Ch 6 Gas Transport Flashcards
What are the two forms in which O2 is carried in the blood?
Dissolved and combined with hemoglobin
The dissolved form obeys Henry’s law, while the combined form involves hemoglobin.
What law describes the amount of dissolved O2 in the blood?
Henry’s law
The amount dissolved is proportional to the partial pressure of O2.
How much O2 is dissolved in blood at a Po2 of 100 mm Hg?
0.3 ml O2·100 ml−1 of blood
This is derived from the fact that for each mm Hg of Po2, there is 0.003 ml O2·100 ml−1.
What is the O2 capacity of normal blood?
About 20.8 ml O2·100 ml−1 of blood
This is calculated based on hemoglobin concentration.
What is the O2 saturation of arterial blood with a Po2 of 100 mm Hg?
About 97.5%
The saturation reflects the percentage of available binding sites occupied by O2.
What is the effect of methemoglobin on O2 release?
Makes it more difficult to release O2 from blood to the peripheral tissues
Methemoglobin is formed when the ferrous ion is oxidized to the ferric form.
What is the P50 value for human blood?
About 27 mm Hg
This value indicates the partial pressure at which hemoglobin is 50% saturated with O2.
What factors shift the O2 dissociation curve to the right?
Increase in H+, Pco2, temperature, and 2,3-diphosphoglycerate
A rightward shift indicates a reduced O2 affinity and increased unloading of O2.
What forms of carbon dioxide are carried in the blood?
Dissolved, as bicarbonate, and in combination with proteins as carbamino compounds
Each form plays a role in CO2 transport from tissues to lungs.
True or False: CO2 is less soluble in blood than O2.
False
CO2 is about 24 times more soluble than O2.
What is the role of 2,3-diphosphoglycerate (DPG) in O2 transport?
Shifts the O2 dissociation curve to the right, enhancing O2 unloading
Increased DPG occurs in chronic hypoxia.
Fill in the blank: The oxygenated form of hemoglobin is referred to as the _______ state.
R (relaxed) state
The deoxygenated form is called the T (tense) state.
What happens to the O2 dissociation curve in the presence of carbon monoxide (CO)?
Shifts to the left
CO competes with O2 for binding sites on hemoglobin, reducing O2 unloading.
What is the main abnormality associated with sulfhemoglobin?
It is not useful for O2 carriage
Sulfhemoglobin is formed when sulfur is incorporated into hemoglobin.
How does fetal hemoglobin (HbF) differ from adult hemoglobin (HbA)?
HbF has a higher oxygen affinity
This characteristic is advantageous in the hypoxic environment of the fetus.
What is the importance of the curved shape of the O2 dissociation curve?
Provides physiological advantages in O2 loading and unloading
The flat upper portion allows for stable O2 loading even with decreased Po2.
What is the first reaction in the formation of bicarbonate in blood?
CO2 + H2O ↔ H2CO3
This reaction is catalyzed by carbonic anhydrase in red blood cells.
What enzyme speeds up the formation of bicarbonate in red blood cells?
Carbonic anhydrase (CA)
This enzyme makes the reaction much faster in red blood cells compared to plasma.
What is the chloride shift?
The movement of Cl− ions into the red blood cell to maintain electrical neutrality as HCO3− moves out
This shift occurs due to the relatively impermeable nature of the cell membrane to cations.
What effect does reduced hemoglobin have on CO2 transport?
It enhances CO2 loading and H+ ion binding
Reduced Hb is a better proton acceptor than oxygenated Hb.
What is the Haldane effect?
Deoxygenation of blood increases its ability to carry CO2
This effect explains the relationship between oxygenation and CO2 transport.
What are carbamino compounds?
Compounds formed by the combination of CO2 with terminal amine groups in blood proteins
The most important protein involved is hemoglobin.
What is the primary form of CO2 in blood?
Bicarbonate (HCO3−)
Approximately 60% of the total venous-arterial difference in CO2 concentration is attributed to bicarbonate.
