Blood gas transport Flashcards
(15 cards)
How is the oxygen content of blood measured and defined
1) O2 partial pressure:
-the partial pressure of O2 within a gas phase that would yield this much O2 in the plasma at equilibrium
2) Total O2 content expressed as mL of O2 per L of blood:
-the volume of O2 carried in each litre of blood —-> including the O2 dissolved in the plasma and O2 bound to Hb
3) O2 saturation:
-the % of total available haemoglobin binding sites that are occupied by oxygen
How does oxygen get from the atmosphere to cells
-O2 inhaled from atmosphere into alveoli within the lungs
-O2 diffuses from alveoli into blood within pulmonary capillaries
-O2 transported in blood, predominantly bound to haemoglobin
-O2 diffuses into cells/tissues for use in aerobic respiration
-CO2 diffuses from respiring tissues
Discuss how CO2 and O2 move throughout the body
Oxygen (O2 ):
At the Lungs: When you breathe in, O2
goes into your lungs. First, it dissolves a little bit in the “water” (plasma) of your blood.
Then, mostly, it hops onto the “boats” (hemoglobin in red blood cells). So, most of the O2 in your blood is carried by these boats (98%), and only a little is dissolved in the plasma (2%).
To the Tissues: When the blood reaches your body tissues that need O2, the O2 “boats” drop off their O2, which then diffuses into the tissues.
Carbon Dioxide (CO2) :
From the Tissues: Your tissues produce CO2 as waste. This CO2 then moves into the blood.
In the Blood: CO2 is transported in three main ways:
Most of it (70%) gets converted into a different form called bicarbonate (HCO3−). Think of it like CO2 changing its shape to travel better.
Some of it (23%) also hops onto the “boats” (hemoglobin), but at a different spot than O2 .
A little bit of it (7%) just dissolves in the “water” (plasma).
At the Lungs: When the blood reaches the lungs, the CO2 (in all its forms) gets converted back into CO2 gas and moves out of the blood into the lungs so you can breathe it out.
Why is haemoglobin critical to O2 transport
Oxygen has low solubility in plasma (0.225mL/L/kPa). 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 – it enables O2 to be concentrated within blood (↑ carrying capacity) at gas
exchange surfaces and then released at respiring tissues.
The vast majority of O2 transported by the blood is bound to
haemoglobin (>98%).
Explain what a leftward shift and rightward shift mean in an oxygen-haemoglobin binding curve
Leftward shift- higher Hb-O2 affinity so Hb binds more O2 at a given PO2
Rightward shift- lower Hb-O2 affinity= Hb binds less O2 at a given PO2
Explain clinical aspects of Hb and O2 transport
Oxyhaemoglobin (Hb-O2) appears red
where as deoxyhaemoglobin (Hb)
appears blue the relative concentrations
determines the colour of blood:
Cyanosis = purple discoloration of the skin
and tissue that occurs when the
[deoxyhaemoglobin] becomes excessive.
Central cyanosis:
* Bluish discoloration of core, mucous
membranes and extremities
* Inadequate oxygenation of blood
* E.g. hypoventilation, V/Q mismatch
Peripheral cyanosis:
* Bluish coloration confined to
extremities (e.g. fingers)
* Inadequate O2 supply to extremities
* E.g. small vessel circulation issues
Explain clinical aspects of Hb and O2 transport: insufficient haemoglobin (anaemia)
-hypoxia can occur despite adequate ventilation and perfusion —-> if the blood is not able to carry sufficient oxygen to meet tissue demands
-causes of anaemia:
-iron deficiency (decrease in production of iron)
-haemorrhage (increased loss of blood)
Explain carbon monoxide poisoning
Hb has >200x affinity for carbon monoxide (CO) than O2 and competes for the same binding site.
