Oxygen and Carbon Dioxide transport in the

blood

Majority of O2 and CO2 is transported in the blood by:

O2 combing with hemoglobinCO2 transformed into

bicarbonate (HCO3¯)

Hemoglobin and O2 transport±99% of O2 transported in blood

is chemically bound to hemoglobin

Hemoglobin is a protein found in red blood cells (erythrocytes)

Each hemoglobin molecule can hold up to 4 O2 molecules

O2 combined with hemoglobin = oxyhemoglobin

O2 that is NOT combined with hemoglobin = deoxyhemoglobin

Oxyhemoglobin

The amount of O2 that can be transported depends on the concentration of hemoglobin

Normal, healthy men = 150g/L of blood

Normal, healthy women = 130g/L of blood

When a hemoglobin molecule is completely saturated with O2 it can transport 1.34ml of O2

Therefore if hemoglobin if full (100% saturated with O2):

Healthy male can transport 200ml of O2

Healthy female can transport 174ml of O2

100% saturation occurs at sea level (low altitude)

Partial PressureThe amount of O2 bound to hemoglobin is directly

related to the partial pressure of O2.

PO2:All gases exert pressure on the walls of their container because the molecules of gas bounce off the walls.

Partial pressure is used to describe a mixture of gases.

Defined as the pressure that any one gas would exert on the walls of the container if it were the only gas present

In the lungs, where the alveoli and capillaries exchange gases, PO2 is high

Therefore O2 binds instantly to hemoglobin.

As the blood circulates to other body tissue the PO2 becomes lower

Therefore hemoglobin releases O2 into the tissue because the hemoglobin cant maintain its full capacity of O2

Oxyhemoglobin Dissociation Curve

In the alveolar capillaries in the lungs:

O2 binding with hemoglobin is called loading

Release of O2 from hemoglobin is called unloading

Loading & unloading are reversible actions: Deoxyhemoglobin + O2 Oxyhemoglobin

Oxyhemoglobin Dissociation Curve An important tool for understanding how our blood

carries and releases oxygen.

Relates oxygen saturation (SO2) and partial pressure of oxygen in the blood (PO2)

Determined by what is called "hemoglobin's affinity for oxygen“

how readily hemoglobin acquires and releases oxygen molecules from its surrounding tissue.

Oxygen hemoglobin dissociation curve

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Oxygen Dissociation Curve

Shows the percentage of O2 that is bound to hemoglobin at each O2 pressure.

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

At rest the body’s O2 requirement is low & only ±25% is unloaded into muscles

At intense exercise, PO2 can reach 20mmHg and 90% of O2 is unloaded into muscles

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Significance of the Flat Portion The flat portion of the curve shows that the PO2 can fall from

100 to 60 mmHg and the hemoglobin will still be 90% saturated with O2

At pressures above 60mm Hg, the dissociation curve is relatively flat.

This means the O2 content does not change much (even with large changes in the partial pressure of oxygen)

E.g. PO2 can fluctuate between 90 – 100mmHg without a large drop in the percentage of hemoglobin that is saturated with O2

*This is important because there is a drop in PO2 with aging and with climbing high altitudes

Haemoglobin Saturation at High Values

Lungs at sea level: PO2 of 100mmHg haemoglobin is 98% SATURATED

Lungs at high elevations: PO2 of 80mmHg, haemoglobin 95% saturated

Even though PO2 differs by 20 mmHg there is almost no difference in haemoglobin saturation.

When the PO2 in the lungs declines below typical sea level values, haemoglobin still has a high affinity for O2 and remains almost fully saturated.

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Significance of Steep PortionPO2 reductions below 40 mm Hg produce a rapid

decrease in the amount of O2 bound to hemoglobin.

When the PO2 falls below 40 mm Hg, the quantity of O2 delivered to the tissue cells may be significantly reduced.

As PO2decrease in this steep area of the curve, the O2 is unloaded to peripheral tissue as hemoglobin’s affinity for O2 diminishes.

Therefore, small changes in PO2 will release large amounts of O2 from hemoglobin.

* This is critical during exercise when O2 consumption is high.

