Bloodhounds weigh from 36 to 72 kg (80 to 160 lbs). They are 58 to 69 cm (23 to 27 inches) tall at the withers. According to the AKC standard for the breed, larger dogs are preferred by conformation judges. Acceptable colors for bloodhounds are black, liver, tan, and red. Bloodhounds possess an unusually large skeletal structure with most of their weight concentrated in their bones, which are very thick for their length. The coat typical for a scenthound is hard and composed of fur alone, with no admixture of hair. The Bloodhound's physical characteristics account for its ability to follow a scent trail left several days in the past. The olfactory bulb in dogs is roughly forty times bigger than the olfactory bulb in humans, relative to total brain size, with 125 to 220 million olfactory receptors. Consequently, dogs have an olfactory sense 40 times more sensitive than that of a human. :246 In some dog breeds, such as bloodhounds, the olfactory sense has nearly 300 million receptors.