Introduction to blood-gas transport notes Flashcards
1. Distinguish between the following terms: minute, alveolar, and dead space ventilation; and anatomic, alveolar and physiologic dead space 2. Specify the partial pressures of O2 and CO2 in the alveoli, mixed venous and arterial blood in normal individuals 3. Using the alveolar ventilation equation, discuss the factors that determine the partial pressure of CO2 in the alveoli and define the terms hyperventilation and hypventilaton 4. Name the factors that affect diffusive transport of a gas f
Dissolution of gases
-when gases are exposed to a liquid (such as blood gas) gas molecules diffuse into the liquid and exist in a dissolved state
Partial pressure of dissolved gas
- dissolved gases also exert a partial pressure
- a gas will continue to dissolve in the liquid until the patial pressure of the dissolved gas= the partial pressure above the liquid
Two forms in which oxygen is transported in the blood
1) Physically dissolved in the plasma (2%) -relatively insoluble in plasma (vs. CO2)
2) Chemically bound to the hemoglobin molecule (Hb) in the red blood cells (98%)
Calculating the amount of physical dissolved oygen
Dissolved O2 (100 ml of blood) = PaO2 (mmHg) x 0.003 Where 0.003 is a constant - for each mmHg of PO2 there is 0.003 ml of O2 per 100ml of blood
Hemoglobin role in oxygen transport
-hemoglobin can combine rapidly and reversibly with O2 to form oxyhemoglobin
What determines the amount of oxygen that hemoglobin can carry
- amount of oxyhemoglobin = a function of partial pressure of oxygen in the blood
- when PaO2 levels are high the reaction shifts to the right to form oxyhemoglobin
How oxygen is released to the tissues
- reaction with hemoglobin is reversible
- i.e. deoxyhemoglobin + O2 oxyhemoglobin
Oxygen carrying capacity- definition
-the maximum amount of O2 that can be carried by hemoglobin
How much O2 can combine with a gram of hemoglobin
-each gram of hemoglobin can combine with 1.34 ml of O2
Normal concentration of hemoglobin in the blood
15 g Hb/100ml of blood (or 150 g/L)
The oxygen carrying capacity of a healthy young adult
20 ml O2 /100 ml of blood (calculate by 15 x1.34)
Oxygen content- definition
- the total amount of O2 in the blood
- incudes O2 bound to hemoglobin and O2 dissolved in blood
Calculating the oxygen content
-generally measured directly but can also be calculated by this equation
CaO2 (ml O2/100 ml blood) = [Hb x1.34 x SaO2] + [PaO2 x 0.003]
Percent saturation
-the proportion of hemoglobin that is bound to O2
-SO2
-a ratio of the quantity of O2 that is bound to hemoglobin to the quantity that can potentially be bound to hemoglobin
SO2 = oxygen combined with Hb/ Oxygen carrying capacity of Hb x100
SaO2
Arterial blood saturation of hemoglobin
Normally 98% at PaO2 = 100 mmHg
SvO2
Saturation of hemoglobin in mixed venous blood
Normally 75% at PaO2 = 40 mmHg
Oxyhemoglobin dissociation curve - what does it show
Relates the oxygen saturation (SO2), the partial pressure of oxygen in the blood (PaO2) and blood oxygen content
S shape of oxyhemoglobin dissociation curve -what does it mean
- increasing affinity of hemoglobin for oxygen as blood PO2 increases
- because as PaO2 is high the reaction shifts to the right to form oxyhemoglobin
Two regions of the S curve
- plateau region
- steep region
Plateau region-what is happening
- represents loading of oxygen in the lungs
- when venous blood returns to lungs has a PO2 of 40 mmHg and hemoglobin is 75% sat with O2
- as blood passes through alveoli it is exposed to PO2 of 100 mmHg in alveoli
- O2 diffuses from alveoli into plasma and hemoglobin becomes 97-100% saturated with O2
Plateau region - why curve is flat
-at partial pressure at or above 60 mmHg flt curve with O2 sats at 90% or higher
-hemoglobin nearly saturated with O2 and even when alveolar PO2 decreases to as low as 60 mmHg (and therefore arterial PO2 decreasing as well) the O2 remains bound to hemoglobin
= safety margin –> ensures O2 content of blood remain high and that tissues receive adequate amounts O2
Steep region - what is happening
- PO2 of tissues typically 40 mmHg
- at this PO2 O2 is released from hemoglobin and enters the tissues
- dissociation curve steep between 20-40 mmHg
- in this steep range a small decrease in PO2 in the tissues will result in the unloading of O2 to the tissues
Consequence of left and right shifts on hemoglobin affinity for O2
- changes of affinity of hemoglobin for O2
a) left shift = increase hemoglobin affinity for O2
