The respiratory system of a dog works with the circulatory system to
provide oxygen to the body. Dogs inhale through their mouth and nose.
Air travels through the trachea and the bronchus into the lungs.
Air intake is controlled by expansion and contraction of the diaphragm.
Most of the space in a dog's thoracic cavity is taken up by the lungs and
et al. 1999).
The ciculatory system of a dog is made up of the heart and blood vessels
1977). The heart pumps blood throughout the body.
A dog’s activity level determines its circulation rate. Twenty percent
of the blood in the system is in the brain at all times. Blood flow
to other parts of the body is regulated by nerves and hormones. Higher
levels of oxygen are provided to the heart and limb muscles during exercise
through increased blood flow to those areas (Whitehead
et al. 1999).
The digestive system of a dog consists of the teeth, esophagus, stomach,
small intestine, and large intestine. The liver and pancreas also
aid in digestion although food does not actually enter either organ.
Food enters the digestive tract through the mouth where food is chewed
before passing into the esophagus. The esophagus leads to the stomach
and has thick, elastic walls that allow the dog to swallow large items.
Glands in the stomach produce acid and enzymes which aid digestion.
The stomach also holds food until it is ready to pass through the pyloric
sphincter into the duodenum, the first part of the small intestine.
Peristalsis moves food through the small intestine where it is mixed with
enzymes from the liver and pancreas. Nutrients are absorbed from
the small intestine into the bloodstream. Fluid is absorbed in the
large intestine. The large intestine also contains bacteria which
help to break down wastes which are then excreted (Whitehead
et al. 1999).
The functions of the excretory system of a dog include the removal of
toxins and metabolic wastes from the bloodstream and regulation of fluid
contents. Wastes are filtered from the blood by the kidneys.
Wastes then pass through ureters into the bladder to be stored until disposal.
Wastes in the form of urine pass out of the body through the urethra in
the penis in males or vulva in females (Whitehead
et al. 1999).
Regulation in dogs involves the endocrine system and the nervous system.
The endocrine system of a dog consists of glands and tissues that produce
hormones. The entire hormonal system is controlled by the pituitary
gland. Hormones play a role in regulation of stress responses, sexual
activity, and blood sugar levels (Whitehead
et al. 1999). A dog's nervous system centers around
the brain and spinal chord. The brain directs impulses to all parts
of the body through a network of nerves branching out from the spinal chord
The dog’s reproductive organs include the testes in the male and the
ovaries in the female. Sperm are produced in the male’s testes and
eggs in the female’s ovaries. Eggs pass from the ovaries to the uterus
through fallopian tubes. The uterus has two horns that meet at the
cervix and give it a distinctive shape. During mating the male ejaculates
sperm which may then fertilize a female's eggs. The average length
of a dog's pregnancy is 63 days (Whitehead
et al. 1999).
Dogs have three types of muscle: smooth, cardiac, and striated.
Smooth and cardiac muscles control involuntary actions. Smooth muscle
is found in large internal organs and cardiac muscle is found in the heart.
Striated muscle controls voluntary actions including those associated with
movement. Movement is achieved when a nerve impulse stimulates muscle
fibers to contract. Tendons attach the ends of muscles to bones.
When a muscle contracts or relaxes it pulls on the bone it is attached
to and causes it to move. Every muscle is paired with another muscle
that exerts an opposite force allowing a dog to achieve precise movements.
Jaw muscles allow a dog to bite and hold things in its mouth. Neck
muscles control movement of the head. Abdominal muscles provide a
thin covering of the abdominal cavity. Tail muscles allow a dog to
raise, lower, or wag its tail. Thigh muscles are used for running
and jumping. Muscles in the foreleg control the limbs and allow the
dog to dig and scratch with its paws. Dogs move from one place to
another in any of four ways: walk, trot, canter, and gallop. Dogs
are also able to swim and some breeds can pull sleds or carts (Whitehead
et al. 1999).
Sight: Dogs eyes are spheres full of fluid
that are held in place by muscles that move the eyes up, down, and from
side to side. Light-sensitive nerves found in the back of the eye
in the retina transmit impulses through the optic nerve to the brain to
be decoded into an image. Also found in the back of the eye is the
tapetum which reflects light. The lens is found near the front of
the eye and is held in place by a muscle that can change the shape of the
lens by contracting. The entire eye is covered by a tough membrane
called the sclera. The cornea is a transparent part of the sclera
that covers the front of the eye. Dogs' eyes are more sensitive to
movement and light than humans' eyes are, but they cannot distinguish colors
or the outlines of objects as well.
Hearing: Dogs' ears consist of four parts: the
ear flap, the ear canal, the middle ear, and the inner ear. The ear
flap is made of cartilage, muscle and skin and varies in shape from breed
to breed. It captures sound and funnels it through the ear canal
to the tympanic membrane. The middle ear contains the smallest bones
in the body: the hammer, anvil, and stirrup. These bones amplify
vibrations from the timpanic membrane and transmit them to the inner ear.
Vibrations enter the spiral-shaped cochlea in the inner ear where they
are converted into signals that are then sent to the brain. The organs
that provide a dog information about the alignment of its head and give
it a sense of balance are also found in the inner ear. Dogs can hear
sounds four times farther away than humans can and can detect sounds in
six-hundredths of a second.
Smell: Moisture on the surface of a dog's noise
helps to dissolve molecules in the air. These molecules then come
into contact with olfactory membranes inside the dog's nose. Nerve
impulses are then sent to the olfactory center in the brain. The
olfactory center of a dog is more than forty times bigger than that of
a human and a dog's sense of smell is estimated to be one million times
more efficient than humans. Dogs also have a vomeronasal organ in
the roof of their mouths which allows them to "taste" certain smells.
This organ transmits information directly to the part of the brain known
as the limbic system, which controls emotional responses.
Taste: A dog's senses of taste and smell are closely
linked and it is possible that dogs gain more imformation about food from
its smell than from its taste. Most of a dog's taste buds are clustered
around the tip of the tongue. Dogs can detect bitter, sweet, salty,
and sour tastes, but their sense of taste is relatively poor and they have
only one-sixth the number of taste buds that humans have (Whitehead
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