5. Gas transport Flashcards
Nomenclature:
Prefix
P: Partial pressure (kPa or mmHg) F: Fraction (% or decimal) S: Hb Saturation (%) C: Content (mL) Hb: Volume bound to Hb (mL)
Nomenclature:
Middle (subscribe)
I: Inspired E: Expired A: Alveolar a: Arterial V: Mixed venous P: Peripheral D: Dissolved
Nomenclature:
Suffix
O2: Oxygen
CO2: Carbon dioxide
N2: Nitrogen
H2O: Water vapour
What is the purpose of breathing?
To maintain O2 delivery to cells undertaking aerobic respiration.
C6H12O6 + 6O2 -> 6CO2 + 6H2O + energy
State Dalton’s Law.
Pressure (P) of a gas mixture is equal to the sum of the partial pressures of gases in that mixture
State Fick’s Law.
Rate of diffusion is directly proportional to diffusion capacity (D) , concentration gradient (P1-P2) and exchange surface area (A)
and inversely proportional to the thickness (T) of the exchange surface.
State Henry’s Law.
At a constant temperature, the amount of a given gas that dissolves in a given type and volume of liquid is directly proportional to the solubility (α) of the gas and the partial pressure (P) of the gas in equilibrium with that liquid.
State Boyle’s Law.
At a constant temperature, the volume (V) of a gas is inversely proportional to the pressure (P) of that gas.
State Charles’ Law.
At constant pressure, the volume (V) of a gas is proportional to temperature (T) of that gas.
Atmospheric gas mixture
N2: 78.2% O2: 20.9% Ar: 0.9% CO2: 0.04% \+ 0.01% inert gases
Barometric pressure at sea level
101.3 kPa
760 mmHg
Therapeutic oxygen
Administration of high O2 mix via nasal cannula or full face mask
May increase fraction of inspired O2 >60%, increasing amount of O2 dissolving in blood
High altitude
Barometric pressure decreases
Gas fractions in inspired air are unchanged, but taking fractions of a lower overall pressure
Equation for partial pressure
Gas fraction (%) X Pressure
Noxious gases
Potentially problematic
Amount of O2 may be critically low
May contain chemicals that interrupt normal physiology e.g. CO
Respiratory conditioning
As air travels through the upper airway it is ‘modified’ to optimise gas exchange and to protect lung tissue.
Mostly occurs in structures with a high blood flow caudally to the trachea (75%) or in the trachea only (25%)
Describe how air changes in respiratory conditioning
The air is warmed to a physiological temperature
The air is humidified to a PH2O of 6.3 kPa (100% saturation)
The air is slowed
The air is mixed with air already in the lungs
At the onset of tidal inspiration lungs aren’t empty and still actively undertaking gas exchange
As fresh air enters it mixes with FRC
This reduces the O2 content and increases CO2 content reaching alveoli
How much oxygen can be dissolved in our bodies?
16 mL/min
What is the normal oxygen consumption at rest?
250 mL/min
Describe the structure of haemoglobin
Tetrameric molecule
4 Hb monomers, each consisting of Haem + Globin
Haem-
Fe2+ at centre of a tetrapyrrole porphyrin ring
Able to reversibly bind with O2
Once bound, undergoes conformational shape making other monomers more receptive to binding O2
-globin
Protein chain. 4 common chains: Alpha Beta Delta Gamma
How are monomers found in haemoglobin?
In 2 pairs
What are the 3 common variants of Hb?
HbA = 2 Hb alpha + 2 Hb beta. 98% HbA2 = 2 Hb alpha + 2 Hb delta. 2% HbF = 2 Hb alpha + 2 Hb gamma. Foetal, trace amounts
Explain why haemoglobin is considered an ‘allosteric’ molecule.
When O2 binds, there is a conformational change which changes the structure and affinity of haemoglobin for O2 meaning that O2 is more likely to bind.
Fully deoxygenated Hb
Tense state
Makes 1st O2 binding difficult
O2 bound Hb
Relaxed state
Increased affinity for O2
What is the phenomenon of increasing affinity of Hb for O2 called?
Cooperative binding
How else does the O2 binding effect Hb?
There becomes a binding site between the 2 beta chains for 2,3-diphosphoglycerate (2,3-DPG)
This is a cofactor in red cell energy production
Pushes the 2 beta subunits into the tense state; promotes O2 unloading
Methaemoglobin
If in high volume skin turns blue
0.5-1% of haemoglobin at any time
Fe3+ instead of Fe2+ so doesn’t bind O2
Oxygen dissociation curve
Across lungs Hb remains almost fully saturated
At respiring tissue, there is a steep relationship between PO2 and Hb saturation (=efficient unloading)
Changes in PO2 in lungs and tissues
Lungs: LARGE change in PO2 = SMALL change in HbO2
Tissues: SMALL change in PO2 = LARGE change in HbO2
What factors displace the ODC leftward? (Increased affinity/ loading)
Decreased temperatures
Alkalosis (Increased pH)
Hypocapnia (Decreased PCO2)
Decreased 2,3-DPG
What factors displace the ODC rightward?
Increased temperature
Acidosis (Decreased pH)
Hypercapnia (Increased PCO2)
Increased 2,3-DPG
What is the P50?
The PO2 on the ODC that corresponds to 50% binding
Found on one of the steepest parts of the ODC
Adult Hb ~ 3.3 kPa
What conditions can stretches the ODC upwards?
Polycythaemia
a condition where concentration of RBCs in the blood is much higher than normal, usually when the Hct/PCV is >55%.
For a given PO2 there is no change in HbO2 saturation but a marked increase in blood O2 content
What conditions can push the ODC downwards?
Anaemia
as there is a lower concentration of haemoglobin, markedly reducing the overall O2-carrying capacity of the blood.