Haemoglobin Saturation at Low Values

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Factors that affect the O2 Dissociation

◦pH - Change in the blood pH◦Temperature- temp increases = the

curve moves to the right◦Carbon Dioxide – increased PCO2

Factors Altering Haemoglobin Saturation

Factors Altering Haemoglobin Saturation (Exercise)

ExampleDuring exercise, the

oxyhemoglobin dissociation curve will shift to the RIGHT

This is because the pH in the body is decreased (from increased lactic acid) AND

Temperature increases during exercise

O2 Transport in muscleMyoglobin:

- Protein that binds with O2

- Found in Skeletal and Cardiac muscle fibers (not in blood)

- Acts as a shuttle to transport O2 from muscle cell membrane to the mitochondria

- Found in large quantities in slow-twitch fibers (high aerobic capacity)

- Smaller amounts in intermediate fibers - Limited quantity in fast twitch fibers

Myoglobin has a similar structure to hemoglobin, but is ¼ weight

Difference in structure = difference in affinity for O2

Myoglobin has a greater affinity for O2: Therefore the myoglobin-O2 dissociation curve is much

steeper

= myoglobin releases O2 at very low PO2 values

NB because PO2 in mitochondria of contracting skeletal muscle can be as low as 1mmHg.

Myoglobin stores O2 = reserve O2 for transition from rest to exercise

At the start of exercise there is a lag time from the onset of muscular contraction and increased O2 delivery to the muscles

Therefore O2 bound to myoglobin before exercise acts as a buffer

so that muscles can receive O2 until the cardiopulmonary system can meet the new O2 demand

At the end of exercise:- myoglobin- O2 stores must be replenished to ensure O2 is available for the next time exercise begins

Therefore O2 consumption above rest contributes to the O2 debt

i.e. O2 consumption continues after exercise has stopped leading to an O2 debt (O2 deficit)

(Anaerobic metabolism of lactate – also called EPOC Post Exercise Oxygen Consumption)

Carbon Dioxide Transport in BloodCO2 transported in the blood by:

1. Dissolved CO2 (±10%)2. CO2 bound to hemoglobin (±20%)3. Bicarbonate (HCO3¯)(±70%)

Dissolvedbound to HbHCO3-

CO2 is converted to bicarbonate in red blood cells:

A high PCO2 causes CO2 to combine with water, forming carbonic acid.

This reaction is rapidly catalyzed by the enzyme Carbonic Anhydrase

The carbonic acid then dissociates into bicarbonate ion and hydrogen ion.

The hydrogen ion then binds with hemoglobin

The bicarbonate ion diffuses out of the red blood cell into the blood plasma

Transport of CO2Carbon Dioxide Transport

H2 CO3 CO2 + H2O

H2 CO3 H+ + H-CO3

CA

Carbonic Acid

Bicarbonate ion

Bicarbonate Ion ExchangeBecause bicarbonate carries a negative charge

(anion), removal of a negatively charged molecule from a cell = electrochemical imbalance

Therefore the negative charge must be replaced

Bicarbonate is replaced by chloride (Cl¯) which diffuses from the plasma into the red blood cell

This is called chloride shift the shift of anions into red blood cells as blood moves through tissue capillaries

When blood reaches the pulmonary capillaries:

PCO2 of the blood is greater than that of the alveolus = CO2 diffuses out of the blood across the blood-gas interface

At the lungs:

Binding of O2 to hemoglobin causes a release of the hydrogen ions (which are bound to hemoglobin) to promote the formation of carbonic acid

H+ + HCO3¯ H2CO3

In conditions where PCO2 is low (at the alveolus), carbonic acid then dissociates into CO2 and H2O

H2CO3 CO2 +H2O

The release of CO2 from the blood into the alveoli is removed from the body in expired gas (CO2 we breathe out)

Revision Questions1. What is the amount of hemoglobin found in a

normal male and female? (2)

2. What is the amount of oxygen carried in a normal male and female? (2)

3. What is partial pressure? (2)

4. Explain the oxghemoglobin dissociation curve, focusing on the use, flat and steep portions (10)

5. What factors affect the oxghemoglobin dissociation curve? (3)

Revision Questions 6. How does exercise affect the oxghemoglobin

dissociation curve? (3)

7. What is myoglobin and where is it found? (4)

8. How is myoglobin different to hemoglobin? (4)

9. What are the differences in myoglobin and oxygen at the start and end of exercise? Why does this happen? (8)

10. What are the way in which carbon dioxide is transported? (3)

11. How is carbon dioxide converted to bicarbonate? (5) 12. Explain bicarbonate exchange. (8)