Oxygen In The Blood Flashcards
Describe the solubility of oxygen
• Oxygen is not very soluble in water
– Solubility factor for O = 0.01 mmol.l-1.kPa-1
– Less soluble than CO2
• [O2]dissolved = solubility x pO2
• At pO2 of 13.3 kPa only 0.13 mmol.l-1 is dissolved
• At rest we need around 12 mmol O2 per minute
• Would need 92 l.min-1 to meet needs
• Cardiac output is 5lmin-1
Even if all the o2 could be extracted, CO would be impossibly high, so we need a chemical reaction to transport oxygen
Describe oxygen binding
- The reaction needs to be reversible
- The oxygen must dissociate at the tissues to supply them
- Many substances will bind oxygen but only some are useful
- Respiratory pigments contain haem group
- Oxygen combines reversibly
Name 2 oxygen binding pigments
• Haemoglobin – present in blood
– Tetramer – binds 4 oxygen molecules
• Myoglobin – present in muscle cells
– Monomer – binds 1 oxygen molecule
What is myoglobin
• Pigment found in muscles • Contains haem • Similar to haemoglobin but only a single subunit • Simple example to consider first
Myoglobin acts as a store for oxygen withinn uncle tissue - can give off o2 when ppo2 in muscle cells is low
Describe dissociation curves
• Using myoglobin as an example
• Can show reversibility of O2 binding as a curve
• Plot of O2 bound vs pO2
• Total content = bound + dissolved
• Saturates because the amount of pigment is limited
But there is not much dissolved
• Binding saturates above a given pO2 • Amount of O2 bound therefore depends on amount of pigment when represented this way
• Can overcome this issue by expressing saturation as a percentage
– Independent of pigment concentration
• So dissociation curves normally
expressed as a percentage of amount bound at saturation
• This is independent of pigment
concentration
How are dissociation curves used
- We can see how much O2 will be bound or given up when moving from one partial pressure to another
- Work out difference in percentage saturations between the two pO2 values
- Take amount bound at full saturation and use percentage to calculate how much given up
Describe harmoglobin
• Molecule of haemoglobin consists of 4 subunits – Tetramer – 2 alpha & 2 beta subunits • Each subunit has one haem and one globin • 4 oxygen molecules can bind to each molecule of haemoglobin
What are the 2 states of Hb
• Low affinity for oxygen in T state (tense)
– Difficult for oxygen to bind
• High affinity for oxygen in R state (relaxed)
– Easier for oxygen to bind
- When pPO2 is low Hb is tense
- So it is hard for the first O2 molecule to bind
- As each O2 molecule binds the molecule becomes more relaxed and binding of the next O2 molecule is easier
Dscribe the Hb dissociation curves
- Initially the relationship between pO2 and binding is shallow
- But as some O2 binds it facilitates further binding
- Curve steepens as pO2 rises
- Then flattens as saturation is reached • This gives a sigmoidal curve
• Hb saturated above 9-10kPa • Virtually unsaturated below 1kPa • Half saturated at 3.5 – 4 kPa • Saturation changes greatly over a narrow range (steep part of curve) • Reaction is highly reversible and depends on pO2 levels
Describe Hb in arterial blood raving the lungs
• Alveolar pO 2
≈ 13.3kPa, therefore Hb is well saturated
• Can calculate oxygen content of arterial blood
• If Hb concentration is normal ≈ 2.2 mmol.l-1
• Each Hb molecule binds four O2 molecules
• Therefore oxygen content = 8.8 mmol.l-1
– If the patient’s lungs are functioning OK, but they are anaemic, pO2 will be
normal, but oxygen content will be lower
Describe Hb in th tissues - how much is unloaded
• Tissue pO2 depends on how metabolically active the tissue is – typically 5 kPa • Hb saturation drops to ≈ 65% • 35% given up (fraction given up = 0.35) • Can calculate amount of O2 given up • 8.8 mmol.l-1 x 0.35 ≈ 3 mmol.l-1
Describe Hb in mixed venous blood
• Mixed venous blood – mixture of blood returning from various tissues • Over half the oxygen is still bound • Could the tissues remove more? • The lower the tissue pO, the more O will dissociate from Hb – lower saturation of venous blood
How low can tissue pO2 get?
• Tissue pO 2
must be high enough to drive diffusion of O2 to cells
• It cannot fall below 3 kPa in most tissues
• However the higher the capillary density, the lower the pO2 can fall (doesn’t have so far to diffuse)
• Very metabolically active tissue will have a higher capillary density
(eg heart muscle)
O2 doesnt have as far to diffuse - can allow ppo2 o all lower - dot have as far to drive diffusion
What is the Bohr shift
• pH effects the affinity of
haemoglobin
• Acid condition shift dissociation curve to right (higher pO2 values)
• ↓pH promotes T-state of Hb (tense
state)
• ↑pH (alkaline) promotes R-state (relaxed)
Bohr shift.- low ph shifts dissociation curve to the right. Hb has lower affinity for o2 - more in tense state
Much more oxygen given off as a result, ,see slide
Describe how temperate and ph change sin tissues affect O2 unloading
• pH is lower in the most metabolically active tissues • So extra O2 is given up • Increased temperature also shifts the dissociation curve • Metabolically active tissues have slightly higher temperature • So extra O 2 is given up
