Outwardly, the members of the Klingon species appear to be similiar to Terran humans and most other humanoids. They tend to be larger in size, both in height and muscle mass, and they tend to reach their full height at an age earlier than most humans, which tends to average between six and six-and-a-half feet.
The most distinguishing feature of the Klingon anatomy is the pronounced bony ridge which runs the centerline of their body, beginning at the tip of the nose and flaring at the forehead where it is most obvious. The pattern continues down the spine, and in many cases the chest, to a lesser degree. The pattern of the forehead ridges varies from Klingon to Klingon, though members of the same geneology tend to have similar inherited features. Additionally, Klingons have short, hair-like spines on their legs and a single, large claw on each foot, the remnant of a dew-claw, an ancestral phalange which is paid homage to in the form of the traditional Klingon boot-spike or claw.
Internally, Klingons differ greatly from humans, benefitting from a redundancy in orgrans called "brak'lul" in their native tongue. They have a large, eight-chambered heart, two livers, and twenty-three rib-pairs; this reinforcement of the chest is often externally visible in the pronounced armor-like ridges on the breast. Their spinal column is also reinforced, visible in the bony ridge along their back. They have double-lined neural pia matter, a secondary synaptic system, and even a small, auxiliary brain at the base of their spinal column, protected by the pelvic girdle, though it's highly unlikely that this organ performs as any more than a redundant control for the autonomous functions of the body.
A substance called "Qachqeb" in Klingon blood cells accelerates healing by rapidly carrying oxygen and needed nutrients to injured areas in a process referred to as the "veK'tal response." This substance is what gives Klingon blood its odd magenta, almost lavender coloration.
Klingon skin tends to be darkly pigmented, usually chestnut in color, though many of the bloodlines from the northern regions tend to be even darker in color. Dark brown and black hair is prevalent in Klingon genetics, though the occasional dirty blonde crops up. Traditionally Klingon males wear their hair long and unkept, and their faces unshaven.
Klingons are, by nature, hunters, and the flame of the predator burns in their blood. They have a highly developed sense of smell and good vision. Their preferred food is flesh, preferably raw or nearly so, and either freshly killed or served while still living. This predatory and aggressive nature is due in large to a hormone called "Qul'Iw" produced in large quantities in both male and female Klingons. Similar to the human hormone testosterone, Qul'Iw production is stimulated by a wide range of experiences: the threat of physical danger, sexual arousal, the visual image of a predatory animal, failure to succeed at a task, sensory experiences evocative of combat, or close proximity to another Klingon producing the hormone. Qul'Iw dissipates much more slowly than it permeates the brain and body. Klingons get angry easily and forgive only with time and effort. Presence of the hormone in the brain not only sharpens reflexes, ramps up the veK'tal response, and shuts off pain receptors; it increases by 13.9% the brain's ability to reason and process sensory input. Thus a Klingon mathematician benefits as greatly from a surge of Qul'Iw as a warrior would, provided he's sufficiently angry and frustrated.
An ancestral passion for violence is deeply rooted in their genetics and their heritage, and has shaped their culture into a true warrior's society. Klingons are an exuberant people, and their preference of activity to passiveness is often, though not always incorrectly, interpreted as aggressiveness or belligerence. Aggression and conflict are natural, preferred states from most Klingons, and they have a long tradition of settling personal disputes through physical confrontation.