Cats are classed as mammals. This means that, like humans, they give birth to live young, which the females feed with milk and care for until they are old enough to look after themselves.

Cats are carnivores (meat eaters) and have become highly adapted predators. Domestic cats are well evolved for hunting small prey such as birds and small mammals like mice, although the occasional adventurous cat may try to hunt larger prey such as young rabbits.

The skeleton

• Provides the internal supporting structure for the body, especially the spine, hind legs and fore legs.
• Protects delicate internal organs:-
  • The skull protects the brain and other sensory organs such as the eyes.
  • The pelvis protects lower abdominal organs such as the bladder and uterus.
  • The vertebrae (spine) protect the nerves that form the spinal cord.
  • The ribs protect all the organs of the chest such as the heart and lungs.
• Enables movement. When the muscles that are attached to the bones around a joint contract, the bones move and the joints bend. It is the strength of the muscles in the cat's hind legs that allows them to jump high and chase prey.
• Contains the bone marrow that produces blood cells.
• Stores important minerals such as calcium and phosphorus.

Although cats have approximately the same number of bones as people, these are shaped differently and have become specially adapted to meet the cat's needs as a predator. A cat's skeleton is strong but very light. They have very flexible spines for increased agility - allowing them to jump and move quickly. The flexibility and strength of their joints allows them to jump great distances without injury and their long tails are important in helping them to stay balanced.

The skin

The skin actually forms the largest organ of an animal's body. It's mainly responsible for protecting the body from infection, physical damage and the loss of heat and water. A cat's skin is much more loosely attached to the underlying structures than a humans, further increasing the cat's flexibility.

A cat's skin is covered in fur (although some cats are bred to be furless). Fur helps keep the cat's body warm and prevent damage to the skin. It can also react to the presence of a threat and by standing on end - making the cat appear larger.

Fur can also have an indirect protective role. In the wild, coat colours may be affected by the cat's environment. Undomesticated cats with tabby coats are more likely to survive and reproduce as the colour of their coats provides camouflage from larger predators and improves hunting success. On the other hand, a black or black-and-white coat may be more prevalent in towns or cities. This is less noticeable now with the influence of selective breeding.

Some parts of the skin or hair have evolved to perform particular functions:-

• The feet are covered by pads of skin that is much thicker than the skin on the rest of the body.
• The whiskers are longer and thicker than normal hairs, and are very sensitive to touch. Apart from the face, they are found on several places on the body and provide cats with information about their surroundings.

The senses

• Sound
  Cats have large ears that they can move around easily, allowing them to pinpoint the origin of faint sounds. The inner ear also works with the brain to help maintain balance.
• Sight
  Cats also have very good eyesight, which has developed to improve their function as hunters. Their large eyes are set in the front of the skull to allow them to judge distance. Unlike the round pupils of humans, cats have elliptical pupils which become thin slits in bright light. A reflective layer at the back of the eye helps to catch all available light to help provide clearer vision at night: as we know cats are very good at seeing their way in the dark. This reflective layer makes a cat's eyes appear to glow at night.
  
  Cats also have an extra eyelid called the nictitating membrane (often referred to as a third eyelid or haw). This will move across the eye under the external eyelids from the centre outwards and helps provide extra protection for the eyes. This nictitating membrane is not usually visible, and indeed if it becomes so, this should be taken as a sign of poor condition or ill health.
• Smell
  Scent is very important to cats. They will use scents to mark territory, to recognise other animals (scent is far more important to cats than sight here) and to communicate with other cats. Smell is detected by nerve endings in the nose and interpreted by the brain. Cats have an additional scent-sensory organ in the mouth, which further improves their sense of smell.
• Taste
  The tongue is covered in taste buds and cats can detect tastes that are sour, bitter and salty. However - unlike humans - cats cannot detect sweet tastes.

The respiratory system

The respiratory system carries air from the nose to the small chambers in the lungs (called 'alveoli'). Via this system the air is warmed, filtered and transported to the lungs, where oxygen is taken in by the body and exchanged with carbon dioxide that is then breathed out.

The cardiovascular system

The cardiovascular system is made up of the heart, veins and arteries, and smaller blood vessels. It is responsible for the transport of blood in the body, carrying oxygen, nutrients and blood cells to where they are needed, and removing waste products (see below). Blood is also important for maintaining body temperature.

The urinary system

The primary function of the urinary system is to control the body's water balance and remove toxins. The kidneys filter the blood and remove excess water and toxins, which are then passed into the bladder and stored until the cat urinates. Chemicals in the urine can be used as a method of scent communication between cats. The kidneys also control levels of other substances in the blood such as electrolytes (sodium and potassium) and produce hormones involved in the control of calcium and production of blood cells.

The digestive system

The digestive system is responsible for the processing of food by the body, breaking it down and absorbing all the nutrients before expelling indigestible elements and other waste products. Digestion starts in the mouth, where cats take in food and begin to chew. Their teeth are specially adapted to their role as hunter. The large, sharp canines are used for grabbing prey and biting, whilst the sharp molars are adapted for tearing meat. The liver and pancreas are also part of this system, with a role in producing factors required to digest food as well as many other vital jobs.

The reproductive system

Uncastrated male cats have two testicles, which sit just below the anus. Males also have a barbed penis, which will be painful for the female during mating. Once sexually mature, females will 'call' regularly - particularly during early spring - and will be receptive to mating. Mating will stimulate the release of eggs. A female can carry several kittens at a time and may even carry kittens with different fathers.

The nervous system

This is responsible for carrying messages to the brain, and from it to the rest of the body, via the spinal cord and nerves. The brain manages all the processes in the body, from breathing to temperature control as well as movement and thought.

The endocrine system

The endocrine system consists of several glands that produce hormones which affect multiple processes in the body. These glands include the thyroid gland, the pancreas, the adrenal glands, parathyroid glands, the ovaries and testicles.