Gas Transport and Exchange

Question 1

State Dalton’s Law.

Answer 1

The partial pressure of a mixture of gases is equal to the sum of the partial pressures of the gases that make up the mixture.

Question 2

State Fick’s Law.

Answer 2

Rate of diffusion is directly proportional to diffusion capacity, concentration gradient and surface area and inversely proportional to the thickness of the exchange surface.

Question 3

State Henry’s Law.

Answer 3

At a constant temperature, the amount of a given gas that dissolves in a given type and amount of liquid is directly proportional to the partial pressure of the gas in equilibrium with the liquid.

Question 4

State Boyle’s Law.

Answer 4

At a constant temperature, volume is inversely proportional to pressure.

Question 5

State Charles’ Law.

Answer 5

At a constant pressure, volume is directly proportional to temperature.

Question 6

Describe how partial pressure of oxygen changes as it passes down the airways.

Answer 6

The partial pressure decreases from 21.3 kPa to 20 kPa to 13.5 kPa in the alveoli.

Question 7

What happens to the air as it passes down the airways?

Answer 7

The air gets warmed, humidified, mixed and slowed as it passes down the respiratory tree

Question 8

What is the normal oxygen consumption at rest?

Answer 8

250 mL/min

Question 9

What is the binding capacity of oxygen to haemoglobin?

Answer 9

1.34 mL/g

Question 10

What is the solubility coefficient of oxygen in blood using mm Hg?

Answer 10

0.003 mL

Question 11

Describe the structure of a normal variant of haemoglobin and of foetal haemoglobin.

Answer 11

HbA2 is a normal variant that consists of two alpha chains and two delta chains (2% of adult haemoglobin)
HbF is present in foetus’ and consists of two alpha chains and two gamma chains.

Question 12

Explain why haemoglobin is considered an ‘allosteric’ molecule.

Answer 12

When oxygen binds, there is a conformational change which changes the structure and affinity of haemoglobin for oxygen meaning that oxygen is more likely to bind.

Question 13

What change occurs in the middle of the haemoglobin tetramer when oxygen binds?

Answer 13

Oxygen binding changes the structure of the middle of the haemoglobin creating a binding site for 2,3-DPG (a glycolytic by-product) - 2,3-DPG production is reflective of metabolism and it binds to the haemoglobin and squeezes out the oxygen (lowers the affinity of haemoglobin for oxygen)

Question 14

What is the name given to the phenomenon where oxygen binding to haemoglobin increases the affinity making more oxygen bind?

Answer 14

Cooperativity

Question 15

What would the consequences be if the oxygen dissociation curve was linear?

Answer 15

A lower haemoglobin saturation would be achieved in the lungs when the partial pressure of oxygen in the lungs is at the lower end of normal. There is also reduced potential to unload oxygen at respiring tissues.

Question 16

What are the benefits of having a sigmoid ODC?

Answer 16

100% haemoglobin saturation can occur at a broad range of partial pressures
Large range and scope for unloading oxygen in the tissues.

Question 17

What is P50?

Answer 17

The partial pressure of oxygen at which haemoglobin is 50% saturated - gives an indication of the shape of the ODC

Question 18

What conditions can shift the ODC to the right?

Answer 18

Conditions brought about by exercise - hypercapnia (high CO2), increased temperature (increased metabolism), acidosis, increased 2,3-DPG.

Question 19

What conditions can shift the ODC to the left?

Answer 19

The opposite conditions - hypocapnia, decreased temperature, alkalosis and decreased 2,3-DPG

Question 20

What conditions can shift the ODC upwards?

Answer 20

Polycythaemia - increase in the haematocrit (could be caused by an increase in the number of red blood cells)

Question 21

What conditions can shift the ODC downwards?

Answer 21

Anaemia - decreased oxygen carrying capacity of the blood

Question 22

How does haemoglobin saturation change in the previous two shifts of the ODC?

Answer 22

It doesn’t change because the haemoglobin is still fully saturated but the amount of oxygen carried by the blood increases.

Question 23

How does carbon monoxide shift the ODC and why?

