Core Ch 7 Gas exchange in humans
7.1 Structures of the respiratory tract
1. Nasal cavity
-hairs in the nostril: filter larger dust particles from the air
-mucus-secreting cells: secrete mucus to (1) trap dust particles, (2) trap
bacteria, (3) moistens the incoming air
-ciliated epithelial cells: contain cilia, which beat to sweep the mucus towards
the pharynxmucus is then swallowed/coughed up (bring away dust
particles & bacteria)
-capillaries: blood warms up the incoming air
2. pharynx and larynx
-larynx is made mainly of cartilage (prevents the larynx from collapsing or
stretching due to pressure changes during breathing)
-when air passing through the larynx causes the vocal cords to vibrate, sound
is produced
3. trachea and bronchi
-trachea is supported by C-shaped cartilages (allow flexible extentions of
oesophagus during breathing)
-bronchi are supported by circular rings of cartilages
-inner walls of them are lined with mucus-secreting cells, ciliated epithelial
cells, capillaries
4. bronchioles and air sacs
-bronchioles are lined with mucus-secreted cells & ciliated epithelial cells (but
no cartilage!!)
-air sacs are surrounded by a network of capillaries
**Structures around the lungs
-lungs are located in the thoracic cavity enclosed by the rib cage(consists of 12
pairs of ribs, sternum and vertebral column) and the diaphragm
-in between each pair of ribs are the intercostals muscles
Within the thoracic cavity:
-pleural membranes(surrounding the lungs) secrete pleural fluidlubricant to
reduce friction between pleural membranes during breathing movement)
7.2.1 How does gas exchange take place?
Uptake of oxygen by the blood:
1. Oxygen is inhaled
2. oxygen dissolves in the water film lining the air sac
3. oxygen diffused into the RBC along the concentration gradient
Removal of carbon dioxide into the air sacs:
1. CO2 diffused out from the capillary along the concentration gradient
2. CO2 dissolves in the water film lining the air sac and diffused into the air sac
3. CO2 is exhaled
7.2.2 Adaptations of air sacs for gas exchange
Feature of air sacs
Adaptation
Large in number
Provides a large surface area for
diffusion
Thin walls (one-cell thick epithelium)
Reduces the diffusion
distanceincreases the rate of
diffusion
Richly supplied with capillaries
Allows rapid transport of gases into
and away from the air sacsmaintain
a steep concentration gradient of
gases between the air sac and the
bloodincreases the rate of diffusion
of gases
Moist inner surface
Allows gases to dissolve in water film
for diffusion
7.2.3 Difference in composition between inhaled air and exhaled air
Inhaled air
Exhaled air
Oxygen
21%
16%
Carbon dioxide
0.03%
4%
Nitrogen
78%
78%
Water vapour
Variable
Saturated
Other gases
1%
1%
Temperature
Lower
higher
7.2.4 How smoking affects breathing?
-Nicotine in smoke causes constriction of bronchi and bronchiolesreduces
the amount of fresh air reaching the air sacs
-chemicals in cigarette smoke inhibit the movement of cilia in the respiratory
tract and stimulate the secretion of excess mucusmucus accumulates in the
respiratory tractnarrows the air passage & reduces the amount of fresh air
reaching the air sacs
-Tar deposits on the surface of air sacsreduces the surface area for gaseous
exchange
-chemicals in cigarette smoke may cause emphysema (walls of air sacs break
downreduces the surface area for gaseous exchange)
7.3.1 Adaptive features of RBC for carrying oxygen
1. fully packed with haemoglobin
allows the RBC to carry a large amount of oxygen
2. biconcave disc shape
provides a large surface area to volume ratio for the diffusion of oxygen
provides a short distance for oxygen to diffuse into or out of the RBC
diffuse into/out of the RBC rapidly
3. no nucleus when mature
provides more room for holding haemoglobin
7.3.2 How is oxygen transported?
∵oxygen concentration is high in the air sac (due to the continuous replacement
of air from the external environment)
∴oxygen in the air sac diffuses into the RBC
oxygen combines with haemoglobin to form oxyhaemoglobin (gives the
blood a bright red colour)
RBC containing oxyhaemoglobin are carried from the lungs to the body
tissue
∵oxygen concentration is low in the body tissues (because the body cells
consume oxygen for respiration to release energy)
