The document discusses gas exchange through the respiratory membrane. It describes how oxygen diffuses into the bloodstream in the alveoli and capillaries while carbon dioxide diffuses out. This process is facilitated by the thin alveolar membranes and large surface area of the lungs, allowing gases to rapidly move across based on partial pressure differences. Several factors like membrane thickness, surface area, gas diffusion coefficient, and partial pressure gradients determine the rate of gas diffusion through the respiratory membrane.

1. DIFFUSION OF GASES THROUGH
RESPIRATORY MEMBRANE
ZEYNAB ATTA
(Physiotherapist to be)

2. INTRODUCTION
• The purpose of the respiratory system is to perform gas
exchange.
• Pulmonary ventilation provides air to the alveoli for this
gas exchange process.
• At the respiratory membrane, where the alveolar and
capillary walls meet, gases move across the membranes,
with oxygen entering the bloodstream and carbon
dioxide exiting.
• It is through this mechanism that blood is oxygenated
and carbon dioxide, the waste product of cellular
respiration, is removed from the body.

3. RESPIRATORY UNIT:
• is composed of a respiratory bronchiole, alveolar
ducts, atria, and alveoli.
• There are about 300 million alveoli in the two lungs,
and each alveolus has an average diameter of about
0.2 millimeter.
• Between the alveoli is a network of interconnecting
capillaries.

4. Respiratory unit

6. Diffusion of Oxygen
• The partial pressure of oxygen is low in the
alveoli compared to the external environment.
• This is due to continuous diffusion of oxygen
across the alveolar membrane and the diluting
effect of carbon dioxide entering the alveoli to
leave the body.

7. Cont…
• Despite this, the partial pressure is still higher
in the alveoli than the capillaries, resulting in a
net diffusion into the blood.
• Once it has diffused across the alveolar and
capillary membranes, it combines with
haemoglobin.
• This forms oxyhaemoglobin which transports
the oxygen to respiring tissues via the
bloodstream.

8. Cont…
• During exercise, blood spends around half the
normal time (1s at rest) in the pulmonary
capillaries due to the increase in cardiac
output.
• However, diffusion of oxygen is complete
within 0.5s of the blood cell arriving in the
capillary, which means that exercise is not
limited by gas exchange.

9. Diffusion of Carbon Dioxide
• The partial pressure of carbon dioxide in the
capillaries is much higher than that in the
alveoli.
• This means that net diffusion occurs into the
alveoli from capillaries. The carbon dioxide
can then be exhaled as the partial pressure in
the alveoli is also higher than the partial
pressure in the external environment.

10. Cont…
• However it is important to note that these
pressure differences are much smaller than
those for oxygen, as carbon dioxide is able to
diffuse more rapidly.
• Carbon dioxide is transported in the blood in
multiple ways; including dissolved, associated
with proteins and as bicarbonate ions.

11. Factors That Affect the Rate of Gas Diffusion Through
the Respiratory Membrane
• the factors that determine how rapidly a gas will pass through
the membrane are
• (1) the thickness of the membrane,
• (2) the surface area of the membrane,
• (3) the diffusion coefficient of the gas in the substance of the
membrane, and
• (4) the partial pressure difference of the gas between the two
sides of the membrane.

12. Clinical Relevance
• Emphysema