A comparison of dog and human anatomy must necessarily involve all of the body—humans and dogs share the same molecular and cellular structures, organs and systems, with major differences showing only in physiology and structure.
The gross anatomy of a dog and a human consists of a set of biological systems—the skeletal and integumentary, digestive, muscular, lymphatic and endocrine, cardiovascular, respiratory, nervous, urinary, male and female reproductive and urinary systems. Each system consists of organs, glands and blood vessels that work together in the animal's body to support particular functions. All mammals have the same biological systems in approximately the same places in differing bodies; varying sizes and shapes commonly being the only distinguishing features.
The skeletal and integumentary systems include all the bones, skin, hair and nails of the mammal body. Protecting the organs and soft tissues from injury and disease is the primary function of both systems, while the skin and hair (or fur) insulates against heat loss.
The digestive system is made up of the mouth, teeth, salivary glands, esophagus, stomach, gall bladder, large and small intestines, pancreas and liver. Food and nutrients are absorbed, disseminated and eventually, eliminated through the digestive system.
The cardiovascular system includes the heart and blood vessels and performs the job of pumping blood containing nutrients and oxygen throughout the entire body.
Hormones (chemical substances that cause other organs in the body to become active) are produced in the thyroid, parathyroid, adrenal and pancreatic glands of the endocrine system. The lymph nodes and vessels of the lymphatic system help to immunize the body against disease. This system also works with the heart and blood vessels to return accumulating body fluids into the blood stream.
The human and dog respiratory systems are made up of the nose, trachea, lungs and the bronchi (the smaller airways) of the lungs. This system is responsible for the intake of oxygen into the body and the elimination of carbon dioxide.
The urogenital system is a combination of the male or female sexual/reproductive organs and the organs of the urinary system—the kidneys, urinary bladder, ureters and urethra. The reproductive organs of both dogs and humans form the sperm and ova needed to produce offspring. Waste products and toxins are removed from the body in urine by the urinary system.
Innately similar in both physiology and function, the anatomy of humans and canines differentiates in ways pertinent to the evolution of each species. The canine eye, which has two types of cone cells, sees a different range of color than the human eye, which has three types. Dog color vision is similar to human red-green color blindness. A human's spinal skeleton has the same number of bones as a dog's, but the canine coccyx (tailbone) has a number of smaller bones attached to it that form the dog's tail.
Because the modern dog evolved from hunting animals, the cortex of the canine brain that controls the sense of smell is much larger than the corresponding part of the human brain. In contrast, the cerebrum -- the thinking, reasoning section -- of the human brain is bigger than that of the dog's.
The adult dog's skull grows 42 teeth, including four extremely prominent canine teeth, due to a carnivorous past. Canine teeth are sharp and pointed to allow the dog to tear and chew meat. Humans begin life with 20 deciduous teeth that fall out and are replaced by 32 adult teeth. Dull, flat human teeth grind grain and chew meat; the tearing function did not evolve because human ancestors were primarily omnivorous.
The reproductive and sexual systems of both species have similar organs, but these organs function in slightly different ways. The penis of the male dog will swell and turn downward after ejaculation, sometimes forcing the male dog to remain attached to the female and allowing the sperm to stay in the female's vagina. The uterus of the female dog has two compartments, or "horns." Unlike a human uterus that only has one compartment, the canine uterus is formed to develop and nurture two or more embryos simultaneously.
Because canine and human anatomy is so similar, much human medicine has been developed through laboratory testing on dogs. Conversely, veterinarians routinely use human medications -- labeled and dosed for animals or off-label -- on their canine patients.