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Inside Your Body Program Content

Cells are the basic unit of life that every living thing starts from and is made up of including humans.  Within our bodies, cells form tissues.  Smooth muscle tissue in the illustration is made of smooth muscle cells attached to one another.  When the muscle cells contract the tissue shortens.  Organs are formed from two or more different layers of tissue like the blood vessel in the illustration.  Organs work together within an organ system.  The circulatory system transports nutrients, oxygen and waste to and from cells within the body.  It consists of the heart, the blood vessels and the blood.

We all start out as a single cell, the fertilized egg.   The genetic information in that cell came from your parentís cells.  The sperm produced in males is a tiny cell that can swim with its whip-like tail called a flagellum.  It is formed by a special cell division process called meiosis that reduces the number of chromosomes in the nucleus to one set, half the number found in our body cells.  The egg cell from the mother is formed by meiosis in the ovaries.  

When the egg and sperm get together under the right conditions, a sperm nucleus moves into the egg cell and combines with the egg nucleus to form a cell that has two sets of chromosomes, one set from the father and one set from the mother.  The fertilized egg makes identical copies of itself by a cell division process called mitosis.  Every cell in your body comes from this original cell and has the same genetic information.  

From a single cell, you become a cluster of hundreds of cells.  After about a week those cells split up and begin to form the tissues that form the organs and organ systems of the developing embryo.  As tissues are formed, the dividing cells differentiate into the muscle cells, nerve cells, skin cells, blood cells and other cells that form your body.  Your nervous system is the first system to form, followed by the digestive organs and other the internal organs.  During the first few weeks you look very strange but after about 8 weeks you are human looking and all of your organs and organ systems are in place.  At this point you are a fetus that is only about two inches long and only weighs a few ounces.  You will remain in the womb for a total of 40 weeks as you develop into a full term baby hundreds of times that size.

Human egg cell

Human embryo four weeks after conception


Human fetus eight weeks after conception



Full-term baby at birth


If you scrape the inside of your cheek with a toothpick you will break cells loose from the tissue lining your cheek.  You can smear that on a microscope slide and see the cells that make up that tissue.  These cells are flat and transparent.  In the tissue they are stacked on top of each other to form a tissue several cell layers thick.  In a tissue smear made like this you will see cells still stuck together and a few cells isolated from the others.  These cells have a nucleus that contains the same genetic information that was found in the fertilized egg cell that they originally came from.  We now have the capability of cloning humans and other animals.  Scientists can take cells like these cheek cells, remove a nucleus and place it in an egg cell that has had its nucleus removed.  The egg cell can be stimulated to undergo mitosis and develop into an embryo that has the exact same genetic makeup as the person the cheek cell came from!

The illustration of the skin shows the arrangement of cells in tissue and the arrangement of tissues in our skin, which is the largest organ of our body.  The cells here are similar to the cells lining the cheek.  New cells are produced by mitosis in the bottom layer and are pushed up to the top to replace the skin as it wears.  In the top layers the cells become very tough and die so that what you see on the outside of your body is dead cells!  These layers of skin cells form the outer tissue of the skin called the epidermis.  There is a second layer of tissue below this that is made of extremely tough protein and contains the blood vessels of the skin and other organs such as hair follicles, sweat glands, and oil glands.  It is called the dermis and it is what is left of animal skin when we cure it into leather.

Dissection of a sheep eye shows the arrangement of tissues in an organ.  The white part of the eye is a tough protein layer called the sclera.  The clear part in front is called the cornea.  It lets light in and plays a big role in focusing.  This is the part of the eye that doctors modify in vision correction surgery such as LASIK surgery.  The back of the eye has fat attached to it to cushion and protect it in the eye socket.  You can also see little bits of muscle that move the eye in the socket. The optic nerve exits the eye in the back, carrying signals from the inside of the eye to the brain.  On the inside of the eye we find the lens and the clear gel that fills up the back of the eye called the vitreous humor.


The front part of the eye has two layers, the outer sclera and cornea along with an inner layer of muscle.  Part of that muscle is the iris that forms the opening called the pupil.  It changes the size of the pupil to regulate how much light comes into the eye.  The iris is also the colored part of the eye.  Another muscle called the ciliary muscle attaches to the lens.  You can see the dark ring around the lens where it attaches.  When it moves it changes the thickness of the lens and this is how your eye adjusts its focus.

The back part of the eye has three layers.  The middle layer inside the sclera is where the blood vessels are located.  It has dark pigment to keep light from reflecting inside the eye.  The inner layer is made up of millions of light sensitive nerve cells wired back to the brain.  This layer is the retina and the cable or wiring carrying the signals to the brain is the optic nerve.  This is the functional part of the eye that gives the brain the information it needs to enable us to see.  This arrangement of different tissues layered one on top of each other is how all of our organs are constructed.  The eye is one of the most complex organs in the body.  It is a sensory organ of the nervous system.

