The three phases of gas exchange are breathing, transport of gases via the circulatory system, and exchange of gases with body cells. Oxygen diffuses from the alveoli into pulmonary capillaries based on the partial pressure gradient, then binds to hemoglobin and is transported to tissues where it diffuses into cells. Carbon dioxide diffuses in the opposite direction, from tissues into blood and from pulmonary capillaries into alveoli. Various factors like pH, temperature, carbon dioxide levels, and 2,3-DPG can shift the oxygen-hemoglobin dissociation curve right or left, facilitating oxygen loading or unloading in the lungs or tissues.

1. QAP20803
Dr. Mohanad R. Alwan
Gas Exchange
and
Transport

2. Copyright © 2005 Pearson Education, Inc. Publishing as Benjamin Cummings
22.1 Overview: Gas exchange involves breathing, transport
of gases, and exchange of gases with tissue cells
• The three phases of gas exchange
MECHANISMS OF GAS EXCHANGE
1 Breathing
O2
CO2
Lung
Circulatory
system
2 Transport
of gases by
the circulatory
system
3 Exchange
of gases
with
body
cells
Capillary
Cell
CO2
O2
Mitochondria
Figure 22.1

3. Concentration and Partial Pressure of
Respired Gases
• Partial pressure = Percentage of
concentration of specific gas × Total pressure
of a gas
• Dalton’s law
– Total pressure = Sum of partial pressure of all
gases in a mixture

4. Ambient Air/Atmosphere
• O2 = 20.93% = ~ 159 mm Hg PO2
• CO2 = 0.03% = ~ 0.23 mm Hg PCO2
• N2 = 79.04% = ~ 600 mm Hg PN2

5. Tracheal Air
• Water vapor reduces the PO2 in the trachea
about 10 mm Hg to 149 mm Hg.

6. Alveolar Air
• Alveolar air is altered by entry of CO2.
• Average alveolar PO2 = 103 mm Hg

7. Movement of Gas in Air and
Fluids
• Henry’s law
– Gases diffuse from high pressure to low
pressure.
• Diffusion rate depends upon
– Pressure differential
– Solubility of the gas in the fluid

8. Pressure Differential
• The difference in the pressure of specific
gases from the capillary blood to the alveoli
dictates the direction of diffusion.

9. Oxygen Loading
• Oxygen diffuses along its
partial pressure gradient,
from the alveolus into the
blood, until equilibrium is
reached
• Equilibrium is reached
within the first third of the
capillary.

11. Oxygen Loading

12. External Respiration
(Pulmonary Gas Exchange)
• Exchange of gases occur between the alveoli and
the blood capillaries surrounding it

13. Solubility
• CO2 is about 25 times more soluble than O2.
• CO2 and O2 are both more soluble than N2.

14. Gas Exchange in
Lungs & Tissues
• Exchange of gases between lungs and blood
and gas movement at the tissue level
progress passively by diffusion, depending
on their pressure gradients.

15. Gas Exchange in the Lungs
• PO2 in alveoli ~ 100 mm Hg
• PO2 in pulmonary capillaries ~ 40 mm Hg
• Result: O2 moves into pulmonary capillaries
• PCO2 in pulmonary capillaries ~ 46 mm Hg
• Average arterial blood gases equal
– PO2 100 mm Hg
– PCO2 40 mm Hg

16. Pulmonary Disease
• Gas transfer capacity may be impaired by
– Thickening of membrane
– Reduction in surface area

17. Gas Transfer in Tissues
• Pressure gradients cause diffusion of O2 into
and CO2 out of tissues.

18. Factors affecting gas exchange
Changes in partial pressure

19. PO2 in Tissues
• At rest
– PO2 = 40 mm Hg
– Venous blood carries ~ 70% of the O2 content
of arterial blood.
– Venous blood carries 15 mL O2 per dL blood.
– Tissues have extracted 5 mL O2 per dL blood.

