There are a number of physical reasons why greyhounds are faster than other breeds:

Greyhounds have very long legs and long spines. Their spines are also extremely flexible, so that the dogs have relatively long strides, and can put their paws onto the ground in such a manner that they can get as much thrust as possible.

The greyhound's "legginess ratio," the length of the dog's leg compared to the depth of the dog's chest, also contributes to the relative speed of the breed.

The dogs have well-developed hindquarter muscles.

Greyhounds have a "double-suspension gait," in which their paws leave the ground twice in each cycle of steps. (The feet of most dog breeds leave the ground only once per cycle.) Such a gait places heavy demands on the dog's body. Like any athlete, a greyhound must train regularly in order to race.

A greyhound's heart is slightly bigger than that of other breeds of dog. Additionally, the left wall of the greyhound heart is also relatively thick. When a greyhound is racing, its cardiac output increases by five times. Additionally, a greyhound's chest is large in order to hold the dog's large heart and lungs.

In a greyhound, more than 11 percent of the body weight is blood. In most other breeds, blood accounts for just 7.2 percent of body weight. Greyhounds tend to have more red blood cells than other dogs, and a greyhound's blood pressure is higher than that of other dogs.

Only 16 percent of a greyhound's body weight is taken up by fat. In other dogs of similar weight, there is more than twice that amount of fat in relation to body weight.

A greyhound's shape is relatively aerodynamic, and its skin is thinner than that of other breeds. Because of this, they are usually muzzled when they are with other dogs, since a relatively small cut can cause a lot of damage.

The hair of the greyhound's coat is shorter as well, and they usually don't shed. There is no undercoat, and the dog's skin does not produce oil, which means that it tends not to have the familiar "doggy odor."