7 Gas exchange in humans
7.1 The human breathing system
1 The human breathing system ĩ⏠䲣䴙Ī consists of the respiratory tract ( ) and
structures that aid breathing:
nasal cavity ( 滣僼) pharynx ( ⑥)
nostril ( 滣⫼) epiglottis ( 㚫⍕)
larynx ( )
bronchiole ( )
intercostal muscle trachea ( )
( )
rib ( )
air sac ()
bronchus
inner pleural
( )
membrane ( )
lung ( )
outer pleural
membrane ( )
pleural cavity () diaphragm
( )
2 The path of air flowing through the respiratory tract into the lungs:
nasal cavity pharynx larynx trachea
bronchus bronchiole (in lungs) air sac (in lungs)
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3 Features of nostrils and the nasal cavity to keep the incoming air clean, moist and warm:
Feature Function
In the nostrils - Hairs - Filter large dust particles
In the lining - Mucus-secreting - Secrete mucus ġto trap dust particles and bacteria
of nasal cells - The mucus also moistens the incoming air
cavity - Ciliated epithelial - Their cilia beat to sweep the mucus towards the
cells pharynx
- Capillaries - Blood in capillaries warms the incoming air
7.2 Gas exchange in the air sacs
1 Gas exchange between the air in the air sac and the blood in the capillary:
deoxygenated blood ( 仢㯏埨)
from pulmonary artery ( ≽傰)
red blood cell
1 Oxygen is inhaled.
3 Oxygen 3 Carbon dioxide
diffuses into 2 Oxygen dissolves
is exhaled.
the red blood in the water film.
cells.
2 Carbon dioxide
epithelium ( )
diffuses into
(one-cell thick)
the air sac.
water film
1 Carbon dioxide
diffuses out from
oxygenated blood ( ) to
the capillary.
pulmonary veins ( )
2 Adaptive features of air sacs for gas exchange:
Feature of air sacs Adaptation for gas exchange
Large in number Provides a large surface area for diffusion
Thin walls (one-cell thick epithelium) Reduce the diffusion distance
Moist inner surfaces Allow gases to dissolve in water film for diffusion
Richly supplied with capillaries A steep concentration gradient can be maintained for
diffusion
Lying close to capillaries Reduces the diffusion distance
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3 Exhaled air contains less oxygen and more carbon dioxide than inhaled air. Exhaled air is
also saturated with water vapour and is warmer than inhaled air.
7.3 Transport of respiratory gases
1 Adaptive features of red blood cells for carrying oxygen:
Feature of red blood cells Adaptation for carrying oxygen
Fully packed with haemoglobin - Allows the red blood cells to carry a large amount of
oxygen
Biconcave disc shape - Provides a large surface area to volume ratio for
the diffusion of oxygen
- Provides a short distance for the diffusion of oxygen
No nucleus when mature - Provides more room for holding haemoglobin
2 The transport of oxygen:
a) In lungs (high oxygen concentration) b) In body tissue (low oxygen concentration)
inhaled air
body cell
oxygen
air sac
capillary
haemoglobin + oxygen oxyhaemoglobin oxyhaemoglobin haemoglobin + oxygen
red blood cell blood flow direction
3 The transport of carbon dioxide:
a) In body tissue (high carbon dioxide concentration) b) In lungs (low carbon dioxide concentration)
exhaled
body cell
CO2
air sac
enzyme 炼 炼 炼 炼
HCO3 + H enzyme CO2 + H2O
+ +
capillary CO2 + H2O H + HCO3 HCO3 HCO3
plasma
red blood cell blood flow direction
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7.4 Ventilation
1 Ventilationġġ involves inhalation and exhalation . It is brought about by the
movements of the rib cage and the diaphragm:
Inhalation Exhalation
Intercostal muscles Contract Relax
Rib cage Moves upwards and outwards Moves downwards and inwards
Diaphragm muscles Contract Relax
Diaphragm Becomes flattened Recoils to dome shape
Volume of thoracic Increases Decreases
cavity
Air pressure in the Becomes lower than atmospheric Becomes higher than atmospheric
lungs pressure pressure
Air flow Rushes into the lungs Forced out of the lungs
2 The change in volume of lungs during inhalation and exhalation:
volume of lungs
inhalation exhalation inhalation exhalation
time (s)
0 2 4 6 8
3 The change in air pressure in the lungs during inhalation and exhalation:
air pressure
in the lungs
inhalation exhalation inhalation exhalation
atmospheric
pressure
time (s)
0 2 4 6 8
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