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FBS 2 > Gaseous Diffusion and Transport > Flashcards

Gaseous Diffusion and Transport Flashcards

1
Q

What does Dalton’s law state?

A

In a mixture of non-reacting gases, the total pressure exerted is equal to the sum of the partial pressures of the individual gases.

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2
Q

Why do people struggle to breathe at high altitudes?

A

The fractional concentration of O2 in the air (FiO2) is unchanged at altitude, but the barometric pressure (Pb) is reduced.

PiO2 = FiO2 x Pb which means that PiO2 falls progressively with increasing altitude.

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3
Q

What is Henry’s law?

A

Describes the influence of pressure on gas solubility.

C = kP

C = concentration of dissolved gas at equilibrium 
k = constant
P = partial pressure of the gas
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4
Q

What does Henry’s law state?

A

That the more partial pressure of a gas is increased, the higher the number of molecules of gas per unit volume which increases the rate at which gas molecules collide with the surface of the liquid so the more will dissolve in liquid.

If this pressure is released, less gas will be held in the solution and bubbles of gas are released.

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5
Q

What is the bends?

A

Where you go from areas of high pressure to low too quickly causing bubbles to appear in bones and joints. This is seen in scuba-divers that ascend too quickly.

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6
Q

What does water vapour pressure depend on?

A

Temperature

Saturation

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7
Q

What does %saturation depend on?

A

How much water the air has been in contact with.

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8
Q

What happens to the saturation of air entering the lungs?

A

As it passes over moist surfaces it becomes 100% saturated.

The pH2O of air in the trachea and lungs is always constant due to a constant body temperature.

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9
Q

Why is alveolar PO2 higher than blood PO2 in the pulmonary capillaries?

A

Because the partial pressure gradient created drives diffusion of O2 through the alveolar-capillary membrane.

At rest, pulmonary capillary PO2 equals alveolar PO2 by about 1/3 along the pulmonary capillary.

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10
Q

How does CO2 equilibriate rapidly across the alveolar-capillary membrane despite having a lower partial pressure gradient?

A

CO2 has a very high solubility coefficient (higher than O2) which means that as CO2 is 23x more soluble than O2, at any given partial pressure it will diffuse much faster than O2.

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11
Q

What is the pressure gradient driving diffusion across the alveolar-capillary membrane?

A

Alveolar pressure (Pa) - mean pulmonary capillary pressure (Pc).

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12
Q

What is the transfer factor of the lungs?

A

The area and thickness of the alveolar-capillary membrane combined to give the diffusing capacity (DLg) which is also known as the transfer factor.

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13
Q

How do you calculate the rate of transfer of a gas?

A

DLg x (Pa-Pc)

This can be done for specific gases e.g. for O2:

Rate of transfer of O2 = DLO2 x (PaO2-PcO2)

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14
Q

How do you calculate DLCO?

A

Because haemoglobin has such a high affinity for CO, when CO is transferred across the alveolar-capillary membrane almost all of it enters Hb which means that the mean pulmonary capillary PCO can be assumed to be zero.

This means that DLCO = CO uptake from the lungs/PACO.

Clincally, a patient inhales a breath containing very low amounts of CO and a tracer gas then the composition of exhaled gas is examined.

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15
Q

In what circumstances would DLCO be reduced?

A
  • Reduction in alveolar-capillary membrane area
  • Increase in thickness of alveolar-capillary membrane
  • Anaemia
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16
Q

In what circumstances would DLCO be increased?

A
  • Increase in pulmonary blood volume i.e. exercise

- Polycythaemia (abnormal increase in Hb in the blood)

17
Q

Why do tissues become hypoxic?

A

Mitochondria in tissues need a PO2 greater than 1mmHg (0.13kPa). If this falls too low, the rate of diffusion becomes too low for the tissue’s demands causing them to become hypoxic.

18
Q

Why do we need haemoglobin?

A

Because the amount of O2 that is capable of dissolving in blood is far too low to sustain life.

19
Q

What is the oxygen capacity of normal blood?

A

200ml/L

20
Q

What is the oxygen capacity of pulmonary venous and arterial blood?

A

Venous = 200ml/L i.e. 100% saturated

Arterial = 150ml/L i.e. 75% saturated

21
Q

How is the O2 dissociation curve affected in anaemia?

A

Downward shift. There is a massive decrease in the oxygen content of blood.

22
Q

What are the two negative effects of CO poisoning?

A

1) Reduces the amount of O2 bound to Hb.
2) Shifts the O2 bindnig curve to the left (increasing Hb affinity for O2) which means that O2 is less readily unloaded to tissues.

23
Q

How does foetal Hb change the O2 dissociation curve?

A

Shifts it to the left. It has a higher affinity for O2 than maternal Hb. This favours O2 moving from the mother’s blood to foetal blood across the placenta.

24
Q

What is cyanosis?

A

Not enough O2 supply causes the amount of deoxyHb in tissue capillaries to increase as a result of hypoxia. DeoxyHb has a blueish tinge which causes discolouration in the tissues.

25
Q

What causes central cyanosis and where is it most obvious?

A

Caused by arterial hypoxaemia.

Buccal mucosa and lips are the most obvious.

26
Q

How much CO2 is dissolved in blood?

A

2.74ml/dl

27
Q

Why is the conversion of CO2 to bicarbonate faster in red blood cells?

A

Because RBCs have carbonic anhydrase.

28
Q

How does bicarbonate leave RBCs?

A

Travels down its concentration gradient to an antiporter where it is exchanged for Cl. This is the chloride shift.

29
Q

How are carbamino compounds produced?

A

By CO2 reacting with NH2 groups on proteins.

30
Q

What are the different ways that CO2 is unloaded from blood and transported to the lungs?

A
  • CO2 dissolves in blood goes along its concentration gradient.
  • Carbamino compounds release the CO2 and diffuses into alveoli.
  • Bicarbonate in plasma is taken up back into RBCs, combines with H+ which comes from Hb and forms carbonic acid. This then dissociates into CO2 and water and diffuses into alveoli.
31
Q

What is the Haldane effect?

A

At any given PCO2, the quantity of CO2 carried is greater in partially deoxygenated blood (venous) than in oxygenated blood (arterial).

This is due to carbamino compounds forming more readily when deoxygenated and Hb binding to H+ more readily when deoxygenated (which favours bicarbonate formation).

32
Q

What is minute ventilation?

A

Tidal volume x frequency of breaths

33
Q

How do you calculate alveolar ventilation?

A

Minute ventilation - dead space ventilation

34
Q

What is the respiratory quotient?

A

The ratio of the CO2 produced in metabolism to the O2 utilised by the body.

35
Q

What is the respiratory quotient dependent on?

A

The exact value is diet dependent because different metabolic fuels generate different amounts of CO2 for each O2 consumed.

36
Q

What is hypocapnia?

A

Low arterial PCO2 as a result of hyperventilation.

FBS 2 (3 decks)

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