Ventilation - perfusion and gas transport (physiology)

Question 1

Define ventilation

Answer 1

The amount of air getting into the lungs (alveolar ventilation)

Question 2

Define perfusion

Answer 2

The blood flow through the pulmonary circulation

Question 3

What is the relationship between ventilation and perfusion?

Answer 3

Ideal relationship is where ventilation = perfusion
(all air getting into alveoli is passed on to circulation)

Question 4

How does ventilation vary throughout the lungs?

Answer 4

Greatest at base and least at apex
(ventilation&raquo_space;> perfusion at apex due to lower blood pressure)

Question 5

How does perfusion vary throughout the lung?

Answer 5

Greatest at base and least at apex
(perfusion&raquo_space;> ventilation at base due to gravity and high blood pressure)

Question 6

Why is ventilation&raquo_space;> perfusion at apex of lung?

Answer 6

Arterial pressure is less than alveoli pressure which then compresses the arterioles.

Question 7

Why is perfusion&raquo_space;> ventilation at base of lung?

Answer 7

Arterial pressure exceeds alveolar pressure so alveolar are compressed.

Question 8

What happens when blood flow is greater than ventilation?

Answer 8

Poor ventilation leads to a build up of PCO2 and a fall in PO2 in alveoli.
Blood flow continues but there is little gas exchange.
A shunt occurs = where there is dilution of oxygenated blood from better ventilated areas. Blood moves from left to right side of the heart without being oxygenated.

Results in pulmonary vasoconstriction (to redirect blood flow to better ventilated areas) and bronchial dilation (to increase ventilation)

Question 9

What happens when ventilation is greater than blood flow?

Answer 9

Results in alveolar dead space. Where the alveolar are well ventilated but due to poor perfusion little gas exchange occurs.

Alveolar PO2 increases and PCO2 falls
Results in pulmonary vasodilation (to increase blood flow ) and bronchial constriction (to reduce ventilation.)

Question 10

Define alveolar dead space

Answer 10

Air just sat in alveoli due to poor perfusion (little gas exchange occurring)

Question 11

Define anatomical dead space

Answer 11

Air sat in the respiratory tract unable to participate in gas exchange as wall of region are too thick.

Question 12

Define physiological dead space

Answer 12

Alveolar DS + anatomical DS

Question 13

What happens to O2 movement between alveoli and blood?
(in terms of partial pressure)

Answer 13

O2 moves down its partial pressure gradient.
Diffusion of O2 stops when PO2 alveolar = PO2 plasma.

Question 14

What happens to CO2 movement between alveoli and blood?
(in terms of partial pressure)

Answer 14

CO2 moves down its partial pressure gradient.
(out of tissue into blood, out of blood into lungs)

Question 15

What is the role of haemoglobin in the transport of O2 in the blood?

Answer 15

O2 diffuses into the blood and binds to Hb which transports it round the body to tissues

There are 4 binding sites per HB
Hb is cooperative
- When one O2 binds in increases affinity for other O2 to bind

Question 16

What is the saturation of Hb in oxygenated and deoxygenated blood?

Answer 16

Oxygenated blood = 98%
Deoxygenated blood = 75%

(Therefore 25% O2 is used in tissues)

Question 17

How quickly does saturation of Hb occur?

Answer 17

Within 0.25 seconds
Occurs quickly as a safety mechanism

Question 18

What is the relationship between O2 and Hb and when is it reversable?

Answer 18

Hb + O2 <> Hb)2

(reversable when PO2 falls)

Question 19

Define cooperativity

Answer 19

Once one O2 binds to Hb the affinity of Hb for O2 increases and so more O2 binds (same process for O2 unloading)

Question 20

Describe the oxygen-haemoglobin dissociation curve

Answer 20

As partial pressure increases percentage of O2 saturation of haemoglobin increases.

Rapid increase from 0 to 40 PO2 then tails of at about 60 PO2 (90% saturation)
Between 60 PO2 and 100 PO2 (alveolar) saturation levels rise to 98%

Question 21

What does this mean for changes in PO2/O2?

Answer 21

You can have a big decrease in PO2 without having a big affect on O2 saturation of Hb

There is little change in O2 saturation until PO2 falls below 60 mmHg
A decrease of PO2 below 60 mmHg results in a large decrease in O2 saturation.

