Respiratory: The Carriage of Oxygen and Carbon Dioxide in the Blood L17 Flashcards
Discuss why dissolved oxygen is an inefficient way to carry blood around the body.
Dissolved oxygen obeys Henry’s LAw, so the amount of dissolved oxygen in the blood is proportional to the partial pressure of oxygen.
For each mmHg of oxygen pressure (PO2), 0.003ml of oxygen is carried dissolved per 100ml of blood.
Arterial pressure of oxygen is 100mmHg, therefore in every 100ml of blood there is only 0.3ml of oxygen at 100mmHg.
Therefore, transporting oxygen dissolved in the blood is very ineffective way of transporting oxygen around the body.
How much oxygen can haemoglobin carry around the body?
One gram of Hb can carry 1.34 mL of oxygen. In adults, blood normally has about 15g of haemoglobin per 100mL. Therefore the maximum amount of oxygen that can be combined with Hb (oxygen capacity) for normal blood is: 1.34 x 15 = 20.1 mL oxygen per 100mL.
What is the calculation to find the total amount of oxygen content of the blood? Taking both oxyhemoglobin and dissolved oxygen into account.
Oxygen concentration in blood = 1.34 x Hb x sat/100 + 0.003 x PO2.
Hb = concentration of Hb in grams per 100mL
Sat = %saturation of haemoglobin (how much oxygen bound)
What is a oxygen-haemoglobin dissociation curve?
The oxygen-haemoglobin dissociation curve is the graph of the oxygen saturation of haemoglobin versus pressure of oxygen in plasma.
The graph is sigmoidal (s-shaped).
The oxygen saturation of haemoglobin is the percentage of the available binding sites that have oxygen attached.
From the curve we see that the oxygen saturation of arterial blood (SaO2) with a PO2 of 100mmHg is about 98%.
The oxygen saturation of venous blood (SvO2) with a PO2 of 40mmHg is about 75%.
Thereby, a big decrease in pressure, only affects %saturation of Hb slightly.
Explain several advantages of the properties of Hb that are demonstrated by the shape of the oxygen-haemoglobin dissociation curve.
The upper, flattened part of the curve shows that moderate changes in PO2 around the normal value in the lung (100mmHg) have only small effects on the percentage of saturation of Hb and therefore on the amount of oxygen carried by arterial blood.
Therefore, minor fluctuations in lung pressure will not have a huge physiological effect on oxygen transport.
The steep part of the curve at lower PO2 shows that small changes in PO2 (once it has dropped below a certain threshold) result in large amounts of unloading oxygen.
This helps with unloading of oxygen to the tissues or other areas when there is low oxygen pressure and thus a need for oxygen.
Give the oxygen saturation calculation.
O2 saturation = (O2 combined with Hb/O2 capacity) x 100
What does a rightward shift on the oxygen-haemoglobin dissociation mean?
Rightward shift on the curve occurs when the affinity of Hb for oxygen decreases.
The oxygen dissociation curve is shifted to the right by increases in H+ concentration (i.e. a decrease in pH - see Bohr effect), PCO2, temperature, and 2,3-DPG.
This results, for example, in more unloading of oxygen from haemoglobin at a given PO2 in the capillaries supplying the tissues.
When does a left shift occur on the oxygen-haemoglobin dissociation curve?
A leftward shift on the oxygen-haemoglobin dissociation curve occurs when there is a high affinity for oxygen for haemoglobin.
The oxygen dissociation curve is shifted to the left by decreases in the same factors that shift it to the right. (i.e DPG, PCO2).
This results, for example, in more loading of O2 onto haemoglobin in the lungs.
Percentage if CO2 transported in dissolved plasma?
10%
What is carbaminohaemoglobin? How is it formed?
