3 Flashcards
(46 cards)
1
Q
Describe partial pressure of a gas in a gas mixture (Dalton’s law)
A
-total pressure of gases= sum of the partial pressures of individual gases Ex. 1/3 O2 and 2/3 N2 Total pressure= ppO2 + ppN2 6atm= 1/3(6) + 2/3(6) 6= 2 +4
2
Q
What is atmospheric pressure?
A
- some of the partial pressures of the gases in the atmospher
- pressure exerted by the wt. of the air above earth in the atmospher
- at sea level: 101.1 kPa = 1atm
- at high altitudes atmospheric pressure is lower (weight of air pressing down is less)
3
Q
What is ambient air composed of?
A
79% N2
21% O2
0.04% CO2
4
Q
How would we calculation the partial pressure of O2?
A
PO2= FiO2 (fraction of O2 in air) x Patm (atmospheric pressure)
PO2= 0.21 x 101 kPa
PO2=21 kPA
5
Q
How does inspired air affect partial pressure?
A
-it is warmed and humidified in the upper respiratory tract
-we add water to the air we breathe in, in the form of water vapour
-so need to subtract water vapour pressure from atm pressure
-SVP= 6.28 kPa
101kPa - 6.28 kPa = 94.28 kPa (new atmospheric pressure)
6
Q
What is the anatomical dead space and how much of the tidal volume fills it?
A
- air conducting space of respiratory tract
- conducting airways extend from the nostrils/nose, nasopharynx, trachea, bronchi to the distal end of terminal bronchioles
- no gas exchange occurs here
- 30% of tidal volume (usually 450ml)
- so 150ml in anatomical dead space
7
Q
How does alveolar ventilation work?
A
- 300ml of new air is added into alveolar ventilation
- typical total air volume is 3L
- fresh air is diluted by old air in lung
- old air constantly has O2 being extracted by blood exchange and CO2 being added
8
Q
List the partial pressures of the conducting airways, alveolar, arteries and veins
A
Conducting airways partial pressure: O2= 20 kPa CO2= 0.04 kPa
Alveoli: O2= 13.3 CO2= 5.3 kPa
Arteries: O2= 13.3 kPa CO2= 5.3 kPa
Veins O2= 5.3 kPa CO2= 6.1 kPa
9
Q
In what direction does gas diffuse in the body?
A
- down the partial pressure gradient
- area of high partial pressure to low partial pressure
- ex: movement of O2 from alveolar air to blood
- ex: movement of CO2 from blood to alveoli
10
Q
Explain partial pressures of gases in a liquid
A
- when inspired gases come in contact with body fluids (made of mostly water), gas molecules will enter fluid and dissolve in the liquid
- at the same time, some gas molecules will leave the liquid and return to gas phase
- at equilibrium the partial pressure of the gas in the liquid is equal to the partial pressure of that gas in the gas phase in contact with the liquid
- equilibrium reached when rate of gas entering water=rate of gas leaving water
- this is first noticed at the alveoli and surfactant lining
- also noticed at the alveolus-capillary border
- therefore PO2 in alveoli=PO2 in blood (13.3kPa)
11
Q
Why is the partial pressure of O2 in alveoli lower than inhaled air and the partial pressure of CO2 in alveoli higher than in inhaled air?
A
- PO2 mixes with residual volume
- effect of O2 diffusing OUT across the alveolar wall
- effect of CO2 entering INTO the alveoli
12
Q
How is partial pressure different from the amount of a dissolved gas?
A
- Henry’s law: amount of a gas that dissolves in a liquid is proportional to the pp of that gas in gas phase AND its solubility coefficient
- amount of gas (mmol/L)= pp x solubility coefficient of gas
13
Q
Define solubility coefficient and what is it for O2?
A
- amount (mmol) of gas that will dissolve in a litre of plasma at body temperature when exposed to a given pp
- solubility coefficient for O2 is 0.01mmol/L
14
Q
What happens if the gas which dissolves combines chemically with the liquid? (Ex. Blood)
A
- content of gas = amount of gas chemically bound + amount of gas in free solution
- O2 binding to Hb does NOT contribute to ppO2 in blood
- chemical reaction must complete before equilibrium is reached and pp is established
- allows blood to carry much ore oxygen (70-fold)
- binding to HB continues until Hb is fully saturated
- after full saturation, O2 continues to dissolve until equilibrium is reached
- at equilibrium pO2 of plasma = pO2 of alveolar air
- 98-99% of O2 bound to Hb and 1-2% O2 dissolved in blood
15
Q
Is O2 or CO2 more easily dissolved?
A
CO2 is more easily dissolved
-diffuses 20 times faster than O2
Molecular weight: CO2 > O2
-larger difference in pp compensates for slower diffusion of O2
-in a diseased lung with lower O2 levels, O2 gas exchange is more impaired than CO2 because slower diffusion rate
-but in hypoventilation higher levels of CO2 in blood since air needs to be delivered to lungs for gas exchange to occur
16
Q
What determine alveolar pO2?
