One lecture, run through the real thing. Nothing edited.

A 24-year-old man is being evaluated after a stab wound to the thigh. His vital signs and laboratory data are used to reason through the sequence by which inspired oxygen reaches the site of aerobic ATP production. Which of the following correctly orders the major steps that oxygen must complete after reaching the alveolus?

1. Alveolar diffusion → hemoglobin binding → convective transport by cardiac output → capillary diffusion → mitochondrial electron acceptance
2. Hemoglobin binding → alveolar diffusion → capillary diffusion → convective transport by cardiac output → mitochondrial electron acceptance
3. Alveolar diffusion → convective transport by cardiac output → hemoglobin binding → mitochondrial electron acceptance → capillary diffusion
4. Capillary diffusion → hemoglobin binding → alveolar diffusion → convective transport by cardiac output → mitochondrial electron acceptance
5. Alveolar diffusion → capillary diffusion → hemoglobin binding → convective transport by cardiac output → mitochondrial electron acceptance

The answer is A: alveolar diffusion → hemoglobin binding → convective transport → capillary diffusion → mitochondrial use. Ventilation delivers oxygen to the alveolus, oxygen diffuses down its partial-pressure gradient into pulmonary capillary blood, and it then binds hemoglobin inside erythrocytes. The heart provides the convective step that moves oxygenated blood to the periphery, where oxygen diffuses across systemic capillaries into cells and serves as the final electron acceptor in oxidative phosphorylation. Choice E is tempting because it also begins with alveolar diffusion, but it places systemic capillary diffusion before hemoglobin loading and before the heart moves the blood, which is out of order. This matches the oxygen pathway on slide 3.

• BHemoglobin cannot bind oxygen before oxygen has diffused from the alveolus into pulmonary capillary blood.
• CHemoglobin loading occurs in the lung before convective transport, and mitochondrial use cannot precede capillary diffusion.
• DSystemic capillary diffusion is the final delivery step, not the first event after inspired air reaches the alveolus.
• EThis inverts the loading sequence by placing tissue capillary diffusion and hemoglobin binding before cardiac transport.

Slide 3, beside the explanation

A student is asked to explain what a blood gas value of PaO2 95 mmHg actually represents in a patient with a hemoglobin of 6 g/dL. Which of the following statements best describes the physiologic quantity that PaO2 measures?

1. The percentage of hemoglobin binding sites currently occupied by oxygen
2. The total quantity of oxygen carried in each deciliter of arterial blood
3. The pressure exerted by oxygen molecules dissolved in plasma, which drives diffusion
4. The quantity of oxygen delivered to the tissues each minute by the circulation
5. The fraction of delivered oxygen that peripheral tissues extract from capillary blood

The answer is C: PaO2 is the pressure generated by oxygen molecules physically dissolved in plasma, and this partial pressure is what drives diffusion into and out of blood. It is not a measure of how much oxygen is present in total. Choice A is the most tempting error because PaO2 and SaO2 move together along the dissociation curve, but saturation describes occupancy of hemoglobin binding sites rather than dissolved gas pressure. Choice B describes CaO2, which additionally requires hemoglobin concentration. In this anemic patient, PaO2 may be entirely normal while total oxygen content is severely reduced. Slide 4 explicitly warns that these three terms are related but not interchangeable.

• AThat defines SaO2/SpO2, the percentage of occupied hemoglobin binding sites, not dissolved oxygen pressure.
• BTotal arterial oxygen per deciliter is CaO2, which requires hemoglobin concentration and saturation, not partial pressure alone.
• DOxygen delivered per minute is DO2, the product of cardiac output and arterial oxygen content.
• EThe extraction fraction is derived from the arterial-venous content difference, a separate concept from partial pressure.

Slide 4, beside the explanation

In a laboratory experiment, purified hemoglobin is exposed to stepwise increases in PO2 and the resulting saturation is plotted. The curve is sigmoid rather than linear, and the steepest portion lies between roughly 20 and 40 mmHg. Which property of the hemoglobin tetramer best accounts for this shape and its physiologic advantage?

