11 gas transport

8/13/2019 11 Gas Transport

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Gas Transport


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Learning Objectives

Covering the the transport of O2and CO2in the blood andtissue fluids.

Know how O2and CO2diffuse in pulmonary capillaries,systemic capillaries and in tissues.

Understand and be able to use the O2-hemoglobindissociation curve.

Know the quantity of O2and CO2delivered by theblood.

Know what causes shifts in the O2-hemoglobindissociation curve.

Know how CO2is transported in the blood andunderstand the Haldane Effect.


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Movement of Gases

Gases move by diffusion, from areas of highpartial pressure to areas of low partialpressure.

- In the alveoli, O2moves from the alveoli (high PO2)to the pulmonary blood (low PO2).

- In the tissues, O2moves from the blood (high PO2)to the tissues (low PO2).

How will CO2diffuse?


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Diffusion of O2in the Pulmonary Capillaries What is the PO2in the arterial

end?

104 mm Hg40 mm Hg = 64 mm Hg.

What is the net direction of O2diffusion in the arterial end?

From the alveolar space into the

blood.

How does the pulmonaryanatomy allow the capillary bloodto reach a PO2of 104 mm Hg soquickly in the venous end?

- Surface area (70 cm2

of respiratorymembrane for 60-140 mL blood).

- Thin respiratory membrane.

- PO2.


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O2Diffusion During Exercise

During exercise, the body may need 20 x more

O2.

How does the exchange in the pulmonary

capillaries meet this need?

- Increased diffusing capacity (increased surface area and

capillaries and improved VA/Q).

- The blood reaches O2saturation quickly (see previous slide).


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Changes in PO2in Cardiovascular

System


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Diffusion of O2in the Systemic Capillaries and

Tissues

The PO2in the arterial blood is ~ 95 mm Hg and PO2in the

interstitial fluid is ~ 40 mm Hg. Thus, the PO2~ 55 mm Hg for

the diffusion of O2into the interstitial fluid.

The PO2in the interstitial fluid is ~ 40 mm Hg and the PO2in

the tissues ~ 23 mm Hg. Thus, the PO2~ 17 mm Hg for the

diffusion of O2into the tissues.


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Blood Flow and Interstitial PO2


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Diffusion of CO2in the Systemic

Capillaries and Tissues

Diffuses in the opposite direction as O2,

because CO2accumulates in the tissues as O2

is consumed.

Note: CO2can diffuse ~ 20 x more rapidly than

O2; so less differences in partial pressures are

required.


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Diffusion of CO2in the Pulmonary

Capillaries

The PCO2in the tissues ~ 46 mm Hg and the PCO2in theinterstitial fluid ~ 45 mm Hg. Thus, the PCO2~ 1 mm Hg for thediffusion of O2into the interstitial fluid.

The PCO2in the interstitial fluid ~ 45 mm Hg and the PCO2in thearterial capillary blood ~ 40 mm Hg. Thus, the PCO2~ 5 mm Hg

for the diffusion of O2into the blood.


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Diffusion of CO2in the Pulmonary

Capillaries


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Blood Flow and the Interstitial PCO2


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Transport of O2by Hemoglobin

Nearly all the O2(~ 97%) is

transported in the blood by

hemoglobin.

One hemoglobin molecule

contains 4 heme prosthetic

groups.

One hemoglobin molecule

can carry 4 O2molecules.


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O2-Hemoglobin Dissociation Curve


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Amount of O2Carried by Hemoglobin (Volumes)

In 100 ml of blood,contains ~ 15 g ofhemoglobin.

Each gram of hemoglobincan carry a maximum of1.34 ml of O2.

Thus, 100 ml of blood cancarry a maximum of 20 mlof O2(15 x 1.34).


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Quantity of O2Released to Systemic Tissues

At 97% hemoglobin saturation(arterial), 100 ml of bloodcarries ~ 19.4 ml O2.

At 75% saturation (venous), 100

ml of blood carries ~ 14.4 ml O2.

Thus, 100 ml of blood delivers ~5 ml of O2under normalcircumstances.

During exercise, 100 ml ofblood can deliver ~ 15 ml O2.


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O2Delivery During Exercise

During exercise, 100 ml of blood can deliver ~ 15 ml

of O2(3-fold increase from normal).

What happens to cardiac output during exercise?

During exercise, the cardiac output increases 6- to 7-fold.

Thus, by increasing O2transport and cardiac output,

there can be a 20-fold increase in O2delivery totissues during exercise.


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Using the O2-Hemoglobin Dissociation

Curve The normal PO2of the alveoli is

104 mm Hg. This results in ahemoglobin saturation of 97%.

What happens to hemoglobinsaturation if the alveolar P

O2

drops to 60 mm Hg, whileclimbing a mountain?

The hemoglobin saturation onlydrops to 89%.

The venous blood PO2only needsto drop to 35 mm Hg (from 40)for 5 ml of O2per 100 ml to bedelivered


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Shifts in the O2-Hemogobin

Dissociation Curve Increases in H+, CO2, and

temperature shift the curve tothe right.

This enhances the release ofO2from hemoglobin and is

called the Bohr Effect. In tissues, the high CO2

increases the [H+] (recall bloodacid/base reactions involvingbicarbonate). Causing theextra release of O2.

In the lungs, the removal ofCO2decreases the [H

+]. Thisincreases the binding of O2tohemoglobin.

What happens to H+, CO2, and

temperature in exercisingmuscle?


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Transport of CO2in the Blood

Under normal conditions, the blood delivers ~4 ml of CO2from the tissues to the lungs ineach 100 ml of blood.

Why only 4 ml of CO2per 100 ml if the blooddelivers 5 ml of O2per 100 ml?

For carbohydrate metabolism, 1 molecule of O2isformed for each CO

2consumed.

When fats are metabolized, some of the O2combines with H+atoms from the fat to form H2O.


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Transport of CO2in the Blood

Most of the CO2in RBCs reacts with H2O, forming

carbonic acid, which dissociates to H+andbicarbonate ion.


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Transport of Bicarbonate to the Plasma

Many of the bicarbonate ions are transported out of theRBC in exchange for Cl-by the bicarbonate-chloridecarrier protein.


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CO2and Hemoglobin

Some CO2combines with

hemoglobin forming

carbaminohemoglobin

(CO2

HbB).

In the lungs, the CO2is

released from CO2HbB.

Hemoglobin also binds

the H+

released from thedissociation of carbonic

acid. This helps buffer

the pH of the RBC.


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Release of bound CO2in the Lungs

How does the CO2that combined with H2O

and hemoglobin get released in the lungs?

Binding of O2to hemoglobin tends to displace CO2from the

blood. This is called the Haldane Effect.


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Haldane Effect

The binding of O2to carbaminohemoglobin

promotes its conversion to hemoglobin and

CO2.

The binding of O2to hemoglobin causes the

release of a H+ion. The H+ion can then

combine with bicarbonate ion to form

carbonic acid, which then dissociates to CO2and H2O.


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CO2and Blood pH

What happens to the blood pH if CO2becomes

elevated?

- It drops, because more carbonic acid is formed.

- Normally, the pH of arterial blood is 7.41 and that

of venous blood is 7.37 (0.04 difference).

- During exercise, the pH of venous blood can drop

by 0.5 units.

11 gas transport

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