Correct answer: In their blood.
Dromedaries, which are by far the most common type of camel, and other camels are capable of accumulating large amounts of water that allow them to survive for several days – and even up to two weeks – without drinking. They can lose more than 30% of their body weight in water, while for most other vertebrates losing 15% is fatal.
They are also capable of filling up again at an unparalleled speed, with volumes that would kill any other mammal: adult dromedary can take in close to 120 liters of water in just ten minutes, regaining that 30% of their body weight in a short space of time.
To endure this internal flooding, camels' red blood cells – or erythrocytes – have a special shape. They are oblong rather than round, which means they still work even when the blood is really thick. Compared with other mammals, camels’ red blood cells are also much more elastic – they can expand up to 240% of their smallest size. This capacity to absorb large amounts of water prevents the cells from rupturing (known as hemolysis) under the pressure caused by a large intake of water into the veins.
While this feature is the primary reason camels can endure extreme cycles of dehydration and rehydration, their bodies have evolved in a number of other ways to make them particularly sparing with water. In sweltering climates, camels can withstand enormous swings in body temperature during the day, from 34°C in the morning to more than 41°C in the heat of the afternoon. Most other species actively regulate their body temperature by sweating, which consumes a lot of water. Camels, moreover, have particularly efficient kidneys that can recover water from the urine and re-inject it into the blood stream. Finally, their nostrils can close in order to recover the water vapor in the air they breathe out.
As for their humps, they're filled with fat and serve primarily as an energy reserve.
B. Ali Zareil Yam & M. Khomeiri, Introduction to Camel origin, history, raising, characteristics, and wool, hair and skin: A Review, Research journal of Agriculture and Environemental Management, 2015