What is the CO2 dissociation curve?
A relationship between Pco2 and total CO2 concentration in blood
It is more linear compared to the O2 dissociation curve.
How does the CO2 concentration change with O2 saturation?
Lower O2 saturation leads to a higher CO2 concentration for a given Pco2
This is a manifestation of the Haldane effect.
What is the Henderson-Hasselbalch equation used for?
To relate pH to bicarbonate concentration and Pco2
The equation is pH = pKa + log(HCO3− / (0.03 × Pco2)).
What indicates a metabolic acidosis in terms of base excess?
Base excess less than -2
This indicates a reduction in bicarbonate concentration.
What causes respiratory acidosis?
An increase in Pco2 due to hypoventilation or ventilation-perfusion inequality
This results in a decrease in the HCO3− / Pco2 ratio.
What compensatory mechanism occurs in the kidney during respiratory acidosis?
Conservation of bicarbonate and increased excretion of H+ ions
This helps to restore the HCO3− / Pco2 ratio.
What causes respiratory alkalosis?
A decrease in Pco2 due to hyperventilation
This increases the HCO3− / Pco2 ratio and elevates pH.
What is the typical response of the kidney to respiratory alkalosis?
Increased excretion of bicarbonate
This helps return the HCO3− / Pco2 ratio back toward normal.
What does a Davenport diagram represent?
The relationships among HCO3−, pH, and Pco2
It illustrates how changes in bicarbonate concentration affect pH.
What is the significance of the pKa value in the Henderson-Hasselbalch equation?
It is typically around 6.1 for carbonic acid
This value helps calculate the pH based on bicarbonate and Pco2.
What occurs during renal compensation in response to high altitude?
Increased excretion of bicarbonate, returning the HCO₃⁻/Pco₂ ratio toward normal.
Renal compensation is slower than respiratory compensation.
What is the typical speed of respiratory compensation compared to metabolic compensation?
Respiratory compensation is typically fast, whereas metabolic compensation is slow.
This distinction is important in understanding acid-base disturbances.
Define metabolic acidosis.
A primary change in HCO₃⁻ that decreases the pH due to accumulation of acids in the blood.
Examples include poorly controlled diabetes mellitus and tissue hypoxia.
What is the primary stimulus for respiratory compensation during metabolic acidosis?
The action of H⁺ ions on peripheral chemoreceptors.
This leads to an increase in ventilation, lowering Pco₂.
What causes metabolic alkalosis?
An increase in HCO₃⁻, often due to excessive alkali ingestion or loss of gastric acid.
Respiratory compensation may be minimal or absent.
In the context of blood-tissue gas exchange, what factors affect the rate of gas transfer?
The rate of transfer is proportional to tissue area and the difference in gas partial pressure, and inversely proportional to thickness.
The blood-gas barrier is less than 0.5 μm thick.
How does O2 consumption in muscles change during exercise?
Additional capillaries open up, reducing diffusion distance and increasing area for diffusion.
CO₂ diffuses much faster than O₂, making O₂ delivery more critical than CO₂ elimination.
What is tissue hypoxia?
An abnormally low Po₂ in tissues, frequently caused by low O₂ delivery.
It can result from various conditions, including pulmonary disease or anemia.
List the four causes of tissue hypoxia.
- Hypoxic hypoxia (low Po₂ in arterial blood)
- Anemic hypoxia (reduced ability of blood to carry O₂)
- Circulatory hypoxia (reduced tissue blood flow)
- Histotoxic hypoxia (toxic substances interfere with O₂ utilization)
Cyanide poisoning is an example of histotoxic hypoxia.
What is the effect of carbon monoxide on the blood?
It reduces the ability of blood to carry O₂, leading to anemic hypoxia.
Carbon monoxide binds to hemoglobin with high affinity, reducing O₂ availability.
What is the O₂ capacity of blood?
The maximum amount of O₂ that can be bound to hemoglobin.
