Carriage of O2 and CO2 in the blood Flashcards
How is oxygen carried?
Carried in two forms
- Dissolved in blood
- Combined with Hb
Oxygen dissolved in blood- what is it affected by?
Amount of gas in solution proportional to temperature (more dissolves at low temperature)
What is the equation for the volume of oxygen in blood at 37 degrees?
Volume of O2 = 0.0232 x PO2
0.0232 is a constant
Does gas dissolve better in cold or hot water?
Amount of gas in solution varies with T and the lower the T the more gas dissolves- more in cold than hot water
What is the equation for oxygen saturation?
HbO2/ HHb+HbO2
Volume of oxygen equation
SO2 x Hb x 1.39 (Hufner constant)
What is the Hufner constant?
Hufner constant- amount of oxygen Hb can carry
What is the structure of Hb?
Primary – 141-146 amino acids per chain
Secondary – globular structure
Tertiary – ‘crevice’ for haem and O2 binding
Quaternary – 4 chains (HbA = 2 x a and 2 x b)
How many global chains, ahem groups, iron atoms and how many oxygen atoms it binds to?
- 4 x globin chains
- 4 x haem groups
- 4 x iron atoms
- and binds 4 x O2 molecules
What state can oxygen bind to Hb?
In R ‘relaxed’ form O2 can access binding site.
What state is oxygen pushed put of Hb?
In T ‘tense’ form O2 pushed out.
What does diphosoglycerate do?
Diphosphoglycerate (DPG)- compound found in RBC which is broken down in glycolysis to provide energy- can control the curve by shifting to make oxygen more available.
Abnormal Hb- absent global chain
Thalassaemia- genetic abnormality
Other globin chains used; often can’t survive
Thalassemia- genetic abnormality when chains (alpha or beta) of Hb can’t be produced- alphas= more severe (often foetus doesn’t make it to birth) patients with beta tend to keep their foetal Hb throughout
Why is alpha thalassaemia more fatal?
Alpha chains are present in adult and foetal Hb.
Abnormal Hb- defective global chain
HbS (sickle cell disease)
Single amino-acid defect; red cells sickle at low PO2
Abnormal Hb- defective Fe atom
Methaemoglobin
Drug induced; metHb does not carry oxygen- oxygen carrying capacity reduced
Abnormal Hb- wrong ligand
CO Hb
Smoking/house fires; CO blocks O2 binding site
What affects Hb- O2 dissociation
Temperature, pH change (Bohr effect) and DPG
Buffer
A buffer is a solution that can minimise changes in the free H+ concentration
and therefore in pH ( pH= - log10 [H+] )
acid ⇌ H+ + base
e.g. H2CO3 ⇌ H+ + HCO3-
Blood buffer systems include what?
Include proteins – carboxyl and amino groups at each end of the chain, and basic/acidic side chains of amino acids
What 3 ways is carbon dioxide carried?
Dissolved blood, carbamino compounds and carbonic acid/ bicarbonate
CO dissolved in blood is dependent on what?
Temperature
Carbamino compounds- what do they do?
- Bound to R-NH2 groups on proteins
* Includes terminal amino group and side chains of lysine and arginine
What is catalyses carbon dioxide and water to dorm carbonic acid?
Carbonic anhydrase
How do RBC manage the amount of H+ and HCO3- ions?
CO2 + H2O ⇌ H2CO3 ⇌ H+ + HCO3-
HCO3- - Pumped out of red cell in exchange for a chloride ion (Hamburger shift)
H+ buffered by Hb
What is the Haldane effect?
Ability of deoxygenated blood to carry more CO2 than oxygenated blood.
What is the Henderson-Hasselbalch equation?
pH= pK+ log10(A-/HA)
The relationship between blood pH, CO2 and HCO3- is described by the Henderson-Hasselbalch equation.
What are the compensation systems?
Respiratory: blood pH regulates ventilation and so controls PCO2 (rapid response)
Renal: excretion of H+ in urine controlled by pH (slow response)
What is acidosis?
Acidosis= blood pH too low
What is alkalosis?
Alkalosis= blood pH too high
If there is a problem with CO2 what would this be called?
Respiratory acidosis/alkalosis
If there is a problem with bicarbonate what would this be called?
Metabolic acidosis/alkalosis
Acid base abnormalities – alkalosis (pH >7.45), low CO2- what is it and why is it caused?
Respiratory alkalosis (low PCO2, normal HCO3-R too low- hyperventilating (anxiety, iatrogenic) Iatrogenic- doctor caused
Acid base abnormalities – alkalosis (pH >7.45), bicarbonate high- what is it what is it caused by?
Metabolic alkalosis (normal PCO2, high HCO3-)
Due to loss of H+ e.g. vomiting; abuse of antacid remedies
Acid base abnormalities – acidosis (pH < 7.35), high CO2, high bicarbonate- what is it and what is it caused by?
Respiratory acidosis (high PCO2; high HCO3- = renal compensation)
Someone with COPD/chest infection (ventilatory failure)- CO2 is high
Acid base abnormalities – acidosis (pH < 7.35), low bicarbonate- what is it and what is it caused by?
Metabolic acidosis (low HCO3-; low PCO2 = respiratory compensation) Not enough bicarb- pH low
Patient is hyperventilating
Can be caused by renal failure; diabetic ketoacidosis; shock (poor tissue perfusion)
Tissues start to produced lactate, lactate build up = metabolic acidosis