20. Arteriovenous O2 Difference
• The a- O2 difference shows the amount of O2
extracted by tissues.
• During exercise a- O2 difference increases up to 3
times the resting value.
v
v

22. Transport of O2 in the Blood
• Two mechanisms exist for O2 transport
– Dissolved in plasma
– Combined with hemoglobin

23. Oxygen in Physical Solution
• For each 1 mm Hg increase, 0.003 mL O2 dissolves
into plasma.
• This results in ~ 3 mL of O2/liter blood.
• With 5 L total blood volume = 15 mL dissolved O2
• Dissolved O2 establishes the PO2 of the blood.
– Regulates breathing
– Determines loading of hemoglobin

24. Oxygen Combined with Hemoglobin
• Each of four iron atoms associated with
hemoglobin combines with one O2
molecule.

25. Oxygen transport: Hb saturation
O2
O2
O2 O2
FULLY SATURATED

26. Oxygen-Carrying Capacity of Hb
• Each gram of Hb combines with 1.34 mL O2.
• With normal Hb levels, each dL of blood
contains about 20 mL O2.

27. Transport of Oxygen
• Oxygen transport
– Only about 1.5% dissolved in plasma
– 98.5% bound to hemoglobin in red blood cells
• Heme portion of hemoglobin contains 4 iron atoms
– each can bind one O2 molecule
• Oxyhemoglobin
• Only dissolved portion can diffuse out of blood into
cells
• Oxygen must be able to bind and dissociate from
heme

29. Myoglobin, The Muscle’s O2
Store
• Myoglobin is an iron-containing globular
protein in skeletal and cardiac muscle.
• Stores O2 intramuscularly
• Myoglobin contains only 1 iron atom.
• O2 is released at low PO2.

30. CO2 Transport
• Three mechanisms
– Bound to Hb (7%)
– Dissolved in plasma ( 15-25%)
– Plasma bicarbonate (70%)

31. CO2 in Physical Solution
• ~ 5% CO2 is transported as dissolved
CO2.
• The dissolved CO2 establishes the PCO2 of
the blood.

32. 32
• Carbon dioxide diffuses (into RBCs) and
combines with water to form carbonic acid
(H2CO3), which quickly dissociates into hydrogen
ions and bicarbonate ions
• In RBCs, carbonic anhydrase reversibly catalyzes
the conversion of carbon dioxide and water to
carbonic acid
Transport and Exchange of Carbon
Dioxide
CO2 + H2O  H2CO3  H+ + HCO3
–
Carbon
dioxide
Water
Carbonic
acid
Hydrogen
ion
Bicarbonate
ion

33. CO2 Transport as Bicarbonate
• CO2 in solution combines with water to
form carbonic acid.
• Carbonic anhydrase
– Zinc-containing enzyme within red blood cell
• Carbonic acid ionizes into hydrogen ions
and bicarbonate ions.

34. CO2 Transport as Carbamino
Compounds
• CO2 reacts directly with amino acid to form
carbamino compounds.
• Haldane Effect: Hb interaction with O2
reduces its ability to combine with CO2.
• This aids in releasing CO2 in the lungs.

35. Anemia Affects Oxygen Transport
• Volume percent (vol%) refers to the
milliliters of oxygen extracted from a 100-
mL sample of whole blood.
• Human blood carries O2 at 14 vol%.
• Iron deficiency anemia reduces O2 carrying
capacity considerably.

37. PO2 and Hb Saturation
• Oxyhemoglobin dissociation curve illustrates
the saturation of Hb with oxygen at various
PO2 values
• Percent saturation = 12 vol% / 20 vol% ×
100 = 60%

38. Oxygen-hemoglobin Dissociation
Curve

39. PO2 in the Lungs
• Hb ~ 98% saturated under normal conditions
• Increased PO2 doesn’t increase saturation.

40. Bohr Effect
• Conditions creating the Bohr effect
– Increased PCO2
– Increased temperature
– Increased 2,3-DPG
– Decreased pH
• Cause a shift to the right of the oxyhemoglobin
dissociation curve

41. Oxygen transport
• Partial pressure PO2
– The most important factor that determines how
much oxygen combines with hemoglobin.
– The greater the PO2, the more oxygen will
combine with hemoglobin, until the Hb become
saturated.

