Merck Manual

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Pet Owner Version

Description and Physical Characteristics of Dogs


John A. Bukowski

, DVM, MPH, PhD;

Susan Aiello


Reviewed/Revised Jul 2011 | Modified Oct 2022
Topic Resources

Although dogs look very different from people, they share many of our body’s characteristics. They have a heart and circulatory system to transport blood, lungs to take in oxygen and rid the body of carbon dioxide, a digestive tract to absorb nutrients from food, and so on. However, it is the differences between dogs and people that are most interesting and that give dogs their unique characteristics as family members.

Dog breeds and sizes

Dog breeds and sizes

Body Size

Dogs come in many shapes and sizes. The smallest breeds include the toy and miniature varieties, such as the Toy Poodle, Papillon, Chihuahua, and Shih Tzu. These dogs usually weigh only 5 to 10 pounds (2.3 to 4.5 kilograms), or even less. Medium-sized dogs include many of the terriers and spaniels, which weigh in the 10 to 50 pound (4.5 to 23 kilograms) range. Larger still are the retrievers, shepherds, and setters, which often weigh 65 to 100 pounds (30 to 45 kilograms). Finally, the giant breeds, such as the Mastiff, Komondor, and Saint Bernard, can approach or exceed 200 pounds (91 kilograms). Of course, sizes vary within breeds, with males usually being larger than females. Mixed-breed dogs include all size ranges.


Dogs have a higher metabolism than people. They breathe faster, pump blood faster, mature faster, and have a higher normal body temperature ( see Table: Normal Canine Physiologic Values Normal Canine Physiologic Values Normal Canine Physiologic Values ). Young dogs seem to have even more energy than children. However, this high metabolism comes with a shorter life span. A common rule of thumb is that 1 dog year equals about 10 to 12 people years for the first 2 years, and then 4 people years (per dog year) after that ( see Table: Dog Years versus People Years Dog Years versus People Years Dog Years versus People Years ). Actual life span depends on health and size, with small breeds generally living longer than larger ones.


Temperature Regulation

Dogs are generally much better at conserving heat than at cooling themselves. In sled dogs, who can survive outdoors even in bitterly cold temperatures, the fur acts as an insulating “blanket” that retains the heat generated by the dog’s high metabolism. However, in hot or humid weather, most dogs have difficulty. Dogs cannot sweat, which is an effective form of evaporative cooling. Instead, dogs lose heat primarily by panting. These rapid breaths (10 times faster than normal) are an attempt to lose heat through evaporation by moving hot, moisture-filled air in and out. During the short, shallow breaths in panting, little air can be exchanged in the lungs. In fact, dogs must stop panting periodically to take a good respiratory breath. Drinking water also helps dogs cool down, and the canine hair coat helps insulate from the sun.

Because the cooling system of dogs is relatively poor, certain summer situations can be dangerous and even life threatening. Sadly, many dogs die of heat stroke every year ( see What to Do at the Scene and How to Transport What to Do at the Scene and How to Transport ). The most common problem is associated with being shut in a parked car. Even with the windows rolled down, the inside of a parked car can quickly reach 150°F (66°C) or more in the summer, which can cause heat stroke and death in a matter of minutes. Other dangerous situations involve being penned or tied out in the sun (without access to shade) or being locked in a poorly ventilated travel crate.

Ways to keep dogs cool during hot weather include air conditioning, spray misters, shade, dips in a wading pool, or gentle spraying with a garden hose. Keeping dogs wet during the heat of the day provides a method of evaporative cooling. Plenty of cool, fresh drinking water should be available at all times.

The Senses

Dogs have the same 5 senses that people do but to very different degrees. Some senses are less developed than in people, with others being extraordinarily more sensitive.


Dogs can see movement and light much better than people. In the retina of the eye, dogs have more of a specific type of cell called a rod, which is good at collecting dim light, so they have better night vision. A reflective layer in the dog’s eye, called the tapetum lucidum, magnifies incoming light. This reflective layer lends a characteristic blue or greenish glint to dogs’ eyes when light (for example, headlights of passing cars) shines into them at night. However, dogs do not have as much visual acuity as people, meaning that they cannot distinguish fine details as well. They also cannot differentiate colors as well because they have fewer of the cells in the retina called cones, which are responsible for color vision. Contrary to popular belief, however, dogs are not completely colorblind.

