Oxygen transport by blood Flashcards
Learning outcomes
- know the significance of central and peripheral cyanosis
- explain the carriage of oxygen in simple physical solution in blood
- explain the carriage of oxygen as oxyhaemoglobin in blood
- draw an oxygen dissociation curve for
- arterial blood
- venous blood
- fetal blood
- anaemic blood
- explain the Fick principle which relates the oxygen extraction from blood and blood flow to oxygen consumption
- know the causes of hypoxia
What is cyanosis?
• The abnormal bluish discolouration of the skin &
mucous membranes due to high levels of deoxygenated haemoglobin
Classification:
- Peripheral
- Decreased blood flow & increased oxygen extraction
- Extremities are blue
• Central
- Low Hb saturation or abnormal Hb
- Lips, mucous membranes, tongue are blue
What is Dalton’s Law?
Dalton’s Law states that the total pressure exerted by the mixture of inert (non-reactive) gases is equal to the sum of the partial pressures of individual gases in a volume of air
1 atm = 760 mmHg or 101.3 kPa
(0.77)x 760 = 560 mmHg (nitrogen- 77% abd in air)
(0.21)x 760 = 160 mmHg (oxygen- 21% abd in air)
What is Henry’s Law?
The partial pressure of a gas in solution is the partial pressure the gas would need in the gaseous phase to equilibrate with that solution.
Henry’s law states that the number of molecules dissolving in a liquid is directly proportional to the partial pressure of the gas
(Note gas conc and partial pressure are not the same, as solubility affects the concentration)
Oxyhaemoglobin/ oxygen saturation
Hb + O2< > HbO2
1 g Hb combines with up to 1.34 mL O2
With 150 g/L, blood contains a max of 200 mL/L O2 bound to Hb
This is known as the oxygen capacity (max amount O2
that can combine with Hb)
Oxygen saturation: Volume of oxygen bound to Hb (mL/L)/ Oxygen capacity (mL/L) x 100%
- 98% of oxygen is carried bound to Hb (201ml/L) . 2% carried in plasma (3ml/L)
po2 is oxygen dissolved in plasma- not relative to Hb
Oxygen dissociation curves
Sigmoidal shaped curve
First oxygen difficult to bind to Hb, after conformational change occurs it is easier
when po2 60mmHg or above this gets more difficult.
Arterial blood nearly 100% saturated with oxygen
Venous blood around 75% saturated with oxygen
A rightward shift shows an Increased P50, or decreased affinity for oxygen: more readily dissociates
- Inc temperature, inc co2 release (Inc PCO2)(Bohr effect) , inc 2,3-DPG (chemical released during glycolysis), reduction in pH (Bohr effect)
A leftward shift shows decreased p50, increased affinity for oxygen- more readily dissociates
- Dec temp, pCO2, 2,3-DPG and increased PH
Cyanosis is the visible appearance of hypoxia Detected clinically when ≥ 50 g/L deoxy-Hb or 33% of the total if [Hb] is 150 g/
Lecture 5 slides 17/18 for diagrams
Fetal ODC feature
Fetal haemoglobin has higher affinity for oxygen- lower P50 than adult Hb
Double bohr effect- mothers Hb to right, fetal Hb to left
Anaemic ODC feature
Major difference in the haemoglobin concentration of blood in anaemic individuals and normal individuals.
- O2 mL per dL significantly different- much lower prevalence of haemoglobin in anaemic patients
- SaO2 not significantly different if those Hb molecules are normal- just not sufficient volume of them
Fick’s principle
Oxygen consumption = Arterio-venous difference x Cardiac output
CaO2- 200ml/L
CvO2- 150ml/L
Oxygen consumption= (200-150)x 5(l/min)= 250ml/min
What is hypoxia and how does it differ between tissues?
Hypoxia is an o2 deficiency within tissues i.e low PaO2
- Hypoxic hypoxia is low o2 uptake in lungs
- Anaemic hypoxia is low o2 uptake in haemoglobin
- Ischaemic/stagnant is low o2 in circulation
- Histotoxic hypoxia is low tissue o2 utilisation
Causes - Hypoxic: high altitude, lung failure
Anaemic: iron deficiency, CO poisoning
Ischaemic: shock, heart failure, embolism
Histotoxic: cyanide poisoning (inhibits mitochondria)