42. Oxygen transport
– The relationship
between the PO2 and
the Hb saturation is not
linear.
– It is a Sigmoid-shaped
curve
The oxygen-hemoglobin dissociation curve

43. Oxygen transport
Effect of pH
• As acidity increases (pH
decreases), affinity of Hb
for O2 decreases
• Increasing acidity
enhances unloading
• Shifts curve to right
• more O2 will release from
Hb.

44. Oxygen transport
Effect of PCO2
• An increase PCO2, shift
curve to right
– Decrease affinity of Hb for
O2
– This enhance O2 release
from blood
– CO2 increase in blood at
tissue level, as CO2
diffuse from cells to blood

45. Chapter 22, Respiratory System 45
pCO2 & Oxygen Release
• As pCO2 rises with
exercise, O2 is
released more easily
• CO2 converts to
carbonic acid &
becomes H+ and
bicarbonate ions &
lowers pH.

46. RBC 2,3-DPG
• 2,3-DPG is a byproduct of glycolysis/
• RBCs contain no mitochondria.
– Rely on glycolysis
• 2,3-DPG increases with intense exercise and may
increase due to training.
• Helps deliver O2 to tissues

47. Oxygen transport
Effect of 2,3-DPG
• 2,3-DPG, is an organophosphate, which is created
in erythrocytes during glycolysis
• High levels of 2,3-DPG shift the curve to the right,
while low levels of 2,3-DPG cause a leftward shift.

48. Oxygen transport
Effect of temperature
• Rise in temperature will
shift the curve to the
right, resulting in more
oxygen to be released
(unloading)
• During hypothermia, more
oxygen remains bound
• Active tissues have
higher temps

50. Time Required for Gas Exchange
• Capillary transit
time is ~0.75 s
• During maximal
exercise, capillary
transit time is ~0.4 s
• Gas exchange during
maximal exercise
not a limiting factor

51. Internal Respiration
• Internal Respiration
• O2 diffuses from systemic
capillaries into cells
• CO2 diffuses from cells into
systemic capillaries.

52. Internal Respiration
• Internal respiration – in tissues throughout body
• Oxygen
– Oxygen diffuses from systemic capillary blood (PO2
100 mmHg) into tissue cells (PO2 40 mmHg) – cells
constantly use oxygen to make ATP
– Blood drops to 40 mmHg by the time blood exits the
systemic capillaries
• Carbon dioxide
– Carbon dioxide diffuses from tissue cells (PCO2 45
mmHg) into systemic capillaries (PCO2 40 mmHg) –
cells constantly make carbon dioxide
– PCO2 blood reaches 45 mmHg
• At rest, only about 25% of the available oxygen is used
– Deoxygenated blood would retain 75% of its oxygen
capacity

53. Internal Respiration Depends on:
1) Available surface area,
which varies in different
tissues.
2) Partial Pressure gradients
3) Rate of blood flow varies
(e.g. metabolic rate of
tissue)

54. Internal Respiration CO2 and O2
Exchange

56. What changes in the oxygen dissociation curve occurs
during carbon monoxide poisoning?
• Because of this higher affinity of hemoglobin for
carbon monoxide than for oxygen, carbon
monoxide is a highly successful competitor that
will displace oxygen even at minuscule partial
pressures.
• The reaction displaces the oxygen molecules
forming carboxyhemoglobin; the binding of the
carbon monoxide to the iron Centre of
hemoglobin is much stronger than that of
oxygen, and the binding site remains blocked for
the remainder of the life cycle of that effected red
blood cell

57. Question:1
Shift of O2-Haemoglobin dissociation curve to
the right is caused by…. (in blood):
A) Decreased hydrogen ions
B) Increased CO2
C) Decreased temperature
D) Decreased BPG

58. Question:5
Oxyhaemoglobin dissociation curve is shifted to
the left by:
A) increase in arterial PCO2
B) acidosis
C) increase in 2,3 DPG
D) fall in temperature