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Toxic injury in which cyanide impairs cellular oxygen use, including through inhibition of cytochrome c oxidase. Oxygen can be available in blood while tissues fail to use it effectively, distinguishing the mechanism from simple lack of inspired oxygen.^[ref]

A cytochrome is a protein containing a haem group whose iron can change oxidation state during electron transfer. The surrounding protein tunes the properties of the haem and its interactions with electron donors and acceptors. The name denotes a broad biochemical class, not one unique enzyme or one universal reaction. ^[ref]

For example, cytochrome c is a mobile carrier between respiratory complexes, while cytochrome c oxidase is a larger terminal complex that transfers electrons toward oxygen. Carrying an electron and pumping protons are distinct operations; not every individual cytochrome performs both. Spectral changes accompanying oxidation and reduction make these cofactors experimentally useful reporters of electron-transfer state.

Cytochrome c oxidase is respiratory complex IV. It receives electrons from cytochrome c and reduces molecular oxygen to water at metal-containing catalytic centers.

This reaction consumes substrate protons and is coupled to additional proton translocation across the membrane. Distinguishing chemical proton consumption from pumped protons is essential in counting the complex's contribution to the proton-motive force.

NADH supplies two electrons to respiratory complex I, then ubiquinone, respiratory complex III, cytochrome c and cytochrome c oxidase; terminal oxygen reduction forms water. Succinate dehydrogenase, or respiratory complex II, supplies another entry to ubiquinone. Respiratory complex I, respiratory complex III and cytochrome c oxidase move protons outward; respiratory complex II does not.^[ref]

For an original electron balance, supply exactly $6\mu \mathrm{mol}$ of NADH to an ideal preparation with no other electron donor or competing electron acceptor. The donor supplies $12\mu \mathrm{mol}$ of electrons. Complete reduction of one oxygen molecule needs four electrons, so the corresponding oxygen consumption is $3\mu \mathrm{mol}$. Adding the two formal half-reactions makes the result explicit:The cancelling electrons determine the donor-to-oxygen ratio. These chemical equations do not count transmembrane protons or determine an ATP yield. Chemical protons consumed in water formation and transported protons have different roles in the ledger.

Succinate is the most plausible substrate. Its oxidation by succinate dehydrogenase, or respiratory complex II, supplies electrons to the ubiquinone pool, bypassing respiratory complex I. Electron passage through respiratory complex III and cytochrome c oxidase supports a lower phosphorylation yield than entry through NADH at respiratory complex I, consistent with an apparent phosphorylation-to-oxidation ratio around $1.5$. The later involvement of a mitochondrial dicarboxylate carrier is additional evidence because succinate is one of its transported substrates. The ratio alone is not a chemically unique identification: other electron-entry routes and altered coupling can also produce a similar yield. Confirm by comparing defined substrates and sensitivity to a succinate dehydrogenase inhibitor.

Poisoning by belladonna characteristically combines delirium with dry secretions and dilated pupils; poisoning by hydrocyanic acid can cause very rapid collapse with failure of breathing. The two patterns are distinctive descriptions, not infallible diagnoses from one symptom.

With belladonna, expect dryness of the mouth and throat, difficulty swallowing, blurred vision and mydriasis. The skin may become hot, dry and flushed; the pulse is often rapid. Restlessness, confusion, visual hallucinations and agitated delirium may be prominent: the person may talk incoherently or pick at imaginary objects. Reduced intestinal activity and urinary retention can accompany the dryness. Severe poisoning can proceed to seizures, depressed consciousness, coma and circulatory failure. This is now understood chiefly as an antimuscarinic poisoning syndrome produced by alkaloids including atropine, which oppose the action of acetylcholine at muscarinic receptors.^[ref]

Hydrocyanic acid is hydrogen cyanide, historically also called prussic acid. Depending on the route and amount, early complaints may include dizziness, headache, anxiety, nausea, weakness and a sense of suffocation. Rapid breathing may give way to slow or failing breathing, followed by seizures, coma, circulatory collapse and death. A sufficiently large exposure may be so rapid that an elaborate sequence of preliminary symptoms is not observed. There is no single obligatory interval from exposure to collapse.^[ref]

In modern terms, cyanide impairs cellular use of oxygen, including inhibition of cytochrome c oxidase in cellular respiration. An apparently oxygenated colour therefore cannot exclude serious poisoning. A bitter-almond odour is an unreliable clue: it may be absent or not perceived, and no examiner should intentionally smell a suspect sample. The historical term “narcotic poison” does not mean that either belladonna or hydrogen cyanide acts like an opioid. The receptor and cellular explanations above are present scientific interpretations, rather than molecular explanations available in 1864.^[ref]