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Respiratory System > 4. Gas Transport and Exchange > Flashcards

4. Gas Transport and Exchange Flashcards

1
Q 
What do the following symbols stand for:
• P
• S
• A
• a
A
  • P - partial pressure
  • S - Hb saturation
  • A - alveolar
  • a - arterial
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2
Q

What does Dalton’s Law state?

A

Partial pressure of a [total] gas mixture is equal to the SUM of the partial pressure of [separate] gases in the mixture

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3
Q

What does Fick’s Law state?

A

Molecules diffuse from regions of high concentration to low concentration at a rate:
• proportional to the concentration gradient, exchange SA and diffusion capacity of the gas
• inversely proportional to the thickness of the gas exchange surface

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4
Q

What does Henry’s Law state?

A

• Constant temperature
• Amount of a given gas
- dissolves in a given type and volume of liquid
• Directly proportional to the partial pressure of that gas
- in equilibrium with that liquid

(bigger solubility coefficient - dissolves more easily)

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5
Q

What do Boyle’s and Charles’ Law both state?

A

Boyle’s Law
• Constant temperature
• Volume inversely proportional to pressure
Charles’ Law
• Constant pressure
• Volume directly proportional to temperature

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6
Q

What do you change if you are supplementing oxygen to someone with a diffusion problem?

A

Steepen the diffusion gradient

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7
Q

How do gases change at a greater altitude?

A
  • Pressure decreases
  • Smaller volume
  • Proportions of gases remain the same
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8
Q

What is the PO2 of oxygen in dry air?

A

21.3 kPa

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9
Q

How does the PH2O change down the conducting airways?

A

Increases as dry air gets warmed, humidified, slowed and mixed with air already in the lungs

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10
Q

What is the PO2 at the respiratory airways?

A
  • 13.5 kPa

* This is 100% saturation

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11
Q

Why can’t we rely on solely dissolved oxygen to keep us alive?

A
  • 0.32mL/dL => 16mL/min
  • Can only dissolve 17mL of oxygen
  • Oxygen consumption (VO2) is around 250mL/min
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12
Q

Describe the structure of haemoglobin

A
  • Monomer has Fe2+ (ferrous iron) at the centre of the tetrapyrrole porphyrin ring
  • Connected to a protein chain (globin)
  • Covalently bonded at the proximal histamine residue
  • Tetramer with 2 alpha and 2 beta chains - HbA
  • HbA2 - normal variant (2%) with 2 alpha and 2 delta chains
  • Foetal (HbF) - trace levels, 2 alpha and 2 gamma chains
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13
Q

Describe the binding of oxygen to haemoglobin

A
  • Haemoglobin has a low affinity for oxygen when not bound
  • Oxygen binds - conformational changes (allosteric), greater affinity
  • Each haem binds one molecule of oxygen
  • Affinity for 4th oxygen is 300x that of the first oxygen
  • Conformational change also occurs in the middle - becomes a binding site for 2,3-DPG (glycolytic by-product, reflective of metabolism)
  • 2,3-DPG decreases the affinity of haemoglobin for oxygen
  • Oxygen can be squeezed out where metabolism is high, so more is available for respiration in these places
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14
Q

What is cooperativity?

A

Describes how haemoglobin changes shape and affinity based on how much oxygen is bound

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15
Q

What is methaemoglobin?

A
  • Fe2+ (ferrous iron) is oxidised to its ferric form (Fe3+) - becoming MetHb
  • Doesn’t bind to oxygen
  • Methaemoglobinaemia can cause functional anaemia (normal Hct but impaired O2 capacity)
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16
Q

What can oxidise Hb into MetHb, and what can be used to correct his?

A
  • Nitrites - oxidise

* Methylene blue - correction

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17
Q

Why is the oxygen dissociation curve not linear?

A
  • HbO2 saturation changes by a proportion that is not large enough in systemic circulation (lower PO2)
  • Little scope for unloading
  • HbO2 saturation changes by a large proportion in pulmonary circulation (high PO2)
  • This causes a large variation in oxygenation in the lungs
18
Q

Why is a sigmoid oxygen dissociation curve better?

A
  • Can go from 76% to 8% saturation in tissues
  • Very high unloading capacity
  • Small change and very high HbO2 saturation in pulmonary circulation
  • Effectively 100% saturation across a big range of alveolar PO2
19
Q

What is P50?

A

Partial pressure of oxygen when haemoglobin is 50% saturated

20
Q

What chemical changes in the respiratory system occur during exercise?

A
  • Increase in temperature
  • Acidosis (lactic acid and excess CO2)
  • Hypercapnia (elevated CO2)
  • Increase in 2,3-DPG
21
Q

How does high energy consumption e.g. exercise, change the oxygen dissociation curve?

A
  • Shifts right
  • Greater unloading of oxygen
  • Therefore lower HbO2 saturation at lower PO2 (systemic)

(hyperventilation decreases CO2 so causes a shift to the left)

22
Q

How does anaemia change the oxygen dissociation curve?

