Transport of Gases

By

Dr. Sumaira Iqbal

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Learning Objectives

• By the end of lecture student should be able to:

– Explain the ways of transport of oxygen and carbon dioxide.

– Draw and interpret the Oxygen hemoglobin dissociation curve.

– Summarize the concept of Bohr’s and Haldane’s effect.

– Draw and interpret the carbon dioxide dissociation curve.

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Introduction

• Movement of gases in the body obeys diffusion

• Diffusion occur when there is difference in partial pressure

• Initial pressure difference that causes oxygen to diffuse into the pulmonary capillary is 104 − 40, or 64 mm hg

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Oxygen Uptake In Exercise

• During strenuous exercise oxygen requirement rises to 20 times

• Reason: increased cardiac output blood stays in pulmonary vasculature for less time almost half

• Safety factor makes the blood saturated with oxygen by the time it leaves the pulmonary capillaries

1. Diffusing capacity for oxygen increases during exercise due to increased surface area of capillaries and nearly ideal ventilation-perfusion ratio in the upper part of the lungs

2. Non-exercising conditions, the saturation of blood one third time in the pulmonary capillary, and later small amount of oxygen is added

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Oxygen Transport

• 97% bound to hemoglobin ---(19.4ml)

• 3% is in dissolved form

• As blood passes through tissues 5ml of oxygen is utilized and 14.4ml of oxygen is still in venous blood (75% saturated)

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Oxygen Transport

• Chemical Combination of O2 to Hb.

• O2 combination -- is with HAEM portion of Hb.

– is loose & reversible.

• O2 binds with Hb — at high PO2 as in pulmonary capillaries(Oxygenation) (O2 Loading)

• O2 released from Hb — at low PO2 as in tissue capillaries (Deoxygenation / Reduction) (O2 unloading)

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Oxygen Transport

• Heme + Globin – Quarternary

– Heme: present as 4 subunits connected by polypeptide globin chains as 2 alpha and 2 beta units(adult)

• Each Heme contains one atom of ferrous iron

Each ferrous iron has ability of binding reversibly to one oxygen molecule (via Oxygenation reaction)

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Oxygen Transport

Vol. of O2 Loading/Combination

• 1.34 ml O2 binds with each gram of Hb.

• With normal Hb. conc. 15 gm/dl,

100 ml of Bl. can bind = 15 x 1.34 ml

= 20.1 ml

Or 20 vol.%.

• • Dissolved O2 = 0.3 ml/100 ml.

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Oxygen Transport

• Total quantity of oxygen bound with hemoglobin is 19.4ml/100ml of blood– 97% saturation

• Passing through tissue capillaries reduces to 14.4ml/100ml of blood– 5ml utilized

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Oxygen Transport

• During heavy exercise, muscle utilize oxygen at a rapid rate

• PO2 in tissues falls to 15mmHg from 40mmHg

• 15mL of oxygen is delivered

• Cardiac Output increases 6-7times, oxygen transport increases 3 times

• 20 times oxygen transport increases in tissues

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Oxygen Transport

Volume of O2 unloading/release.

• 5 ml O2/100ml of blood or 250 ml/min O2 is transported to the tissues.

During exercise

• 5 times increased binding capacity

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Utilization coefficient

• Percentage of the blood that gives up its oxygen as it passes through the tissue capillaries-- utilization coefficient

• Normal value-- 25 %

• During strenuous exercise, it increases to 75 to 85%

• Local tissue areas where blood flow is very slow/metabolic rate is very high, utilization coefficients becomes 100%

• All the oxygen is to the tissues

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Oxygen deliver to Tissues

• In normal conditions at 40mmHg 5ml of oxygen /100 ml of blood is required while passing through the tissue capillaries

• During heavy exercise, 20 times more oxygen to be delivered to the tissues

• For this little further decrease in tissue PO2 is required

1. Steep slope of the dissociation curve

2. Increase in tissue blood flow caused by the decreased pO2

• Small fall in pO2 causes large amounts of extra oxygen to be released from hemoglobin

• Oxygen to the tissues is delivered efficiently at a pressure between 15 to 40mmHg 19

Changes in Atmospheric Pressure

• When pO2 decreases to 60mmHg– Hb saturation becomes 89% still will give 5ml of oxygen to the tissues

– Venous blood pO2 will become 35mmHg

• When pO2 increases to 100mmHg– Hb saturation becomes 100% still will give 5ml of oxygen to the tissues

– Venous blood pO2 will become few mmHg greater than normal

Thus explains oxygen buffering of the blood hemoglobin system

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Oxygen Hemoglobin Dissociation Curve

• Demonstrates an increase in the percentage of hemoglobin bound to oxygen as PO2 increases in blood--- percent saturation of hemoglobin

• Bohr effect is a physiological phenomenon first described in 1904 by the Danish physiologist Christian Bohr

• Bohr’s effect --- response to increase in CO2 and H ions enhance the release of oxygen from blood to tissues

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Oxygen Hemoglobin Dissociation Curve

