Rules2

And They Shall Know No Fear. The oldest and surest of the Astartes gifts: a battle-brother does not break. This rule is always in force. A fighter with it can never be Pinned by incoming fire, standing where a mortal soldier would throw himself flat. He automatically passes any Nerve test he is called to make, holding steady while lesser warriors rout around him. And once each round, should he be knocked Prone, he may Stand Up for free rather than spending an action to do so. Together these make the Marine a warrior who fights on through fire, terror and wounds that would shatter the will of anything less than transhuman — the durability of the Adeptus Astartes carried not in his armour but in his unbreakable mind.

Astartes Physiology. Beneath the plate a battle-brother is barely recognisable as a man. Two hearts, a lattice of grafted organs, ossified bone and skin cured against fire and blade make him a creature no unmodified human can match. So the line Marine stands at Toughness 5 with three Wounds, able to take blows that would kill a mortal outright and keep fighting. The Scout, whose implants are not yet complete, is the one exception, fielding the lighter Toughness 4 and two Wounds of a brother still in the making. The strength of a Chapter is written into its warriors' flesh: the body endures what the enemy cannot overcome.