Answer 23

Carbon monoxide shifts the curve downwards and leftwards. It moves downwards because it binds irreversibly to haemoglobin meaning that there is less haemoglobin available to bind to oxygen. It moves leftwards because if only a few of the 4 oxygen binding sites on the haemoglobin are occupied by oxygen, then the oxygen that is bound to haemoglobin will be less likely to dissociate, thus increasing the affinity of the haemoglobin for oxygen.

Question 24

Describe the shape of the ODC of myoglobin and foetal haemoglobin and why this shape is needed for their function.

Answer 24

Myoglobin is a store of oxygen in the muscles and it has a very high affinity for oxygen because it must be able to extract oxygen from the circulating blood.

Foetal haemoglobin also has a high affinity because it needs to be able to steal oxygen from the mother’s blood.

Question 25

What is the PO2 of blood arriving at the respiratory exchange surface?

Answer 25

5.3 kPa

Question 26

Why does the Hb saturation of the blood decrease from 100% at the respiratory exchange surface to 97% in the systemic circulation?

Answer 26

This is because of the bronchial circulation joining the pulmonary circulation before returning to the left side of the heart. This also causes a decrease in partial pressure from 13.5 kPa to 13.3 kPa

Question 27

What are the changes in concentration of oxygen and saturation that take place at the tissues?

Answer 27

20.3 –> 15.1 mL/dL

97% –> 75%

Question 28

Define oxygen flux and state the usual oxygen flux at rest.

Answer 28

The overall amount of oxygen being deposited in the tissues.
Oxygen flux = 20.3 - 15.1 = around 5 mL/dL

Normal cardiac output = 5 L (50 dL) so oxygen flux is:
250 mL/min

Question 29

Describe the reaction of carbon dioxide with water.

Answer 29

Carbon dioxide reacts with water to produce carbonic acid (H2CO3). This is a slow reaction.

Question 30

Why does this reaction take place faster in the red blood cells?

Answer 30

The red cells have carbonic anhydrase, which catalyses this reaction and allows it to occur at a rate 5000 times greater than in the cytoplasm.

Question 31

Which transporter moves the bicarbonate produced in the red blood cell into the plasma?

Answer 31

AE1 transporter

Question 32

The AE1 transporter also allows the influx of which ion? What is the term given the this movement of ions?

Answer 32

Chloride ions move in - this is called chloride shift

Question 33

What effect does the influx of chloride (antiport with bicarbonate via AE1) have on the red blood cell?

Answer 33

As an HCO3- is moving out, a chloride (also negatively charged) must move in to maintain the electrochemical neutrality. The chloride also draws water in with it, which is used to react with carbon dioxide.

Question 34

How does carbon dioxide binds to proteins and what does it form?

Answer 34

To prevent a decrease in intracellular pH, the excess H+ can be buffered by the globin chains of haemoglobin - certain residues within the globin chain are active proton acceptors.

Some of the intraerythrocytic carbon dioxide binds to haemoglobin - not at the oxygen binding site but at the AMINO group at the N terminus forming an NHCOOH end - this is called CARBAMINOHAEMOGLOBIN

Question 35

What is the net CO2 flux?

Answer 35

52-48 mL/dL (+4 mL/dL) - total of 200 mL of CO2 produced per minute

Question 36

Why are total oxygen consumption and total carbon dioxide production not equal?

Answer 36

Because some of the water is lost in metabolic water production.

Question 37

What is pulmonary transit time?

Answer 37

The time that blood is in contact with the exchange surface/respiratory membrane - usually around 0.75 s

Question 38

What is the Haldane effect?

Answer 38

The amount of carbon dioxide that binds to the amine end of proteins forming carbaminohaemoglobin depends on the amount of oxygen that is bound to the haemoglobin - this is another allosteric effect. Increasing oxygen binding means less carbaminohaemoglobin.

Question 39

What is the ventilation perfusion mismatching of the lungs?

Answer 39

The ventilation and perfusion is greater at the inferior parts of the lungs. At the top, alveoli are stretched due to gravity so are larger. V/Q at the base tends towards zero, V/Q at the apex tends towards infinity.

At the bottom of the lungs you get wasted perfusion - At the top we get wasted ventilation.