Oxygen in blood

Question 1

Why do we need Hb to transport O2?

Answer 1

O2 is not soluble enough, if we were to rely on this alone delivery of O2 would not meet the demands of the tissues

Question 2

What is haemoglobin known as?

Answer 2

Metalloprotein
Respiratory pigment

Question 3

What does O2 combine with?

Answer 3

Combines with iron in haem reversibly

Question 4

Haemoglobin structure

Answer 4

4 subunits - 2 alpha, 2 beta
Each subunit has haem group
Each haem can bind 1 O2 molecule

4 oxygen molecules can be carried by 4 haem groups in 1 haemoglobin

Question 5

What is oxygen saturation (SaO2) vs partial pressure of O2?

Answer 5

O2 saturation is the percentage of Hb bound to O2
Partial pressure is the amount of O2 dissolved in the blood

Question 6

two states which Hb exists in

Answer 6

T state - tissue state
R state - resp state

Question 7

T state of Hb

Answer 7

Tense state
Low affinity for O2 (eg in TISSUES when delivering O2, difficult for O2 to bind)

Question 8

R state of Hb

Answer 8

Relaxed state
High affinity for O2 (eg in pulmonary capillaries RESP, easy for O2 to bind)

Question 9

What is haemoglobin cooperativity?

Answer 9

A molecular rearrangement of haem group so that iron is more accessible to O2
Most difficult to bind first O2 and then structure changes so as more O2 binds affinity increases

Question 10

What does cooperativity mean for Hb O2 dissociation curve?

Answer 10

Initial O2 is difficult to bind - takes high increase in pO2 to bind
Curve steepens as pO2 rises and flattens as saturation is reached

= SIGMOIDAL CURVE

Question 11

When is Hb saturated?

Answer 11

Between 9-10 kPa O2

Question 12

When is Hb half saturated?

Answer 12

Between 3.5 - 4 kPa
(unsaturated below 1)

Question 13

What do O2 saturation actually tell us?

Answer 13

percentage of Hb bound to O2 - so have no idea of how much Hb is in the blood, just whether or not this is bound

Question 14

Oxygen in the blood in anaemia

Answer 14

Partial pressure and saturation of O2 are normal in the blood
Oxygen content is not as there is decreased Hb (less Hb carrying O2)

Question 15

Oxygen content in haemoglobin

Answer 15

8.8mmol/L - each haem binds 2.2mmol/L x 4

(when fully saturated)

Question 16

Total O2 content calculation

Answer 16

Need amount dissolved + amount on Hb
If O2 is dissolving (eg in normal PO2) then Hb must be saturated

soooo… 8.8mmol/L + (13.3x0.01) = 8.93mmol/L

Question 17

What is hypoxaemia vs hypoxia?

Answer 17

Hypoxaemia - low partial pressure of O2 in arterial blood eg usually not all Hb is saturated (in anaemia this is normal)

Hypoxia - low oxygen levels relative to need of bodies tissues

Question 18

Causes of hypoxia

Answer 18

Shock
Peripheral vasoconstriction

Peripheral arterial disease and congestive heart failure with low CO - use O2 faster than delivered

Secondary to anaemia - low Hb so oxygen content = low (even though PAO2 is normal)

Question 19

Typical tissue pO2

Answer 19

5kPa - but depends how metabolically active tissue is

Question 20

How low can tissue pO2 get?

Answer 20

Cannot fall below 3kPa
(but tissues with high capillary density, pO2 can fall lower)

Question 21

When does capillary density increase?

Answer 21

If tissue is very metabolically active
High altitude environment

Question 22

Hb in venous blood and tissues

Answer 22

Low saturation of Hb as has delivered O2 to tissues
Is tense so does not want to bind to O2
Lower tissue pO2 = more O2 dissociate from Hb

Question 23

Mixed venous blood pO2

Answer 23

6kPa - blood from R heart –> lungs
(peripheral venous blood is 5.3kPa typically)

Question 24

What is the Bohr effect?

Answer 24

Acidic conditions (increase in CO2) shifts Hb O2 curve to the right
Promotes T state - lower affinity for O2
(eg in metabolically active tissues)

Question 25

What else causes right shift of Hb O2 curve?

Answer 25

Lower affinity for O2:
Increased temp
Increased 2,3-DPG
Increased H+

Question 26

What causes left shift of HbO2 curve?

Answer 26

Higher affinity for O2:
Decreased temp
Decreased 2,3BPG
Decreased H+
CO

Question 27

Why does increase 2,3-DPG cause lower affinity for O2 of Hb (shift curve to right)?

Answer 27

2,3-DPG is an intermediate of RBC glocolysis
Rapidly consumed

In hypoxaemia RBC production of this increases so this facilitates O2 unloading to tissues

Question 28

What is maximal unloading of O2?

Answer 28

Where pO2 falls to low level or there is increased metabolic activity meaning environment is acidic and higher temp

70% of bound o2 can be given up (at rest is usually only 30%)

Question 29

Pathological shifting of Hb O2 curve with carbon monoxide - what happens?

Answer 29

Carbon monoxide has 200x affinity for Hb
Reacts to form CO-Hb
Reduces O2 transport as takes up space on Hb
Increases affinity for O2 in Hb that is unaffected - means that O2 doesn’t dissociate in tissues and these die

Question 30

When is carbon monoxide poisoning fatal?

Answer 30

If HbCO >50%
(does not decrease PaO2)

Question 31

Symptoms of carbon monoxide poisoning

Answer 31

Children at increased risk
Headache
Nausea
Vomiting
Slurred speech
Confusion
Few resp symptoms initally

Question 32

What is peripheral cyanosis

Answer 32

Cyanosis at peripheries - in darker skin will need to look at nail beds
Due to poor circulation

Question 33

Central cyanosis

Answer 33

On tongue/around lips/mucous membranes
Due to poorly saturated blood in systemic circulation

Question 34

What causes cyanosis?

Answer 34

Bluish colour due to unsaturated haemoglobin (<85/90%)
Deoxygenated Hb is less red than oxygenated

Question 35

What is pulse oximetry?

Answer 35

Detects Hb saturation using red and infrared light (non invasive sats probe)

Detects difference in absorption of light between oxygenated and deoxygenated Hb

Question 36

Problem with pulse oximetry

Answer 36

Only detects pulsatile arterial blood levels
Cant detect tissue O2 or non pulsatile venous blood
May not be able to differentiate Hb-CO
Does not give into about Hb levels
Less accurate on darker coloured skin

Question 37

What is an arterial blood gas?

Answer 37

partial pressure of O2 in arterial blood (DISSOLVED O2)
Expressed in kPa
Also gives pCO2, pH and bicarbonate

Question 38

What do you need for arterial blood gas/

Answer 38

Invasive - need arterial blood sample
Usually from radial artery (less common is femoral)