Our bodies are made up of eleven systems, each of which has been designed to fulfill different functions:

Digestive. This is the system that breaks down food and absorbs its nutrients. The digestive tract is a long system of tubes that run from the mouth to the anus. It includes the esophagus, stomach, small intestine, and large intestine. The liver and the pancreas manufacture special enzymes to help break down food.

Muscular. This system provides the body with movement. There are three types of muscle: skeletal, cardiac, and smooth. Skeletal muscles attach to bones, and are voluntary—they are consciously controlled by the nervous system. Cardiac muscles cause the heart to pump blood, and are involuntary—they contract automatically. Smooth muscles are also involuntary, and cause movement in other organs; these are the muscles that push the food that we eat down the esophagus and into the stomach.

Integumentary. The integumentary system includes skin, which provides sensory receptors and protects the body.

Excretory. This system gets rid of various body wastes. It includes the sweat glands (where the body excretes unnecessary salts), kidneys (which filter the blood), and urinary tract.

Reproductive. The reproductive system allows for the continuation of life. Gametes from the male’s sperm and the female’s egg combine to form a zygote—a unique combination of genes which no other human being has!

Circulatory. This is the system that our blood flows through. It carries nutrients and oxygen to all the organs of the body, and carries away wastes.

Respiratory. The respiratory system provides the body with oxygen, and it expels carbon dioxide from the body. The nasal passage, trachea, bronchial tubes, lungs, and alveoli are involved in this process. Inhaled oxygen is broken down in the alveoli (tiny air sacs in the lungs) and then passed into the capillaries, where it travels into the bloodstream. In the same way, carbon dioxide from the blood is passed back into the alveoli, and exhaled from the body.

Skeletal. The skeletal system provides structure for the body and protection for the body’s internal organs. Bones, ligaments, joints, and the skull are all part of the skeleton.

Immune. The body’s immune system fights disease. The lymphatic system is the main defense mechanism within this system. A liquid called lymph flows over the tissues and carries off harmful bacteria, which is filtered out in the lymph nodes. White blood cells, which are produced in our bones’ marrow, are another important part of the body’s defenses.

Nervous. The brain, spinal cord, and nerves work together to coordinate the body’s actions. Our senses are also part of this system, allowing us to see, taste, smell, touch, hear, and feel pressure and pain.

Endocrine. The endocrine system is made up of the chemical messengers which control many bodily activities: hormones. Hormones initiate many bodily processes, such as reproduction, growth, and digestion.

To learn more about the body’s systems, we recommend the Systems and Structures chart book.

Blood and the Circulatory System

Did you know that blood contains more than just blood cells? Our bloodstreams carry red blood cells, white blood cells, platelets, and plasma. Red blood cells, or erythrocytes, carry oxygen through the body and make up a little over 44% of our blood content. White blood cells, which are part of the immune system, fight disease by attacking and devouring harmful cells. Platelets initiate blood clotting by pinching together damaged arteries. White blood cells and platelets together make up less than 1% of our blood. Liquid plasma, which makes up 55% of our blood, carries nutrients to the rest of the body.

Veins carry deoxygenated blood to the heart, and arteries carry blood out from the heart. Tiny capillaries connect veins and arteries to body tissues. Arteries have thick cell walls and are tough and elastic in construction, to deal with the pressure of the blood being pumped through them. Veins have thinner walls and are not as elastic.

Human hearts have four chambers. Deoxygenated blood—which needs a fresh supply of oxygen—is brought into the first chamber, the right atrium. The blood is pumped into the right ventricle, and from there it is pumped to the lungs. In the lungs, the blood receives oxygen. From the lungs, the oxygenated blood is brought back to the heart. The blood passes through the left atrium into the left ventricle, and from there it is pumped through the rest of the body.

Body —> right atrium—> right ventricle —> lungs —> left atrium —> left ventricle —> body

Graphic used with permission from www.advocatehealth.com.

To measure your pulse rate, place your second and third fingers over your wrist on the palm side, in the slight depression beneath your thumb. Gently move your fingers up and down until you find the pulse. You shouldn’t have to push down on your wrist. Count the number of beats that you feel in one minute’s time.

