1. Diffusion
D.A. Asir John Samuel, BSc (Psy), MPT (Neuro Paed),
MAc, DYScEd, C/BLS, FAGE
Lecturer, Alva’s college of Physiotherapy,
Moodbidri
Dr.Asir John Samuel (PT)
2. Transport of O2 and CO2
• O2 from alveoli into pulmonary blood in
combination with Hb
• Presence of Hb in RBC allows blood to
transport 30 to 100 times as much O2
transported in dissolved form
• Increases CO2transport 15-20 fold
Dr.Asir John Samuel (PT)
3. Diffusion
• Gases moves from one point to another by
pressure difference from first point to next
• O2 diffuses from alveoli into pulmonary capillary
blood because PO2in alveoli is greater than in
pulmonary blood
• Higher PO2 in capillary blood than tissues causes
O2 to diffuse into surrounding cells
Dr.Asir John Samuel (PT)
4. Diffusion
• When O2 is metabolised in cells, intracellular
CO2 rises to a high value
• CO2 diffuses into tissue capillaries
• Similarly diffuses out of blood into alveoli
because CO2 in pulmonary capillary blood is
greater than in alveoli
Dr.Asir John Samuel (PT)
5. Uptake of O2
• PO2 of gaseous oxygen in alveolus averages
104 mm Hg
• PO2 of venous blood entering pulmonary
capillary at arterial end averages 40 mm Hg
• Initial pressure difference (104 – 40 = 64)
causes O2to diffuse into pulmonary capillary
Dr.Asir John Samuel (PT)
6. Transport of O2
• About 98% of blood enters left atrium is
oxygenated upto PO2 of about 104 mm Hg
• Another 2% of deoxygenated blood of PO2 of
about 40 mm Hg enters directly from bronchial
circulation – shunt flow
• Both blood combines – venous admixture of blood
• Blood pumped from Lt side of heart fall to about
95 mm Hg Dr.Asir John Samuel (PT)
7. Diffusion from peripheral capillaries
• PO2 when arterial blood reaches peripheral
tissues is still 95 mm Hg
• PO2 in interstitial fluid that surrounds tissues
averages only 40 mm Hg
• This tremendous pressure difference causes
O2 to diffuse rapidly (95 – 40 = 55)mm Hg
Dr.Asir John Samuel (PT)
8. Tissue capillaries to tissue cells
• Intracellular PO2 ranges about 23 mm Hg
• Tissue capillaries and tissue cells pressure
difference (40 – 23 = 17) mm Hg causes O2 to
diffuse rapidly
• 1 to 3 mm Hg of O2 pressure is normally
required for all support of chemical processes
• 23 mm Hg is more adequateDr.Asir John Samuel (PT)
9. Transport in blood
• About 97% of O2 transported from lungs to
tissues is carried in chemical combination with
haemoglobin (Hb) in RBC
• Remaining 3% is transported in dissolved state
in water of plasma and cells
• Under normal conditions, O2 is carried to
tissues almost entirely by Hb
Dr.Asir John Samuel (PT)
10. Oxygen with Hb
• O2 combines loosely and reversibly with
haeme portion of Hb
• Each Hb molecule contains 4 Hb chain
containing 1 atoms of iron each
• Each atom binds with 1 molecule of O2
• Each Hb molecule carries 4 molecules of O2(8
O2 atoms) Dr.Asir John Samuel (PT)
11. Oxygen with Hb
• When PO2 is high, as in pulmonary capillaries,
O2 binds with Hb
• But, when PO2 is low as in tissue capillaries, O2
is released from Hb
• This is basis for O2 transport from lungs to
tissues
Dr.Asir John Samuel (PT)
12. Amount of O2 combine with Hb
• Blood of normal person contains about 15 g of
Hb/100 ml of blood
• Each gram of Hb combines with 1.34 ml of O2
• 15 x 1.34 = 20.1 gram
• Hb in 100 ml blood caries about 20 ml of O2
when 100 % saturated
Dr.Asir John Samuel (PT)
13. Amount of O2 released
• At 97% saturated blood, 19.4 ml/100 ml
• Reduced to 14.4 at PO2 40 mm Hg
• Ultimately tissue receives 5 ml/100 ml blood
as is PO2 23 mmHg
Dr.Asir John Samuel (PT)
14. O2 –Hb dissociation curve
• oxygen–hemoglobin dissociation curve
relates percentage saturation of the O2
carrying power of hemoglobin to the PO2
• Sigmoid shape
• Demonstrates progressive increase in % of Hb
bound with O2 as blood PO2 increases, is called
per cent saturation of Hb
Dr.Asir John Samuel (PT)
16. O2 –Hb dissociation curve
• Blood leaving lungs and entering systemic
arteries usually has PO2 of about 95 mm Hg
