All About the Human Body

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, allof which are constantly working together and 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.

• 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 located in the ear; it is .1 inch long.

• 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.

• 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 ten 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.

Circulatory System

The circulatory system is the body's transport system. It is made up of a group of organs that transport blood throughout the body. The heart pumps the blood and the arteries and veins transport it. Oxygen-richblood leaves the left side of the heart and enters the biggest artery, called the aorta. The aorta branches into smaller arteries, which then branch into even smaller vessels that travel all over the body. When blood enters the smallestblood vessels, which are called capillaries and are found in body tissue, it gives nutrients and oxygen to the cells and takes in carbon dioxide, water, and waste. The blood, which no longer contains oxygen and nutrients, then goesback to the heart through veins. Veins carry waste products away from cells and bring blood back to the heart, which pumps it to the lungs to pick up oxygen and eliminate waste carbon dioxide.

Digestive System

The digestive system is made up of organs that break down food into protein, vitamins, minerals, carbohydrates, and fats, which the body needs for energy, growth, and repair. After food is chewed and swallowed, it goes down the esophagus and enters thestomach where it is further broken down by powerful stomach acids. From the stomach the food travels into the small intestine. This is where your food is broken down into nutrients that can enter the bloodstream through tiny hair-like projections. The excessfood that the body doesn't need or can't digest is turned into waste and is eliminated from the body.

Endocrine System

The endocrine system is made up of a group of glands that produce the body's long-distance messengers, or hormones. Hormones are chemicals that control body functions, such as metabolism, growth, and sexual development. The glands, which include the pituitary gland, thyroid gland, parathyroid glands, adrenal glands, thymus gland, pineal body, pancreas, ovaries, and testes, release hormones directly into the bloodstream, which transports the hormones to organsand tissues throughout the body.

Immune System

The immune system is our body's defense system against infections and diseases. Organs, tissues, cells, and cell products work together to respond to dangerous organisms (like viruses or bacteria) and substances that may enter the body from the environment. There are three types of response systems in the immune system: the anatomic response, the inflammatory response, and the immune response.

• The anatomic response physically prevents threatening substances from entering your body. Examples of the anatomic system include the mucous membranes and the skin. If substances do get by, the inflammatory response goes on attack.
• The inflammatory system works by excreting the invaders from your body. Sneezing, runny noses, and fever are examples of the inflammatory system at work. Sometimes, even though you don't feel well while it's happening, your bodyis fighting illness.
• When the inflammatory response fails, the immune response goes to work. This is the central part of the immune system and is made up of white blood cells, which fight infection by gobbling up antigens.About a quarter of white blood cells, called the lymphocytes, migrate to the lymph nodes and produce antibodies, which fight disease.

Lymphatic System

The lymphatic system is also a defense system for the body. It filters out organisms that cause disease, produces white blood cells, and generates disease-fighting antibodies. It also distributes fluids and nutrients in the body and drains excess fluidsand protein so that tissues do not swell. The lymphatic system is made up of a network of vessels that help circulate body fluids. These vessels carry excess fluid away from the spaces between tissues and organs and return it to the bloodstream.

Muscular System

The muscular system is made up of tissues that work with the skeletal system to control movement of the body. Some muscles – like the ones in your arms and legs – are voluntary, meaning that you decide when to move them. Other muscles, likethe ones in your stomach, heart, intestines and other organs, are involuntary. This means that they are controlled automatically by the nervous system and hormones – you often don't even realize they're at work.

The body is made up of three types of muscle tissue: skeletal, smooth, and cardiac. Each of these has the ability to contract and expand, which allows the body to move and function.

• Skeletal muscles help the body move.
• Smooth muscles, which are involuntary, are located inside organs, such as the stomach and intestines.
• Cardiac muscle is found only in the heart. Its motion is involuntary

Nervous System

The nervous system is made up of the brain, the spinal cord, and nerves. One of the most important systems in your body, the nervous system is your body's control system. It sends, receives, and processes nerve impulses throughout the body. These nerveimpulses tell your muscles and organs what to do and how to respond to the environment. There are three parts of your nervous system that work together: the central nervous system, the peripheral nervous system, and the autonomic nervous system.

• The central nervous system consists of the brain and spinal cord. It sends out nerve impulses and analyzes information from the sense organs, which tell your brain about things you see, hear, smell, taste, and feel.
• The peripheral nervous system includes the craniospinal nerves that branch off from the brain and the spinal cord. It carries the nerve impulses from the central nervous system to the muscles and glands.
• The autonomic nervous system regulates involuntary action, such as heart beat and digestion.

Reproductive System

The reproductive system allows humans to produce children. Sperm from the male fertilizes the female's egg, or ovum, in the fallopian tube. The fertilized egg travels from the fallopian tube to the uterus, where the fetus develops over a period of ninemonths.

Respiratory System

The respiratory system brings air into the body and removes carbon dioxide. It includes the nose, trachea, and lungs. When you breathe in, air enters your nose or mouth and goes down a long tube called the trachea. The trachea branches into two bronchialtubes, or primary bronchi, which go to the lungs. The primary bronchi branch off into even smaller bronchial tubes, or bronchioles. The bronchioles end in the alveoli, or air sacs. Oxygen follows this path and passes through the walls of the air sacs and bloodvessels and enters the blood stream. At the same time, carbon dioxide passes into the lungs and is exhaled.

Skeletal System

The skeletal system is made up of bones, ligaments, and tendons. It shapes the body and protects organs. The skeletal system works with the muscular system to help the body move. Marrow, which is soft, fatty tissue that producesred blood cells, many white blood cells, and other immune system cells, is found inside bones.

Urinary System

The urinary system eliminates waste from the body in the form of urine. The kidneys remove waste from the blood. The waste combines with water to form urine. From the kidneys, urine travels down two thin tubes called ureters to the bladder. When the bladderis full, urine is discharged through the urethra.