Hb Structure, Function and Gas Transport Flashcards
What determines how much oxygen can be carried in the blood (5)?
What can causethe amount of O2 dissolved in the blood to increase?
With high levels of inhaled oxygen the amount dissolved in the blood can become more significant.
What drives the movement of gases between the alveoli and the blood?
Partial pressures of CO2 and O2 in the respective environments. The gases will move from one environment to the other based on their respective pressure gradients.
When can perfusion limited gas exchange be transformed to diffusion limited?
During heavy exercise (with increase in cardiac output) or with a diffusion barrier the normal perfusion limitation can transform to diffusion limitation.
What are the ways that CO2 can be carried in the blood?
CO2 leaves tissues and enters blood. 5% of it remains dissolved and unchanged in the blood. 95% of it is taken up into the RBC and then 5% of it binds to Hb in the RBC. The remaining 90% of it is converted to H2CO3, which then rapidly decomposes to H+ + HCO3-. The proton can bind to Hb, making it have lower affinity for O2 so it will release more O2 to the tissues. The HCO3- is then pumped out of the RBC by a transporter that uses Cl-.
What does a pulse oximeter measure?
Whether or not hemoglobin has something bound to it. It DOES NOT distinguish between what is bound to hemoglobin. Thus, if a person has things other than oxygen bound to Hb, their pulse ox can appear normal and their arterial blood gas can reveal hypoxemia.
Concentration of a gas in solution depends on its […] in the solution
Partial pressure and solubility
What is the Bohr Effect?
Shift in oxyhemoglobin dissociation curve to left or right
What is the haldane effect?
The less O2 that is bound to Hb, the higher the affinity the Hb molecule has for CO2 and vice versa.
CO2 is able to diffuse across the alveolar/capillary membrane 20x faster than O2. However, these gases take the same amount of time to reach equilibrium with regards to gas exchange across this membrane. Why is this?
Although CO2 diffuses faster, the pressure gradient for O2 is larger so there is a larger driving force for O2 to cross the membrane. As such, they diffuse in about the same amount of time due to different driving factors.
If CO enters the blood, will it expert a partial pressure?
NO - Carbon monoxide has very high affinity for hemoglobin. If CO is introduced into the blood, very little exists in the plasma exerting a partial pressure and virtually all of the CO is bound to the hemoglobin.
How do you determine the partial pressure of a gas?
Each gas contributes to the total pressure in proportion to its concentration. I.e. if the total pressure of the gas is 760mmHg and the gas is 21% oxygen, then oxygen’s partial pressure is (760*0.21) = ~160mmHg
What does the CO2 dissociation curve look like?
What are the various forms that a gas can take in solution?
Dissolved gas
Bound to carrier (hemoglobin)
Chemically modified (CO2 being converted to HCO3-)
CO2 is […] more soluble in aqueous phase then O2
20 x
How many mL of O2 are “unloaded” to tissues during gas exchange in capillary beds of tissues?
5mL
In fully oxygenated blood there is about 20mL of O2 per 100mL of blood, in fully deoxygenated blood there is about 15mL of O2 per 100mL of blood
How do oxygen and carbon dioxide move from the alveoli into the blood?
Via diffusion - they are freely soluble in membranes
What things decrease the P50 of the oxyhemoglobin dissociation curve (i.e. increase affinity of hemoglobin for oxygen)?
Decreasing temp
Decreasing partial pressure CO2
Decreasing concentration of 2,3 BPG
Decreasing [H+]
What is the equation that describes alveolar respiratory membrane diffusion?
Why is an O2 sat of 90, 91, 92 concerning?
A patient with those levels of % oxygen has a partial pressure of about 60mmHg in their blood. If their blood saturation drops further, they are approaching an area of the dissociation curve where further deoxygenation can trigger a massive loss of oxygen binding due to the cooperative nature of oxygen binding to hemoglobin. This would put them at risk for becoming hypoxic and suffering tissue damage.
What is the PO2 and PCO2 in tissues before gas exchange occurs with oxygenated blood?
23mmHg
46mmHg
Why is the partial pressure of oxygen in the systemic arterial blood supply 100mmHg and the partial pressure of CO2 in the systemic arterial blood is 40mmHg?
