Blood Gases and Oxygen Curve Flashcards
Oxygen Dissociation Curve
The oxygen dissociation curve graphically illustrates the % of Hb (left -side) that is chemically bound to oxygen at each oxygen pressure (PaO2)…(bottom). On the right-side of the graph, a second scale is included that gives the CaO2.
The curve is s-shaped with a steep slope between 10 and 60 mmHg and a flat portion between 70 and 100 mmHg
The steep portion of the curve shows that oxygen rapidly combines with hemoglobin as the Pa02 increases
-Beyond this point (60 mmHg), a further increase in the Pa02 produces only a slight increase in oxygen-hemoglobin bonding…in fact…
Because the hemoglobin is already 90% saturated at a Pa02 of 60 mmHg, an increase in the Pa02 from 60 to 100 mmHg elevates the total saturation of the hemoglobin by only 7%
Clinical Significance of the Flat portion of the curve
the Pa02 can fall from 100 to 60 mmHg and the hemoglobin will still be 90% saturated with oxygen…. thus…
the upper curve plateau illustrates that hemoglobin has an excellent safety zone for the loading of oxygen in the lungs
the flat portion also means that increasing the Pa02 beyond 100 mmHg adds very little additional oxygen to the blood
a small additional amount of oxygen does however continue to dissolve in the plasma as
the Pa02 rises (Pa02 x .003) = dissolved O2
A reduction of PaO2 to below 60 mmg produces a RAPID decrease in the amount of oxygen bound to hemoglobin
therefore, when the PaO2 continues to fall below 60 mmHg, the quantity of oxygen delivered to the tissue cells may be significantly reduced
P 50
-A common point of reference on the oxygen dissociation curve is the P50.
-The P50 represents the partial pressure at which the hemoglobin is 50% saturated with oxygen, typically 26.6 mm Hg in adults.
- The P50 is a conventional measure of hemoglobin affinity for oxygen.
Shifts in P 50
-In the presence of disease or other conditions that change the hemoglobin’s oxygen affinity and, consequently, shift the curve to the right or left, the P50 changes accordingly.
-An increased P50 indicates a rightward shift of the standard curve, which means that a larger partial pressure is necessary to maintain a 50% oxygen saturation, indicating a decreased affinity.
-Conversely, a lower P50 indicates a leftward shift and a higher affinity.
Factors that Shift the Oxygen dissociation Curve
—Temperature—
• As the body temperature increases, the curve noves to the right
• Exercise, which produces an elevated temperature, enhances the release of oxygen as blood flows through the muscle capillaries
• As the body temperature decreases, the curve shifts to the left
• Their Pa2 is normal, but oxygen is not readily released from the hemoglobin
This mechanism partly explains why an individual’s lips, ears and fingers appear blue while swimming in very cold water
Factors that Shift the Oxygen dissociation Curve
PH
As the blood ph decreases, the oxygen dissociation curve shifts to the right
This enhances the unloading of oxygen at the cellular level, because the pH decreases in this area as carbon dioxide moves into the blood
In contrast, as the blood pH increases the curve shifts to the left
This mechanism facilitates the loading of oxygen onto hemoglobin as blood passes through the lungs, because the ph increases as carbon dioxide moves out of the blood and into the alveoli
Factors that Shift the Oxygen dissociation Curve
Carbon Dioxide
•As the PaCO2 level increases, the oxyhemoglobin saturation decreases, shifting the curve to the right
•As the PaCO2 level decreases, the curve is shifted to the left
•The effect of PaCO2 and pH on the oxyhemoglobin curve is known as the
BOHR EFFECT
2,3-Diphosphoglycerate
-The RBC’s contain a large quantity of the Substance 2,3-DPG
-2,3-DPG is formed by the RBC’s during anaerobic glycolysis the dissolution glucose molecules into small forms of energy….ATP)
• Hemoglobin’s affinity for oxygen decreases as the 2,3-DPG level increases, thus, the effect of an elevated concentration of 2,3-DPG is to shift the curve to the right
•Decreased 2,3-DPG levels shifts the curve to the left
Right Shift of the Oxygen Dissociation Curve
-Decreased PH
-increased Temperature
-increased PaCO2
-increased 2,3 DPG
A right shift of the curve means hemoglobin’s affinity for oxygen has decreased…. thus…
• The hemoglobin unloads oxygen at the tissue level readily
Left Shift of the Oxygen Dissociation Curve
-increased PH
-Decreased Temperature
-Decreased PaCO2
-Decreased 2,3 DPG
A left shift of the curve means hemoglobin’s affinity for oxygen has increased… thus….
•The hemoglobin loads oxygen in the lungs readily
Carbon Dioxide Transport and Acid/ Base Balance
— Respiratory Quotient R/Q= .8
—tissue cells consume about 250 ml of oxygen and produce about 200 ml of carbon dioxide each minute
—the newly formed carbon dioxide is transported from the tissue cells to the lungs by six (6) different mechanisms
— (3) are in the plasma
—(3) are in the red blood cells (RBC’s)
-Plasma
1) Carbamino compound (bound to protein) approx.1%
2) Bicarbonate (HCO3)…initially the CO2 combines with water in a process called HYDROLYSIS… this process forms Carbonic acid (H2CO3) which in turn breaks down to form HCO3 and H+…this hydrolysis reaction in the plasma is very slow, therefore, the amount of HCO3 and H+ are very small…approx. 5%
3) Dissolved CO2…this portion of the CO2 transport system in the venous blood is what we measure to assess the patient’s PaCO2…i.e., ABG…approx. 5%
-Red Blood Cells
1) Dissolved CO2… in the intracellular fluid of the red blood cells…accounts for about 5% of the total CO2 released in the lungs
2) Carbamino Hgb…a compound formed by the combination of CO2 and Hemoglobin…approx. 21%
3) Bicarbonate (HCO3)…most of the carbon dioxide is transported from the tissue cells to the lungs in this form….approx. 63%
-How is the Dissolved CO2 That Enters the RBC’s Converted to HCO3?
-dissolved CO2 entering the RBC undergoes hydrolysis
(CO2 combines with H20)
—normally a slow process in the plasma, but it is greatly enhanced in the RBC by the enzyme carbonic anhydrase
CO2 + H20»>H2C03 (carbonic acid)»>Н+ + НСО3
• —the rapid hydrolysis causes the RBC to become saturated with HCO3
…to maintain equilibrium between the RBC/Plasma, the excess HCO3 diffuses out of the RBC
How is the Dissolved CO2 That Enters the RBC’s Converted to HCO3?
once in the plasma
—once in the plasma, the HCO3 combines with Na (sodium), which is normally in the plasma in the form of NaCl (sodium chloride)
• —the HCO3 is then transported to the lungs as NaHC (sodium bicarbonate) in the plasma of the venous blood
-as HCO3 moves out of the RBC, the (CI) which has been liberated from the NaCi molecule moves into the RBC to maintain electric neutrality
—this movement is known as the Chloride Shift or the Hamburger Phenomenon
