Respiratory Physiology IV

Question 1

How does the oxygen partial pressure change as you move from air entering the main bronchi to the partial pressure of oxygen in the blood at the tissues?

Answer 1

It decreases considerably.

Question 2

What does Henry’s Law state?

Answer 2

The amount of a given gas dissolved in a given type and volume of liquid (e.g. blood) at a constant temperature is proportional to the partial pressure of the gas in equilibrium with the liquid.

Question 3

What happens to the concentration of gas in the liquid phase if the partial pressure in the gas phase is increased?

Answer 3

It increases proportionally.

Question 4

What does the partial pressure of a gas in solution mean?

Answer 4

The partial pressure of a gas in solution is its partial pressure in the gas mixture with which it is in equilibrium.

Question 5

What does Henry’s Law mean for the amount of oxygen dissolved in the blood?

Answer 5

It is proportional to the partial pressure of oxygen.

Question 6

Is the amount of oxygen dissolved in blood alone enough to meet the resting oxygen consumption of our body?

Answer 6

No, only 1.5% of the total blood oxygen concentration is dissolved in the blood.

Question 7

What is the value for normal oxygen concentration in arterial blood and what is the normal haemoglobin (Hb) concentration?

Answer 7

200 ml/L oxygen concentration.

150 g/L haemoglobin (Hb) concentration.

Question 8

How do the percentages of oxygen carried bound to haemoglobin and in the dissolved form compare?

Answer 8

Bound to haemoglobin - 98.5%.

Dissolved in blood - 1.5%.

Question 9

How many haem groups are in each haemoglobin molecule and how many oxygen molecules can each group bind?

Answer 9

Four haem groups per molecule.

Each can bind one molecule of oxygen.

Question 10

When is haemoglobin considered fully saturated?

Answer 10

When all four haem groups are bound to a molecule of oxygen.

Question 11

What is the primary factor which determines the percentage saturation of haemoglobin?

Answer 11

The partial pressure of oxygen.

Question 12

Describe the structure of haemoglobin.

Answer 12

Four sub-units (haem groups): two beta and two alpha chains.

Each haem group contains an iron ion which binds to an oxygen molecule.

Question 13

Describe the shape of the oxygen-haemoglobin dissociation curve.

Answer 13

“Sigmoid curve.”

Initial increase in % saturation is slow but then this increase gets greater.

Slowly begins to tail off to a plateau.

Question 14

How does oxygen concentration of the blood change as haemoglobin concentration is changed? Why is this?

Answer 14

An increase in haemoglobin concentration results in a large increase in blood oxygen concentration.

A decrease in haemoglobin concentration results in a large decrease in blood oxygen concentration.

This is due to the majority of oxygen in the blood being bound to haemoglobin (98.5%) so its concentration has a great influence on oxygen concentration.

Question 15

What is the formula to calculate the oxygen delivery index, DO2I?

Answer 15

DO2I = CaO2 x CI.

where CaO2 = oxygen content of arterial blood (ml/L);
CI = cardiac index (L/min/m^2).

Question 16

What does the cardiac index do?

Answer 16

Relates the cardiac output to the surface area of the body (i.e. size of the individual).

(Normal range = 2.4 - 4.2 L/min/m^2.)

Question 17

What is the formula to calculate the oxygen content of arterial blood, CAO2?

Answer 17

CaO2 = 1.34 x [Hb] x SaO2.

where 1.34 is the amount of oxygen in ml one gram of haemoglobin can carry when fully saturated;
[Hb] = haemoglobin concentration in gram/L;
SaO2 = %Hb saturated with oxygen.

Question 18

What three factors can impair oxygen delivery to the tissues?

Answer 18

1. Respiratory disease.
2. Heart failure.
3. Anaemia.

Question 19

What two factors does the partial pressure of inspired oxygen depend upon?

Answer 19

1. Total pressure (i.e. atmospheric pressure).

2. Proportion of oxygen in the gas mixture (~21% in the atmosphere).

Question 20

Why does decreased partial pressure of inspired oxygen/respiratory disease reduce oxygen delivery to the tissues?

Answer 20

These can both decrease arterial PO2 and hence decrease Hb saturation with oxygen as well as dissolved oxygen in the blood.

Question 21

Why does anaemia reduce oxygen delivery to the tissues?

Answer 21

Anaemia decreases the Hb concentration of the blood and hence decreases the oxygen content of the blood.

Question 22

Why does heart failure reduce oxygen delivery to the tissues?

Answer 22

Heart failure decreases cardiac output.

Question 23

What occurs when one oxygen binds to a haem group of a haemoglobin molecule? What significance does this have for the oxygen-haemoglobin dissociation curve?

Answer 23

The haemoglobin’s affinity for further oxygen binding increases.

The curve is therefore sigmoid as initially binding of oxygen is slower but then the affinity increases with more oxygen binding.

It also flattens as the partial pressure of oxygen further increases as all sites are becoming occupied.

Question 24

What is the physiological significance of the flat upper portion of the oxygen-haemoglobin dissociation curve?

Answer 24

This means that a moderate fall in alveolar partial pressure of oxygen will not greatly diminish oxygen loading of haemoglobin.

Question 25

What is the physiological significance of the steep lower part of the oxygen-haemoglobin dissociation curve?

Answer 25

This means that the peripheral tissues get a lot of oxygen for a small drop in capillary partial blood pressure.

Question 26

What is the Bohr Effect?

Answer 26

A shift of the oxygen-haemoglobin binding curve to the right.

Question 27

What three four changes cause the oxygen-haemoglobin curve to shift to the right?

Answer 27

1. Increase in partial pressure of carbon dioxide.
2. Increase in hydrogen ion concentration.
3. Increase in temperature.
4. Increase in 2,3-biphosphoglycerate (2,3-BPG).

Question 28

What influence does the Bohr Effect have on oxygen delivery to tissues?

Answer 28

Increased release of oxygen by conditions at the tissues.

Question 29

How does the structure of foetal haemoglobin (HbF) differ from adult haemoglobin in structure?

Answer 29

It has two alpha and two gamma (as opposed to beta) sub-units.

Question 30

What does foetal haemoglobin (HbF) interact less with in red blood cells?

Answer 30

2,3-biphosphateglycerate (2,3-BPG).

Question 31

What does the different structure of foetal haemoglobin (HbF) mean for its affinity for oxygen? What does this do to the oxygen-haemoglobin dissociation curve for HbF?

Answer 31

It increases its affinity for oxygen.

The curve is shifted to the left.

Question 32

What advantage does the increased affinity of foetal haemoglobin (HbF) for oxygen give during foetal development?

Answer 32

It means that even if the partial pressure of oxygen in the mothers blood is low, it is still possible to transfer oxygen to the foetus.

Question 33

Describe the shape of the myoglobin dissociation curve.

Answer 33

The myoglobin is shown on the graph to very quickly reach 100% saturation as partial pressure of oxygen increases initially, and quickly reaches a plateau.

“Hyperbolic curve”.

Question 34

Why is the oxygen-myoglobin dissociation curve hyperbolic?

Answer 34

Only one molecule of oxygen binds to each molecule of myoglobin and so myoglobin becomes saturated quickly.

i.e. absence of the cooperativity seen in haemoglobin binding with oxygen.

Question 35

Does myoglobin release oxygen when the partial pressure of oxygen is low or high?

Answer 35

(Very) low.

Question 36

What is the purpose of myoglobin?

Answer 36

Provides a short term storage of oxygen for anaerobic conditions.

Question 37

What does the presence of myoglobin in the blood indicate?

Answer 37

Muscle damage.