Blood Gas transport Flashcards
what happens to gases carried in the blood before they are transported in other forms?
gases carried in the blood first dissolve in the plasma before mostly being transported in other forms
can alveolar air dissolve straight into haemoglobin?
no - there are a series of steps
how much of the oxygen transported by the blood is bound to Hb?
98%
is co2 also transported in the blood bound to Hb?
only a small proportion of co2 is transported bound to Hb, but at a different site to o2
how is the majority of co2 transported, and why?
because co2 is more soluble, a greater proportion is transported just dissolved in the plasma
what does partial pressure within the blood indicate?
partial pressure within the blood refers to how much of that gas is dissolved in the plasma, doesn’t necessarily indicate how much is bound to haemoglobin
Why is haemoglobin critical to o2 transport?
oxygen has low solubility in plasma, and in order to dissolve the amount of O2 needed to supply tissues, an impossibly high alveolar PO2 would be required
The presence of haemoglobin overcomes this problem- organisms have evolved carrying proteins with high oxygen binding affinity
Hb it enables O2 to be concentrated within blood (↑ carrying capacity ie. larger volume of o2 can be transported within circulatory fluid) at gas exchange surfaces and then released at respiring tissues.
what 2 things greatly increases the number of oxygen molecules that can be carried?
- The multiple (4) binding sites present on haemoglobin
- The high concentration of the protein within the large number of red blood cells present within the blood
what 3 things can be used to define/measure the o2 content of blood?
1) O2 partial pressure (PaO2, kPa)
≈ “the partial pressure of O2 within a gas phase (at a gas-liquid interface) that would yield this much O2 in the plasma at equilibrium”
2) Total O2 content (CaO2), expressed as mL of O2 per L of blood (ml/L),
3) O2 saturation (SaO2 = measured directly in arterial blood, SpO2 = estimated by pulse oximetry), expressed as %,
define o2 saturation
“What % of total available haemoglobin binding sites are occupied by oxygen?”
define total o2 content
“what volume of O2 is being carried in each litre of blood, including O2 dissolved in the plasma and O2 bound to Hb?”
what can the relationship between O2 concentration, partial pressure and saturation be shown by?
the ODC
oxygen-dissociation curve
what does the oxygen-haemoglobin dissociation curve illustrate?
relationship between how much O2 partial pressure the blood is exposed to and how much O2 is actually bound to haemoglobin
describes the affinity between oxygen and haemoglobin
state and explain the shape of the oxygen-haemoglobin curve and why it looks this way:
Sigmoidal (s) shape, because you have an acceleration of the curve due to cooperate binding
Once the first O2 binds to a molecule of haemoglobin (haem group) it makes it easier for subsequent oxygen molecules to bind until they’re full
This can only go on for so long, because at a certain point you start to fill up and saturate the available binding sites, so you inevitably get a plateau at the point at which saturation reaches 100%
oxygen affinity is itself affected by the number of oxygen molecules bound
Why is haemoglobin so effective at transporting O2 within the body?
1) The structure of Hb produces high O2 affinity
(2) The concentration of haem groups & Hb contained in RBCs enables high carrying capacity
(3) The oxygen-haemoglobin binding curve shifts to offload oxygen to demanding tissues
(4) Hb O2 affinity changes depending on the local environment, enabling O2 delivery to be coupled to demand
explain the importance of the fact that the structure of Hb gives it a high O2 affinity
means a high level of Hb-O2 binding and saturation is achieved at a relatively low PO2
You need to go to extremely hypoxic levels to start getting really substantial effects on haemoglobin saturation
explain the benefits of having 4 binding sits on Hb and having such a high conc of Hb in RBC’s
(each haem group has 4 binding sites, and 270 Hb molecules per RBC and 5 billion RBC’s per ml of blood)
so, overall there are a very large number of o2 binding sites enabling it to carry a huge amount of o2 per unit of blood
explain why the oxygen-haemoglobin binding curve shifts to offload oxygen to demanding tissues
The degree of which Hb binds to oxygen is affected by physiological factors (PCO2, pH, temperature and [2,3-DPG])
These factors affect the number of o2 molecules bound to haemoglobin by altering the structural confirmation of the Hb molecule and changing O2-Hb affinity
This has the effect of shifting the oxygen-haemoglobin dissociation curve to the right or left
explain what a leftwards shift and a rightward shift on the curve indicate:
leftward - higher Hb-o2 affinity
(Hb binds more o2 at a given Po2)
rightward - lower Hb-o2 affinity
(Hb binds less o2 at a given Po2)
describe the Bohr effect
The effect of CO2 and pH on Hb-O2 affinity
In respiring tissue carbon dioxide and 2,3-DPG (product of respiration) levels increase as well as pH, shifting the curve to the right
Less oxygen is bound to Hb as the oxygen is being released to tissue that needs that oxygen
in resting tissue state what are co2 levels are like and what is the relevance of this?
In resting tissue, there are low levels of CO2 and so the Hb binds to more oxygen
give examples of how Hb o2 affinity changes depending on the local environment, enabling o2 delivery to be coupled to demand
1) in the lungs, there are high amounts of o2 and low amounts of co2 leading to an increased pH and increased o2 saturation, curve shifts to left
2) in resting tissues, you have a lower partial pressure of o2, resulting in o2 saturation decreasing- curve shifts back to normal place
3) working tissue e.g. exercise, lots of metabolism going on these tissues naturally produce lots of lactic acid, co2 and 2,3-DPG, so curve shifts to the right
- these things influence Hb structure so it has a lower Hb-o2 affinity (so Hb will give off oxygen relative to what it would do normally)
how does myoglobin act?