A
- the rate at which O2 is taken up by the blood and the rate at which it is replenished by alveolar ventilation
- balance between perfusion and ventilation
- keeps the pp of O2 in alveolar gas normal
- hypo or hyperventilation will change the alveola pO2
17
Q
What determines alveolar PCO2?
A
- rate at which the CO2 enters the alveoli from blood and the rate at which it is removed from alveolar gas by ventilation
- pp of CO2 in alveolar gas is stable
- hypo or hyperventilation will change the alveolar PCO2
18
Q
What is mixed venous blood and how is it significant?
A
- mixed from SVC, IVC and coronary veins
- contains CO2 and O2
- PO2= 6 kPa PCO2= 6.1 kPa
- varies with metabolism-ratio of carbohydrates to fats eaten (respiratory quotient)
- alveolar PO2 > PO2 in mixed venous blood
- alveolar PCO2 < PCO2 in mixed venous blood
- so O2 will diffuse into blood and CO2 out
19
Q
What factors affect rate of diffusion?
A
- area available for exchange
- resistance to diffusion
- gradient of pp
- thickness (distance molecules must diffuse)
- rate of diffusion is inversely proportionate to thickness and directly proportionate to SA
- rate= [A x D x (p1-p2)]/T
- properties of the individual gas (solubility, molecular weight)
20
Q
What are the diffusion barriers that alveolar air must cross to get to RBC?
A
- fluid film lining alveolus
- epithelial cell of alveolus
- interstitial space
- endothelial cell of capillary
- plasma
- red cell membrane
- 5 cell membranes
- 3 layers of cytoplasm
- 2 layers of tissue fluid and plasma
21
Q
What factors affect rate of gas diffusion in disease?
A
thickness of the membrane/space
-increase as a result of oedema fluid in the interstitial space and in alveoli
-lung fibrosis: increased thickness of alveolar and capillary membranes and interstitium
Surface area of the membrane
-decreased by removal of an entire lung
-emphysema: decreased surface area
Diffusion coefficient of the gas
-CO2 always diffuse much faster than O2
-so, diffusion of O2 is affected when pO2 is low
-Diffusion of CO2 is not affected since PCO2 is normal until LATE STAGE disease
22
Q
What diseases cause diffusion defects?
A
- interstitial lung disease: characterized by excessive deposition of collagen in the interstitial space, with thickening of alveolar walls and lengthening of the diffusion pathway; slows gas exchange
- pulmonary oedema: fluid in the interstitum and alveolus increases the length of the diffusion pathway; talkers longer for rate of diffusion
- emphysema: destruction of alveolar walls result in large air spaces, reduces total surface area available for gas exchange
23
Q
How is diffusion resistance measured by the CO transfer factor
A
- calculated by measuring CO uptake following a single maximal breath of a gas mixture containing air
- inhaled CO used b/c of high affinity for Hb
- all CO entering blood binds to Hb so hardly any in plasma
- therefore concentration gradient of PaCO across the alveolar capillar membrane is maintained and stays the same for the entire time blood remains in contact with alveolar gas
- amount of CO transferred from alveoli to the blood is an estimate of the diffusion resistance of the barrier
24
Q
How does atmospheric pressure differ when high in air and when underwater?
A
- pressure lower when higher in air
- pressure much higher when underwater
25
How much O2 is dissolved at pO2 of 13.3kPa?
0.13mmol/L
26
How much dissolved gas do we need at rest per minute?
12mmol per minute
27
What are the requirements for oxygen binding?
- reaction needs to be reversible
- O2 must dissociate at the tissues to supply them
- many substances will bind to oxygen but only some are useful
- respiratory pigments contain haem group
- O2 combines reversibly
28
What are the two oxygen binding pigments?
Hb: present in blood
-tetramer: binds 4 O2 molecules
Myoglobin: present in muscle cells
-monomer: binds 1 oxygen molecule
29
What is cardiac output at rest and the maximum cardiac output?
