Exchange of gases

1. Ms.Anu Sebastian: B.Pharm; M.Pharm
Assistant Professor,
Dept. Of Pharmacology
Nirmala College Of Pharmacy
Muvattupuzha,Ernakulam,Kerala

2.  Exchange of gases takes place by diffusion across a
semipermeable membrane from a region of higher
concentration to a region of lower concentration until
equillibrium is reached.
 Exchange of gases takes place at two levels.
 EXTERNAL RESPIRATION
 External respiration or pulmonary gas exchange is the diffusion
of O2 from air in the alveoli of the lungs to blood in pulmonary
capillaries and the diffusion of CO2 in the opposite direction.

3.  External respiration in the lungs converts deoxygenated blood
coming from the right side of the heart into oxygenated blood
that returns to the left side of the heart.
 As blood flows through the pulmonary capillaries, it picks up
O2 from alveolar air and unloads CO2 into alveolar air.
 O2 diffuses from alveolar air, where its partial pressure is 105
mmHg, into the blood in pulmonary capillaries, where PO2 is
only 40mmHg in a resting person.
 Diffusion continues until the PO2 of pulmonary capillary blood
increases to match the Po2 of alveolar air,105 mmHg.

4.  While O2 is diffusing from alveolar air into deoxygenated blood,
CO2 is diffusing in the opposite direction.
 The PCO2 of deoxygenated blood is 45mmHg in a resting person
and the PCO2 of alveolar air is 40mmHg.Because of this difference
in PCO2, CO2 diffuses from deoxygenated blood into the alveoli,
until the PCO2 of the bloood decreases to 40 mmHg.
 The number of capillaries near alveoli in the lungs is very large and
the blood flows slowly enough through these capillaries, so there is
sufficient time available for the exchange of gases to take place.
 Thus, CO2 diffused into the alveoli is exhaled during expiration &
O2 of the blood is returned to the left atrium.

5.  INTERNAL RESPIRATION
 The exchange of O2 & CO2 between systemic capillaries
and tissue cells is called internal respiration or systemic gas
exchange.
 The PO2 of blood pumped into systemic capillaries is
higher(100mmHg) than the PO2 in tissue cells (40mmHg at
rest) because cells constantly consume O2 to produce
energy.
 Due to this pressure difference, O2 diffuses out of the
capillaries into tissue cells & blood PO2 drops to 40 mmHg
by the time the blood exits systemic capillaries.

6.  While O2 diffuses from the systemic capillaries into
tissue cells, CO2 diffuses in the opposite direction.
Since the PCO2 in the blood is less than PCO2 in
tissue cells(as the cells constantly produce CO2).
 Hence oxygen diffuses from the blood through the
capillaries into the tissues & CO2 diffuses from the
tissue into the blood in the systemic capillaries.
 The deoxygenated blood then returns to the heart & is
pumped to the lungs for another cycle of external
respiration.

7. Fig: Exchange of Gases

8. Factors affecting Gas Exchange:
 Partial pressure difference of the gases.
 Alveolar PO2 must be higher than blood PO2 for oxygen to
diffuse from alveolar air into the blood.
 The rate of diffusionn is faster when the difference between
PO2 in alveolar air & pulmonary capillary blood is higher
and vice-versa.
 Surface area available for gas exchange.
 Surface area of the alveoli is huge (about 70m2).In addition,
many capillaries around each alveolus, is so many tha as
much as 900mL of blood is able to participate in gas
exchange at any instant.

9.  Diffusion Distance
 The respiaratory membrane is very thin, so diffusion occurs
quickly .
 Molecular weight & solubility of the gases
 O2 has a lower molecular weight than CO2, it could be expected
to diffuse across the respiratory membrane about 1.2 times faster.
 However the solubility of Co2 in the fluid portions of the
respiratory membrane is about 24 times greater than that of O2.
 Taking both of these factors factors into account, net outward CO2
diffusion occurs 20 times more rapidly than net inward O2
diffusion.

10. THANK YOU