Resp: Oxygen Exchange Flashcards
What are the features of oxygen binding?
- Reaction has to be reversible
- Oxygen must dissociate at the tissue to supply them
- Oxygen combines reversibly
What are 2 examples of oxygen binding pigments?
- Haemoglobin: Tetramer to bind 4 oxygen molecules
- Myoglobin: binds 1 oxygen molecule
What is myoglobin?
- Pigment found in muscles
- Contains 1 subunit of haem
Why is myoglobin not a good carrier of oxygen?
- It will not give up oxygen at the tissues due having a high affinity for oxygen even at low partial pressure.
- It acts as a storage molecule that will give up oxygen if the oxygen in the tissue gets very low. Also acts as a pigment for the muscle giving it the red appearance
What is the structure of haemoglobin?
- Tetramer consisting of 2 alpha and 2 beta subunits
- Each subunit has a haem group and a globin group
- 4 oxygen molecules bind to each molecule of haemoglobin
What are the forms of haemoglobin?
- Low affinity T state
- High affinity R state
What happens to haemoglobin when the pO2 is low?
The haemoglobin shift to the low affinity T state so it is harder for the first O2 molecule to bind
What happens as each O2 molecules bind to the haemoglobin?
The molecule becomes more relaxed and the binding of the next O2 molecule is easier
What is the shape of the haemoglobin dissociation curve?
Sigmoidal curve
What are the features of the haemoglobin dissociation curve?
- Saturation changes greatly over a narrow range
- Reaction is highly reversible and depends on pO2 levels
What happens to the oxygen content and pO2 if the patient is anaemic?
- pO2 is normal
- Oxygen content is much lower
What is the oxygen content of blood when the haemoglobin is saturated (13.3 kPa)?
How would you work out the amount of oxygen given up?
-2.2 mmol/l if the Hb concentration is normal
Each haemoglobin bind 4 oxygen molecules
2.2X4 = 8.8 mmol/l
Amount of oxygen given up = ((100 - Saturation at Partial pressure in the site)/100) X 8.8
What happens to the dissociating at tissue with a lower pO2 and the result of this on venous blood?
- Increased dissociation
- Lower saturation of venous blood
What is the saturation of haemoglobin in venous blood?
-Over half the oxygen is still bound
What is the adaptation of very metabolically active tissue to allow them to receive sfficnet oxygen?
Very high capillary density so that pO2 will fall lower due to decreased diffusion distance
What is the Bohr effect?
- pH affects the affinity of haemoglobin
- Acid condition shifts dissociation curve to the tight
- Lower pH promotes gift to the T state and higher ph promotes a shift to the R state
Why is the Bohr effect beneficial to the metabolically active tissues?
-pH is lower in most metabolically active tissue so extra O2 is given up
What is maximum unloading of oxygen?
- Occurs in tissues where pO2 can fall to a low level
- In conditions where increased metabolic activity results in more acidic environment and higher temperature
- Under these conditions about 70% bound oxygen can be given up
In extreme exercise , metabolism can increase 10x but the cardiac output only goes up by 5x. What supplies the tissues with oxygen?
Tissues have improved extraction of oxygen.
This is due to a number of factors
What is the function of 2,3 BPG?
Increased 2,3-BPG shift the Hb dissociation curve for O2 to the right
This allows more O2 to be given up to tissues because of a shift in the curve
What is the result of carbon monoxide poisoning?
- Reacts with Hb to form COHb
- Increased affinity for unaffected subunits for O2
- Therefore O2 is not given up at tissues
- This is fatal if the HbCO is greater than 50%
What is hypoxaemia?
Low pO2 in arterial blood
What is hypoxia?
Low oxygen levels in the body and tissues
What is cyanosis?
- Bluish coloration due to unsaturated haemoglobin
- Can be peripheral due to poor local circulation or systemic due poorly saturated blood in systemic circulation
Why is cyanosis difficult to detect sometimes?
- Poor lighting
- Skin colouration
What is pulse oximetry?
- Detection of level of Hb saturation by detection of difference in absorption of light between oxygenated and deoxygenated Hb
- It only detect the pulsatile arterial blood
- Venous blood and blood in tissues is ignored
What is the limitation of pulse oximetry?
- It doesn’t say how much haemoglobin is present
- It will not detect anaemia but just how well saturated a person blood is
Why does cyanosis appear blue?