Because these are the partial pressures of oxygen and carbon dioxide respectively in the alveoli. Gas exchange occurs until the partial pressures in the alveoli and the capillaries are equal, so that’s why the pressures are the same
Pressure of a gas acting on the surface of blood vessels is proportional to the […] of the gas
Concentration (in moles)
What volume of O2 is transferred from the lungs to the tissues?
5mL
- How much O2 can be carried per gram of Hb?
- How much O2 is dissolved in the blood and not bound to Hb?
- What equation describe the relationship between the [O2], the amount of O2 bound to Hb and the amount of O2 dissolved in the blood?
- 1.34 mL O2 / 1 gram Hb
- 0.003mL O2 / 100mL of blood / mmHg
- CaO2 = (SaO2 % x Hb g/dl x 1.34 ml O2/ g Hb) + (0.003 ml O2/100ml/1 mm Hg x Pao2 mmHg)
Takeaway: 98% of O2 in blood is bound to Hb
What is meant by the term “perfusion limited gas exchange”?
As RBCs travel through the capillary beds of the lungs from the venous side to the arterial side, they reach full oxygenation (i.e. partial pressure O2 of 100mmHg) fairly early in their path of travel. Thus, the limitation on how much oxygen can be delivered to the body is not on the rate of diffusion of the gases but rather on the amount of blood that perfuses through the capillaries. To increase oxygen delivery to the periphery, cardiac output must be increased.
In fact, this equilibrium occurs in about one third the time (0.25 seconds) it takes for the blood to move through the capillary (0.75 seconds).
What things increase the P50 of the oxyhemoglobin dissociation curve (i.e. decrease affinity of hemoglobin for oxygen)?
Increasing temperature
Increasing partial pressure CO2
Increasing [2,3 BPG]
Increasing [H+]
What factors affect the rate at which oxygen and carbon dioxide will diffuse from the alveoli into the blood?
Partial pressure of gasses in each environment
Area across which they are diffusing
Distance across which they must diffuse
Diffusion coefficient
V = (delta_P)(A)(diffusion constant) / (distance)
In the alveoli, the partial pressure of oxygen is 100mmHg and the partial pressure of carbon dioxide is 40mmHg. In venous blood, the partial pressure of oxygen is 40mmHg and the partial pressure of carbon dioxide is 46mmHg. The pressure differential for oxygen is 60mmHg whereas carbon dioxide it is only 6mmHg. This would lead to the assumption that oxygen diffuses faster due to the larger concentration gradient. However this is not the case. Why?
CO2 is 20x more soluble than oxygen so it can still diffuse well even with a small driving force of only 6mmHg pressure differential
What is the respiratory quotient and where does it derive from?
R = CO2 exhaled / O2 consumed = 0.8
Derived from looking at CO2 dissociation curve and oxygen dissociation curves. For every 5 molecules of oxygen that are consumed, 4 molecules of CO2 are exhaled, thus giving 4/5 or 0.8.
What determines the partial pressure of oxygen in the blood?
Determined by the oxygen that is dissolved in the blood NOT the oxygen that is bound to hemoglobin
What is meant by the term “diffusion limited gas exchange” and when is this seen clinically?
This term means that the oxygenation level of the blood is limited by the diffusion of the gas itself. This is seen in people who have fibrosis or scar tissue in the interstitial space between the alveoli epithelial cells and the capillary endothelial cells or in the fused basement membrane. The build up of this tissue increases the distance the gas must diffuse across and increases the resistance of the gas to diffusion, so the blood is never able to completely equilibrate with the alveoli and the partial pressure of oxygen in arterial blood is decreased.
Fill in the missing information indicated by the arrows as well as the information in the table.
- What is methemoglobin?
- How does oxygen bind to methemoglobin?
- What impact does methemoglobin have on the oxyhemoglobin dissociation curve?
- Why can methemoglobinemia lead to hypoxia?
- The form of hemoglobin with ferric iron is termed methemoglobin.
- If iron is in the ferric form (Fe3+ ) it cannot bind oxygen and the other oxygen binding sites in the molecule demonstrate higher affinity for oxygen than normal-driving the oxyhemoglobin dissociation curve to the left.
- There is reduced circulating oxyhemoglobin and an increased affinity of the bound oxygen in methemoclobin for Hb so less is released to the tissues from these molecules.
