A blowhole is the hole through which a whale breathes. It is anatomically the same structure as our nostrils, although it has slowly worked its way to the top of the head, making it easier for the whale to breathe with minimal protrusion above the surface of the sea.

The nasal passages have been extended to travel to the top of the head, but in all whales the blowhole emerges in front of the cranium - the equivalent of the top of their forehead. In the toothed whales the skull is often concave, and the nasal passages may travel behind the 'melon', an bulging organ composed of fat; this gives the appearance of the blowhole emerging at the top of the head, as if it extended through the center of the skull.

It is notable that in the whales the trachea only connects to the blowhole, and not the mouth. They can breathe only through the blowhole, and the mouth is only used for eating. This also means that all sounds -- whale song and clicks for echolocation -- come from the blowhole and related structures, rather than the mouth. The nasal passages are connected to various air sacs, branches, folds, and plugs, which allow for the production of a wide range of sounds. The shape of the whale's skull and the various oil-, wax-, and air-filled sacs therein all contribute to produce carefully shaped and directed cries. This bulky added anatomy comes at the cost of the turbinate bones; having lost these structures, whales have a very limited sense of smell, and the toothed whales have lost theirs altogether.

The toothed whales (Odontoceti), which includes the dolphins, have only one blowhole; the other is completely closed up, although the nasal passage beneath remains. This closed-off passage appears to play a role in producing clicks needed for echolocation. The singular blowhole is usually a crescent shape, with the points of the crescent facing forward. Some toothed whales may have asymmetrical blowholes, such as the sperm whale, which has an S-shaped blowhole which emerges at the left front corner of their head. The sperm whale blowhole also has thick muscular flaps known as 'monkey lips' to help it produce its loud calls.

The baleen whales (Mysticeti), have two paired blowholes set in a V-formation, with the point of the V facing the front, which are placed far forward on the head. The baleen whales are more likely to have external structures around their blowholes; the bowhead whale has a muscular bulge called a 'stack', and the blue whale and humpback whale have ridges in front of their blowholes known as splash-guards.

Blowholes are rimmed by muscle and cartilaginous fiber; they are closed when the muscles are relaxed, and are pulled open when the whale surfaces. As whales reach the water surface to breathe, they will forcefully expel air through the blowhole. Along with the air, water droplets and mucus are expelled; in addition, the warm wet air from the animal's lungs may condense in cold air, resulting in a visible and often impressive spray known as the 'blow'.