Session 3 - Properties Of Gases & Oxygen In Blood

Question 1

What is the ideal gas equation?

Answer 1

P= n R.T

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Question 2

What is boyle’s law?

Answer 2

The pressure (P) of a gas is inversely proportional to its volume (V).
(If temp + number of molecules is constant in a closed system)

Question 3

What is the partial pressure of a mixture of non-reacting gases?

Answer 3

Total pressure exerted = sum of the partial pressures of the individual gases.
(E.g. 1/3 oxygen, 2/3 Nitrogen - add pressures together)

Question 4

What is atmospheric pressure? What is its value?

Answer 4

The pressure exerted by the weight of the air above the earth in the atmosphere, at SEA LEVEL.

101 Kilopascals (kPa)
= 1 atm
= 760 mmHg

Question 5

What happens to atmospheric pressure at higher altitudes?

Answer 5

Atmospheric pressure is lower at higher altitudes.

Question 6

What happens when inspired gases come into contact with body fluids?

Answer 6

Gas molecules enter the liquid and dissolve.

Water molecules evaporate, entering the air.

Question 7

What is saturated vapour pressure? Where does this occur? What is the value?

Answer 7

SVP - Pressure of water molecules in completely vapour saturated air.

In the upper Respiratory Tract.

6.28kPa at body temperature.

Question 8

When is air said to reach saturated vapour pressure?

Answer 8

When no more water molecules can be contained in the air.

When water molecules leave + enter water at the same rate.

Question 9

How does the saturated vapour pressure affect the other gases?
How are pressures calculated?

Answer 9

Pressure of the other gases is no longer 101kPa.
Now 101-6.28 kPa.
But in the same ratios.

Can be calculated still by using the ratios, e.g. (101-6.28) x 20.9% = 19.8kPa) for Oxygen, etc.

Question 10

When does partial pressure of a dissolved gas in a liquid become the same as the pressure of the gas in the air?

Answer 10

When the rate of gas molecules leaving + entering the water are the SAME.

Question 11

How is partial pressure different from the amount of a gas dissolved in a liquid?
What is the equation for gas in liquid?

Answer 11

Amount of gas dissolved = partial pressure x solubility coefficient of gas

So if pO2 is 13.3kPa, and O2 coefficient in plasma is 0.01/mol/L/kPa

Then 0.01 x 13.3 = 0.13 mol dissolved.

Question 12

What happens to the components of blood in the alveoli when oxygen diffuses?

Answer 12

Initially oxygen enters plasma, dissolved, enters RBC and binds to Haemoglobin.

Once all Hb is fully saturated, 02 continues to dissolve into plasma until equilibrium is reached.

Here the partial pressure of O2 is the same in the plasma, and alveoli air.

Question 13

When partial pressure of the blood of oxygen is given, what does this value actually mean?

Answer 13

The partial pressure of the DISSOLVED O2, in the plasma.

Not Hb bound O2

Question 14

What are the normal partial pressures of the components in air?

Answer 14

101 kPa = 1 atm
O2 = 20.9 kPa
CO2 = 0.03 kPa
N2 = 78 kPa

Question 15

Why is alveolar pO2 13.3kPa, and CO2 5.3kPa, despite the partial pressures being different in air?

Answer 15

• Inhaled air is mixed with residual volume.
• O2 is being taken up, CO2 given up by blood.
• blood/ alveolar equilibrium reached.

Question 16

What is the problem with breathing at altitude?

Answer 16

Same volume of air breathed, 500ml, but contains much fewer oxygen molecules, due to reduced kPa.
E.g. climbing mount everest would require oxygen!`

Question 17

Why is haemoglobin essential in blood for oxygen transport?

Answer 17

Solubility of oxygen in plasma is very low, 0.01 mmol per kPa.
Haemoglobin is required to bind oxygen and allow more to be delivered.

Question 18

What is the difference between haemoglobin and myoglobin?

Answer 18

Haemoglobin - in blood, tetramer, binds 4 oxygen molecules.
Myoglobin - in muscle, monomer, binds 1 oxygen.
(myoglobin has higher affinity for oxgyen than haemoglobin)

Question 19

How is oxygen binding represented in dissociation curves?

