvements Intermediate. Work Excellent!
needed. harder.