The human gas exchange system

1
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
2
The Gas Exchange Surface
3
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
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)

4
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
5
Gas Exchange occurs across the Alveolar Epithelium
This thin wall is surrounded by an extensive
network of capillaries whose walls are also
composed of flattened, squamous epithelium
6
Gas Exchange occurs across the Alveolar Epithelium
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
7
Gas Exchange occurs across the Alveolar Epithelium
This electron micrograph shows a portion of human
lung
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