Rest: 5L/min
Max: 25L/min
30
Describe myoglobin as a respiratory pigment
- found in muscles
- contains haem
- one single subunit
- myoglobin dissociation curve (right angle type)
- saturates easily because the amount of pigment is limited
- binding saturates above a given pO2
- amount of O2 bound depends on amount of pigment when represented this way (more pigment = more binding)
- can overcome this issue by expressing saturation as a percentage (independent of pigment concentration); saturation curve
LOOK AT DIAGRAM
- can see how much O2 will be bound or given up when moving from one partial pressure to another
- work out difference in percentage saturation’s between the two pO2 values
- take amount bound at full saturation and use percentage to calculate how much given up
31
Describe haemoglobin as a respiratory pigment
- is a tetramer (consists of 2 alpha and 2 beta subunits)
- each subunits contains one haem and one globin in order to bind 4 O2 molecules
- T state (tense): difficult for O2 to bind
- R state (relaxed): easier for O2 to bind
- when pO2 is low Hb is tense
- hard for first O2 molecule to bind, but as each O2 binds the molecule becomes more relaxed and binding of next O2 molecule is easier
- dissociation curve is more sigmoidal
- initially binding is shallow, but curve steepens as pO2 rises and flattens as saturation is reached
- Hb is saturated above 9-10 kPa
- virtually unsaturated below 1kPa
- half saturated at 3.5-4 kPa
- saturation changes greatly over a narrow range
- reaction is highly reversible and depends on pO2 levels
32
Describe Hb in arterial blood leaving the lungs
- since alveolar pO2 is 13.3 kPa the Hb is well saturated
- can calculate oxygen content of arterial blood
- if Hb concentration is normal then is is 2.2mmol/L(4)= 8.8mmol/L
33
What happens to saturation and content if pt. Is anaemic?
- pO2 will be normal so saturation will be normal
| - but O2 content will be less
34
Describe Hb in tissues
- tissue pO2 depends on how metabolically active the tissue is (typically 5kPa)
- Hb saturation drops to about 65%
- 35% given up
- can calculate amount of O2 given up
- 8.8 mmol/L x 0.35 = 3mmol/L
35
Describe Hb in venous blood
- mixed venous blood
- over half the oxygen is still bound
- the lower the tissue pO2, the more O2 will dissociate from Hb (lower saturation of venous blood)
- if tissues are metabolically active, pO2 will be lowered in order to gain more O2
36
How low can tissue pO2 get?
- tissue pO2 must be high enough to drive diffusion of O2 to cells
- cannot fall below 3kPa in most tissues
- higher the capillary density, lower the pO2 can fall (doesn’t have so far to diffuse)
- very metabolically active tissues will have higher capillary density (ex. Heart muscle)
37
Explain the Bohr shift
- effect of pH on the affinity of Hb
- acid condition shift dissociation curve to right (higher pO2 values)
- decrease in pH promotes T state of Hb and shifts curve right (Hb has lower affinity for O2)
- increase in pH promotes R state
38
How is the Bohr shift in tissues?
- ph is lower in most metabolically active tissues
- so extra O2 is given up
- increased temperature also shifts dissociation curve to the right
- metabolically active tissues have slightly higher temp. So extra O2 is given up
39
Describe maximum unloading of oxygen
- maximal unloading occurs in tissues where pO2 can fall to low level
- also in conditions where increased metabolic activity result in more acidic environment and higher temperature
- under these conditions about 70% bound O2 can be given up
40
How is O2 given up in mixed venous blood?
- over the whole body about 27% of O2 from arterial blood is given up
- this can increase in exercise
- there is an oxygen reserve (used during extreme exercise)
- in extreme exercise you can increase metabolism by 10x but CO only goes up by 5x
- improved extraction of O2 by the tissues
41
What is the significance of 2,3 BPG?
- RBC normally contains 5mM of 2,3 BPG
- 2,3 BPG levels increase with anaemia or at altitude
- increased 2,3 BPG shifts Hb dissociation curve for O2 to right
- allows more O2 to be given up to tissues b/c of shift in curve
- 2,3 BPG levels drop in stored blood due to refrigeration
- limits how much O2 can be given up at tissues but not a problem clinically
42
How does CO poisoning affect oxygen intake
- CO reacts with Hb to form COHb
- increased affinity for O2 subunits
- thus wont give up O2 at the tissues
- fatal if HbCO is >50%
43
What is the difference between hypoxaemia and hypoxia?
Hypoxemia: low pO2 in arterial blood
Hypoxia: low O2 levels in body or tissues
-if pO2 levels are low, not all the Hb will be saturated
-if Hb levels are low, not enough O2 will be present in the blood
-conditions such as shock can reduce blood flow
-peripheral vasoconstriction can cause peripheral hypoxia b/c not enough O2 getting to tissues
-tissues using O2 faster than it is delivered such as in peripheral arterial disease and Raynaud’s disease (vasoconstriction to peripheries)
44
What is cyanosis?
- bluish colouration due to unsaturated Hb
- deoxygenated Hb is less red than oxygenated Hb
- can be peripheral due to poor local circulation
- or central due to poorly saturated blood in systemic circulation (such as from cardiac defect or lung problem)
- can be difficult to detect due to poor lighting and skin colouration
45
What is pulse oximetry?
- detects level of Hb saturation
- detects difference in absorption of light between oxygenated and deoxygenated Hb
- only detects pulsation arterial blood
- ignores levels in tissues and non-pulsation venous blood
- DOES NOT say how much Rb is present
- device has a light source, light detector and microchip, clips onto finger and is painless
46
What is arterial blood gas and electrolyte analysis?
- requires an arterial blood sample (as opposed to venous blood sample obtained by venepuncture)
- obtained by arterial puncture usually from the radial artery
- invasive technique
