Physiology - Gas exchange and transport Flashcards
What are salt bride interactions?
Interactions that hold Hb subunits together in deoxygenated state
- Globin subunits rigid => oxygenation difficult
What is the consequence of O2 binding to a subunit?
Changes the conformational shape of globin
=> breakage of salt-bridge
=> more O2 binding
=> more O2 uptake
=> further breakage of salt bridges
What is the significance of the sigmoid shaped O2 dissociation curve?
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
What is the Bohr effect?
O2 dissociation curve shifting to the right
When does the Bohr effect occur?
When:
- increased CO2
- increased temperature
- decrease pH
What is the benefit of the bohr effect?
More O2 released in the tissue when exercising
- due to reduced affinity for O2
What is the mechanism responsible for the Bohr effect?
Chloride shift
Increased salt bridge formation as H+ charges globin surface => O2 release
What effect does BiPhosphoGlycerate on the O2 dissociation curve?
Shifts curve to the left
What is BiPhosphoGlycerate?
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.
Why is foetal BPG different to adult BPG?
Foetal Hb as a much higher affinity for O2
Easier for foetal Hb to pick up O2 at low placenta pO2
When does the 02 dissociation curve shift to the left?
Increased pH
Decreased CO2
Decreased temperature
What are the three ways of CO2 transportation out of the body?
Ionisation of Carbonic acid (70%)
Carbamino compounds (20%)
Dissolved in plasma (10%)
What enzyme converts CO2 into carbonic acid, and where does it take place?
Carbonic anyhydrase
In the RBC
What is the chloride shift?
Cl- ions move into RBC to maintain electrical neutrality (due to H+ in the cell) which replaces HCO3- which diffuses out
Why does the chloride shift occur?
H+ build up would lead to stoppage of HCO3- production
What is the haldane effect?
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
How are carbamino compounds formed?
Rapid combination of CO2 and terminal amino groups on proteins
Most prevalent protein in blood is Hb => Co2 + Hb
=> carbaminohaemoglobin
How long does plasma take to become saturated with CO2?
Very quickly
Where in the lungs does diffusion of gases take place?
Alveolar-capillary rmembane
How many alveoli in the body and what size?
300 million (0.2 mm diameter)
Where is alveolar epithelium located and what does it contain?
Sits betwee neighbouring alveoli and rests on basement membrane
Consists of:
- pulmonary capillaries
- elastin
- collagen
What forms the alveolar-capillary membrane?
The alveolar epithelium and the capirally epithelium
- gases diffuse through this
- very thin apart from the elastin/collagen areas
What are the functions of type 1 alveolar cells (pneumocytes)?
Line alveoli
Gas diffusion
What are the functions of type 2 alveolar cells (pneumocytes)?
Alveolar surfactant (lining fluid) - large nuclei, microvilli
What factors affect diffusion across a membrane?
g = k(A/T)(P1 - P2)
g = rate of gas transfer A = area T = thickness P1 - P2 = partial pressure gradient K = constant
What pathologies may reduce diffusion capacity by reducing exchange area?
Emphysema
PE
Lung resection
Pulmonary fibrosis
What pathologies may reduce diffusion capacity by increasing membrane thickness?
Pulmonary oedema
Pulmonary fibrosis
How does exercise increase diffusing capacity?
Increase pulmonary blood volume which increases effective area
How long does it take ofr blood to become saturated with O2 in the alveoli?
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
What is the Va/Q ratio?
Alveolar ventilation / perfusion ratio
- ratio of 1 most efficient
Where in the lung are ventilation values and perfusion values greatest, and why?
Ventilation slightly greater at base of lung
Perfusion greater at base of lung
- due to gravity
Where in the lung are Va/Q ratios smallest and largest?
Largest at apices (~ 3)
Smallest at base (~ 0.7)
