Lecture 4 slides

Transport of oxygen and carbon dioxide in

blood and body fluids

July 13th, 2011

Ann Raddant, B.S.

Pressures of gases in water and tissues

• Henry’s Law

• Solubility coefficient depends on physical and chemical attraction or repulsion to water molecules

Molecule Solubility coefficient

Oxygen 0.024

Carbon dioxide 0.57

More soluble = lower partial pressure

Blood P

_O2

: 100mmHg

Today’s Topics

• Diffusion of respiratory gases from the alveolus to the level of the systemic capillary and back to the lung

• The two mechanisms by which oxygen in carried in the blood: dissolved and bound to hemoglobin

• Oxygen-hemoglobin dissociation curve

• The three forms by which carbon dioxide is carried in the blood

• The basics of acid-base control using the Henderson-Hasselbach equation and Davenport diagrams for

Uptake of oxygen by pulmonary blood

• PO2 gradient between alveolus

and pulmonary capillary • RBC transit time

– Safety factor

• Increased flow during exercise is easily accommodated

• Pathological thickening of

membranes can limit O2 transfer

– Fick’s Law (increased surface area) – Decreased rate of diffusion

P

_O2

in arterial blood

Of blood entering the left heart:

Oxygen: from blood to cell: 2 step process

Diffusion of oxygen from capillaries to the interstitial fluid

• PO2 capillary = 95

• PO2 interstitial fluid = 40

Increased blood flow will increase interstitial fluid PO2

Oxygen: from blood to cell: 2 step process

Diffusion of oxygen from interstitial fluid to cells

• PO2 interstitial fluid = 40

• PO2 intracellular = 5 – 40 (shown as 23 here)

C_O2 diffusion from peripheral tissues back to alveoli

CO

2

is a waste product of many metabolic reactions

• P

CO2

intracellular = 46

• P

CO2

interstitial = 45

CO2 diffusion from peripheral tissues back to alveoli

• ΔP can be lower for CO

₂

than O

2

because it

diffuses so much faster than O

2

• Increased blood flow will reduce venous P

CO2

Transport of O

₂

in the blood

1. Dissolved oxygen – 3%

– Low solubility limits the concentration of O2 that can be transported dissolved in blood

2. Hemoglobin (Hb) – 97%

– Contained within red blood cells (RBC’s) – Each Hb molecule contains 4 chains

Oxygen capacity

• ~ 20 ml O

₂

/100ml blood

• O

2

capacity is affected in diseases such as anemia

and polycythemia

– anemia: decreased number of RBC’s or

decreased amount of Hb in blood

Volumes per cent:

common expression of

a solution’s concentration

Volumes per cent:

common expression of

a solution’s concentration

Saturated blood: 20% O2

15 grams Hb per 100ml blood

1.34 ml O2 per 1g Hb

Oxygen-Hemoglobin Dissociation Curve

Small change in PO2 here won’t impair Hb loading

Utilization coefficient

• Percentage of blood that gives up its

oxygen as it passes through the tissue

capillaries

Utilization coefficient

Arterial blood Venous

blood

5ml O₂ per 100ml

blood

5ml O₂ per 100ml of blood used 20ml O₂ = starting amount

Utilization coefficient

Arterial blood

Venous blood during exercise

15ml O₂ per 100ml

blood

15ml O₂ per 100ml of blood used 20ml O₂ = starting amount

Factors that shift the

Oxygen-Hemoglobin dissociation curve

P

_O2

%

H

b

S

at

u

ra

ti

o

n

The Bohr Effect:

CO

₂

and H

+

ions interact with Hb

and reduce its affinity for O

2

• Shifting the curve to the right enhances the release of O2

– Lower saturation at the same PO2

• Shifting the curve to the left enhances loading of O2

R: release

L: loading

R: release

L: loading

Factors which can shift the curve to the

right

• Decreased pH

– Increased [H

+

]

• Increased CO

₂

• Increased temp

• Increased DPG

• Exercise

DGP: 2,3-diphosphoglycerate

Conditions of low tissue O

₂

lead

to generation of more DGP

• High altitude

• Airway obstruction

Factors which can shift the curve to the

left

• Increased pH

– Decreased [H

+

]

Transport of CO

₂

in the blood

1. Dissolved carbon dioxide – 7%

– Obeys Henry’s law, by CO2 is 20x more soluble than O2

2. Bicarbonate (HCO

3-

) – 70%

– carbonic anhydrase - CA

3. Carbamino compounds – 23%

The chloride shift

• HCO

₃-

diffuses out of RBC down its

concentration gradient

• Cl

-

moves into RBC in order to balance total

CO

₂

dissociation curve

The Haldane Effect

• Binding of O

₂

with hemoglobin tends to displace

CO

₂

from the blood

– Opposite of Bohr effect

The Haldane Effect

In peripheral tissues

(top):

• Reduced Hb (no O

₂

) is a

better proton acceptor –

binds H

+

, shifts CO

2

-

The Haldane Effect

In the lungs

(bottom):

• O

₂

-Hb is a bad proton

acceptor – promotes H

+

release, shifts CO

₂

-

Displacement of O

2

by CO

• Hemoglobin has a

much higher binding

affinity for CO then

oxygen – small

Respiratory exchange ratio

Transported in every 100ml of blood:

• 5ml O2

• 4ml CO2

R changes in response to metabolism

• Carbs: R = 1.0

• Fat: R = 0.7

Acid Base Balance

Lung excretes 10,000 mEq/day of carbonic acid every day

Bicarb buffer is critical for maintenance of blood pH

Take home points

• Oxygen and carbon dioxide move

between blood to tissue based on partial pressure gradients

• Most oxygen is transported bound to

hemoglobin, while most carbon dioxide is transported as bicarbonate

• Many factors can affect the binding of oxygen to hemoglobin