TRANSPORT OF OXYGEN & CARBON

DIOXIDE in blood and body tissues chapter 40

DR FARZANA

objectives

At the end of this chapter students must be able to Explain partial pressures of gases Describe transport of O2 in the body Draw the oxygen dissociation curve and explain significance of flat and steep portions

Name and explain the factors which shift oxygen dissociation curve to right and left Explain transport of CO2 in the body write briefly on Bohr’s

effect, Haldane effect and chloride shift

Respiratory Gas Transportation

Oxygen Transport Delivery System

• Objective– Efficient Supply of Oxygen to tissues

according to their metabolic needs.

• Factors affecting Oxygen delivery i. Amount of Oxygen entering lungs

(Atmospheric Air PO2

ii.Adequacy of Pulmonary Gas Exchange (Respiratory Membrane)

iii.Blood Flow to the Tissue (Local Regulating Factor)

iv.Oxygen Carrying Capacity of Blood (Amount and Type of Hb)

Oxygen Transport• O2 carried by red blood cells

(erythrocytes)– Hemoglobin (Hb)– Normal hemoglobin concentration is 150

g/L or 15g/dL– Carries 65 times more O2 than plasma

• Dissolved O2 in plasma– Low capacity to carry or transport O2

Forms of Oxygen Transport

• Chemical combination with Hb– Oxygenation 97%

• In dissolved state 3%– Amount of dissolved related directly

to partial pressure (PO2)– 0.0003 ml O2/100 ml/mmHg

• Arterial blood –PO2 104mmHg

Chemical Combination of O2 to Hb

• Oxygenation• Loose, Reversible, Binding of O2 to

Haem in Lungs Hb 4 + 4O2 Hb4O8• Volume of O2 Carried (Hb Bound)

- 1.34 ml/g Hb- Normal Hb 15 g/dl- 20 ml/dl

The transport of oxygen in blood- Haemoglobin

• Hb + 4O2 → Hb(O2)4

• Over 95% of O2 is carried in this way

• Red blood cell (erythrocyte)

Oxygen Saturation & Capacity

• Up to four oxygen molecules can bind to one hemoglobin (Hb)

• Ratio of oxygen bound to Hb compared to total amount that can be bound is Oxygen Saturation

• Maximal amount of O2 bound to Hb is defined as the Oxygen Capacity

Uptake of O2 by Pulmonary Capillary Blood

Oxygen Transport Mechanism - DnaTube.com - Scientific Video and Animation Site.flv

Uptake of O2 by Pulmonary Capillary Blood

Uptake of O2 by Pulmonary Capillary Blood

• Arterial PO2 = 40 mmHg• Alveolar PO2 = 104 mmHg• Difference = 104-40 = 64 mmHg• Rapid rise in PO2 as blood passes

through the capillaries and becomes equal to alveolar PO2. Therefore

• Venous PO2 =104 mmHg

Uptake of O2 by Pulmonary Capillary Blood

during exercise

• During strenuous exercise= O2 required 20 times the normal

• Increased COP = stay of blood in capillaries is decreased to half normal

Cont:• Due to safety factor increased diffusing capacity blood becomes saturated in

initial 1/3rd of capillary and little O2 enters the blood in subsequent part of capillary• During exercise with shortened

stay of blood in capillaries, blood is fully oxygenated

Transport of Oxygen in the Arterial Blood

• 98% of blood enters the left Atrium Oxygenated up to a PO2 of about 104 mmHg

• Shunt Flow: 2 percent Shunt Flow

• Venous admixture of blood PO2 change 104 to 95 mmHg

Effect of Venous Admixture

Transport of Oxygen from the Lung to the Body

Tissue Diffusion of Oxygen from the

Alveoli into the Pulmonary Blood due to difference in partial pressure of Oxygen (Po2).

