Physiology - Gas exchange and transport

Question 1

What are salt bride interactions?

Answer 1

Interactions that hold Hb subunits together in deoxygenated state
- Globin subunits rigid => oxygenation difficult

Question 2

What is the consequence of O2 binding to a subunit?

Answer 2

Changes the conformational shape of globin

=> breakage of salt-bridge

=> more O2 binding

=> more O2 uptake

=> further breakage of salt bridges

Question 3

What is the significance of the sigmoid shaped O2 dissociation curve?

Answer 3

gradual curve at very low pO2
- Hb entirely deoxygenated (rare)

steep curve in tissues (at low pO2)
- pO2 given up easier by Hb
[] due to increased salt bridges

gradual curve in lungs (at high pO2)
- high pO2 => O2 readily accepted by Hb
[] even if p02 is reduced e.g. altitude

Question 4

What is the Bohr effect?

Answer 4

O2 dissociation curve shifting to the right

Question 5

When does the Bohr effect occur?

Answer 5

When:

• increased CO2
• increased temperature
• decrease pH

Question 6

What is the benefit of the bohr effect?

Answer 6

More O2 released in the tissue when exercising

- due to reduced affinity for O2

Question 7

What is the mechanism responsible for the Bohr effect?

Answer 7

Chloride shift

Increased salt bridge formation as H+ charges globin surface => O2 release

Question 8

What effect does BiPhosphoGlycerate on the O2 dissociation curve?

Answer 8

Shifts curve to the left

Question 9

What is BiPhosphoGlycerate?

Answer 9

A by-product of glycolysis in RBC

Without BPG, Hb would be saturated at 20mmHg

BPG levels vary over time, increase at high altitude

BPG important for anaemia etc.

Question 10

Why is foetal BPG different to adult BPG?

Answer 10

Foetal Hb as a much higher affinity for O2

Easier for foetal Hb to pick up O2 at low placenta pO2

Question 11

When does the 02 dissociation curve shift to the left?

Answer 11

Increased pH

Decreased CO2

Decreased temperature

Question 12

What are the three ways of CO2 transportation out of the body?

Answer 12

Ionisation of Carbonic acid (70%)

Carbamino compounds (20%)

Dissolved in plasma (10%)

Question 13

What enzyme converts CO2 into carbonic acid, and where does it take place?

Answer 13

Carbonic anyhydrase

In the RBC

Question 14

What is the chloride shift?

Answer 14

Cl- ions move into RBC to maintain electrical neutrality (due to H+ in the cell) which replaces HCO3- which diffuses out

Question 15

Why does the chloride shift occur?

Answer 15

H+ build up would lead to stoppage of HCO3- production

Question 16

What is the haldane effect?

Answer 16

For any pCO2, [CO2] in deoxygenated blood < than [CO2] in oxygenated blood

=> blood can take up more CO2 from tissues even if blood is deoxygenated

Also allows CO2 to be released in lungs for expiration if Hb in the lungs is oxygenated

Question 17

How are carbamino compounds formed?

Answer 17

Rapid combination of CO2 and terminal amino groups on proteins

Most prevalent protein in blood is Hb => Co2 + Hb
=> carbaminohaemoglobin

Question 18

How long does plasma take to become saturated with CO2?

Answer 18

Very quickly

Question 19

Where in the lungs does diffusion of gases take place?

Answer 19

Alveolar-capillary rmembane

Question 20

How many alveoli in the body and what size?

Answer 20

300 million (0.2 mm diameter)

Question 21

Where is alveolar epithelium located and what does it contain?

Answer 21

Sits betwee neighbouring alveoli and rests on basement membrane

Consists of:

• pulmonary capillaries
• elastin
• collagen

Question 22

What forms the alveolar-capillary membrane?

Answer 22

The alveolar epithelium and the capirally epithelium

• gases diffuse through this
• very thin apart from the elastin/collagen areas

Question 23

What are the functions of type 1 alveolar cells (pneumocytes)?

Answer 23

Line alveoli

Gas diffusion

Question 24

What are the functions of type 2 alveolar cells (pneumocytes)?

Answer 24

Alveolar surfactant (lining fluid)
 - large nuclei, microvilli

Question 25

What factors affect diffusion across a membrane?

Answer 25

g = k(A/T)(P1 - P2)

g = rate of gas transfer
A = area
T = thickness
P1 - P2 = partial pressure gradient
K = constant

Question 26

What pathologies may reduce diffusion capacity by reducing exchange area?

Answer 26

Emphysema
PE
Lung resection
Pulmonary fibrosis

Question 27

What pathologies may reduce diffusion capacity by increasing membrane thickness?

Answer 27

Pulmonary oedema

Pulmonary fibrosis

Question 28

How does exercise increase diffusing capacity?

Answer 28

Increase pulmonary blood volume which increases effective area

Question 29

How long does it take ofr blood to become saturated with O2 in the alveoli?

Answer 29

0.25 s

• at rest blood spends 0.75s within alveoli => excess time
• during exercise time may reduce to 0.25s
  [] affects lung disease patients during exercise first

Question 30

What is the Va/Q ratio?

Answer 30

Alveolar ventilation / perfusion ratio

- ratio of 1 most efficient

Question 31

Where in the lung are ventilation values and perfusion values greatest, and why?

Answer 31

Ventilation slightly greater at base of lung
Perfusion greater at base of lung

• due to gravity

Question 32

Where in the lung are Va/Q ratios smallest and largest?

Answer 32

Largest at apices (~ 3)

Smallest at base (~ 0.7)