JELILAT KAREEM T. FACULTY NO: 127546. PHYSIOLOGY SEMINAR- 14TH DECEMBER 2016. MEDICAL UNIVERSITY OF PLEVEN.
Physiology of respiration
PHYSIOLOGY OF RESPIRATION
BY: KAREEM JELILAT T.
PHYSICAL PRINCIPLES OF GAS EXCHANGE
❖Diffusion of Oxygen and Carbon dioxide through the respiratory
❏ The first step in the respiratory process is the supply of fresh air to the alveolar (alveolar
❏ The second step involves the exchange of oxygen and carbon dioxide across the pulmonary
capillaries, with subsequent transport of these gases between the lungs and systemic tissues
throughout the body.
❏ Finally, gas exchange across the systemic capillaries delivers oxygen to tissue cells and
removes carbon dioxide from the tissue cells.
❏ The process of diffusion is the random motion of molecules intertwining their way in all
directions through the respiratory membrane and adjacent fluids.
❏ However, in respiratory physiology, one is concerned not only with the basic mechanism by
which diffusion occurs but also with the rate at which it occurs and this is a much more
complex problem, requiring a deeper understanding of the physics of diffusion and gas
In order to understand the mechanisms and processes involved in the
diffusional exchange of oxygen and carbon dioxide, certain relevant
physical and chemical properties of gases must be considered; bringing us
to the partial pressure of gases.
In respiratory physiology, one deals with mixtures of gases, mainly of
oxygen, nitrogen, and carbon dioxide.The rate of diffusion of each of these
gases is directly proportional to the pressure caused by that gas alone,which
is called the partial pressure of that gas.
The concept of partial pressure is explained below:
❖ It is a well known fact that atmospheric air contains approximately
79% nitrogen, 21% oxygen, negligible amount of carbon dioxide,
water vapour and inert gases.
❖ The total pressure of this mixture at sea level averages 760 mm Hg.
❖ According to Dalton, in a mixture of different gases, the pressure exerted by an
individual gas in the mixture is independent of the pressure exerted by the other
gases in the mixture.
❖ These individual pressures called partial pressures is denoted by Pgas is directly
proportional to both the concentration and the temperature.
❖ For example: PN2=79/100 * 760
❖ Similarly the partial pressure of oxygen is determined using the same formula
except that 21 is used in place of 79 to get 160mm Hg as the partial pressure of
PARTIAL PRESSURE GRADIENT
The partial pressure gradient is the difference in partial pressures between
the capillary blood and the surrounding structures.
It exists between the alveolar air and pulmonary blood; and exists between
systemic capillary blood and surrounding tissue cells.
In the lungs, partial pressure gradient drives the exchange of respiratory
gases (oxygen and carbon dioxide) between alveolar air and blood flowing in
the pulmonary capillaries. Moreover, in the systemic tissues, partial pressure
gradient drives the exchange of O2 and CO2 between systemic capillary
blood and the surrounding tissue cells.
DIFFUSION OF GASES THROUGH THE
As a result of the extensiveness of the capillary plexus, the flow of blood in
the alveolar wall has been described as a “sheet” of flowing blood. Thus, it is
obvious that the alveolar gases are in very close proximity to the blood of the
Further, gas exchange between the alveolar air and the pulmonary blood
occurs through the membranes of all the terminal portions of the lungs, not
merely in the alveoli themselves. All these membranes are collectively known as
the respiratory membrane, also called the pulmonary membrane.
The figure below shows the ultrastructure of the respiratory membrane drawn in cross
section on the left and a red blood cell on the right. It also shows the diffusion of oxygen from
the alveolus into the red blood cell and diffusion of carbon dioxide in the opposite direction.
The following layers should be
noted on the respiratory membrane:
1. A layer of fluid lining the alveoli
and containing surfactant that
reduces the surface tension of the
2. The alveolar epithelium which is
Composed of epithelial cells.
3. An epithelial basement membrane
4. A thin interstitial space between the alveolar epithelium and the capillary membrane
5. A capillary basement membrane that in many places fuses with the alveolar
epithelial basement membrane.
6. The capillary endothelial membrane.
The average diameter of the pulmonary capillaries is only about 5 micrometers,
which means that red blood cells must squeeze through them. The red blood cell
membrane usually touches the capillary wall, so that oxygen and carbon dioxide need
not pass through significant amounts of plasma as they diffuse between the alveolus
and the red cell and this increases the rapidity of diffusion.
n epithelial basement mem
Factors That Affect the Rate of Gas Diffusion Through the
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.
Diffusing Capacity of the Respiratory Membrane
The ability of the respiratory membrane to exchange a gas between the alveoli and the
pulmonary blood is expressed in quantitative terms by the respiratory membrane diffusing
capacity, which is defined as the volume of a gas that will diffuse through the membrane
each minute for a partial pressure difference of 1 mmHg.
All the factors discussed earlier that affect diffusion through the respiratory membrane
can affect this diffusing capacity.
In the average young man, the diffusing capacity for oxygen under resting conditions
averages 21 ml/min/mm Hg.
The diffusing capacity for carbon dioxide has never been measured because of the
following technical difficulty: Carbon dioxide diffuses through the respiratory membrane
so rapidly that the average Pco2 in the pulmonary blood is not far different from the Pco2
in the alveoli; the average difference is less than 1 mm Hg- and with the available
techniques, this difference is too small to be measured.