Po2 of Alveoli > Po2 of Pulmonary Capillary Blood

Po2 of Pulmonary Capillary Blood > Po2 in the tissues

Gas Content of Blood (Hb = 14.5 g/dl)

• Arterial Blood - 100 ml of blood combines with 19.4ml of O2

– Po2 95 mmHg– %Hb saturation 97%

• Venous Blood - 100 ml of blood combines with 14.4ml of O2

– Po2 40 mmHg – % Hb saturation 75%

Oxygen Utilization Coefficient

– Thus 5ml of O2 is transported by each 100 ml of blood through tissues per cycle

– During exercise ….increased cellular O2 utilization ….decreased interstitial PO2…..15 mmHg

– Venous blood 100ml combines with 4.4 ml of O2 ( sat 20% , PO2 18mmHg)

– Thus 15 ml of O2 is transported by each 100 ml of blood through tissues per cycle

Diffusion of O2 from peripheral capillaries in to

tissue fluid

Volume of O2 Delivered (In Tissues)

• 5ml/dl/min • 250 ml/5L/ min• Increases three times in exercise Factors Affecting affinity of Hb for O2

• Temperature • PH of Blood• Hb concentration• 2, 3 DPG• CO

Summary of Gas Exchange in Lungs &

Tissues

Effect of Blood Flow & Rate of O2 Consumption on Tissue

PO2

Diffusion of o2 from peripheral capillaries to

the tissues• O2 used by the cells, PO2 in

peripheral tissue cells remains lower in the peripheral capillaries

• There is considerable distance between capillaries and cells.

• Therefore cellular PO2 ranges b/w 5-40mmHg average 23mmHg

Cont:

• Only 1-3mmHg of O2 pressure normally required for full support of chemical processes that incorporates O2 in the cell

• Low intracellular PO2 of 23 mmHg is enough and provides large safety factor.

Role of o2 in hb transport

• 97% by blood Hb• 3% by plasma• When PO2 is high in pulmonary

capillaries……O2 binds with Hb• When PO2 is low in tissue

capillaries……O2 released from Hb

PO2 40mmHg

% Sat 75%

Vol% 14.4

PO2 95mmHg

% Sat 97%

Vol% 19.4

Effect of Blood PO2 on Quantity of OxyHb

The Oxygen hemoglobin Dissociation Curve

Reveals the amount of haemoglobin saturation at different PO2 values.

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Oxygen Dissociation Curve

• The O2 dissociation curve graphically illustrated the percentage of Hb that is chemically bound to O2 at each O2 pressure.

• The curve is S-shaped with a steep slope between 10 and 60 mm Hg and a flat portion between 70 and 100 mm Hg.

• The flat and steep portions of the curve each have a distinct clinical significance.

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Significance of the Flat Portion

• The flat portion of the curve shows that the P02 can fall from 100 to 60 mmHg and the Hg will still be 90% saturated with 02

• At pressures above 60mm Hg, the standard dissociation curve is relatively flat. This means the oxygen content does not change significantly even with large changes in the partial pressure of oxygen.

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Significance of Steep Portion

• PO2 reductions below 60 mm Hg produce a rapid decrease in the amount of O2 bound to hemoglobin.

• Clinically, when the PO2 falls below 60 mm Hg, the quantity of O2 delivered to the tissue cells may be significantly reduced.

• As oxygen partial pressures decrease in this steep area of the curve, the oxygen is unloaded to peripheral tissue readily as the hemoglobin’s affinity diminishes.

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The P50

• A common point of reference on the oxygen dissociation curve is the P50.

• The P50 represents the partial pressure at which the hemoglobin is 50% saturated with oxygen, typically 26.6 mm Hg in adults.

• The P50 is a conventional measure of hemoglobin affinity for oxygen.

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Shifts in the P50• In the presence of disease or other

conditions that change the hemoglobin’s oxygen affinity and, consequently, shift the curve to the right or left, the P50 changes accordingly.

• An increased P50 indicates a rightward shift of the standard curve, which means that a larger partial pressure is necessary to maintain a 50% oxygen saturation, indicating a decreased affinity.

• Conversely, a lower P50 indicates a leftward shift and a higher affinity.

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Factors that effect the 02 Dissociation

– pH- Change in the blood pH– Temperature- as temperature

increases the curve moves to the right

– 2,3 Diphosphoglycerate-Increases 2,3 DPG results in decreased affinity

– Carbon monoxide

272727

2727

2727

5050 ------

Decreased affinityUnloading

Increased affinityLoading

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Clinical Significance of Shifts

• Individuals with PaO2’s within normal (80-100) limits are rarely afected by shift changes.

• However, when a patients PaO2 falls below 80, a shift to the right or left can have remarkable effects on the hemoglobin’s ability to pick up and release oxygen.

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Right Shifts

• Right shift decrease the loading of oxygen onto Hb at the A-C membrane.