-Deoxygentated haemoglobin is less red that oxygenated haemoglobin
What are the features of carbon dioxide when compared with oxygen
It is more soluble
It reacts chemically with water
React with haemoglobin as well at a different site
2.5 times as much in arterial blood
CO2 control is more important for pH than for transporting it from the tissues to the lungs. True/False
True
What is the pH range that arterial blood must be kept in?
7.35-7.45
How does CO2 interact with arterial blood?
- Reacts with water in plasma to form carbonic action
- Reacts with water in red blood cells.
- It is not there as a waste product
What happens to carbonic acid in the blood?
Dissociates quickly to hydrogen ions and hydrogen carbonate ions.
The reaction that form carbonic acid from CO2 and water is irreversible. True/False
False. It reversible and rate of reaction depends on amount of reactants and products
What does the pH of plasma depend on?
-Depends on how much CO2 reacts to form H+ (dissolved CO2 pushes the reaction to the right and HCO3- pushes the reaction to the left)
This depends on dissolved CO2 and concentration of hydrogen carbonate
What determines how much CO2 dissolved in the plasma?
-Partial pressure of CO2
What happens to plasma pH when pCO2 rises?
Becomes more acidic
What happens to the plasma pH when the pCO2 falls?
It will become more alkaline
What is the determining factors for dissolved CO2?
pCO2 of alveoli which is controlled by rate of breathing
What does high HCO3- prevent from happening in the blood?
Prevent nearly all dissolved CO2 from reacting by shifting the equilibrium
What determines the pH of arterial blood?
Ratio of HCO3- and pCO2.
What is the Henderson-Hasselbalch equation?
pH=pK+log([HCO3-])/(pCO2 X 0.23))
What enzyme speeds up the reaction that speeds up hydrogen carbonate production in red blood cells?
Carbonic anhydrase
How do the red blood cells produce hydrogen carbonate?
- H+ ions bind to the negatively charged Hb inside the red blood cells
- Chloride-bicarbonate exchanger transports HCO3- out of red blood cells which is left front he reaction between CO2 and H2O.
Eythrocytes control concentration of HCO3- in plasma. True/False
False. They merely produce HCO3-.
What determine the amount of HCO3- that is produced by the erythrocytes?
Binding of H+ to haemoglobin
What is the main determant of plasma hydrogen carbonate? pCO2 or erythrocytes ?
Most of the HCO3- comes from the red blood cells
What is the role of the kidney in controlling HCO3-?
The kidney controls the amount of HCO3- by varying the excretion of bicarbonate. Concentration present in blood is controlled by the kidneys
How does hydrogen carbonate buffer extra acid?
Acids react with HCO3- to produce CO2. Therefore the bicarbonate decreases.
CO2 produced is removed by breathing and pH changes are minimised
What determine arterial pCO2?
Alveolar pCO2 which determine how much CO2 is dissolved. This therefore affects pH.
pCO2 is higher in venous blood than arterial blood. True/false
True. It is returning from metabolically active tissue so more CO2 is dissolved
What does the buffering of H+ by the haemoglobin depend on?
Level of oxygenation
What happens to the amount of H+ ions that can bind to Hb as more O2 binds to Hb?
The haemoglobin switches into the R state
-Less H+ ions bind as a result
What happens to the amount of H+ ions that can bind to Hb as less O2 binds to Hb?
The haemoglobin switches to the T state
-More H+ ions bind
How does the amount of CO2 increase in plasma in the venous system?
How does this affect the pH?
- Less O2 bound to Hb so haemoglobin switches to the T state
- More H+ ions bind to Hb
- More HCO3- can be produced and is exported to the plasma
- Therefore more CO2 is present in plasma in venous system.
-More HCO3- is also present so therefore ratio is similar. Small change in plasma pH as both CO2 and HCO3- have increased
What happens when venous blood arrives at the lungs?
- Hb picks up O2 and goes into R-state
- Causes Hb to give up the extra H+ it took on at the tissues
- H+ reacts with HCO3- to form CO2
- CO2 is breathed out
How are carbamino compounds formed?
- CO2 binds directly to amine groups on the globulin on Hb.
- This contribute to the CO2 transport but it is not part of the acid base balance
- This is CO2 given up at the lungs
Why are more carbamino compounds formed at the tissues?
-The pCO2 is higher and unloading of oxygen facilitates binding of CO2 to haemoglobin
What are the forms that CO2 is transported in?
- Dissolved CO2
- Hydrogen carbonate
- Carbamino compounds
