The brak'lul is a Klingon term for a characteristic redundancy in Klingon physiology. All vital bodily functions are protected by a redundant organ or system. Klingons possess two livers, an eight-chambered heart, 23 ribs, two stomachs and a double-lined neural pia mater. This allows Klingons to survive injuries that normally kill other humanoids. Some Federation scientists, such as Doctor Toby Russell, believed it to be a good design in practice, but also noted that the extra redundancy meant there was also much more chance for a system to fail or cause problems.

The redundancies due to the brak'lul helped Lieutenant Worf survive in 2368 after a risky surgery involving technology known as the genitronic replicator. (TNG: "Ethics")