-Decreased affinity• The total oxygen delivery may be much

lower than indicated by a particular Pao2 when the patient has some disease process that causes a right shift.

• Right shift curves enhance the unloading of oxygen at the tissue level.

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Left shift

• Left shift curves enhance the loading capability of oxygen enhance the loading capability of oxygen at the A-C membrane.

• The total oxygen delivery may be higher than indicated by a particular PaO2 when the patient has some disease process that cause a left shift.

• Left shift curves decreases the unloading of oxygen at the tissue level.

Factors Altering Haemoglobin Saturation

Exercise

Factors affecting Disassociation

BLOOD TEMPERATUREtemperature

• reduces haemoglobin affinity for O2

• hence more O2 is delivered to warmed-up tissue

•Respiratory Response to Exercise

CARBON DIOXIDE CONCENTRATION

CARBON monoXIDE CONCENTRATION

• Interferes with O2 transport because it has 200 times more affinity for Hb

• Competes with O2 for same sites for binding and decreases functional Hb

BLOOD pH

2,3 DPG – 2,3 diphosphoglycerate

2,3 diphosphoglycerate, generated by glycolysis during anaerobic metabolism, binds to Hb and decreases affinity for O2

Stored blood

• Packed red blood cells (PRBC) used for virtually all blood transfusions are stored cold and have significantly diminished levels of 2,3 DPG

• Hb in stored blood would initially show a left shift in the Hb-O2 disassociation curve

Venous Hb Affinity Shift• Shift to the right

reducing the affinity for O2 below Po2 of 70 mmHg

• Shift occurs because of rising Pco2 & [H ions] – Bohr effect = an increase in [H+] decreases Hb’s affinity for O2

• Enhances the quantity of O2 released in systemic capillaries

• Increases delivery of O2 to tissues

Hemoglobin & Myoglobin

• Myoglobin is single chained heme pigment found in skeletal muscle

• Myoglobin has an increased affinity for O2 (binds O2 at lower

Po2)• Mb stores O2

temporarily in muscle

Carbon Dioxide• Volatile waste product of cellular

metabolism - Intracellular Pco2 46 mmHg

– Interstitial Pco2 is 45 mmHg– Diffuses into systemic capillaries with

arterial Pco2 40 mmHg

– Venous Pco2 is 45 mmHg

– Alveolar air Pco2 is 40 mmHg

Mechanisms of CO2 Transport

Complex mechanism of CO2 transport

Carbon dioxide transported by blood in three forms:

1. Dissolved directly in blood2. Bicarbonate ion (HCO3-) & Carbonic acid

(H2CO3)3. Bound to hemoglobin & plasma proteins

MOST CO2 TRANSPORTED

AS BICARBONATE (HCO3-)*

• When CO2 molecules diffuse from the tissues into the blood, 7% remains dissolved in plasma and erythrocytes, 23% combines in the erythrocytes with deoxyhemoglobin to form carbamino compounds, and 70% combines in the erythrocytes with water to form carbonic acid, which then dissociates to yield bicarbonate and H+ ions

Chloride shift and reverse Chloride shift

• Most of the bicarbonate then moves out of the erythrocytes into the plasma in exchange for Cl- ions & the excess H+ ions bind to deoxyhemoglobin. The reverse occurs in the pulmonary capillaries and CO2

moves down its concentration gradient from blood to alveoli.

Carbon dioxide transport

Differences between Bohar and Haldane Effects

• BOHAR EFFECT1.It is the effect by

which the presence of CO2 decreases the affinity of Hb for O2

• HALDANE EFFECT1.It is the effect by

which combination of O2 with Hb displaces CO2 from Hb

2. Was postulated by Bohr in 1904

3. Occurs at tissues and systemic capillaries

4. In tissues ……metabolism

↑PCO2 & ↓ PO245 mmHg 40

mmHg

2. Described by Jhon Scott Haldane in 1860

3. Occurs at alveolar and pulmonary capillary blood

4. Hb+O2…..HbO2 HbO2 has low

tendency to combine with CO2

• CO2 enters the blood and O2 released from blood to tissues…..Shifting O2 disosiciation curve to right…O2 to tissues

• O2+Hb….H+ and CO2

• H+ + HCO3-….H2CO3….H2O + CO2…..Released from blood to alveoli