Factors effecting affinity of Hb for O2

i. Temp

ii. pH of Blood

iii. Conc. of 2,3-DPG

iv. Hb. Conc.

v. CO

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Oxygen-Hemoglobin Dissociation Curve

Significance/Features• Characteristic sigmoid shape• Progressive increase in percent saturation of Hb (O2- carrying

power) with O2 as PO2 increases.• Flat upper portion assists in diffusion of O2 across blood-gas barrier

in lung i.e loading of blood with O2.• Small fall in PO2 of Alveolar gas hardly affects the O2 content of

Arterial blood.• Steep lower part of the curve means “Peripheral tissues can

withdraw large amount of O2 for a small drop in capillary O2” i.e it assists diffusion of O2 into the tissue cells.

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CO poisoning• O2 content of blood

– Reduced, CO has binding capacity 250 times more

• PO2 of blood

– May be normal

– Blood is bright red

– No obvious signs of hypoxemia

– Cyanosis absent

– Feedback mechanism to hypoxia Absent

– Disorientation & unconsciousness before becoming aware

– CO pressure 0.6mmHg can be lethal

TREATMENT:

• Pure oxygen

– Hyperbarric Oxygen therapy

– Displaces CO

• Carbon dioxide therapy increases ventilation

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CO2 DIFFUSION

• CO2 is the end-product of aerobic metabolism.

• Produced almost entirely in the mitochondria where the PCO2 is the highest

• Elimination of CO2

• At each point in gas transport chain, CO2 diffuses in exactly the opposite direction to O2 diffusion.

• CO2 diffuses 20 times as rapidly as O2

• Pressure differences required for CO2 diffusion far less than O2 diffusion

Carbon dioxide transport

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Carbon Dioxide Exchange

(fast)

PLASMA

CO2 Transport

4ml of CO2 is transported in 100ml of blood

• In three ways

1. Dissolved in solution ---- 7% (0.3ml)

2. Buffered with water as carbonic acid --- 70%

3. Bound to proteins, particularly hemoglobin ---- 23%

Dissolved Form

• 0.3ml in this form to lungs

• PCO2 of venous blood 45mmHg (2.7ml)

• PCO2 of arterial blood 40mmHg (2.4ml)

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Buffered With Water As Carbonic Acid

• 70% of CO2 transport from tissues to lungs.

• Dissolved CO2 in blood reacts with water to form Carbonic Acid

CO2 + H2O H2CO3

• Carbonic Anhydrase (C.A.) present RBCs accelerate it 5000 fold both ways & markedly time required

*

• Carbonic acid dissociates into H+ & HCO3H2CO3 H + HCO3

• Most of H+ combine with Hb (powerful buffer) & HCO3 diffuse out of RBCs into plasma in exchange for Cl - Band 3 HCO3/Cl carrier protein in RBC membrane

• Cl content of RBCs - CHLORIDE SHIFT

• Special bicarbonate chloride carrier protein

• Chloride content greater in RBCs

• 1st ion moves out of cell before 2nd ion moves inwards - most other ion pumps simultaneous exchange 2 ions

Buffered With Water As Carbonic Acid

CO2 Bound As Carbaminohemoglobin

• 23% of total CO2 transport (1.5ml)

• CO2 reacts directly with Hb to form the carbaminoHb (Hgb.CO)

• Combine with amine radicals

• Reversible - very loose bond CO2 easily released into alveoli where less PCO2 in pulmonary capillaries

• Small quantity of CO2 reacts with plasma proteins - less significant (quantity of proteins 1/4th that of Hb)

HALDANE EFFECT

• Binding of O2 with Hb tends to displace CO2 from blood

• Quantitatively more important in promoting CO2 transport than is Bohr effect in promoting O2 transport

• Acidic hemoglobin (BIND WITH OXYGEN )– Less tendency to combine with CO2, displacing more CO2

– Excess H+ ions combine with HCO3 release CO2 and is dissipated in air from alveoli

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CO2 Dissociation Curve

• Narrow range

– Arterial blood----40mmHg

– Venous blood----45mmHg

• Normal concentration--- almost 50 volume percent

• In tissues becomes 52% and in lungs 48%• Haldane effect ---- upon binding O2 Hb releases CO2 from the blood

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CO2 Equilibrium Curve• HbCO2• Different than HbO2 curve

– Relationship is linear– Venous blood transports more

CO2 than arterial blood• CO2 equilibrium is affected by O2

saturation of Hb• Normal concentration of CO2 in

blood is 50 volumes %, only 4 volumes % is exchanged during transport from tissues to lungs

• In tissues 52 volume %• In lungs 48 volume %

PO2 = 100mmHg

PO2 = 40mmHg

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Respiratory Exchange Ratio(R)

• Ratio of carbon dioxide output to oxygen uptake

• Average value is 0.825

• Carbohydrate metabolism R rises to 1.00

• Fats metabolism R becomes 0.7

• Depends on metabolism

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THANK YOU

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