b) shift right = decrease hemoglobin affinity for O2
Factors that cause right shift
1) Increase in temperature
2) Increase in PCO2
3) Decrease in pH
4) Increase 2,3, DPG (DPG = 2,3 diphosphoglycerate)
What is 2,3 DPG -when do its levels increase
- an organic phosphate
- a byproduct of anaerobic metabolism of glucose in RBCs
- levels increase with chronic hypoxia (high altitude, chronic lung disease)
Bohr effect
- effect of CO2 and pH (H+) on hemoglobins affinity for O2
- both CO2 and H+ bind directly to hemoglobin and induce conformational change that will decrease hemoglobin’s affinity for O2
Factors that shift the oxyhemoglobin curve to the left
-CO monoxide poisoning
Effect of CO on oxyhemoglobin curve
- competes for same binding sites on hemoglobin as oxygen (heme-binding sites)
- affinity for CO is 200x that of O2 (will bind the same amount of hemoglobin as O2 at a partial pressure 200x lower)
- CO-hemoglobin reaction is reversible and dependent of PCO
- high concentrations of CO wil shift the reaction t the right and favor the formation of HbCO
-CO also increases hemoglobins affinity for O2 and shifts the oxyhemoglobin dissociation curve to the left - making it more difficult to unload oxygen to metabolically active tissues
Arterial PO2 in CO poisoning
-will remain the same as the O2 diffusion gradient has not changed
Cause of severe tissue hypoxia with CO poisoning
-PaO2 levels are normal -therefore no feedback mechanism to detect arterial oxygen levels have decrease -lead to severe tissue hypoxia
Best treatment for CO poisoning
- breathe 100% oxygen
- high oxygen concentration will drive off the CO and favor the formation of oxyhemoglobin
Anemia
Reduced number of circulating RBC’s
Reduced level of hemoglobin
Affect of anemia on oxygen transport in the blood
-Patients with anemia and normal lungs will have a normal PaO2 an normal SaO2 but reduced O2 content
Why SaO2 levels remain constant in anemic patients
Because content and capacity are proportionally reduced
Colors of hemoglobin
a) oxygenated
b) deoxygenated
c) carboxyhemoglobin
a) bright red
b) blueish
c) bright, cherry red
Cyanosis -definition
-arterial blood with >5g Hb
-results in a bluish/purple discoloration of nail beds an mucous membranes
(absence of cynosis does not indicate normal oxygen transport) -anemic patient with low O2 in the blood may not have sufficient deoxygenated Hb to appear cyanotic
Transport of CO2 by the blood -3 forms
1) Physically dissolved in the plasma (5-10%)
2) Physically dissolved as bicarbonate ions (60%)
3) combined with hemoglobin as carbamino protein (30%)
What drives CO2 into the blood
-the high PCO2 in the tissues
What determines the amount of CO2 dissolved in the plasma
-dependent on the PCO2
Solubility of CO2 in plasma vs. O2
-CO2 is 20x more soluble than oxygen
Formation of carbonic acid
- in RBCs CO2 combines with H2O to form H2CO3 which then becomes H+ and HCO3- (spontaneous)
- this reaction is accelerated by the enzyme carbonic anhydrase
Chloride shift
- As HCO3- accumulates in the cells it will passively diffuse out of the cell using a bicarbonate-chloride carrier
- because HCO3= is negatively charged chloride diffuses in from the plasma to maintain electrical neutrality
Fate of H+ produced in RBCs and its consequences
-does not move out of RBCs because of low permeability of the membrane
In tissues:
-most of the H+ is buffered by hemoglobin: H+ + HbO2 –> Hb + O2
-as H+ binds to hemoglobin it decreases the oxygen binding affinity (shifting oxygen hemogobin curve to the right))
-this promotes O2 unloading for delivery to the tissues and favors loading of CO2 in RBCs to be carried to the lungs for expirtion
In pulmonary capillaries:
-oxygenation of hemoglobin favors the dissociation of H+ from hemoglobin
-therefore CO2 is unloaded from RBCs and released from the alveolus
Carbon dioxide dissociation curve
- relationship between the PCO2 and the total CO2 content in the blood (in all 3 forms)
- nearly a straight line function in the range of normal arterial blood PCO2
Haldone effect
- a higher PO2 will shift the curve down and to the right
- allows the blood to load more CO2 when in the tissues (low PO2 and high PCO2) and to unload more CO2 in the lungs (high PO2 and low PCO2)
- CO2 dissociation curve is influenced by state of oxygenation of the Hb because:
1. Deoxygenated hemoglobin is better at combining with H+ and in turn assisting the blood to load more CO2 from the tissue
2. Deoxygenated hemoglobin is better at combining with CO2 to form carbamino compounds
