Anatomists are people who study the human body.

Everyone is unique. We have different skin colors, hair colors, body shapes and sizes — but we all look alike inside. If you could peek inside your own body, what would you see? Hundreds of bones, miles of blood vessels, and trillions of cells, all of which are constantly working together, doing all kinds of different things.

Skin

Main job: To protect your internal (inside) organs from drying up and to prevent harmful bacteria from getting inside.

How much: The average person has a total of six pounds of skin.

Main layers:

• Epidermis: Outer layer of skin cells, hair, nails, and sweat glands.
• Dermis: Inner layer of living tissue, containing nerves and blood vessels.

Skin facts: Your skin …

… is flexible so that you can bend and stretch.

… feels heat, cold, pain, pressure, moisture, irritation, and tickles because it has nerves.

… heals itself when wounded.

… keeps heat in on cold days and releases it as perspiration on hot days.

… is a watertight container for your body.

Bones

The largest bone in the body is the femur, or thigh bone; it is 20 inches long in a 6-foot-tall person.

Main job: To give shape to your body.

How many: At birth you had more than 300 bones in your body. As an adult you'll have 206, because some fuse together.

The smallest bone is the stirrup bone, in the ear; it is .1 inch long.

Kinds of Bones

• Long bones are thin; they are found in your legs, arms, and fingers.
• Short bones are wide and chunky; they are found in your feet and wrists.
• Flat bones are flat and smooth, like your ribs and shoulder blades.
• Irregular bones, like the three bones in your inner ear and the vertebrae in your spine, come in many different shapes.

Joints

Bones don't bend. It is the joint that allows two bones next to each other to move.

Main job: To allow bones to move in different directions.

Ligaments

Main job: These bands of tough tissue hold joints together. They are strong and flexible.

Muscles

Every day, the average person's muscles work as hard as if they were placing 2,400 pounds on a 4-foot-high shelf.

Main job: To make involuntary or voluntary body movement possible.

How many: Your body has more than 650 muscles. Each muscle does only two things: contract when being used and expand when resting.

Kinds of Muscles

• Skeletal muscles move your bones. They are called voluntary muscles because you decide when to move them. You have more than 400 voluntary muscles.
• The job of the cardiac muscle, or heart, is to pump blood through your body. The cardiac muscle is involuntary; it never stops working during your lifetime.
• Smooth muscles control your internal movements, such as moving food around in your intestines. These muscles are also found in the blood vessels, where they assist the flow of blood. Smooth muscles are involuntary.

Tendons

Your fingers are mostly powered by muscles in your palm and wrist.

Main job: To hold your muscles to your bones.

Tendon fact: Tendons look like rubber bands.

Viscera

This term refers to the organs, including the trachea or windpipe, lungs, liver, gallbladder, spleen, stomach, large intestine, small intestine, and bladder, that fill your body's chest and abdominal cavity. They belong to many different systems: respiratory, digestive, and urinary.

Main job: To provide your body with food and oxygen and to remove waste.

How many: The viscera are made up of 10 organs.

Glands

Main job: To manufacture substances that help your body to function in various ways.

Kinds of Glands

• Endocrine glands make hormones, which tell the different parts of your body when to work.
• Oil glands keep your skin from drying out.
• Salivary glands make saliva, which helps to digest carbohydrates in your mouth and aids in swallowing.
• Sweat glands make perspiration, which regulates your body temperature.

Cells

There are 26 billion cells in a newborn baby and 50 trillion cells in an adult.

Main job: To perform the many jobs necessary to stay alive, such as moving oxygen around your body, taking care of the fuel supply, communications, and waste removal.

Some Different Cells

• The egg is the largest human cell. Once it is fertilized, all other cells begin forming.
• Bone cells help build your skeleton by secreting the fibers and minerals from which bone is made.
• Fat cells store fat. They can shrink or grow. Once you have them you can't get rid of them.
• Muscle cells are organized into muscles, which move body parts.
• Nerve cells pass nerve messages around your body.
• Red blood cells carry oxygen around your body.
• White blood cells fight disease.