20% of CO2 transported as carbaminohaemoglobin HbCO2. Carbamino compounds can be formed by the combination of CO2 with terminal amine groups in blood protein, the most important of which is the globin of haemoglobin: Hb.NH2 + CO2 Hb.NH.COOH
This reaction occurs rapidly without an enzyme, and reduced deoxygenated Hb can bind more CO2 as carbaminohaemoglobin than as oxygenated Hb.
Therefore the unloading of oxygen in the peripheral capillaries facilitates the loading of CO2 (a phenomenon known as the Haldane effect), while oxygenation has the opposite effect.
How does CO2 get transported by bicarbonate? Discuss the reaction of CO2 to form bicarbonate. Also explain chloride shift.
70% of CO2 is transported as bicarbonate.
CO2 + H2O H2CO3 H+ + HCO3-
The first reaction happens very slowly in the plasma, but it can happen quickly within red blood cells due to the presence of the enzyme carbonic anhydrase.
The second reaction happens quickly without an enzyme.
When the concentration of the ions generated by the second reaction increases in the red blood cell, the bicarbonate ions diffuse out.
Because cell membrane is relatively impermeable to cations, the hydrogen ions do not diffuse out.
Therefore, in order to maintain electrical neutrality, chloride ions (Cl-) move into the redblood cell from the plasma: a process known as the chloride shift.
Explain the Bohr Effect right shift.
The Bohr effect causes a right shift of the oxygen dissociation curve, whereas the Haldane effect causes a left shift of the carbon dioxide dissociation curve.
The Bohr effect:
Increased CO2 = O2 unloads more easily.
Metabolism = Produces CO2
Active tissues = more O2 unloading
The Bohr effect refers to the effect of changes in CO2 and H+ on oxygen carriage by haemoglobin: increases in their concentrations cause more oxygen to be unloaded from haemoglobin (i.e. the oxygen dissociation curve mores to the right).
An example of the Bohr effect would be actively metabolising tissues (exercise) creating higher concentrations of CO2 in the local environment. More CO2 = further unloading of oxygen, gives cascade effect.
Explain the Haldane effect
Haldane effect - Deoxygenation of blood increases CO2 carriage.
There is initial oxygen unloading from Hb due to e.g metabolising tissue. This leaves deoxyHb.
H+ (floating around from bicarbonate buffer) can attach to deoxy Hb, allowing more CO2 to bind to H-Hb.
In addition, this also increases conversion of CO2 to HCO3- since when form H-Hb removing one of the products (H+) of the reaction, drives equilibrium forward.
I.e metabolising tissue removes more O2 from blood, which means blood can ‘store’ more oxygen.
In lungs lots of oxygen around, adds to Hb - less CO2 can be stored as HCO3- -leaves blood as CO2-exhaled.
Explain the Bohr effect - leftshift.
Decreased PCO2 e.g when it is removed from blood in lungs.
Any particular PO2 will have higher Hb saturation and O2 content i.e Hb ‘picks up’ O2 which is what you want in lungs.
Bohr effect improves both pick up and delivery of O2 by Hb.
Normally Bohr effect: High CO2 = unload oxygen
But at lungs, there is low CO2, so more loading of oxygen.
Discuss the relationship between Bohr effect and Haldane effect.
Bohr effect is about the fact that high carbon dioxide (from metabolising tissues) leads to more oxygen unloading from Hb.
The Haldane effect is about this now low oxygen level at the tissues, causes more carbon dioxide loading onto Hb to form carbaminohaemoglobin (for carbon dioxide transport). This is assisted by the reaction of CO2 + H2O H2CO3 H+ + HCO3-
which is basically saying when the H+ attaches to Hb to form H-Hb, the reaction drives forward to make more HCO3- and H+ using the CO2 from metabolising tissues. It is more likely for chemical reasons that CO2 will attach onto the H-Hb rather than O2. So O2 gets used further instead for metabolising, and CO2 further attaches onto H-Hb. The H+ can be used for carbaminohaemoglobin for CO2 transport and the HCO3- is also a big factor in CO2 transport.