O₂ saturation is the proportion of binding sites occupied by O₂.
What shifts the O₂ dissociation curve to the right?
Increases in Pco₂, H⁺, temperature, and 2,3-diphosphoglycerate.
A right shift indicates reduced O₂ affinity of hemoglobin.
How is most CO₂ transported in the blood?
In the form of bicarbonate, with smaller amounts as dissolved and carbamino compounds.
This is crucial for maintaining acid-base balance.
What determines the acid-base status of blood?
The Henderson-Hasselbalch equation, particularly the ratio of bicarbonate concentration to Pco₂.
Acid-base abnormalities include respiratory and metabolic acidosis and alkalosis.
What is the typical Po₂ in some tissues?
Less than 5 mm Hg.
This low level is critical for ensuring adequate diffusion of O₂ to mitochondria.
What happens to Po₂ between adjacent open capillaries during O₂ diffusion?
Po₂ falls as O₂ is consumed by tissue.
This can lead to critical or inadequate O₂ levels for metabolism.
What is the expected Po₂ in skeletal muscle cells during exercise?
Closest to 3 mm Hg.
Myoglobin in muscle cells helps maintain O₂ levels.
Describe the acid-base status of a patient with arterial Po₂ of 50 mm Hg, Pco₂ of 60 mm Hg, and pH of 7.35.
Partially compensated respiratory acidosis.
The values indicate a respiratory issue with some metabolic compensation.
What is the clinical significance of a patient presenting with shortness of breath and a history of dark stools?
Possible anemia due to gastrointestinal bleeding, requiring further evaluation.
The low hemoglobin level indicates significant blood loss.
What would you expect for the oxygen concentration in mixed venous blood of a patient with carbon monoxide poisoning?
Reduced oxygen concentration.
Despite high arterial saturation, tissue utilization is impaired.
In carbon monoxide poisoning, what is the expected arterial Po₂?
Reduced arterial Po₂.
However, oxygen saturation may appear normal due to carboxyhemoglobin.
What is carboxyhemoglobinemia?
A condition resulting from carbon monoxide binding to hemoglobin.
It leads to reduced O₂ delivery despite normal O₂ saturation.
What are the expected changes after a blood transfusion in a patient with low hemoglobin?
- Increased arterial Po₂
- Increased arterial oxygen saturation
- Increased oxygen concentration of mixed venous blood
These changes occur due to the increase in red blood cell mass.
What was the arterial Po2 of the patient in the morning?
72 mm Hg
This value was unchanged compared to blood gas results from the preceding day.
What physiologic change would be expected with a Po2 of 72 mm Hg?
Increase in arterial oxygen saturation
This is one of the potential physiologic changes that could occur.
What are the possible physiologic changes expected with a low arterial Po2?
- Decrease in carbon dioxide production
- Decrease in the shunt fraction
- Increase in arterial oxygen concentration
- Increase in the P50 for hemoglobin
What does an arterial blood gas reveal for pH in this case?
7.48
This indicates a possible state of alkalosis.
What is the PCO2 level found in the arterial blood gas?
45
This value can indicate respiratory compensation or failure.
What is the HCO3 level in the arterial blood gas results?
32
This suggests a metabolic component to the acid-base status.
Which clinical situation could account for a pH of 7.48?
Anxiety attack
This is one of the potential conditions that can lead to respiratory alkalosis.
Which clinical situation is least likely to account for a pH of 7.48?
Opiate overdose
Opiate overdose typically causes respiratory acidosis.
Fill in the blank: A pH of 7.48 may suggest _______.
[alkalosis]
True or False: Severe chronic obstructive pulmonary disease would likely result in a pH of 7.48.
False
COPD typically leads to respiratory acidosis.
Which of the following could lead to a high HCO3 level: Vomiting or Uncontrolled diabetes mellitus?
Vomiting
Vomiting can lead to metabolic alkalosis and increased HCO3.
