Diffusion

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)

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


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)

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


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


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


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


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 adequate Dr.Asir John Samuel (PT)

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)

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)

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


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


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


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)




• 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%


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)

Factors affecting it

• pH

• Temperature

• Concentration of 2,3-biphosphoglycerate

(BPG; 2,3-BPG)


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


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


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


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)


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


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



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


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)

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)

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


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 dioxide Dr.Asir John Samuel (PT)

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)

Chloride shift


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 s Dr.Asir John Samuel (PT)

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Diffusion

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