Alveolus and plasma
Direction
of diffusion
Red blood cell (RBC)
The CO2 that diffused out of the RBC into an
alveolus is exhaled.
A
o
o
Carbaminohemoglobin (Hb-CO2) unbinds, yielding CO2 and hemo-
globin (Hb). The CO2 diffuses out of the RBC into an alveolus.
O2 diffuses from alveolus into plasma and RBC.
O2 •••►
O2 binds to Hb to become oxyhemoglobin (Hb-O2) and displaces H+.
Bicarbonate (HCO3-) diffuses from plasma into
RBC.
HCO3- •••►
HCO3- combines with hydrogen ions (H+) released from Hb to form
carbonic acid (H2CO3).
The CO2 that diffused out of the RBC into an
alveolus is exhaled.
◄••• CO2
The enzyme carbonic anhydrase converts H2CO3 into CO2 and H2O.
The CO2 diffuses out of the RBC into an alveolus.
Cl-
To maintain ionic balance in the wake of the HCO3- influx, chloride
ions (Cl-) move from the RBC into the plasma.
During internal respiration, oxygen diffuses from red blood cells into tissues, and carbon dioxide diffuses out.
Tissue cells and plasma
Direction
of diffusion
Red blood cell
CO2 diffuses from tissue cells into RBC.
o
O
T
CO2 reacts with Hb-O2 to form Hb-CO2, displacing O2.
A
o
The displaced O2 from the RBC diffuses into the plasma and across
the capillary membrane into the tissue cells.
◄••• HCO3-
The enzyme carbonic anhydrase combines H2O and CO2 to form
H2CO3, which breaks down spontaneously into HCO3- and H+.
H+ binds to Hb in the RBC.
HCO3- diffuses from RBC into plasma.
To maintain ionic balance in the wake of the HCO3-
efflux, Cl- moves from the plasma into the RBC.
o
Gases Are Exchanged at the Blood Capillaries 385
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