Question 22

What factors affect the oxygen-haemoglobin dissociation curve? (4)

Answer 22

pH
PCO2
Temperature
DPG

Question 23

How does pH affect oxygen-haemoglobin dissociation curve?

Answer 23

An increase in pH
- Increases the affinity of Hb for O2.
- Reduces O2 delivery to peripheral tissues as Hb hangs onto it

A decrease in pH
- Bohre effect
- aids O2 unloading in peripheral tissues by reducing affinity of Hb for O2

Question 24

How does temperature affect oxygen-haemoglobin dissociation curve?

Answer 24

Increase in temperature
- Bohre effect
- Aids O2 unloading in peripheral tissues by reducing affinity of Hb for O2

Decrease in temperature
- Increase Hb affinity for O2
- Reduces O2 delivery to peripheral tissues as Hb onto it.

Question 25

How does PCO2 affect oxygen-haemoglobin dissociation curve?

Answer 25

Increase in PCO2 - Bohre effect - Aids O2 unloading in peripheral tissues by reducing affinity of Hb for O2 Decrease in PCO2 - Increase Hb affinity for O2 - Reduces O2 delivery to peripheral tissues as Hb onto it.

Question 26

How does DPG affect oxygen-haemoglobin dissociation curve?

Answer 26

Reduces affinity of Hb for oxygen Increases O2 delivery to periphery tissues Regions of hypoxia produce more DPG

Question 27

What is anaemia?

Answer 27

When oxygen carrying capacity of the blood is compromised

Question 28

What causes anaemia?

Answer 28

Iron deficiency Loss of RBCs Vit B deficiency (treated with B12)

Question 29

How does anaemia show PaO2 determines but is independent of total blood O2 content?

Answer 29

PO2 is NORMAL despite the low O2 content in anaemic patients The saturation of Hb is normal but don't have as much Hb so hence the low O2 content.

Question 30

In what forms is CO2 carried in the blood? (and in what proportions)

Answer 30

7% - remains dissolved in plasma 23% - carbamino compounds (CO2 combines with deoxyhaemoglobin in erythrocytes). 70% - bicarbonate ions and H+ ions (CO2 and water combine in erythrocytes to form carbonic acid which then dissociates)

Question 31

What is the action of carbonic anhydrase in CO2 transport?

Answer 31

Aids in the conversion of carbon dioxide to carbonic acid and bicarbonate ions. (catalyses the conversion of CO2 into carbonic acid which then hydrolyses into H+ ions and bicarbonate ions)

Question 32

What factors favour CO2 unloading to the alveoli at the lungs?

Answer 32

Increased acidity

Question 33

What is the difference between partial pressure and gas content?

Answer 33

Partial pressure is the force driving gas into/out of solution. Gases do not travel in gaseous phase in blood, if so bubbles = fatal air embolism

Question 34

How is foetal Hb different to HbA and why is this helpful?

Answer 34

It has a higher affinity for O2 than HbA Helpful so that foetus can get O2 from the mother.

Question 35

How is myoglobin Hb different to HbA and why is this helpful?

Answer 35

It has a higher affinity for O2 than HbA Allows myoglobin to extract O2 from HbA and so muscles can get more O2 out of the blood.

Question 36

What are the 5 types of hypoxia?

Answer 36

Hypoxaemia hypoxia Anaemic hypoxia Stagnant hypoxia Histotoxic hypoxia Metabolic Hypoxia

Question 37

What happens in hypoxaemia hypoxia?

Answer 37

Reduction in O2 diffusion at lungs either due to decreased PO2 (atmos) or tissue pathology

Question 38

What happens in anaemic hypoxia?

Answer 38

Reduction in O2 carrying capacity of blood due to anaemia (RBC loss, iron/vit B deficiency)

Question 39

What happens in stagnant hypoxia?

Answer 39

Heart disease results in inefficient pumping of blood to lungs and around the body

Question 40

What happens in histotoxic hypoxia?

Answer 40

Poisoning prevents cells utilising O2 delivered to them e.g. carbon monoxide/cyanide poisoning.

Question 41

What happens in metabolic hypoxia?

Answer 41

Oxygen delivery to the tissues does not meet the increased O2 demand by cells. Occurs during exercise, can be acute and short and is reversable If a pathological problem then it is chronic