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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, and a double-lined neural pia mater. (TNG: "Ethics"; VOY: "Lineage") 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. (TNG: "Ethics") Some geneticists also speculated that Klingons evolved a third lung to afford them more stamina on the battlefield. (VOY: "Lineage")

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")

In 2373, Tom Paris doubted B'Elanna Torres' claims of nausea, as he believed that Klingons couldn't suffer from nausea, having a redundant stomach. She assured him both stomachs were unhappy. (VOY: "Macrocosm")

In 2377, The Doctor took issue with Torres' wish to delete entire DNA sequences of her Klingon-Human hybrid daughter's genetic code, giving the genes responsible for these redundancies as one notable example. When Torres protested that they were unnecessary, he assured her that they had a purpose. She asked him if her daughter required a third lung to survive, and he agreed that she didn't. However, he also pointed out that it could confer certain advantages, giving increased stamina as one example. Torres then protested that her daughter would not be a Klingon warrior, which he countered by saying that not only did she not know what her daughter would grow up to be, but that other people, such as athletes, would also benefit from greater lung capacity. (VOY: "Lineage")