Cyanide stops the formation of enzyme after it is swallowed by someone. It destroys the natural qualities of body enzymes and makes a solid complex by binding with them.
A small amount of cyanide can show its action within a minute. Only 50 or 60 milligram cyanide is enough to kill someone. Hydrogen cyanide is a gaseous form of cyanide that contains H-C=N and the solid form of cyanide known as solid salts include the cyanide ions (H=N). Hydrogen cyanide has several uses. It can kill insects and rodents found in warehouses, ships and railway cars and sometimes it is visible on certain types of fruit trees.
When cyanide infuses into our bloodstream, it combines instantly with iron atom, resulting to halt the formation of haeme section of the Haemoglobin. This phenomenon engages iron atoms and stops them to deliver oxygen to every section of our body. It causes oxygen deficiency in tissues, which is known as Hypoxia. This Hypoxia is also called histotoxic hypoxia as it happens due to the poisonous material. It acts on the brain first and causes unconsciousness within 10 to 20 seconds and leads to death in 4 to 5 minutes if it is left untreated.
Cyanide creates a stable complex by combining with the oxidation enzyme which halts the oxidation process of glucose inside a cell. There are some enzymes in our body cells that release electrons to reduce oxygen atom in the cell. Those electrons get engaged by cyanide and can’t take part in the reduction process.
After this process, all the cells in our body start dying and cause breathing problem which further leads to the sudden failure of respiratory organ.
Since cyanide acts immediately or within a minute, doctors don’t get enough time to admit antidote. However cyanide poisoning can be treated with sodium nitrate and sodium thiosulphate, but doctors need enough time. It can be possible if patient consumes a low amount of cyanide.
Source: http://myqacorner.blogspot.in/2012/02/how-does-cyanide-cause-rapid-death.html