Download Gas Exchange and Pulmonary Circulation

Gas Exchange and Pulmonary
Circulation
Gas Pressure
• Gas pressure is caused by the molecules
colliding with the surface.
• In the lungs, the gas molecules are colliding
with the surfaces of the respiratory passages
and alveoli.
• Higher concentrations of gas will produce
more collisions and cause a higher pressure.
• This idea of pressure applies to gases whether
in air or water.
Diffusion
Gases diffuse from an area of high concentration to an area of low concentration.
It is based on the probability of freely moving molecules.
Direction of Diffusion
• The net diffusion is determined by the
difference between the partial pressures.
• If the partial pressure of O2 is greater in the
alveolar air than in the blood, the net
diffusion of O2 will be into the blood.
Diffusing Capacity
• Diffusing capacity is a
measure of how well as
gas diffuses across the
respiratory membrane.
• It is defined as the
volume of a gas that will
diffuse through the
membrane each minute
for a partial pressure
difference of 1 mm Hg.
Diffusing Capacity for O2
• Diffusing capacity for O2 is ~ 21 ml/min/mm
Hg in the average young man.
• Multiply this by the mean pressure difference
(11 mm Hg) and one obtains the amount of O2
diffusing through the respiratory membrane
each minute. In this example, 230 ml O2/min.
Dalton’s Law of Partial Pressure
• The total gas pressure is the pressure caused by all the gas
molecules colliding with the surface.
• The partial gas pressure is the pressure exerted by 1 gas
species alone. Written as PO2 (partial pressure of O2), PCO2
(partial pressure of CO2).
Atmospheric Air Partial Pressures
Nitrogen
597 mm Hg
78.62 %
Oxygen
159 mm Hg
20.84 %
Carbon
Dioxide
0.3 mm Hg
0.04 %
Water
3.7 mm Hg
0.5 %
Total
760 mm Hg
100 %
• The rate of diffusion of a gas molecule is directly proportional
to its partial pressure.
Henry’s Law
• When a mixture of gasses is in contact with a
liquid each gas will dissolve in the liquid in
proportion to its partial pressure.
Solubility Coefficient
• The higher the solubility, the higher the
solubility coefficient and the lower the partial
pressure for a given concentration.
Comparing Atmospheric and Alveolar Air
• In the alveoli:
O2 is constantly being absorbed into the blood.
CO2 is diffusing into the alveolar air.
Air is humidified compared to atmospheric air.
Rate of Alveolar Removal
• The alveolar air is replaced slowly. During normal
ventilation, ~1/2 of the gas is removed in 17 sec.
• The slow replacement of alveolar air prevents sudden
changes in [blood gas].
Partial Pressure of O2 in Alveoli
• Alveolar PO2 depends on:
- The rate of O2 absorption into the blood.
- The rate of entry of new O2 during ventilation.
Why does the alveolar partial pressure of O2 not increase above 150 mm Hg?
Partial Pressure of CO2 in Alveoli
• Alveolar PCO2 depends on:
- The rate of CO2 excretion from the blood.
- The rate of removal of CO2 during ventilation.
Respiratory Membrane
• Gas exchange between
the alveolar air and
pulmonary blood occurs
through the alveolar
ducts and alveoli.
• For gas exchange to be
efficient their must be a
match between the
amount of gas reaching
the alveoli (ventilation)
and the blood flow in the
capillaries (perfusion).
Respiratory Membrane
In healthy lungs the
alveolar membrane
and the capillary wall
are only about 1 cell
thick, so gas exchange
can occur easily.
Factors Affecting Diffusion through the
Respiratory Membrane
• Thickness of the membrane.
• Surface area of the membrane.
• Diffusion coefficient.
• Difference in partial pressure.
Hemoglobin
• Remember that O2 from the lungs is carried
by red blood cells.
• On every red blood cell is an iron containing
heme group.
• Each hemoglobin molecule can bind with 4
molecules of O2
Hemoglobin
• A hemoglobin with an oxygen is called an
oxyhemoglobin. (HbO2)
• A hemoglobin that has released it’s oxygen is
called a reduced or deoxyhemoglobin. (HHb)
Hemoglobin
• The rate at which Hb binds or releases O2 is
regulated by the following:
•
•
•
•
Partial Pressure
Temperature
Blood pH
Concentration of organic Chemicals
CO2 Transport
• A normal body cell produces 200 ml of carbon
dioxide each minute.
• Blood transports CO2 from the tissues to the
lungs in three forms
– Dissolved in the plasma (7-10%)
– Bound to hemoglobin (roughly 20%)
– As a bicarbonate ion in plasma (70%)
From CO2 to Bicarbonate
• CO2 enters the plasma and then enters the
RBC to be turned into Bicarbonate.
• When CO2 enters the blood cell it combine
with water to make carbonic acid.
• Carbonic acid is unstable and quickly
disassociates into hydrogen ions and
bicarbonate.
The Bohr Effect
• When the Hydrogen ions are released they
bind with Hb (hemoglobin) and cause the
release of O2.
• The Bicarbonate is released back into the
plasma and carried to the lungs.
From Bicarbonate to CO2
• Once in the lungs the Bicarbonate returns to
the RBC and the whole process is reversed
producing CO2, which you exhale.
Buffer System
• The process of turning carbonic acid into
bicarbonate and visa versa is how your body
deals with pH shifts.
• If too many H+ are present bicarbonate in the
plasma will bond with it forming carbonic
acid.
• If H+ are too low then carbonic acid will
disassociate and release the hydrogen ions.
Acidosis/Alkalosis
• Too much CO2 in the blood will result in more
carbonic acid and there fore a lower pH level.
• If prolonged this will cause acidosis and organ
failure can occur.
• Not enough CO2 and the blood pH will rise
causing Alkalosis.