The following is an extract from the e book "Caring for Dogs"  written by staff of ACS Distance Education -click for more information on the book 


Today dogs can vary greatly in size, shape and colouration. However, all dog breeds are descended from the wolf and share many physical and biological characteristics. All dogs are classified as the same species Canis familiaris. They all have a heart linked to a circulatory system, which transports blood and oxygen around the body, a central nervous system, digestive system and a sexual reproductive system. 


Physiological Characteristics

Circulation
The circulatory system in dogs consists of the heart and connected blood vessels. The purpose of the circulatory system is to transport blood to organs, muscles and tissues. The activity level of the dog will influence the rate at which its heart pumps blood around the body.  During exercise, the heart will beat at a faster rate than when a dog is at rest. The size of a dog will also establish the heart rate. In general:

• Large breed – 60 – 100 beats per minute
• Small breed - 100 – 150 beats per minute

Respiration
The respiratory system in dogs works in conjunction with the circulatory system to transport oxygen around the body. Dogs breathe in oxygen through their nose and mouth. Oxygen then travels via the trachea and bronchus to the lungs. A muscle below the lungs called the diaphragm contracts rhythmically creating a vacuum which causes the lungs to pull in air. When it returns to its former shape, the air is forced out of the lungs again through the trachea.  Most dogs will breathe at a rate of 14-16 breaths per minute.

Thermoregulation
Thermoregulation is the ability of an animal to regulate its body temperature. Dogs are mammals and are therefore warm-blooded or endothermic. This means that dogs are able to control their body temperature to a degree. The normal body temperature for dogs is around 38 – 39.2ºC (100-103ºF).  Dogs use various methods to help maintain their body temperature. These are:

• Panting – when hot, this is a dog’s main method of cooling the body. Panting increases air flow over the moist surfaces of the upper respiratory tract. This leads to increased evaporation and ventilation. Panting breaths are generally 10 times faster than normal breathing.
• Coat – The coat of the dog can insulate it from heat loss or gain.  Guard hairs can be raised to assist with thermoregulation. Most dogs will shed their coat twice a year. Shedding will remove hairs not needed in summer and prepare a thicker coat for the cooler winter months.
• Drinking water – dogs can cool their bodies by drinking water.
• Sweating – dogs have sweat glands in the pads of their feet. However, these are not a main tool for thermoregulation.
  
  

Metabolism
In general, dogs have a higher metabolism than humans. They breathe, pump blood and mature faster than humans. They also have a higher average body temperature. As the dog’s metabolism is much higher than humans they tend to age at a different rate. It is generally thought that dogs will age 10 to 12 human years for ach of the first 2 years of its life and then 4 human years each year after that.  

Smaller dogs tend to live longer than larger dogs. A small dog will often live to over 16 years while a medium to larger dog will generally live between 10 to 13 years. Giant dogs, such as the Great Dane have a life span of less than 10 years.


Mouth and Teeth
The mouth contains jaws, teeth, tongue and salivary glands. The purpose of the jaw is to hold the teeth for grasping and tearing food. The salivary glands secrete saliva to help lubricate food for swallowing. The tongue helps to guide food to the back of the mouth and throat and for licking water and small pieces of food. 

Dogs have an organ on the roof of their mouth which assists in tasting smells. Smell and taste are closely linked and dogs gain most of their information from these two senses. Dogs actually have only 1/6th of the amount of tastebuds as humans. Therefore their sense of taste is reduced.


Ears 
The ear in dogs is concerned with hearing and the sense of balance in the animal.  It is divided into four parts: the ear flap, the ear canal, the middle ear, and the inner ear.


Eyes
The front of the eyeball is known as the cornea. It is transparent in order to allow light to enter the main part of the eye. The inside of the eyeball is lined with a tissue comprised of specialised cells called the retina. The retina is very sensitive to light impulses. The retina in dogs has a specific rod cell that can collect dim light.  Impulses are received on the retina, transmitted through the optic nerve to the cerebrum part of the brain and there they are turned into visual images. 

The lens of the eye is a bi-convex structure, shaped like this ().  The lens is attached to the eye by very fine fibres of ligament.  At one end of the lens, the fibres join the ciliary body, which is controlled by the ciliary muscle. Dogs also possess a reflective layer in the eye which magnifies incoming light. This reflective layer is called the tapetum lucidum and gives the dog a blue or green eye shine when seen by lights at night. 

The eye of a dog is more sensitive to light and movement than a human eye due to the presence of the rod cell. However, they are not able to distinguish fine details that the human eye can. They also have difficulty differentiating colours, but are not colour-blind as was thought to be the case.

Dogs possess a unique feature on the eye called a nictitating membrane. It is known as a third eyelid and is light-pink in colour. It is found in the inside corner under the eyelids but can be extended to protect the eye when needed. 


Nose 
Dogs are renowned for having an acute sense of smell. It is believed that their sense of smell is around a thousand times more sensitive than humans. They can detect and differentiate between extremely low levels of odours. This is due to the design of the nose and the highly developed olfactory senses (i.e. sense of smell).

The dog’s nose has two nostrils that lead to a nasal cavity for inhaling air and odours. The nasal cavity contains ethmo-turbinate bones which are lined with olfactory receptor cells. These receptors are located in the olfactory epithelium which also has a large supply of olfactory nerves that connect to the olfactory lobe within the dog’s brain. 

The nose of a dog is usually cool and moist. This moisture is secreted from a mucous gland in the nasal cavity. Dogs use sniffing to increase identification of odours. This is different to their regular breathing as they use short rapid inhalations and exhalation to force air into the olfactory epithelium.