Answer 19

Saturation (%)
and kPa of oxygen (x axis)
Percentage saturation of the haemoglobin / myoglobin.
Can be used to work out how much given up etc.

Question 20

Describe the different states of haemoglobin.

Answer 20

Tense (T) state = Low affinity - hard for O2 to bind.

Relaxed (R) state = High affinity - easy for O2 to bind.

Question 21

How does pO2 affect haemoglobin affinity?

Answer 21

Low pO2 = Hb in tense state.
Hard for first molecule to bind.
After first molecule, Hb more relaxed, easier for subsequent molecules to bind.

Question 22

What is the shape of the haemoglobin dissociation curve?

Answer 22

Sigmoidal - initially binding is shallow.
Curve steepens, as pO2 rises.
Flattens as saturation reached.

Question 23

How can oxygen content of arterial blood be calculated? How is this different with anaemia?

Answer 23

Alveolar pO2 = 13.3kPa
Normal Hb concentration around 2.2 mmol/L
Each Hb binds 4 O2.
So oxygen content = 8.8 mmol/L
In anaemia, the lungs may be functioning ok, so pO2 is normal, but oxygen content lower.

Question 24

What is the typical value of pO2 at the tissues?

Answer 24

Around 5kPa

Question 25

Thinking about the pO2 at the tissues, how much oxygen is given up at the tissues? (assuming haemoglobin levels are normal and saturated)

Answer 25

Hb saturation drops to around 65% (at 5kPa in tissues)
so 35% given up.
0.35 x 8.8mmol/L = around 3mmol/L of blood.

Question 26

What is the lowest partial pressure of oxygen a tissue can get to?

Answer 26

Tissue partial pressure of oxygen must be high enough to drive oxygen into the cells.
Can’t fall below 3 kPa in most tissues.

Question 27

How does pH affect the dissociation curve of haemoglobin?

Answer 27

• pH affects affinity.

- pH promotes R-state

Question 28

How is pH important in oxygen delivery?

Answer 28

pH lower in metabolically active tissues.

So more oxygen is given up due to bohr shift.

Question 29

How does temperature affect the affinity of haemoglobin for oxygen?

Answer 29

Higher temperature promotes R-state.

So in slightly warmer metabolically active tissues, more oxygen is given up.

Question 30

What is maximum unloading of oxygen?

Answer 30

Where the maximum (~70%) oxygen is unloaded at tissues where:

• pO2 can fall to a low level.
• Increased metabolic activity causes more acidic and higher temp environment.

Question 31

Over the whole body, how much oxygen is given up from arterial blood?

Answer 31

Only around 27% of oxygen.

Oxygen reserve, for exercise etc.

Question 32

What are the effects of 2,3-BPG in blood?

Answer 32

Around 5mM of it in RBC.
Causes decreased affinity of haemoglobin for oxygen, allowing more oxygen to be given up at tissues.
(shifts curve right)

Question 33

When will 2,3-BPG levels change?

Answer 33

• At altitude/ anaemia - more 2,3-BPG

- Levels drop in stored refrigerated blood. (not usually a problem clinically)

Question 34

How does carbon monoxide poisoning affect affinity?

Answer 34

• CO reacts with Hb forming COHb with higher affinity.
• Increases affinity of unaffected subunits for oxygen.
• Therefore won’t give up oxygen at tussues.
• Fatal of HbCO > 50% -

Question 35

What is the difference between hypoxemia and hypoxia?

Answer 35

Hypoxemia = Low pO2 in arterial blood.

Hypoxia = Low oxygen levels in body or tissues

Question 36

What is cyanosis, how is it detected?

Answer 36

A bluish colouration due to unsaturated haemoglobin (less red in colour).

1) Peripheral due to poor local circulation or
2) Central (mouth, tongue, lips etc) due to poorly saturated blood in systemic circulation.

Question 37

What are the limitations of using pulse oximetry to detect saturation of haemoglobin?

Answer 37

Pulse oximetry only measures % saturation of haemoglobin in pulsatile arterial blood.
Doesn’t detect amount of haemoglobin (e.g. anemia)
Ignores levels in tissues, and venouse blood.