The brak'lul was a Klingon term for a characteristic redundancy in Klingon physiology. All vital bodily functions were protected by a redundant organ or system. Klingons possessed two livers, an eight-chambered heart, twenty-three ribs, two stomachs, three lungs, two urinary tracts, and a double-lined neural pia mater. (TNG: "Ethics"; VOY: "Lineage", DIS: "Will You Take My Hand?") This allowed Klingons to survive injuries that normally killed other humanoids. Some scientists, such as Doctor Toby Russell, believed it to be a good design in theory, 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 genetronic replicator. (TNG: "Ethics")