Flashcards in Resp Session 4 Deck (60):
What are the two respiratory pigments used to carry oxygen and what in their structure allows them to do this?
Myoglobin and haemoglobin which both contain haem groups that can combine reversibly with O2
What is myoglobin?
Monomer present in myocytes which gives them their red colour
What is haemoglobin?
Tetramer (2 alpha and 2 beta subunits) present in blood which gives it its red colour
How do the two different states of haemoglobin vary?
T state has low affinity for oxygen
R state has high affinity for oxygen
Does the dissociation curve for either myoglobin or haemoglobin tell you how much of oxygen is present?
No, they do not show how much of either pigment is present
Why is myoglobin good for oxygen storage in cells of active tissues?
It is fully saturated at 5 kPa and half saturated at 1 kPa so is fully saturated at arterial pO2 and doesn't released oxygen until pO2 drops significantly
What causes the sigmoidal shape of the haemoglobin binding curve?
Describe Hb in the arterial blood leaving the lungs.
Alveolar pO3 ~13.3 kPa so Hb is well saturated and carries four O2, therefore at Hb=2.2 mmol per litre, oxygen content is 8.8 mmol per litre
How is pO2 and oxygen content affected by anaemia?
If anaemic pts have normally functioning lungs, pO2 is normal but oxygen content is low
What usually happens to Hb saturation in the tissues?
Decreases to 65% but this depends on the pO2 of the tissue determined by its metabolic activity
What saturation does mixed venous blood have and when is this altered?
>50% but will decrease if tissue pO2 is lowered
What determines how low tissue pO2 can get?
Needs to be high enough to drive oxygen diffusion to cells so in most tissues cannot fall below 3 kPa
Increased capillary density allows tissue pO2 to fall lower
What is the capillary density of cardiac muscle?
Each myocyte has its own capillary
What effect does increasing pH have on haemoglobin?
Causes it to adopt R state
Why does decreasing pH cause Hb to adopt its T state?
H+ ions bind voraciously causing it to change shape and promote oxygen dissociation
Why is Bohr shift favourable?
pH is lowest in most metabolically active tissues due to carbonic, lactic and sulphuric acid production so ensures oxygen delivery to tissues with highest demand
What is the effect of increasing temperature on Hb?
Shifts dissociation curve to R as it promotes oxygen dissociation
What is the benefit of the action of increasing temperature on oxygen dissociation from Hb?
Metabolically active tissues have a slightly higher temperature so extra oxygen can be released to meet higher demands
What percentage increases in oxygen dissociation is seen with the Bohr shift?
What is maximum unloading?
Where ~70% of oxygen can be given up in tissues with extremes metabolic rates that can tolerate low pO2 and have an environment which is more acidic and higher temperature
Why is there an oxygen reserve in mixed venous blood?
So cardiac output doesn't have to change as much if the demand for oxygen dissociation from the mixed blood rises above the ~27% usually dissociated
When do levels of 2,3-BPG increase in the RBCs?
What is the action of 2,3-BPG on Hb?
Changes how it behaves so dissociation curve becomes more of a rectangular hyperbola
How is 2,3-BPG synthesised?
Shunt in the glycolytic pathway
How does carbon moodier exposure lead to decreased oxygen transport?
CO+Hb--> carboxyhaemoglobin and this increases the affinity of unaffected subunits for oxygen therefore it is not released at the tissues
When does carbon monoxide poisoning become fatal?
If HbCO >50%
What is cyanosis?
Bluish colouration due to unsaturated Hb close to the skin
What causes peripheral cyanosis?
Poor local circulation
What causes central cyanosis?
Poorly saturated blood in systemic circulation
How does pulse oximetry measure blood O2?
LED emits a range of wavelengths to detect differences in absorption of light between oxygenated and deoxygenated Hb
What are the disadvantages of using pulse oximetry to measure blood O2?
Only detects pulsatile blood so ignores venous and tissue O2 levels
Doesn't indicate how much Hb is present so in anaemic pts will paean normal
What is the benefit of using ABG to measure blood O2?
Can be used to investigate possible causes including acid-base disturbance or electrolyte imbalance
What are the disadvantages of using ABB to measure blood O2?
When is ABG used to assess blood O2 content?
In acutely ill pts with suspected acid-base of electrolyte disturbance (not used for O2 sats alone)
What two features does the reaction of oxygen binding need to have in order to allow for effective transport?
Reaction must be reversible
Must use molecular oxygen so it can dissociate at tissues and be used
Why is there more CO2 present in the plasma than O2?
It is more soluble and reacts with water
Does CO2 react with Hb?
Yes but at a different site to oxygen
How does the arterial blood content of CO2 compare to that of O2?
2.5x as much CO2
What is the main function of CO2 in the blood?
Control of blood pH
What does the amount of dissolved CO2 on arterial blood depend directly on?
What is the determining factor of the amount of CO2 dissolved in blood?
Rate of breathing controlling alveolar pCO2
What is the purpose of the high [HCO3-] in the plasma?
Stops nearly all dissolved CO2 from reacting so the pH of plasm is alkaline
What are most HCO3- associated with in the blood and why?
Na+ to stop it becoming too acidic
What is the Hendersson-Hasselbalch equation?
pH = pK + (log([HCO3-]/(pCO2xsolubility factor))
What is the usual ratio of [HCO3-]/dissolved CO2 meaning blood pH is 7.4?
What increases the reaction rate of hydrogen carbonate production in RBCs?
Why does the reversible for HcO3- production in RBCs proceed in the forward direction?
Mop of products
What does the amount of HCO3- production in RBCs depend on?
H+ binding to Hb which depend on O2 binding
How are the products of HCO3- production by RBCs removed?
HCO3- reacts with lactic, keto and sulphuric acid
CO2 E moved by breathing
How does Hb in the RBCs favour HCO3- production?
Binds to H+ which removes a product
How is HCO3- produced by RBCs?
In RBC: CO2+H20 H+ + HCO3-
HCO3- moved out by cotransporter which moves Cl-
HCO3- + H+ H2O and CO2
What happens in HCO3- production at tissues?
Less O2 on Hb --> Hb in T state so more H+ binds --> more HCO3- can be produced --> more CO2 in plasma due to HCO3- exportation
Why is there only a small change in pH between arterial and venous blood despite the relatively large changes in dissolved CO2 and HCO3- amounts?
Because both have increased so ratio remains almost constant
What happens to Hb on arrival of venous blood at the lung?
In R state so gives up extra H+ from tissues which reacts with plasma HCO3- to form CO2 which can be breathed out
What role do carbamino compounds have in CO2 transport?
CO2 can bind directly to proteins e.g. amine groups on globin of Hb for transport not acid-base balance
Why are more carbamino compounds formed at the tissues?
pCO2 is higher and O2 unloading from Hb facilitates CO2 to Hb binding
Can CO2 in carbamino compounds formed at tissues be breathed out?
How do the amounts of CO2 in arterial and venous blood compare?
Arterial has less dissolved in plasma, lower [HCO3-] in plasma, less dissolved in RBCs, lower [HCO3-] in RBCs and less carbaminos present
How is transported CO2 calculated?
Total in venous - total in arterial