∴oxyhaemoglobin in RBC breaks down into oxygen and haemoglobin
oxygen diffuses into the body cells and the blood becomes purplish red
7.3.3 How is carbon dioxide transported?
∵CO2 concentration is high in the body tissue (CO2 is continuously produced by
body cells through respiration)
∴CO2 produced by body cells diffuses into RBC
reacts with water to from hydrogencarbonate ions
hydrogencarbonate ions diffuse out of the RBC and are carried by the plasma
to the air sacs of lungs
∵CO2 concentration is low in the air sacs (it is continuously removed through
exhalation)
∴hydrogencarbonate ions carried by the plasma diffuses into the RBC and break
down into CO2 and water
CO2 diffuses into the air sacs and is exhaled
7.4 Ventilation
1. Inhalation
-intercostal muscles contract
-ribs move outwards and upwards
-diaphragm muscles contractdiaphragm becomes flattened
-volume of thoracic cavity increases
air pressure in the lungs decreases (becomes lower than the atmospheric
pressure)
air rushes into the lungs, lungs inflate
2. Exhalation
-intercostal muscles relax
-ribs move inwards and downwards
-diaphragm muscles relaxrecoils to the dome shape
-volume of thoracic cavity decreases
air pressure in the lungs increases (becomes higher than the atmospheric
pressure)
air is forced out of the lungs, lungs deflate
Question bank
1. It is known that smoking can inhibit the beating of cilia. Suggest how smoking
may result in a higher chance of influenza infection. {CE 10 Q8}
-Due to the inhibition of beating of cilia, smoking reduces the efficiency of
removal of influenza virus (1)
-trapped in the mucus on the respiratory tract (1)
2. Explain how the constant and high blood oxygen content in the aorta can be
achieved during vigorous exercise. {CE 06 Q10b(v)}
-During vigorous exercise, there is an increase in ventilation rate/rate and
depth of breathing (1)
-the oxygen content in air sac increases (1)
-the diffusion gradient across the wall of air sacs increases/this increases the
rate of diffusion of oxygen into the blood (1)
thus maintaining the blood oxygen content of the aorta at a constant and high
level
3. Explain the importance of the water film in gaseous exchange. {CE 04 Q1c(i)}
-oxygen in air dissolves in the water film (1)
-so that it can diffuse readily through the wall of air sac into the blood
capillary (1)
4. SARS patients may have fluid accumulated in the air sacs. Explain how the
accumulation of fluid may affect the oxygen content of blood of the patients.
-the accumulation of fluid increases the distance for diffusion/reduces the
surface area for dissolving oxygen (1)
-hence decreases the rate of diffusion of dissolved oxygen into the blood
capillary (1)
-thus the oxygen content of the blood decreases/becomes lower than
normal(1)
5. Suggest an explanation for the reduced oxygen supply to the foetus when
mother smokes. {CE 03 Q2(b)}
-tar in cigarette smoke deposits on the surface of the air sacs in the mother’s
lungs (1)
-as a result, less oxygen can be absorbed into the mother’s blood, hence
reducing the oxygen supply to the foetus (1)
6. Explain why the lung will collapse if pleural membrane is punctured in an
accident. {CE 91 Q3(c)}
-air leaks in (1)
-the lung collapse due to its own elasticity (1)
7. Cigarette smoke may inhibit the beating of cilia in the respiratory tract.
Explain how this could reduce the efficiency of the lungs in gaseous
exchange. {CE 91 Q3(c)}
-dust particles cannot be removed from the respiratory tract/more dust
particles enter the lungs (1)
-thereby reducing the surface area for gaseous exchange (1)
Respiration & Gas Exchange in Human (★★★★★)
1. Gaseous Exchange in Alveolus (the process; structural adaptations of
alveolus)
{DSE 16 P1-11, CE 00-1(c), CE 05-4(a)}
2. Effects of smoking on gaseous exchange
-Tar (deposits on the wall of alveolus, thus reducing surface area for gaseous
exchange)
-CO (decreasing oxygen carrying capacity)
-Nicotine/tar (lung cancer, bronchitis, asthma, emphysema)
-Surface area of alveoli in smoker vs non-smoker
3. Mechanism of Breathing {AL 99 PIA-7(b), AL 02 PIIA-3(b), AL 07 PIB-9}
(a) Significance of increasing the rate & depth of breathing during exercise
(b) Control of breathing (medulla oblongata, the effect of CO2 on the rate &
depth of breathing) {CE 03-1(a) 2(b)(i)}
(c) Inspiration/Expiration {DSE 17 P1-5, CE 01-4(b)}