Session 3 Flashcards
Why is this dangerous
Gas embolism
Q. SVP depends only on temperature. At body temperature of 37ºC the SVP of water is ? kPa (47 mmHg) and is independent of total pressure.
- Despite the addition of water vapour, the total pressure of the humidified air in the airways remains 101kPa, because this gas is continuous with the outside air, and pressure will equilibrate to atmospheric pressure. In the airways, water vapour contributes 6.28 kPa to the pressure, while the other gases together account for = ?
Since the other gases remain in the same proportions as in dry air (i.e. O2 20.9%, N2 78%), the pO2 of the humidified air = ?
- 6.28
- 94.28 kPa (101 -6.28 = 94.28 kPa
(100 – 6.28) x 20.9% = 19.8 kPa
- Gas dissolves in body fluids. Dissolved gas molecules also exert pressure in the liquid
Equilibrium is reached when: - The pressure it exerts is called?
- At equilibrium?
- rate of gas molecules entering water = rate of gas molecules leaving the water
(Think of alveolus)
- Partial pressure: pressure exerted by the dissolved gas in the liquid
- partial pressure of the dissolved gas in the liquid = the partial pressure of the gas it is exposed to in the gas phase
another term used for this pressure in the liquid is TENSION
Calculate the content of oxygen and carbon dioxide in plasma using their solubility coefficients, and partial pressure LO
- Partial pressure is different from the amount of a dissolved gas in plasma.
Amount of gas dissolved =
What is the solubility coefficient of O2?
- Using the info above and in this instance the po2 of blood is 13.3 kPa to work the amount of O2 dissolved
- What happens after oxygen has entered the capillary downs its pressure gradient to provide our tissues with oxygen? (5)
- Amount of gas dissolved = partial pressure x solubility coefficient of gas
O2 = 0.01 /mmol/L /kPa at 37°C
- 0.01 x13.3 = 0.13
Plasma has 0.13 mmol dissolved oxygen/per litre
- If gas REACTS (e.g. O2 binding to Hb) with a component of the liquid in addition to dissolving this reaction must complete before partial pressure is established.
- O2 passes down its partial pressure gradient and enters the plasma & dissolves in it
- dissolved O2 does not remain in the plasma but first binds to Hb in RBC
- Process continues until Hb fully saturated (each Hb molecule binds to 4 O2 molecules)
- After Hb is fully saturated, O2 continues to dissolve till equilibrium is reached
- At equilibrium, pO2 of plasma = pO2 of alveolar air
- If gas REACTS (e.g. O2 binding to Hb) with a component of the liquid in addition to dissolving this reaction must complete before partial pressure is established.
- Where is oxygen located in the blood ?
- The pO2 is a measure of?
- Which oxygen supplies tissues first?
- The oxygen that is dissolved in the plasma. The O2 bound to Hb is then replenishes the oxygen removed in the plasma.
- It is Hb bound and dissolved in plasma
- Dissolved O2 in the blood
Q. Explain the different partial pressures of O2 and CO2 observed in inspired air, alveolar air, mixed venous blood and arterial blood LO
A. Inspired air has an equal pressure to atmospheric so has the same gas ratios, alveolar air has
lower oxygen as it is diffusing into the capillary bed and because some oxygen has mixed in the liquid
in the alveoli
Inhaled air becomes saturated with water vapour in the upper respiratory tract. This water vapour
displaces some oxygen and nitrogen so that percentage of oxygen drops to about 20%, with a drop
in pO2 drops to about 19.8 kPa. In the alveoli oxygen is constantly diffusing into capillary blood
flowing past. Alveolar pO2 is determined by the rate of removal of O2 by the blood and the rate of
replenishment of O2 by alveolar ventilation.
The balance between perfusion and ventilation, keeps the partial pressure of oxygen in the alveolar
gas stable at its normal value of 13.3 kPa Alveolar pCO2 is similarly determined by the balance
between the rate at which CO2 enters the alveoli from blood and the rate at which it is removed
from alveolar gas by ventilation. The partial pressure of CO2 in the alveolar gas is stable at its
normal value of 5.3 kPa.
The mixed venous blood reaching the pulmonary capillaries has a pO2 of 6 kPa and a pCO2 of kPa.
Gas exchange taking place at the alveolar capillary membranes allows to blood to equilibrate to with
alveoli air; hence blood leaving the alveoli has a pO2 of 13.3 kPa and a pCO2 of 5.3kPa; which is the
same as that of alveolar air.
- State the normal PO2 and PCO2 in alveolar air, arterial blood and mixed venus blood LO
- Alveolar air:
pO2: < 13.2
pCO2: > 5.2
Arterial blood:
pO2: 13
pCO2: 5.3 kPa
Mixed venous blood different organs have different metabolic rates so different contents of oxygen would have been used):
pO2: 6
pCO2: 6
A. Inhaled air has mixed with residual volume &
O2 being taken up and CO2 being given up by pulmonary capillary blood
Alveolar air composition stays constant around this level
• The blood equilibrates with alveolar air
Also (look in the group work)
What is the composition of alveolar air LO
Describe the layers making up the diffusion barrier at the air-blood interphase LO
- diffusion through gas to alveolar wall
- 5 cell membranes
- 3 layers of cytoplasm
- 2 layers of tissue fluid
– epithelial cell of alveolus
– tissue fluid
– endothelial cell of capillary
– plasma
– red cell membrane
Describe the role of diffusion resistance in gas exchange. State and explain the difference in the diffusion rates of O2 and CO2. LO
Q. State and explain the difference in the diffusion rates of O2 and CO2.
A. CO2 is 21 times faster than O2
Because…
The resistance is not the same for the two gases. For most of the barrier (the cells, membranes and
fluid) the rate of diffusion is affected by the solubility of the gas in water, and carbon dioxide diffuses
much faster, because it is significantly more soluble that oxygen.
Overall, carbon dioxide diffuses 21 times as fast as oxygen for a given gradient. This means that
anything affecting diffusion will only change oxygen transport, as that is limiting.