Measure the pulse rates of your children while they are resting. Then measure their pulse rates again after each has exercised for one minute and then ten minutes. Compare the results and explain that exercise increases the body’s demand for oxygen, which results in increased heart and pulse rates.

The Brain and the Nervous System

Our nervous system enables our body to communicate. The central nervous system includes the brain and the spinal cord. The peripheral nervous system includes the nerves that extend from the brain and the spinal cord. ‘Peripheral’ means ‘by the surface or the edge’.

The brain of an average-sized adult weighs about three pounds, yet the amount of information it contains is amazing! The cerebrum—the large, upper part of the brain—orchestrates memory, thought, and learned behavior. The cerebrum’s surface is called the cerebral cortex, and is convoluted—patterned with intricate twists. The cerebellum—the underneath part of the brain—coordinates voluntary muscle action and balance. The brain stem connects to the spinal cord and controls involuntary activities.

The cerebrum is divided into two hemispheres, or halves, which are connected by a communication bridge called the corpus callosum. Each hemisphere controls the actions of the opposite side of the body. In human brains and some animal brains, there are certain specific skills that are controlled by each hemisphere. In human brains, the left hemisphere controls temporal (time) judging skills, rhythm processing, mathematics skills, and both spoken and written language skills. The right hemisphere controls pattern-matching, hand-eye coordination, facial recognition, non-speech-sound processing, and music skills. Logic or artistic ability can be on either side, depending on which skills a person is strongest in.

Experiments and research have been done which demonstrate where certain thought activities take place. The split-brain phenomena is an indicator of which skills are located in each hemisphere. Split-brain patients have had their corpus callosum severed, usually to cure epileptic seizures. This means that the two hemispheres can no longer communicate with each other. Research on split-brain patients has shown that what the right hemisphere knows can only be communicated non-linguistically—without using written or spoken language. However, what the left hemisphere knows can be expressed using language, indicating that language skills are contained in the left hemisphere.

Nerve cells, called neurons, are the communications cells of the nervous system. Hair-like tentacles, called dendrites, that extend from the cell body, are the neuron’s receivers. The axon, a tail-like structure on the cell body, is the neuron’s transmitter. The synapse is the connection point between two neurons or a neuron and a muscle or gland cell.

Synapse —> dendrite—> cell body —> axon —> synapse

Our skin has pain receptors, both mechanoreceptors that that allow us to feel pressure and touch, and thermoreceptors that allow us to feel heat and cold.

With all of our senses, an electrochemical signal is passed between neurons and travels through the nerves, spinal cord, and brain.

Have your child hold his thumb and index finger two inches apart while you drop a ruler to fall between them. The distance the ruler falls before he stops it with his thumb and finger indicates his reaction time. Repeat three to four times with each child. Record the results. Try the experiment again while the child recites addition or multiplication facts. Compare the results, and explain that our reaction time increases when we are distracted.

Noteworthy Scientist: William Harvey (1578-1657)

William Harvey was born in England on April 1, 1578, during the reign of Queen Elizabeth I. As a young man, he attended college in Cambridge and received a Bachelor of Arts in 1597. After that he studied medicine at the University of Padua, in Italy. When he returned to England in 1602, he got another medical degree from Cambridge. Around the same time, Harvey married Elizabeth Brown, the daughter of one of Queen Elizabeth’s physicians.

Harvey received a fellowship at the Royal College of Physicians, and gave medical lectures. He was also physician to Saint Bartholomew’s Hospital in London, until 1643. In 1618 Harvey was appointed as a physician to James I and continued to serve as doctor to the royal family, under Charles I, until the English Civil War in 1642.

Most scientists at that time thought that ‘nutritive’ blood was made in the liver, and that ‘vital’ blood was made in the heart. They also thought that the heart sucked blood into itself, rather than pumped it. Harvey learned from dissecting animals and cadavers that this was not possible. He proved that the heart is a pump, and that blood is pumped back through the heart and body in a closed system, rather than used up by the body.

His work, On the Movement of the Heart and Blood in Animals, was published in 1628, in Latin. The foundation for the book came from college lectures that he had given, beginning in 1615. Harvey wrote another book in 1651, Essays on the Generation of Animals, which was the foundation of modern embryology (the study of an embryo’s development from conception to birth). Harvey died in 1657.