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
O2in 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)
CO2 diffusion from peripheral tissues back to alveoli
CO
2is a waste product of many metabolic reactions
• P
CO2intracellular = 46
• P
CO2interstitial = 45
CO2 diffusion from peripheral tissues back to alveoli
• ΔP can be lower for CO
2than O
2because it
diffuses so much faster than O
2• Increased blood flow will reduce venous P
CO2Transport of O
2in 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
2capacity 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
2and 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
2lead
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
2in 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
2dissociation curve
The Haldane Effect
• Binding of O
2with hemoglobin tends to displace
CO
2from 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