A severely anaemic patient may still have a normal pulse oximetry because they can still fully saturate their Hb, the problem is that it isn’t enough.
P50 does not change in these circumstances.
How does carbon monoxide shift the ODC and why?
CO shifts the curve downwards and leftwards.
Downwards because it binds irreversibly to haemoglobin meaning that there is less haemoglobin available to bind to O2. (less capacity to bind O2)
Leftwards because if only a few of the 4 O2 binding sites on the haemoglobin are occupied by O2, then the O2 that is bound to haemoglobin will be less likely to dissociate, thus increasing the affinity of the haemoglobin for bound O2.
Haemoglobin affinity for CO
250X greater than that for O2
Describe the shape of the ODC of myoglobin (Mb) and why this shape is needed to function.
Curve shifted to extreme left
Myoglobin is a storage molecule for O2 in the muscles
Much much greater affinity for O2 than adult HbA to extract O2 from circulating blood and store it
Describe the shape of the ODC of foetal haemoglobin and why this shape is needed to function.
Curve shifted left
Has a greater affinity for O2 than adult Hb.
Because HbF must be able to preferably bind O2 that is already bound to the maternal Hb within the placenta.
Structure of myoglobin
Monomeric molecule
i.e. 1 haem group, 1 protein chain and 1 molecule of bound O2
What gives meat its fresh red/pink colour?
High concentration of Mb in muscle
Methaemoglobinaemia ([MetHb] > 1%) is a type of functional anaemia
Hct and PCV in such patients normal, but O2-carrying capacity impaired.
Recessive condition, blue tinge to skin
What is the PO2 of blood arriving at the respiratory exchange surface?
5.3 kPa
Why does the Hb saturation of the blood decrease from 100% at the respiratory exchange surface to 97% in the systemic circulation?
Because of the bronchial circulation joining the pulmonary circulation before returning to the left side of the heart.
Also dilutes PaO2 from 13.5 kPa to 12.7 kPa
What promotes diffusion of O2 from plasma into endothelial cells?
Diffusion gradient
Tissue PO2 is lower than PaO2
What are the changes in concentration of oxygen and saturation that take place at the tissues?
20.3 -> 15.1 mL/dL
97% -> 75%
Define oxygen flux and state the usual oxygen flux at rest.
The overall amount of O2 being deposited in the tissues.
Oxygen flux = 20.3 - 15.1 = ~ 5 mL/dL
Normal cardiac output = 5 L/min (50 dL) so oxygen flux is: 250 mL/min
What is the major difference between O2 and CO2 transport?
CO2 is much more soluble and diffuses into plasma very quickly
Describe the reaction of carbon dioxide with water.
Carbon dioxide reacts with water to produce carbonic acid (H2CO3)
Dissociates into H+ and HCO3-
Slow, but can cause pH to fall below 7.4
Why does the conversion of CO2 to H2CO3 take place faster in RBCs?
RBCs have carbonic anhydrase, which catalyses this reaction and allows it to occur at a rate 5000x greater than in the cytoplasm.
What happens when plasma PCO2 begins to rise?
CO2 diffuses into RBCs
Which transporter moves the bicarbonate produced in the red blood cell into the plasma?
AE1 transporter
The AE1 transporter allows the influx of which ion? Why? What is the term given the this movement of ions?
Chloride ions move in
To maintain membrane electroneutrality
This is called chloride shift
What is the influx of chloride into RBCs associated with? Why is this useful?
An influx of water
Keeps cell hydrated, as water is being pumped out of the cell in the form of bicarbonate
How does carbon dioxide binds to proteins and what does it form?
To prevent a decrease in intracellular pH, excess H+ can be buffered by the globin chains of haemoglobin - certain residues within the globin chain are active H+ acceptors. Some intraerythrocytic CO2 binds to haemoglobin - not at the O2 binding site but at the AMINE group and N-terminal of the globing chains forming an NHCOOH end. Forms CARBAMINOHAEMOGLOBIN (HbCO2)
What is the net CO2 flux?
52-48 mL/dL (+4 mL/dL)
Total of 200 mL of CO2 produced per minute
Why are total oxygen consumption and total carbon dioxide production not equal?
Because some of the water is lost in metabolic water production.
What is pulmonary transit time?
Time that blood is in contact with the respiratory exchange surface
~0.75 s
What is the Haldane effect?
Amount of CO2 that binds to the amine end of proteins forming carbaminohaemoglobin depends on the amount of O2 that is bound to the haemoglobin - an allosteric effect.
Increasing O2 binding means less carbaminohaemoglobin.
CO2 unloading
CO2 in solution will diffuse into the alveoli 1st
This triggers the reversal of all of the other binding mechanisms.
Bicarbonate will re-enter erythrocytes and re-associate with H+ to form H2CO3, which the carbonic anhydrase enzyme will convert back into CO2 and H2O.
What is the ventilation perfusion mismatching of the lungs?
Ventilation and perfusion is greater at the inferior parts of the lungs.
V/Q at the base tends towards 0
V/Q at the apex tends towards infinity.
What causes low perfusion at the apex of the lung?
Lower intravascular pressure (gravity)
Less recruitment
Greater resistance
Lower flow rate
What causes low ventilation at the apex of the lung?
PPL is more negative
Greater transmural pressure gradient
Alveoli larger and less compliant
What causes high perfusion at the base of the lung?
Higher intravascular pressure (gravity)
More recruitment
Less resistance
Higher flow rate
What causes high ventilation at the base of the lung?
PPL is less negative
Smaller transmural pressure gradient
Alveoli smaller and more compliant
What are the consequences of V/Q at the base and apex?
Base: Wasted perfusion
Apex: Wasted ventilation