1. Binding of one oxygen molecule increases the affinity of the remaining heme sites, steepening oxygen release as tissue PO2 falls
2. Each of the four heme groups binds oxygen independently, producing proportional saturation across all partial pressures
3. Dissolved plasma oxygen saturates first, and hemoglobin only begins to bind once plasma capacity is exceeded
4. Hemoglobin irreversibly binds oxygen at high PO2 and requires enzymatic cleavage for release at the tissue level
5. Hemoglobin releases oxygen only when the intracellular 2,3-BPG concentration falls below a threshold value

The answer is A: cooperativity. Oxygen binding to one heme group induces a conformational change that raises affinity at the remaining sites, generating the sigmoid curve. The plateau lets arterial saturation stay high despite fluctuating alveolar PO2, while the steep segment near tissue PO2 values means a small drop in PO2 unloads a comparatively large amount of oxygen. Choice B is the tempting incomplete answer: four independent binding sites would yield a hyperbolic, not sigmoid, relationship and would lose the steep unloading advantage. The P50 of approximately 26-27 mmHg sits within this steep region, as shown on slide 6.

• BIndependent, non-interacting binding sites would generate a hyperbolic curve without the steep tissue-unloading segment.
• CDissolved oxygen and hemoglobin binding occur simultaneously; plasma does not saturate first in a sequential fashion.
• DOxygen binding to hemoglobin is reversible and non-enzymatic, as expressed by Hb + O2 ↔ HbO2.
• E2,3-BPG modulates affinity but is not a threshold switch required for any unloading to occur.

Slide 6, beside the explanation

A 27-year-old man is running a marathon on a hot day. In his quadriceps capillary beds, muscle temperature is 39.5°C, local PCO2 is elevated, and lactate accumulation has lowered local pH. Which change in the oxygen-hemoglobin dissociation curve occurs in these capillaries, and what is its consequence?

1. ARightward shift with a higher P50, increasing oxygen release to the exercising muscle
2. BRightward shift with a lower P50, increasing oxygen release to the exercising muscle
3. CLeftward shift with a lower P50, increasing oxygen release to the exercising muscle
4. DLeftward shift with a higher P50, decreasing oxygen release to the exercising muscle
5. ENo shift, with oxygen release increasing solely because of the higher cardiac output

A 62-year-old woman has a hemoglobin of 10 g/dL, an SaO2 of 0.90, and a PaO2 of 60 mmHg. Using the arterial oxygen content equation taught in this lecture, which of the following is closest to her calculated CaO2?

1. A8.2 mL O2/dL
2. B10.4 mL O2/dL
3. C12.2 mL O2/dL
4. D13.4 mL O2/dL
5. E20.0 mL O2/dL

Two patients in a physiology demonstration each have an SpO2 of 98% and a PaO2 of 100 mmHg. Patient A has a hemoglobin of 15 g/dL and Patient B has a hemoglobin of 7.5 g/dL. Which statement best explains why their arterial oxygen contents differ by nearly half despite identical oximetry?

1. ASaturation reports the fraction of binding sites occupied, whereas content also depends on how many binding sites exist
2. BSaturation is unreliable at any hemoglobin below 10 g/dL, so Patient B's reading is falsely elevated
3. CThe dissolved oxygen term becomes proportionally larger in anemia and lowers total measured content
4. DAnemia shifts the dissociation curve leftward, reducing the oxygen that hemoglobin can bind at a given PO2
5. EReduced hemoglobin lowers PaO2 in the pulmonary capillary, which in turn lowers arterial content

A 58-year-old man with decompensated heart failure has a hemoglobin of 15 g/dL, an SaO2 of 98%, an arterial oxygen content of 20 mL O2/dL, and a cardiac output of 2.5 L/min. Which of the following best approximates his whole-body oxygen delivery?

1. A250 mL O2/min
2. B500 mL O2/min
3. C750 mL O2/min
4. D1000 mL O2/min
5. E2000 mL O2/min

An instructor presents two patients with markedly reduced oxygen delivery of approximately 500 mL O2/min. Patient 1 has Hb 7.5 g/dL, SaO2 98%, and cardiac output 5 L/min. Patient 2 has Hb 15 g/dL, SaO2 98%, and cardiac output 2.5 L/min. Which principle does this comparison best illustrate?

