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 give 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.

Question 8

How much oxygen can be dissolved in out bodies?

Answer 8

17 mL at 0.34 mL/dL

Question 9

What is the normal oxygen consumption at rest?

Answer 9

250 mL/min

Question 10

What is the binding capacity of oxygen to haemoglobin?

Answer 10

1.34 mL/g

Question 11

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

Answer 11

0.003 mL

Question 12

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

Answer 12

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 13

Explain why haemoglobin is considered an ‘allosteric’ molecule.

Answer 13

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 14

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

Answer 14

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 15

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

Answer 15

Cooperativity

Question 16

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

Answer 16

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 17

What are the benefits of having a sigmoid ODC?

Answer 17

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

Question 18

What is P50?

Answer 18

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

Question 19

What conditions can shift the ODC to the right?

Answer 19

Conditions brought about by exercise - hypercapnia, increased temperature, acidosis, increased 2,3-DPG

Question 20

What conditions can shift the ODC to the left?

Answer 20

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

Question 21

What conditions can shift the ODC upwards?

Answer 21

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

Question 22

What conditions can shift the ODC downwards?

Answer 22

Anaemia - decreased oxygen carrying capacity of the blood

Question 23

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

Answer 23

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

Question 24

How does carbon monoxide shift the ODC and why?

Answer 24

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 25

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

Answer 25

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 26

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

Answer 26

5.3 kPa

Question 27

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

Answer 27

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 28

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

Answer 28

20.3 --> 15.1 mL/dL | 97% --> 75%

Question 29

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

Answer 29

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 30

Describe the reaction of carbon dioxide with water.

Answer 30

Carbon dioxide reacts with water to produce carbonic acid (H2CO3)

Question 31

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

Answer 31

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 32

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

Answer 32

AE1 transporter

Question 33

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

Answer 33

Chloride ions move in - this is called chloride shift

Question 34

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

Answer 34

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 35

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

Answer 35

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 36

What is the net CO2 flux?

Answer 36

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

Question 37

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

Answer 37

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

Question 38

What is pulmonary transit time?

Answer 38

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

Question 39

What is the Haldane effect?

Answer 39

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 40

What is the ventilation perfusion mismatching of the lungs?

Answer 40

The ventilation and perfusion is greater at the inferior parts of the lungs. V/Q at the base tends towards zero, V/Q at the apex tends towards infinity.