Nictating membrane, dog

Nictating membrane, dog

A unique feature of the dog eye is the nictitating membrane, which is also called the third eyelid. This additional eyelid is a whitish pink color, and it is found under the other eyelids in the inside corner (near the nose) of the eye. The third eyelid extends up when needed to protect the eyeball from scratches (for example, while traveling through brush) or in response to inflammation.


The ear canal of the dog is much deeper than that of people and creates a better funnel to carry sound to the ear drum. The average dog can hear about 4 times better than the average person, including sounds at higher frequencies than can be detected by the human ear. Dogs are also better at distinguishing the direction of a sound, which is an adaptation useful for hunting. Unfortunately, this deeper ear canal predisposes dogs to ear problems. Grease, wax, and moisture can build up in the ear, leading to inflammation and infection. Floppy ears or hair within the ears further limit ventilation, making matters worse. This is why many dogs need frequent preventive ear cleaning.

Smell and Taste

Dogs have an extraordinarily acute sense of smell; it is about a million times more sensitive than that of people. They can detect odors at extremely low levels and can distinguish odors that are subtly different. This is why dogs are able to sniff out drugs and explosives at airports, search for human victims at disaster sites (including victims deep under water), and follow the scent track of criminals.

Odor molecules dissolve in the moisture that coats the inside of the canine nose. Signals are then sent from the olfactory membranes in the nose to the olfactory center of the brain, which is 40 times bigger in dogs than in people.

Dogs also have an organ on the roof of the mouth that allows them to “taste” certain smells. As in people, taste and smell in dogs are closely linked. However, dogs gain much more information about food from smell than from taste. Dogs have only about one sixth the number of taste buds that people do, and their distinct sense of taste is actually quite poor.


Dogs have most of the same muscles, tendons, joints, and ligaments as people. All 4 of the dog’s limbs are maximized for locomotion, from a steady walk to a rapid sprint. In many respects, dogs run like horses, and have the same 4 gaits: walk, trot, canter, and gallop. The canine bones that are comparable to the long bones of our hands and feet are located in the dog’s lower legs. The angular hock in the hind legs is comparable to the ankle in people. Most dogs can swim, although some breeds specifically developed for swimming (for example, retrievers) can swim better than others (such as Bulldogs).

Leg bones, dog

Leg bones, dog

Pads and Nails

The canine paw contains specialized structures that help the dog move over different surfaces. The bottom of the paw is covered by thick, resilient pads that become callused after years of steady wear in direct contact with the ground. These pads protect the paw and help provide a secure grip on many types of surfaces. The toenails help provide traction while running and are also used for digging. Canine toenails are thick, brittle structures made up of a protein called keratin (just like hair). A large blood supply runs down the middle and feeds the cuticle (or “quick”) of the growing nail. Avoiding these blood vessels when trimming toenails can be difficult, especially when the nails are dark. Regardless, keeping nails trimmed is important because nails that snag or break during running or jumping can cause considerable bleeding and pain. Broken nails should be examined by a veterinarian, who can trim away the fractured part of the nail, treat the wound to stop any bleeding, and prevent infection. Dogs have rudimentary equivalents of human thumbs called dew claws that are found on the middle side of the front paws or lower front legs. Dew claws have no function, but they commonly snag and break. Dew claws should also be trimmed periodically to prevent snagging and to keep them from curling around and growing into the foot. They are commonly removed in very young puppies or as an additional surgical procedure when dogs are spayed or neutered.

Nails, dog

Nails, dog

Skin and Hair

Canine skin has several layers, including an outer epidermis that is constantly being replaced and an inner dermis that contains nerves and blood vessels. Canine skin is thinner and much more sensitive than human skin. Dogs should be bathed only with shampoos made specifically for pets. Shampoos and other topical products for people can be irritating to canine skin and should be avoided.

