Improved Toughness( Monster Manual IV, p. 203)
You are significantly tougher than normal.
Base Fortitude save bonus +2,
You gain a number of hit points equal to your current Hit Dice. Each time you gain a Hit Die (such as by gaining a level), you gain 1 additional hit point. If you lose a Hit Die (such as by losing a level), you lose 1 hit point permanently.
A fighter can select Improved Toughness as one of his bonus feats.