• Usual O2 saturation of systemic arterial blood
is about 97%
• In normal venous blood, PO2 is about 40 mm
Hg and saturation is about 75%
Dr.Asir John Samuel (PT)
17. Combing O2 with Heme
• Combination of the first heme in the Hb
molecule with O2 increases the affinity of the
second heme for O2
• Oxygenation of the second increases the
affinity of the third, and so on
• So that the affinity of Hb for the fourth O2
molecule is many times that for the first
Dr.Asir John Samuel (PT)
18. Factors affecting it
• pH
• Temperature
• Concentration of 2,3-biphosphoglycerate
(BPG; 2,3-BPG)
Dr.Asir John Samuel (PT)
19. Effect on pH
• Rise in temperature or a fall in pH shifts the
curve to the right
• When the curve is shifted in this direction, a
higher PO2 is required for hemoglobin to bind a
given amount of O2
Dr.Asir John Samuel (PT)
20. Effect on temperature
• A fall in temperature or a rise in pH shifts the
curve to the left
• Lower PO2 is required to bind a given amount
of O2
Dr.Asir John Samuel (PT)
21. P50
• A convenient index for comparison of such
shifts is the P50, the PO2 at which hemoglobin
is half saturated with O2
• The higher the P50, the lower the affinity of
hemoglobin for O2
Dr.Asir John Samuel (PT)
22. Bohr effect
• Decrease in O2 affinity of hemoglobin when the
pH of blood falls is called the Bohr effect
• Deoxygenated hemoglobin (deoxyhemoglobin)
binds H+ more actively than does oxygenated
hemoglobin (oxyhemoglobin)
Dr.Asir John Samuel (PT)
23. Bohr effect
• pH of blood falls as its CO2 content increases,
so that when the PCO2 rises, the curve shifts
to the right and the P50 rises
• Hemoglobin's oxygen binding affinity is
inversely related both to acidity and to the
concentration of carbon dioxide
Dr.Asir John Samuel (PT)
24. Effect on 2,3-BPG
• 2,3-BPG is very plentiful in red cells
• Formed from 3-phosphoglyceraldehyde, a
product of glycolysis via the Embden–
Meyerhof pathway
• HbO2 + 2,3-BPG ↔ Hb- 2,3-BPG + O2
Dr.Asir John Samuel (PT)
26. Effect on 2,3-BPG
• Increase in the concentration of 2,3-BPG shifts
the reaction to the right, causing more O2 to
be liberated
• Acidosis inhibits red cell glycolysis
• 2,3-BPG concentration falls when the pH is
low
Dr.Asir John Samuel (PT)
27. Myoglobin
• Myoglobin is an iron-containing pigment
found in skeletal muscle
• Resembles hemoglobin but binds 1 rather
than 4 mol of O2 per mole
• Rectangular hyperbola rather than a sigmoid
• Because its curve is to the left of the
hemoglobin curve, it takes up O2 from Hb in
the blood Dr.Asir John Samuel (PT)
28. CO2 transport
• Solubility of CO2 in blood is about 20 times
that of O2
• More CO2 than O2 is present in simple solution
at equal partial pressures
• CO2 that diffuses into red blood cells is rapidly
hydrated to H2CO3 because of the presence of
carbonic anhydrase
Dr.Asir John Samuel (PT)
29. CO2 transport
• H2CO3 dissociates to H+ and HCO3
–
• Some of the CO2 in the red cells reacts with
the amino groups of hemoglobin and other
proteins (R), forms carbamino compounds
Dr.Asir John Samuel (PT)
30. Haldane effect
• Deoxyhemoglobin binds more H+ than
oxyhemoglobin does and forms carbamino
compounds more readily
• Binding of O2 to hemoglobin reduces its affinity
for CO2
• Deoxygenation of the blood increases its ability to
carry carbon dioxide while oxygenated blood has
a reduced capacity for carbon dioxideDr.Asir John Samuel (PT)
31. Chloride shift
• HCO3
– content of red cells is much greater
than that in plasma
• As the blood passes through the capillaries,
about 70% of the HCO3
– formed in the red
cells enters the plasma
• Excess HCO3
– leaves the red cells in exchange
for Cl–
Dr.Asir John Samuel (PT)
33. Chloride shift
• Process is mediated by anion exchanger 1
• It is a major membrane protein in RBC
• Because of this chloride shift, the Cl– content
of the red cells in venous blood is significantly
greater than that in arterial blood
• Chloride shift occurs rapidly and is essentially
complete within 1 sDr.Asir John Samuel (PT)