∴ ↑CO-Hb =↓O2 capacity
Carboxyhaemoglobin has cherry red pigmentation, hence hypoxia
occurs in the absence of cyanosis
How and why does transport of CO2 differ to O2
CO2 has a higher H2O solubility than O2 does – therefore a greater % of CO2 is transported simply dissolved in plasma (CO2 ≈
7%, O2 ≈ 1%)
CO2 binds to Hb at different sites than O2 (R–NH2 residues at the end of peptide chains, forming carbamino-Hb, R-NHCOOH)
and with decreased affinity. Thus, a lower % of CO2 is transported
in this manner (≈ 23%).
CO2 reacts with water to form carbonic acid, which accounts for
the majority (≈70%) of CO2 transported.
Explain the haldane effect
-venous blood carries more CO2 than arterial blood
CO2 produced by tissues. If excess dissolved CO2 cannot be released (e.g.
outside lung), then oxygenation of blood enables less CO2 to be transported.
CO2 accumulation = acidosis
CO2 removed by the At lung, oxygenation of blood lung enables greater CO2 release
Explain blood gas transport in tissues
CO2 is produced by respiring cells and
dissolves in the plasma + enters RBCs.
Conversion of CO2 + H2O to H2CO3
within RBCs (catalysed by carbonic
anhydrase)
The effective removal of CO2 by (2)
enables further CO2 to diffuse into the
RBC (& more can then enter the plasma).
H2CO3 ionises to HCO3- + H+. The
RBC cell membrane is impermeable to
H+, therefore H+ cannot leave
Accumulation of H+ within cell, and
cessation of (2), is prevented by deoxy-
Hb acting as a buffer and binding H+.
Movement of O2 into tissues from RBCs
↑[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 blood gas transport in the lungs
Low PACO2, creates a diffusion
gradient for CO2 to diffuse out of the
blood into the airspace.
Increased PAO2 leads to O2-Hb
binding. O2-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 entry the RBC in
exchange for Cl-.
The net result of these effects is transport of
O2 and CO2 interact:
* Deoxygenated blood carries more CO2
* Oxygenation of blood causes CO2 to leave
(both points = “the Haldane effect”).
Explain how the Bohr effect happens
a) Binding of O2 to Hb induces a structural change (to Hb),
reducing Hb affinity for CO2 and H+
* Therefore deoxygenated blood carries more CO2 at any given
PCO2 (the Haldane effect)
b) Binding of CO2 (or H+) to Hb induces a (different) structural
change (to Hb), reducing Hb affinity for O2
* Therefore Hb releases more O2 at any given PO2 when CO2
and/or H+ levels rise (the Bohr effect)
Explain the relationship between PCO2 and [H2CO3]
) Accumulation of
CO2 (hypoventilation)
2)↑[H2CO3]
CO2 +H2O
1) Removal of CO2
(hyperventilation)
H2CO
2)↓[H2CO3]
3)↑[H+]
(acidosis)
H+ +3)↓[H+]
HCO3- (alkalosis)
This means that ↑CO2 = ↑H+ (acidity, ↓pH).
As the lungs play a role in regulating CO2 levels, they also therefore
contribute to acid-base balance.
Furthermore, signs of respiratory and metabolic distress can be diagnosed & interpreted from analysis of ABG and pH.
Give a summary all the key points
Oxygen solubility in water (plasma) is low. Dissolved O2 only
accounts for ≈1% of O2 in blood. >98% is carried bound to Hb.
CO2 is transported in 3 ways: 7% dissolved in plasma, 23% bound as
carbamino-Hb, and 70% converted to H2CO3 / HCO3-
.
Anaemia and CO poisoning both decrease the O2-carrying capacity of
blood by decreasing available Hb-O2 binding sites (by reducing [Hb]
and by increasing [Hb-CO], respectively). Blood O2 content decreases
whilst saturation of O2 binding sites remains normal.
Increased levels of H+, CO2, temperature and 2,3-DPG (all products
of anaerobic respiration) all decrease Hb-O2 affinity (shifting the ODC
to the left, “the Bohr effect”) – this helps match O2 release to demand.
O2 binding to Hb displaces CO2 and H+, reducing the total CO2
capacity of blood (as less dissolved CO2 can be converted to H2CO3).
Thus less oxygenated, venous blood carries more CO2 than more
oxygenated arterial blood (“the Haldane effect”)
