In which two ways in O2 present in the blood?
- dissolved in the plasma
2. carried by haemoglobin molecules (98%)
Describe the structure of hemoglobin
-Four subunits, each subunit is a heme molecule combined with a polypeptide chain
-2 Alpha chains, 2 beta (collectively name globin)
-Each subunit binds an iron ion (this is what O2 binds to)
-Therefore a single hemoglobin molecule can bind 4 oxygen molecules
O2 + Hb ⇌ HbO2
-deoxyhemoglobin (Hb) or oxyhemoglobin (HbO2)
What is percentage Hb Sat? What factors affect it?
Percentage Sat = O2 bound to Hb /oxygen-carrying-capacity X 100
- Blood O2
- subunits of Hb are held together by electrostatic bonds, the binding of one oxygen molecules breaks some of these bonds, this leaves more binding sites exposed, this increases the binding affinity further… - Amount of Hb in blood (low Hb is known as anemia)
Describe the oxygen-hemoglobin curve
- Steep slope between 10 and 60mmHg
- Relatively flat between 70 and 100mmHb (O2 binding produces only a small increase in further binding)
- this plateau is important, eg at high altitudes changes in O2 saturation will only cause a slow decrease in O2 binding*
How does alveoli/capillary and capillary/cell mantain o2 conc grads?
O2 moves from alveoli to capillary, Hb uptakes O2 maintaining conc grad
-mitochondria in cells use o2, so cellular o2 is less than surrounding o2 - o2 in plasma diffuses out , so o2 in hemoglobin dissociates and diffuses across
What is the effect of carbon monoxide binding to hemoglobin?
It has a high affinity (210 X that of oxygen) for hemoglobin
-this means it reduces the amount of o2 that combines with hemoglobin in pulmonary capillaries due to competition
(there is also no reflex increased ventilation)
What is the affect of 2,3-diphospohglycerate (DPG) presence in the blood?
-binds with hemoglobin altering the confirmation of the molecule (this is a form of allosteric modulation)
-an increase in DPG con causes dissociation curve to shift to the right
-this means hemoglobin has a reduced affinity of o2
(temp and acidity have the same effect) -each of these factors is higher in tissue capilarry blood than in arterial blood
-DPG is produced during glycolysis
(DPG is released at high altitudes so that more O2 is available to the body)
What is the affect of fetal hemoglobin presence in the blood?
- shifts dissociation curve to shift to the left
- increased affinity for o2
- high uptake of O2 across placental diffusion barrier
How do Co2 and H+ conc affect hemoglobin affinity?
Co2 and H+ bind to globin portion of hemoglobin altering the confirmation of the molecule (this is a form of allosteric modulation)
-an elevated temp also alters its conformation
why does a more metabolically active cell lower hemoglobin affinity for o2?
- PCO2 or acidity (H+ conc) is higher in capillary due to co2 entering blood from tissues and release of acids like latic acid
- temp = tissue metabolism
more metabolically active cell = lower affinity = more o2 release
Why do erythrocytes contain large quantities of DPG
- no mitochondria
- rely on glycolysis
- this releases DPG
- enhanced O2 unloading due to hemoglobins reduced affinity
How is Co2 carried in the blood?
10%
-It is more soluble than O2 so more is dissolved in the plasma and erythocytes
25-30%-
- forms carbaminohemoglobin by reacting reversibly
- bonds with the amino groups of hemoglobin (deoxyhemoglobin has a higher affinity for CO2 than oxyhemoglobin)
60-65%
- converted to HCO3-
- converted to carbonic acid via carbonic anhydrase, this enzyme is present in erythrocytes but not in plasma
- carbonic acid then dissociates very rapidly
- once HCO3- is formed it is transported from erythrocytes to the plasma in exchange for a Cl ion (this is known as the CHLORIDE SHIFT)
*This reactions are driven to the left when CO2 begins to diffuse from pulmonary capillaries into the alveoli, releasinf CO2
What does total-blood carbon dioxide contain?
- Blood dissolved in plasma
- Co2 in carbaminohemoglobin
- Co2 that has been converted to HCO3-
What happens to H+ that is generated by CO2 becomes HCO3-
-It is buffered by hemoglobin (uptake)
HbO2 + H+ ⇌ HbH + O2
-deoxygenated hemoglobin has a high affinity than oxygenated hemoglobin
(only a small amount of H+ remains free, this is why venous blood is more slightly more acidic than arterial blood)
-as hemoglobin passes the lungs and binds to oxygen it releases the H+
-H+ binds with CO3- to form carbonic acid and then CO2 and H2O
What is respiratory acidosis?
Increased arterial H+ concentration due to carbon dioxide retention
-this can occur during hypoventilation or in a diseased state preventing normal Co2 elmination
What is respiratory alkalosis?
Hyperventilation means that there is more ventilation so there is a decrease in arterial PCO2 and arterial H+ concentration
What decreases Hb affinity?
- increased H+ conc (acid)
- increase PCO2 (acid)
- increased time
- increased DPG (produced during glycolysis)
