Carbon dioxide transport Flashcards
What is the first thing that happens to gases carried in the blood?
They’re dissolved in the plasma before being transported in other forms.
During gas exchange at the lungs:
- O2 is dissolved in plasma
- O2 binds to Hb
For CO2 removal:
- CO2 is transported as HCO3- or bound to Hb
- The CO2 is dissolve in plasma (PaCO2)
- CO2 exchanged at lungs
CO2 produced by tissues:
- CO2 dissolved in plasma
- CO2 transported as HCO3- or bound to Hb
O2 diffusion into tissues:
- O2 bound to Hb
- O2 dissolved in plasma
- O2 diffuses into tissues
What does it mean if CO2 has a higher H2O solubility than O2?
How do you calculate concentration?
A greater % of CO2 is transported simply dissolved in plasma (CO2 -7%, O2-1%)
Concentration = Partial pressure x Solubility
How does CO2 binding to Hb differ from O2-Hb binding?
CO2 binds to Hb at different sites than O2 does
It binds to R-NH2 residues at the end of the peptide chains, forms carbamino-haemoglobin, R-NHCOOH).
This binding happens with decreased affinity, thus a lower % of CO2 is transported in this manner (23%)
What is the product formed when CO2 reacts with H2O?
Carbonic acid (HCO3-). This accounts for the majority of CO2 transported.
CO2 + H2O -> H2CO3 (this is reversible)
H2CO3 -> H+ + HCO3 (this is reversible)
What happens to carbonic acid once it forms?
It partially ionises to form H+ and HCO3-.
How is carbon dioxide transported from respiring tissues to the lungs?
- CO2 is produced by respiring cells and dissolves in the plasma + enters RBCs
- CO2 and water are converted into H2CO3 within red blood cells, and this reaction is catalysed by carbonic anhydrase
- The conversion of CO2 into H2CO3 enables further CO2 to diffuse into the RBC (and more will then be able to enter into the plasma) as the conversion means CO2 has effectively been removed
- Due to Le Chatelier’s principle, dissolved CO2 is pulled out of the plasma and into storage as bicarbonate. This CO2 lost via conversion to HCO3 is then replaced by fresh CO2 produced by tissues, thus the overall amount of CO2 carried within the blood increases. Conversely, when oxygenation of blood increases, Hb-H+ binding is reduced, and the reverse effect occurs, decreasing CO2 carrying capacity
- H2CO3 ionised to HCO3- and H+. The RBC cell membrane is impermeable to H+, therefore H+ is unable to leave
- H+ accumulates within the cell, leading to the conversion of CO2 + H2O into H2CO3 to be halted. This is prevented by deoxyhaemoglobin acting as a buffer and binding to H+. O2 moves into tissues from red blood cells, therefore there are increased levels of deoxyhaemoglobin, enabling more CO2 to be transported.
- The increased levels of HCO3- creates a diffusion gradient for HCO3- to leave the cell. It’s exchanged for CL- to maintain electric neutrality
Why does venous blood carry more CO2 than arterial blood?
This is the Haldane effect
Venous blood is usually deoxygenated (bar pulmonary veins), and arterial blood is generally oxygenated (bar pulmonary arteries).
Deoxyhaemoglobin has a higher affinity for CO2 and H+ than oxyhaemoglobin does, therefore if there are increased levels of oxyhaemoglobin, less CO2 is carried
What happens if excess CO2 dissolved cannot be released?
Oxygenation of blood enables less CO2 to be transported, leading to accumulation of CO2, resulting in acidosis
At the lungs, oxygenation of blood enables greater CO2 release.
Why might rapid O2 therapy in hypercapnic individuals with COPD be dangerous?
Hypercapnic = Elevated CO2 levels in blood
Starting supplemental O2 therapy too quickly is dangerous for patients with COPD as oxygenation of their blood enables it to carry less CO2 due to the Haldane effect.
Due to the fact COPD patients experience chronic hypoventilation, CO2 build up, and they’re already able to carry more oxygen due to the low levels of oxygen in their blood.
When oxygen levels suddenly increase, CO2 is displaced from the blood, and the blood transports less CO2 bound to Hb and as bicarbonate. This can result in dangerous acidaemia due to the high levels of CO2 in the blood. COPD patients are unable to remove this excess CO2 due to their lungs not functioning well enough to do so.
How is carbon dioxide transported at the lungs?
- Low partial pressure of CO2 creates a diffusion gradient for CO2 to diffuse out of the blood into the airspace
- An increased O2 partial pressure leads to O2-Hb binding. O2-Hb binds less H+ than deoxyhaemoglobin, leading to increased levels of 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-.
Describe the Bohr effect
The Bohr effect describes Hb’s lower affinity for oxygen secondary to increases in the partial pressure of CO2 and decreased blood pH. This lower affinity enhances the unloading of oxygen into tissues to meet the oxygen demand of the tissue
CO2 and pH affect O2-Hb affinity
Increased CO2 levels decrease O2 haemoglobin binding affinity and decrease the amount of O2 bound to Hb at a given PO2.
The level of CO2 present mediates this effect directly and indirectly via its relationship with blood pH following conversion to carbonic acid (i.e. more CO2 = more H+ = lower pH = decreased O2-Hb affinity = less O2 carried).
This occurs because when CO2 and H+ bind to the Hb molecule, they induce a conformational change in the Hb molecules, resulting in a change to the structure of the O2 binding site, altering O2-Hb binding affinity
Describe the Haldane effect
Complementary process by which CO2 transport by Hb is influenced by O2.
Deoxygenation of blood carries more CO2
Oxygenation of blood causes CO2 to leave
When O2 binds to Hb, it induces a conformational change in the structure of Hb, reducing its affinity for CO2 and H+, resulting in greater amounts of oxygenated Hb.
What relationship is important in acid-base balance?
The carbonic acid-bicarbonate buffer system (The relationship b/w PCO2 and H2CO3.)
Changes in pH are resisted by converting excess H+ to H2O and CO2.
The excess CO2 can then be removed at the lungs by increasing ventilation.
If pH increases, ventilation can be reduced to increase partial pressure of CO2, and yield more H+.
How do the lungs and kidneys maintain blood pH homeostasis?
By regulating PaCO2 and HCO3- respectively.
The kidneys regulate HCO3- via regulating reabsorption/excretion in glomerular filtrate
The lungs regulate PaCO2 by regulating ventilation
pH ∝ HCO3-/PaCO2
What do increasing levels of HCO3- do?
Increasing levels of HCO3- removes H+ from the system.
This means that blood pH is determined by the ratio of HCO3- to PaCO2, and changes in either results in disruptions in pH:
↑PaCO2= ↓ pH (unless [HCO3-] changes proportionally, in the opposite direction)
↓PaCO2= ↑ pH (unless[HCO3-]changes proportionally, in the opposite direction)
↑[HCO3-] =↑ pH (unlessPaCO2changes proportionally, in the opposite direction)
↓[HCO3-] =↓pH (unlessPaCO2changes proportionally, in the opposite direction)
