Oxygen transport

Adult hemoglobin has four globin chains, two alpha and two beta, and each carries one heme group with an iron ion at its center. Each iron can bind one oxygen molecule.

• One globin chain wrapped around four iron ions:
• Correct: Four globin chains, each holding one heme group with an iron ion:
• Two heme groups joined by a single protein chain:

Dissolved oxygen = PO2 × solubility = 100 × 0.003 = 0.3 mL per 100 mL. That is far too little to supply the tissues, which is why hemoglobin carries almost all the oxygen.

• About 20 mL:
• About 3 mL:
• Correct: About 0.3 mL:

pH is a logarithmic measure of H+ concentration, and a lower pH means more H+. A fall of 0.2 pH units means the H+ concentration rose about 1.6-fold.

• Correct: It rose:
• It fell:
• It stayed the same:

Bound: 14 × 1.34 × 0.98 = 18.4 mL. Dissolved: 100 × 0.003 = 0.3 mL. Total: 18.7 mL per 100 mL.

• About 13.7 mL per 100 mL: 13.7 multiplies hemoglobin by saturation and forgets the 1.34 mL each gram carries.
• Correct: About 18.7 mL per 100 mL: Correct. 18.4 bound + 0.3 dissolved = 18.7 mL per 100 mL.
• About 0.3 mL per 100 mL: 0.3 mL is only the dissolved oxygen.
• About 30 mL per 100 mL: 30 mL would need far more hemoglobin; 14 g can carry at most about 18.8 mL.

The fetal curve lies to the left: at a low PO2 like 25 mm Hg, fetal hemoglobin is well above 50% saturated while adult hemoglobin is just under 50%. Maternal hemoglobin unloads on its steep part, and fetal hemoglobin picks up that oxygen at the same PO2.

• Correct: Fetal hemoglobin is more saturated at the same low PO2: Correct. Higher affinity lets fetal hemoglobin load where adult hemoglobin unloads.
• Adult hemoglobin is more saturated at every PO2, so it pushes oxygen out: The adult curve lies below the fetal curve at low PO2, not above it.
• The two curves are identical, so oxygen moves by chance: The curves clearly differ at low PO2, where the fetal curve is to the left.
• Fetal blood has a higher PO2 than maternal blood: Oxygen moves from maternal to fetal blood, so fetal PO2 is lower, not higher.

Saturation falls from 98% to 35%, a drop of 63 percentage points. As a share of what arrived: 63 ÷ 98 ≈ 0.64, about 64%, nearly three times the 23% released at rest.

• About 35%: 35% is the saturation left in the blood, not the share released.
• About 23%: About 23% is the share released at rest, when tissue PO2 is 40 mm Hg.
• About 98%: Hemoglobin does not release all its oxygen; at 20 mm Hg it is still about a third saturated.
• Correct: About 64%: Correct. (98 − 35) ÷ 98 ≈ 64%.

Working muscle raises carbon dioxide, H+ and temperature. Through the Bohr effect and heat, hemoglobin's affinity falls and the curve shifts right, so hemoglobin gives up more oxygen exactly where it is being used.

• PCO2 of the blood in the muscle capillaries: up. Faster aerobic respiration makes more carbon dioxide, which diffuses into the blood.
• Blood pH in the muscle capillaries: down. Carbon dioxide forms carbonic acid, which releases H+, and working muscle adds acid of its own, so H+ rises and pH falls.
• Hemoglobin's affinity for oxygen: down. More carbon dioxide, more H+ and higher temperature all stabilize hemoglobin's tense shape: the curve shifts right.
• P50: up. A lower affinity means a higher PO2 is needed to half-saturate hemoglobin.
• Oxygen released per 100 mL of blood: up. A lower tissue PO2 on the steep part of the curve, plus a right shift, lowers the saturation of blood leaving the muscle.

A standard two-light oximeter cannot distinguish carboxyhemoglobin from oxyhemoglobin, so it reads near normal. PO2 measures only dissolved oxygen, which carbon monoxide does not change. Meanwhile, much of the hemoglobin is carrying carbon monoxide. A CO-oximeter measures carboxyhemoglobin directly.

• Carbon monoxide raises both the dissolved PO2 and the oximeter's saturation reading: Carbon monoxide does not raise PO2; it leaves it normal while cutting the oxygen carried on hemoglobin. The oximeter reads high because it mistakes carboxyhemoglobin for oxyhemoglobin.
• The finger probes were placed on cold fingers: Cold fingers give weak or failed readings, but they do not explain a normal PO2 as well.
• The furnace fumes contain extra oxygen: Faulty burning makes carbon monoxide because there is too little oxygen, not extra oxygen.
• Correct: Oximeters read carboxyhemoglobin as oxyhemoglobin; PO2 ignores hemoglobin: Correct. Neither measurement sees the hemoglobin occupied by carbon monoxide.

Equal content does not mean equal delivery. Carbon monoxide on some heme sites holds the rest of each hemoglobin in its high-affinity shape, so patient A releases less of the oxygen she carries. Patient B's curve is normal, or shifted right by extra 2,3-BPG.

• Patient B, because anemia lowers PO2: Anemia lowers oxygen content, not arterial PO2, which depends on the lungs.
• Neither, because their oxygen contents are equal: Content is equal, but unloading is not; the shape of the curve decides how much is released.
• Patient B, because fewer red blood cells mean slower delivery: Delivery depends on the oxygen content and how readily hemoglobin releases it; patient B's hemoglobin releases normally.
• Correct: Patient A, because carbon monoxide also shifts the curve left: Correct. The left shift impairs unloading in patient A.

Cyanide blocks the electron transport chain, so cells cannot use the oxygen delivered to them. Oxygen stays in the blood, and venous PO2 stays high. That is histotoxic hypoxia.

• Hypoxemic hypoxia: Hypoxemic hypoxia means a low arterial PO2, and his is normal.
• Anemic hypoxia: Anemic hypoxia means low oxygen content, and his is normal.
• Stagnant hypoxia: Stagnant hypoxia comes from slow flow, which lets tissues take more oxygen and lowers venous PO2; his venous PO2 is high.
• Correct: Histotoxic hypoxia: Correct. Cells that cannot use oxygen leave it in the venous blood.

Saturation is a percentage of the hemoglobin present. Her 7 g is 99% loaded, but her oxygen content is only about 7 × 1.34 × 0.99 + 0.3 ≈ 9.6 mL per 100 mL, half of normal. That is anemic hypoxia.

• Her blood is carrying a normal amount of oxygen to her tissues: A full load on half the normal hemoglobin carries about half the normal oxygen.
• Her pulse oximeter must be faulty: A 99% reading is accurate here: nearly every hemoglobin molecule she has is loaded.
• Her lungs are failing to load oxygen: Her lungs load her hemoglobin almost completely, as the 99% shows. The problem is the amount of hemoglobin.
• Correct: Her hemoglobin is nearly full, but there is too little of it: Correct. Saturation is not content.