Our nervous system controls our body.  It receives sensory information from the eyes and other sense organs, analyzes it, and responds by controlling our body through motor nerves.  In this sheep brain you can see the tough outer covering called the meninges that protects the brain.    The sheep head shows how the skull protects the brain.  It also shows some of the different parts of the brain.  The cerebrum is what sets humans apart from other animals with our ability to think and reason.  The cerebellum coordinates muscle function.  The mid brain and brain stem control our body functions that we do not control consciously such as heart rate and breathing.  The spinal cord begins where the brain stem exits the skull.  The spinal cord from a cow shows the spinal nerves that exit from the cord to the body. 

A fetal pig is a baby pig that was killed when its mother was slaughtered for meat.  Sometimes these are sold to companies that preserve them with chemicals so that they can be used for dissection.  All mammals are about the same in the arrangement of their internal organs.  Pigs are very much like humans because they are omnivores and their digestive organs are very similar to those of humans.   There are two really large cavities or openings where most of our internal organs are located.  The thoracic cavity contains the heart and lungs and the abdominal cavity contains digestive and other organs.  A wall of muscle called the diaphragm separates these.  We will look at some of the organs in the fetal pig by organ system.

The respiratory system gets oxygen into our blood.  The larynx contains our vocal chords and prevents food from entering the trachea that carries air to and from our lungs.   When the trachea reaches the lungs it branches into smaller and smaller air tubes within the lungs that finally end in microscopic air sacs called alveoli.  These are surrounded by microscopic blood vessels called capillaries that pick up oxygen from the air in the alveoli.  Breathing moves fresh air into the alveoli.  Most of the work in breathing is done by the diaphragm.  It pushes down on the abdomen and opens up the chest cavity.  If you put your hand on your abdomen as you breath you will feel it move out as the diaphragm pushes down on it.

The circulatory system carries nutrients and oxygen to the cells of the body and picks up the carbon dioxide and waste that they produce.  The heart is the pump that circulates the blood through the blood vessels.  It is located between the lungs in the thoracic cavity.   It is a hollow organ made mostly of muscle.  In the sheep heart, we see where blood flows into the chambers and the muscle contracts to squeeze it out through the blood vessels exiting the heart.  There are two sets of chambers inside of the heart that are separated by a wall of muscle.  One side pumps blood from the body to the lungs to pick up oxygen at the same time as the other side pumps the oxygenated blood coming back from the lungs to the body.  It is actually two pumps working at the same time!  A system of one-way valves keeps the blood moving through the heart in one direction.     

In the fetal pig, we can also see the blood vessels carrying blood through the neck to the brain.  The sheep heart shows the path one of the coronary arteries that supply blood to the heart.  These are the arteries that get blocked when people have a heart attack.  Blood is the other componant of the circulatory system.   Under the microscope you can see the many red blood cells that carry oxygen and the scattered larger and darker white blood cells that are part of our bodyís immune system

The digestive system begins with our mouth where we chew our and food and mix it with saliva.  We swallow it in little balls that travel down a muscular tube called the esophagus to our stomach.  The pictures show the location of the esophagus underneath the trachea in the neck and below the heart in the thoracic cavity.  

The fetal pig shows the location of the stomach underneath the liver in the abdominal cavity.  The adult pig stomach shows how thick the muscle of the stomach is and where the food enters the stomach from the esophagus and exits the stomach into the small intestine.  The diaphragm has a hole in it to allow the esophagus to pass from the thoracic cavity into the abdominal cavity.  The stomach holds lots of food so that we can eat a meal.  In humans it can expand from less than a cup to a gallon!  It also breaks the food down by mixing it with gastric juice that contains acid strong enough to melt your skin.  After churning is acid for a few hours your food is a liquid that passes through a valve into your small intestine one squirt at a time.   

The small intestine is the longest and most important organ of digestion.  Food is broken down by digestive enzymes here and absorbed into the blood.  Tiny molecules from your food actually move into the walls of the intestine and into the microscopic blood vessels or capillaries found there.  In the fetal pig, you can see some of the larger the blood vessels that travel out to the small intestine to pick up these digested nutrients and transport them to the liver.  The small intestine empties the food that is left into the large intestine.  Some nutrients, vitamins and water are absorbed here but most of the remaining food becomes the waste that is expelled from your body as feces.  The large intestine of the fetal pig is arranged differently than the human large intestine but it has the same structure and function. 

The liver is also a digestive organ but food does not go there directly.  The nutrients absorbed into the blood travel there and are processed before they go to the rest of your body.  The liver makes fats and proteins that your body needs and maintains your blood sugar at a constant level by storing sugar after you eat and releasing it as you need it.  The little bag on the back of the sheep liver is the gall bladder. It stores the waste from the liver which is a yellow substance called bile.  Bile is released into the small intestine to help with the digestion of fat.  When problems occur with the liver your skin turns yellow as your body attempts to excrete the bile through the skin.

The urinary system filters waste from the blood and excretes it as urine.  The kidneys are located on either side of your lower back.  One kidney in the fetal pig has been dissected away from the back and is held only by the two large urinary blood vessels and the tube called the ureter that carries urine to the bladder.  The other side is not dissected and shows how the kidneys are stuck to the back.  The bladder is simply a muscular storage bag that holds the urine.  Another tube called the urethra drains the bladder.  The bladder from an adult pig shows how thick the muscular wall is.  As the bladder stretches the wall gets thinner.

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