Lecture 17

what 2 forms is O2 transported in

dissolved O2

combined with haemoglobin

dissolved O2

for each mmHg PO2 only 0.03ml og O2 is dissolved

so arterial blood with PO2 ~100mmHg contains only 3ml dissolved O2/litre (0.3ml of O2 dissolved per 100ml of blood)

so dissolved O2 is very inefficient for transport as we need about 250ml O2/min

haemoglobin

protein complex woth 4 subunits and a haem group attached to each subunit

red from iron of haem

lack of iron → form of anaemia, affects O2 binding/carrying capacity

structure of the haem

each haem can bind to one O2 molecule

4 haem and globin = Hb

O2 forms an easily reversible combination with Hb to give oxyhaemoglobin

binding depends on PO2

oxygen-haemoglobin dissociation curve

with increased partial pressure of oxygen, there is an increased amount of haemoglobin bound to oxygen

haemoglobin saturation in arterial blood is around 100% at around 100mmHg

in venous blood there is around 75-85% of Hb saturated at around 40mmHg

P50

the partial pressure of oxygen at which 50% of haemoglobin is bound to oxygen - pretty low

shape of oxygen-haemoglobin dissociation curve

sigmoidal, not linear

advantages of a sigmoidal oxygen haemoglobin curve

1. upper flat part of the curve - moderate changes in PO2 around the normal value (~100mmHg) have only small effects on the % saturation and therefore the amount of O2 carried by arterial blood i.e. some reserve capacity

2. steep part of curve at lower PO2 - helps with loading of Hb in lungs and unloading of O2 to the tissues. so small changes in PO2 result in large changes in small amount of O2 bound to Hb 2

O2 carrying capacity

how much O2 COULD the blood carry - max amount

normal blood has about 150g of Hb per litre

one gram of Hb can combine with 1.34 ml O2

so O2 capacity = 200ml/litre of blood

O2 content

how much O2 is the blood actually carrying

O2 content = O2 capacity x saturation (+dissolved)

O2 content of blood (Ml O2/litre blood) = [1.34 x Hb x Sat/100] + 0.03 x PaO2

O2 content of arterial blood

assumes 150g of Hb/L, PaO2 100mmHg, SaO2 98%

using equation to find O2 content = [1.34 × 150 × 98/100] + 0.03 × 100

~200 ml O2/litre blood (arterial O2 content)

O2 content of venous blood

assuming 150g of Hb/litre, PvO2 40mmHg, SvO2 74%

using equation to fin O2 content = [1.34 × 150 × 74/100] +0.03 × 40

so O2 content of venous blood ~150 ml O2/litre blood

arterial - venous O2 difference

a-v difference = amount of O2 extracted by tissues

arterial content = 200ml O2/litre blood

venous content = 150ml O2/litre blood

so a-v difference = (200-150) = 50ml O2/litre blood i.e. 50ml O2 was extracted from each litre of blood by the tissues and used in metabolism

total O2 extracted by tissues

how many litres/min flow to tissues? - determined by cardiac output

if CO = 5L/min then total O2 extracted by tissues = 250ml/min (VO2)

oxygen extraction during exercise

more oxygen is extracted and used by the tissues when exercising

the a-v O2 difference will increase

e.g. normal is 50 and exercise could be 150

saturation curve has the capcity to provide this extra O2 extraction

but if low Hb (i.e. anaemia) then will cause problems

bohr effect overall - right and left shift

right shift = release

left shift = loading

right shift of O2-Hb curve

increased P50 and decreased O2 affinity

favours the release of O2 from Hb to tissues (offloading)

natural right shift occurs as blood flows through the capillaries of the tissues (high metabolic demand - high CO2 and H+) to facilitate O2 release

increased 2,3-BPG, increased H+, increased temperature

release

left shift of O2-Hb dissociation curve

decreased P50 and increased O2 affinity

favours the binding of O2 to Hb (onloading)

natural elft shift occurs as blood flows through the lung capillaries (less CO2 and H+)

facilitates the uptake of O2 from the alveoli into the blood

decreased 2,3-BPG, decreased H+, decreased temperature

loading

RBC 2,3-DPG

by-product of glycolysis

RBCs contain no mitochondria so rely on glycolysis

2l3-DPG increases with intense exercise, altitude, and due to devere lung diseases of anaemia

helps deliver O2 to tissues (due to rightward shift of Hb saturation curve which allows more O2 to be released from Hb at a particular PO2 - increase unloading)

anaemia

saturation curve stays the same

O2 content is reduced - e.g. [Hb] = 76 → 100ml)2/L

exercose problems from a-v difference e.g. can’t remove 150ml/L when only have 100

carbon monoxide

has high affinity to Hb - 250 times the affinity of O2 for Hb

small amounts of CO can tie up large proportion of Hb in the blood, making it unavailable for O2 carriage - so less O2 content

shifts curve to the left - more difficult to unload O2 to tissues

smoking → arterial CO increases

oxygen-haemoglobin dissociation curve

what 3 forms is CO2 transported in

1. dissolved in plasma - 20 times more soluble than O2 (10%)

2. as bicarbonate (70%)

3. combined with proteins as carbamino compounds (20%)

CO2 uptake and O2 liberation in systemic capillaries

CO2 in capillaries either exhaled as dissolved CO2, or goes into RBC where it can be dissolved, to turn into carbamino Hb or bicarbonate which can be transported out of the RBCs to the lungs

Cl- also moves into the cell

haldane effect

deoxygenation of blood increases CO2 transport

Hb affinity for CO2 increases when it is not bound to oxygen

RBC vs tissues gas exchange

RBC:

haldane - Hb oxygenation facilitates CO2 unloading

bohr - decreased CO2 facilitates O2 loading

tissue:

bohr - increased CO2 facilitates O2 unloading

haldane - unoxygenated Hb facilitates CO2 loading

transport of O2 and CO2 in lungs and peripheral tissues