Transcript gas_exchange - biologypost

The Human Gas Exchange System
The human gas exchange system
consists of the nasal passages, the
pharynx or throat, the larynx or
voice box, the trachea, the right
and left bronchus and the lungs
Bronchioles
Larynx
Trachea
(with rings of cartilage)
Left lung
Ribs
Right
bronchus
Section through
ribs
Intercostal
muscles
Diaphragm
(a powerful sheet of muscle
separating the thorax from the abdomen)
The Gas Exchange Surface
The bronchioles divide
many times forming
respiratory bronchioles,
which in turn divide to
to form alveolar ducts
that terminate in groups
of sacs – the alveoli
A single alveolus
Alveolar
duct
Respiratory
bronchioles
Alveoli
Each alveolus is a
hollow, thin-walled
sac that is surrounded
by a dense network of
capillaries and is the
site of gas exchange
in the lungs
The Gas Exchange Surface
As deoxygenated blood from the body tissues flows through the network of
capillaries surrounding each alveolus, oxygen diffuses into the blood and carbon
dioxide diffuses from the blood into the alveolus; oxygenated blood travels from
the lungs to the left of the heart for delivery to the body tissues
Gases are exchanged across the alveoli by diffusion
According to Fick’s Law...
Rate of diffusion =
surface area x difference in concentration
thickness of exchange surface
Maximum rate of diffusion of respiratory gases is achieved by:
• the large surface area presented by the alveoli (there are about 350 million
alveoli in the two lungs presenting an enormous surface area of
approximately 90 square metres – about the area of a tennis court)
• the large differences in concentration of metabolites between the alveoli
and the blood capillaries
• the thinness of the diffusion barrier (alveolar and capillary walls provide
a total thickness of only 0.005 mm)
Gas Exchange occurs across the Alveolar Epithelium
The exchange of gases between the lungs and the circulating blood takes place
across the thin-walled alveoli whose walls consist of squamous epithelium
This photomicrograph shows the thin walls and large surface area
displayed by the alveoli in human lung tissue
ALVEOLI
Gas Exchange occurs across the Alveolar Epithelium
The wall of the alveolus is composed of squamous epithelium together with
its basement membrane
squamous
cells
basement membrane
This thin wall is surrounded by an
extensive network of capillaries whose
walls are also composed of flattened,
squamous epithelium
Gas Exchange occurs across the Alveolar Epithelium
capillary
The thinness of the capillary and alveolar walls, together
with their basement membranes, provides a barrier
between the alveolar air and the blood in the capillaries
of approximately 0.3 mm
The thinness of this barrier is one of the features that
aids rapid diffusion of gases into and out of the blood
Gas Exchange occurs across the Alveolar Epithelium
This electron micrograph shows
a portion of human lung
O2
CO2
The short diffusion
path for efficient
exchange of gases
between the air in
the alveoli and the
blood in the capillaries
is provided by the thin
squamous epithelium
layers of these tissues
Alveolar
wall
Capillary
wall
Red blood
cell
Nucleus of
capillary
capillary and alveolar walls