Venoms and poison are substances that are very harmful. The only difference between them lies in how they enter an organism. Venom is a more specific term referring to substances produced by tissues of an organism. It needs an apparatus to get injected. For instance, venomous snakes like viper produce venom in glands in its head and injects through its hollow long pointed tooth.
Poisonous organisms like poisonous plants or animals are harmful when eaten (fish like puffers) or touched (frogs, toads). The poisonous ones do not deliver the toxin directly like how the venomous ones do. Most poisonous animals do not produce poison themselves but store them from their food. Many species of fish are poisonous because of what they eat.
There are many kinds of poisons like mercury, rat poison, arsenic, cyanide, paint thinners, venom from some species of spider, snakes etc. These have a different combination of toxic substances. They work in different ways in order to disturb the working of different organs causing mild to severe side effects or even prove fatal.
The toxins mix with the blood and get carried to all parts of the body. All cells in the human body have cell walls. These walls are membranes that enclose the cell allowing movement of water and nutrients within and outside the cell. Poisons of any type cause the cell walls to become so rigid. Now the cells cannot perform their basic task of rebuilding or detoxifying themselves. Also the transport of oxygen, nutrients via the blood vessels to each and every cell is affected. Without sufficient oxygen and other nutrients, cells die. Also body is not able to fight back because red blood cells, white blood cells become rigid and cannot work. This results in failure of organs in the body causing death.
Antidotes are substances that are used in treating poisoning. There are different antidotes for different kind of poisoning. Lots of interesting researches are going on in the modern medical industry trying to use ‘poison’ as an antidote. Proteins taken from poisonous plants are being used to treat cancer and heart attacks.