Myoglobin act as an O2 reservoir within muscle tissue-only releases O2 at low PO2
what is the difference between myoglobin and haemoglobin?
Myoglobin in muscles has a much higher affinity for oxygen than haemoglobin
-provides a reservoir of oxygen for respiring muscles when they need it.
when does myoglobin give off o2?
only gives off o2 when o2 partial pressures start to become extremely low
Why does myoglobin release o2?
provides a little bit of o2 as an extra supply to ensure muscle contraction activity can actually continue for a long period of time
does adult or foetal haemoglobin have a higher affinity for o2? give a reason for your answer
foetal haemoglobin
- it has a higher affinity so it can effectively ‘steal’ O2 from maternal Hb
- this means when the foetal and maternal blood supplies come into contact with each other within the placenta you naturally get oxygen moving from mum to foetus
what colour does oxyhaemoglobin appear?
red
what colour does deoxyhaemoglobin appear?
blue
what determines determines the colour of blood?
the relative concentrations of dexoxy Hb and oxygen Hb
what is cyanosis?
purple discolouration of the skin and tissue that occurs when the [deoxyhaemoglobin] become excessive
what are the types of cyanosis?
2 types
- central
- peripheral
define central cyanosis and when it occurs:
- Bluish discolouration of central features of the body (core, mucous membranes and extremities)
- Inadequate oxygenation of blood
- eg. hypoventilation, V/Q mismatch (tends to be because of a major defect)
define peripheral cyanosis and when it occurs:
- Bluish colouration confined to extremities (fingers)
- Inadequate oxygen supply to extremities
- eg. small vessel circulation issues
even if there is adequate perfusion and ventilation, hypoxia can still occur - why is this?
hypoxia can still occur if the blood isn’t able to carry sufficient oxygen to meet tissue demands.
name some causes of anaemia?
(insufficient RBC’s or haemoglobin)
• Iron deficiency (↓ production)
• Haemorrhage (↑ loss)
signs of anaemia
- tiredness
- pale conjunctiva
give the equation connecting concentration, pressure and solubility
concentration = pressure x solubility
where does co2 bind on haemoglobin
(different site to o2)
R–NH2 residues at the end of peptide chains, forming carbamino-Hb, R-NHCOOH
what is the difference between Hb’s interactions with oxygen and carbon dioxide?
- The interaction between haemoglobin and carbon dioxide has a lower affinity than the reaction between haemoglobin and oxygen.
co2 reacts with water to form….
carbonic acid
-this accounts for the majority (approx 70%) of CO2 transported
CO2 + H2O –> H2CO3 H+ + HCO3-
(reversible)
what is the Haldane effect?
oxygenated blood has a reduced affinity for carbon dioxide
why does venous blood carry more CO2 than arterial blood?
because there is a difference in the amount of oxygenation
deoxyhaemoglobin has a higher affinity for co2 and H+ ions than oxygenated haemoglobin does
what does oxygenation of blood at the lungs enable?
greater co2 release
when oxygen molecules bind to Hb what changes?
haemoglobin structure is changed which makes haemoglobins affinity for CO2 and H+ lower, so it will bind to them less
what happens to the co2 and H+ when oxygenation of blood increases (i.e. Hb binds to them less)?
some of the co2 is kicked off the RBC’s and will enter the plasma
some of the H+ ions previously bound to haemoglobin will be kicked off and converted back into carbonic acid
why is oxygenation of blood and co2 being kicked off a good thing when it happens at the lungs?
the co2 will enter to alveolar space and will be expelled from the body
why might oxygenation of blood and co2 being kicked off not be a good thing when it happens in tissues?
if excess co2 cannot be released this means co2 could build up in the tissues, because oxygenation means less co2 can be transported, and co2 accumulation is known as acidosis
explain how CO2 enters tissues and what happens:
- CO2 is produced by respiring cells and dissolves in the plasma and enters RBC’s
- Conversion of CO2 and H2O to H2CO3 within RBCs (catalysed by carbonic anhydrase)
- The effective removal of CO2 through the process in (2) enables further CO2 to diffuse into the RBC (and more can then enter the plasma)
- H2CO3 ionises to HCO3- and H+. The RBC membrane is impermeable to H+, therefore H+ cannot leave
- Accumulation of H+ within cell, and therefore cessation of (2), is prevented by deoxy-Hb acting as a buffer and binding H+. Movement of O2 into tissues from RBCs therefore increases [deoxy-Hb] and enables more CO2 to be transported.
- The increased [HCO3-] creates a diffusion gradient for HCO3- to leave the cell. It is exchanged for Cl- to maintain electrical neutrality
explain how CO2 enters the lungs and what happens:
- Low PACO2, creates a diffusion gradient for CO2 to diffuse out of the blood into the airspace
- Increased PAO2 leads to O2-Hb binding. oxy-Hb binds less H+ than deoxy-Hb, increasing free [H+]
- Increased free [H+] leads to increased H2CO3 and ultimately CO2 which contributes to CO2 plasma saturation
- The changing equilibrium of carbonic acid reaction also leads to decreased [HCO3-], as it binds the free H+. This creates a diffusion gradient that allows HCO3- ions to enter the RBC in exchange for Cl-.
how does anaemia limit blood oxygen transport?
the number of total oxygen-Hb sites is reduced
does deoxygenated blood carry more CO2 or O2?
Deoxygenated blood carries more CO2
what does oxygenation of blood cause?
causes CO2 to leave
