A patient with severe anemia (hemoglobin 5 g/dL) has a pulse oximetry reading of 99%. Which statement best describes their oxygen delivery?
AOxygen delivery is normal — saturation of 99% indicates the blood is carrying maximal oxygen
BOxygen delivery is critically reduced — hemoglobin concentration dominates total oxygen content
COxygen delivery is slightly reduced — dissolved oxygen partially compensates
DOxygen delivery cannot be assessed without knowing arterial PO₂
Oxygen content is CaO₂ = (1.34 × Hgb × SaO₂) + (0.003 × PaO₂). At Hgb = 5 g/dL and SaO₂ = 99%, the hemoglobin-bound component is only 1.34 × 5 × 0.99 ≈ 6.6 mL/100 mL — roughly one-third of normal. Pulse oximetry measures saturation, not content. The common error is equating 'fully saturated' with 'adequate oxygen delivery'; the hemoglobin concentration term dominates, so severe anemia is life-threatening even with normal SaO₂.
Question 2 Multiple Choice
The sigmoidal shape of the oxyhemoglobin dissociation curve, as opposed to a simple hyperbolic shape, provides which physiological advantage?
AIt allows hemoglobin to bind the maximum number of oxygen molecules at any PO₂ above zero
BIt keeps hemoglobin highly saturated across a wide range of alveolar PO₂ while still allowing substantial unloading in tissues
CIt increases the total oxygen-carrying capacity per gram of hemoglobin
DIt prevents any oxygen from being released until the tissues are severely hypoxic
The flat upper portion of the sigmoid (PO₂ 70–100 mmHg) means hemoglobin stays highly saturated even at altitude or with mild hypoventilation — a useful safety margin. The steep middle portion (PO₂ 20–60 mmHg) is where most unloading occurs in tissues. A hyperbolic curve (like myoglobin's) would stay saturated in tissues and unload poorly. Cooperativity produces this S-shape, which is not merely aesthetic — it is functionally essential.
Question 3 True / False
A rightward shift of the oxyhemoglobin dissociation curve, caused by tissue acidosis (Bohr effect), increases oxygen loading in the lungs.
TTrue
FFalse
Answer: False
A rightward shift reduces hemoglobin's affinity for oxygen — it promotes unloading in tissues, not loading. In the lungs (where pH is higher and CO₂ lower), the curve is shifted leftward, favoring loading. The Bohr effect is a fine-tuning mechanism that matches unloading to metabolic demand: acidic, CO₂-rich, active tissues extract more oxygen from each passing hemoglobin molecule.
Question 4 True / False
The majority of oxygen carried in arterial blood is dissolved directly in plasma rather than bound to hemoglobin.
TTrue
FFalse
Answer: False
At normal arterial PO₂ of 100 mmHg, dissolved oxygen contributes only ~0.3 mL/100 mL of blood. Hemoglobin-bound oxygen at normal hemoglobin levels contributes ~19–20 mL/100 mL — roughly 66 times more. Dissolved oxygen matters clinically only in extremes: hyperbaric oxygen therapy raises dissolved O₂ enough to sustain metabolism without functional hemoglobin, which is why it treats carbon monoxide poisoning.
Question 5 Short Answer
Why can a patient with severe anemia have critically low oxygen delivery despite a normal or near-normal pulse oximetry reading? Explain using the oxygen content equation.
Think about your answer, then reveal below.
Model answer: Pulse oximetry measures SaO₂ (the fraction of hemoglobin that is saturated), not total oxygen content. Oxygen content is CaO₂ = (1.34 × Hgb × SaO₂) + (0.003 × PaO₂). The hemoglobin concentration term dominates: with severe anemia (low Hgb), even fully saturated hemoglobin carries far less oxygen per deciliter of blood than normal. Dissolved oxygen (the second term) is trivially small at normal PO₂. So oxygen delivery — the product of cardiac output and content — is critically reduced despite a reassuring saturation reading.
This is one of the most clinically important distinctions in respiratory physiology. Saturation tells you what fraction of available hemoglobin is loaded; content tells you the actual amount of oxygen per unit volume. Both the 'how full' (saturation) and the 'how many' (hemoglobin concentration) terms matter. Treating only the saturation is like knowing a fuel tank is 99% full while ignoring that the tank is 1/3 its normal size.