Canine fur grows from hair follicles in the skin. Dogs have compound hair follicles, with a central (guard) hair surrounded by 3 to 15 secondary hairs growing out of the same pore. Sebaceous (oil) glands within the skin lubricate the hair, keeping the coat shiny and water resistant. Hair growth is controlled by several factors, including nutrition, hormones, and time of year. In general, dogs shed at a slow steady rate all year round, with periods of increased shedding in the spring and fall. Shedding replaces hair gradually, without bald patches (which can be a sign of illness and should be investigated).

The main functions of the hair coat are to protect the skin and to help regulate temperature. Fur traps air, which provides a layer of insulation against the cold. Small muscles attached to the guard hairs allow dogs to raise these hairs, which improves air trapping. Dogs also raise their hackles as a threatening gesture in response to danger.

Different breeds of dogs have different types of hair coats. Breeds from northern climates (such as Huskies and Malamutes) have a soft, downy undercoat that provides better insulation in cold weather. Water breeds (retrievers, for example) have more long and stiff guard hairs to protect the skin and undercoat from harsh environmental conditions. Water breeds also have ample oil secretions to lubricate the hair. Breeds from warmer climates have shorter coats designed only to shade the skin. Poodles have very fine, curly hair that sheds far less than that of other breeds.

Teeth and Mouth

Dog teeth

Dog teeth

Like their wolf ancestors, dogs are carnivores with teeth designed for rending and tearing meat. They have 28 deciduous (baby) teeth that are replaced by 42 permanent (adult) teeth between 2 and 7 months of age ( see Table: Canine Adult Dentition Canine Adult Dentition Canine Adult Dentition ). The different types of teeth have specialized functions, depending on their position in the mouth. The front teeth, which include the 12 incisors and 4 large canine teeth (eye teeth), are designed for grasping and tearing. The rearward premolar and molar teeth grind food into smaller pieces that can be swallowed.


The mouth also contains the salivary glands, which secrete saliva that lubricates the food and begins digestion. The tongue helps guide food to the back of the throat and is important for licking up small food pieces and lapping up water. Dogs also lick as a sign of affection or subservience, or both.

Digestive and Urinary Tracts

The gastrointestinal tract includes the stomach, the small intestine, and the large intestine (colon). This system digests food into useful nutrients, absorbs water, and eliminates waste. Digestive problems often show up as vomiting or diarrhea, which can have many causes, including viral infections; worms; stress; or ingestion of bones, sticks, or other foreign material.

The urinary system eliminates nitrogenous wastes from protein breakdown and helps control fluid levels. Waste products are filtered by the kidneys and then sent through the ureters to the urinary bladder for storage. Urine is passed out of the body through the urethra. In males, the urethra doubles as a channel for sperm during copulation. Urinary infections are much more common in females and usually show up as frequent dribbles of urine that may be tinged with blood.

Both urinary and digestive problems are often associated with straining while urinating or defecating. At first glance, it may be difficult for dog owners to tell the source of the problem. Therefore, it is important to watch your dog while it eliminates and to note the character and color of the urine and feces. Your veterinarian may request a sample of the urine or feces, or both. Diarrhea usually consists of frequent, soft or runny feces that may be a different color (often yellow, gray, or black) than usual. Any sign of blood in the feces calls for veterinary attention. Repeated, unproductive attempts to pass a bowel movement can be a sign of serious constipation or bowel obstruction, which can be an emergency. Prompt veterinary attention is needed if the dog has a tense, painful abdomen or is passing only small amounts of bloody, gel-like feces.

Anal Glands

Rump rubbing or “scooting” is usually associated with impacted anal glands, although it can be confused with a digestive problem. The anal glands are located in a layer of muscle at the 4 and 8 o’clock positions around the anus. These scent glands contain a foul-smelling secretion that is normally expressed during a bowel movement. The secretions often thicken, which can plug the duct, causing pressure and irritation that can lead to infection. Many dogs need to have their anal glands manually emptied by their veterinarian on a regular schedule.

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