1. ADelivery is determined by the product of flow and content, so a proportional fall in either term impairs delivery equally
2. BDelivery is determined chiefly by arterial saturation, with flow and hemoglobin acting only as modifiers
3. CDelivery is preserved whenever arterial saturation remains above 95%, regardless of hemoglobin or flow
4. DDelivery depends primarily on dissolved oxygen, which is why supplemental oxygen corrects both scenarios
5. EDelivery falls only when arterial oxygen content and cardiac output are reduced simultaneously

A hemodynamically monitored patient has a cardiac output of 5 L/min, an arterial oxygen content of 20 mL O2/dL, and a mixed venous oxygen content of 15 mL O2/dL. Which of the following pairs correctly gives this patient's oxygen consumption and resting extraction ratio?

1. A100 mL O2/min; approximately 10%
2. B250 mL O2/min; approximately 25%
3. C250 mL O2/min; approximately 75%
4. D500 mL O2/min; approximately 50%
5. E1000 mL O2/min; approximately 100%

A 24-year-old woman presents 12 hours after significant postoperative bleeding. Heart rate is 118/min, blood pressure is 102/66 mmHg, SpO2 is 99% on room air, and hemoglobin is 7.0 g/dL. She reports fatigue and dizziness. Which mechanism best explains how her tachycardia partially preserves tissue oxygen delivery?

1. AIt increases cardiac output, offsetting the reduced arterial oxygen content in the delivery equation
2. BIt raises arterial oxygen saturation toward 100%, compensating for the reduced hemoglobin mass
3. CIt increases dissolved plasma oxygen by raising pulmonary capillary transit pressure
4. DIt shifts the dissociation curve leftward, allowing hemoglobin to load more oxygen per gram
5. EIt stimulates acute erythropoiesis, restoring hemoglobin-binding capacity within hours

The same postoperative patient with hemoglobin 7.0 g/dL and SpO2 99% on room air is placed on a nonrebreather mask, raising her PaO2 from 100 to 400 mmHg. Assuming saturation stays at 99%, approximately how much does her arterial oxygen content increase, and what does this imply?

1. AApproximately 0.9 mL O2/dL, a minimal gain because the dissolved term contributes little relative to hemoglobin-bound oxygen
2. BApproximately 4.0 mL O2/dL, a substantial gain because dissolved oxygen scales with the fourfold rise in PaO2
3. CApproximately 9.0 mL O2/dL, restoring content to normal because supplemental oxygen replaces hemoglobin capacity
4. DApproximately 0.9 mL O2/dL, a minimal gain because supplemental oxygen also lowers saturation through a right shift
5. ENo measurable change, because dissolved oxygen does not contribute to arterial oxygen content at any PaO2

A patient in early cardiogenic shock has a cardiac output that has fallen from 5.0 to 3.0 L/min while arterial oxygen content remains 20 mL O2/dL. Whole-body oxygen consumption is unchanged at 250 mL O2/min. Which of the following mixed venous oxygen content values is most consistent with this state, and by what mechanism?

1. A18.3 mL O2/dL, because reduced flow lengthens capillary transit and lowers extraction
2. B20.0 mL O2/dL, because extraction is fixed and consumption is met entirely by dissolved oxygen
3. C15.0 mL O2/dL, because the extraction ratio is fixed at 25% regardless of flow
4. D11.7 mL O2/dL, because tissues extract a greater fraction of delivered oxygen to maintain consumption
5. E5.0 mL O2/dL, because oxygen consumption must fall proportionally with delivery

A patient with chronic kidney disease and a hemoglobin of 8 g/dL has adapted over months to her anemia. Compared with a healthy control at the same PaO2, her erythrocyte 2,3-BPG concentration is elevated. What is the direct functional consequence for hemoglobin in her systemic capillaries?

1. AIncreased oxygen affinity, so hemoglobin retains oxygen and venous oxygen content rises
2. BDecreased oxygen affinity, so hemoglobin unloads more oxygen at any given tissue PO2
3. CIncreased total oxygen-carrying capacity, so arterial oxygen content approaches normal
4. DDecreased oxygen affinity in the lungs specifically, so arterial saturation falls markedly below 90%
5. ENo change in affinity, since 2,3-BPG regulates erythrocyte glycolysis rather than hemoglobin binding