A
  • Downwards shift
  • The HbO2 concentration remains ‘the same’ as y-axis just becomes smaller
  • Comparing the oxygen carrying capacities
  • Pulse oximetry can’t compare the Hct, only saturation of the total
23
Q

How does polycythaemia (increased Hct) change the oxygen dissociation curve?

A
  • Upwards shift
  • More erythrocytes - higher oxygen capacity - larger y-axis
  • Thicker blood - slower blood flow - oxygen delivery impeded
24
Q

How does Carbon Monoxide poisoning change the oxygen dissociation curve?

A
  • Downwards and leftwards shift
  • Decreased oxygen capacity - smaller y-axis
  • Increased affinity (for CO than O2)
  • Reduces available haemoglobin or holds onto oxygen tighter
25
Q

How is the oxygen dissociation curve different for foetal haemoglobin and myoglobin?

A

Foetal
• Sharper increase
• Greater affinity than adult HbA to extract oxygen

Myoglobin (monomeric protein)
• Hyperbolic curve
• Very sharp increase, reaching 100% saturation at a low PO2
• Present for when the muscle needs oxygen rapidly

26
Q

How oxygenated is blood arriving in the pulmonary circulation?

A
  • Mixed venous blood
  • Not deoxygenated
  • Around 75% oxygen bound
  • Arrives at the exchange surface at around 5.3 kPa
27
Q

Describe the oxygen gradient in the red blood cell

A
  • Plasma > intraerythrocytic oxygen concentration
  • Oxygen moves into the red blood cell
  • Occupies the final binding spot in haemoglobin
28
Q

What is the oxygen saturation of the blood when it reaches the tissues?

A

97%

29
Q

Why is the blood diluted in the pulmonary system?

A

• Pulmonary system has 2 circulations:
- own blood supply (to stay alive)
- pulmonary blood supply for oxygenation
• Circulation of own blood supply drains into the pulmonary circulation before returning to the left atrium

30
Q

How does the concentration and saturation of oxygen change at the tissues?

A
  • 20.3 - 15.1 mL/dL

* 97 - 75%

31
Q

What is oxygen flux?

A
  • Overall amount of oxygen being deposited
  • -5 mL/dL
  • This equals a consumption of 250 mL of oxygen per minute in the body
32
Q

What happens to CO2 when it enters circulation?

A
  • CO2 is more soluble than oxygen - dissolves in plasma easily
  • CO2 + H2O => H2CO3 (carbonic acid)
  • H2CO3 => H+ + HCO3- (dissociates into proton and bicarbonate - slow as there are no enzymes)
  • Same reaction occurs in red blood cells
  • Rate of bicarbonate production is 5000 times greater - carbonic anhydrase catalyses formation of carbonic acid
  • Chloride shift - as bicarbonate ions leave, chloride ions enter to maintain the resting membrane potential (AE1 transporter)
  • Chloride entering draws water in with it, which is used to react with CO2
  • Water needed to prevent dehydration too

(process also acts as a pH buffer)

33
Q

How does CO2 interact with proteins in the blood

A

Binds to the amine end of haemoglobin => Carbaminohaemoglobin (HbCO2)

34
Q

How do proteins control red blood cell pH

A
  • Excess protons

* Negatively charged amino acids e.g. histidine are good proton acceptors

35
Q

How does CO2 production compare to O2 consumption?

A
  • +4mL/dL net increase in CO2 concentration
  • 200mL of CO2 produced a minute
  • Not equal to the 250mL oxygen consumption
  • Some water produced is lost in metabolic water production
36
Q

Describe the CO2 dissociation curve

A
  • As PCO2 increases, blood CO2 content increases (more linear than oxygen)
  • As blood loses oxygen, haemoglobin carries more CO2 - allosteric behaviour
37
Q

What is the Haldane Effect?

A

Describes how the amount of CO2 that binds to the amine end of haemoglobin protein chain changes depending on how much oxygen is bound

38
Q

What is the respiratory membrane?

A

Areas where the alveolar cells and endothelial cells of the capillaries are close enough for exchange to take place

39
Q

What is the pulmonary transit time?

A
  • Time during blood cells are in contact with the respiratory membrane
  • Total - 0.75s
  • Gas exchange complete - 0.25s
  • CO2 reaches equilibrium - 0.1s
40
Q

Why can exercise lead to hypoxaemia?

A
  • Increased CO and pulmonary blood flow
  • Blood not in contact with respiratory membrane for long enough
  • Not enough time to reach 100% oxygen saturation

(CO2 crosses the membrane much more easily and faster)

41
Q

How does ventilation in the apex compare to the base of the lungs?

A 
Apex
• Less ventilation
• Less blood perfusion (gravity)
• Ventilation outweighs perfusion
• V/Q (ventilation/perfusion) tends towards infinity
Base
• More ventilation
• Better perfusion
• Perfusion outweighs ventilation
• V/Q tends towards zero
42
Q

How does alveolar pressure, venous pressure and arterial pressure compare in the apex, middle and base of the lungs?

A
  • Apex - alveolar pressure > arterial pressure > venous pressure
  • Middle - arterial pressure > alveolar pressure > venous pressure
  • Base - arterial pressure > venous pressure > alveolar pressure

Respiratory System (18 decks)

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