The Endocrine System

In this topic you will learn about how the endocrine system uses chemicals to control many things.
Certain chemicals in the body react only when they reach a target. Hormones are chemicals that control body functions by influencing how cells work. Only when hormones reach their target can they cause changes in your body. A gland that produces hormones is called an endocrine gland. These glands make up your endocrine system, which, like the nervous system, is a control system of the body.
Endocrine glands are not connected like the parts of the digestive system. They are scattered around the body. Each endocrine gland makes one or more hormones. Every hormone seeks out a target organ—the place in the body where the hormone acts.
The human body makes more than 50 different hormones. They control body activities like growth. Only tiny amounts are needed because hormones are powerful substances. Too little or too much of a hormone can upset how the body works. The body depends on a process called biological feedback to determine when to release a hormone and when to stop.
The pituitary gland is often called the master gland. Some of the hormones it makes control how other endocrine glands in the body work. The pituitary gland is located at the base of the brain.
The thyroid gland is controlled by the pituitary gland. It produces two hormones that affect the rate at which cells use oxygen and release the energy in food. The sum of all chemical reactions that occur in the body is called its metabolism.
The parathyroid glands are four pea-sized glands embedded in the back of your thyroid gland. They make a hormone that helps control how much calcium is in your blood.
The two adrenal glands sit on top of the kidneys. The outer part of each adrenal gland produces 30 different hormones. The inner part of the adrenal gland produces adrenaline. This powerful hormone's main job is to help you deal with emergencies.
The reproductive system starts to develop when the pituitary gland releases hormones that target the reproductive organs. These organs produce sex cells. Ovaries are the female reproductive organs that produce eggs. Testis are the male reproductive organs that produce sperm. These organs also function as endocrine glands. They produce sex hormones.
Above your pituitary gland is a part of the brain called the hypothalamus. The nerve cells in the hypothalamus make hormones that tell your pituitary gland which of its hormones to start or stop.
  

The Nervous System

In this topic you will learn about how the nervous system controls your ability to hear, smell, touch, and think.
Electrical signals, or impulses, are sent through nerve cells. Impulses flash along individual nerve cells, or neurons. Each neuron has three main parts—a cell body, dendrites, and an axon. Dendrites are branching nerve fibers that carry impulses toward the cell body. An axon is a nerve fiber that carries impulses away from the cell body.
When an impulse reaches the tip of an axon, another nerve cell can't pick it up directly. That's because there is a gap between the neurons. This gap is called a synapse, and it is only a fraction of an inch wide. When an impulse reaches the tip of an axon, the axon releases chemical droplets. The chemical connects the neurons by carrying the impulse across the gap to the dendrites on the next nerve cell.
There are three kinds of neurons—sensory, motor, and associative. The function of the sensory neuron is to collect information from stimuli (singular, stimulus) and send it to the brain and spinal cord. A stimulus is anything that causes a response. Motor neurons carry commands from the brain and spinal cord to the muscles and glands. For example, when you make a muscle, your brain sends impulses along motor neurons to your biceps muscle. Other motor neurons can speed up your heartbeat and help move food through your intestines. Associative neurons pass impulses from sensory to motor neurons. Most of the neurons in your brain are associative. They are found only inside the brain and the spinal cord.
Your brain and your spinal cord are your central nervous system. All other nerves are your outer, or peripheral, nervous system.
The bones running down the middle of your back are vertebrae. Stacked together, your vertebrae make up your backbone, or spine. Running through the holes in the center of your spine is your spinal cord. Your spinal cord is the biggest and thickest bundle of nerve cells in your body. Your spinal cord helps protect you when you are in danger. It can make you do something automatically. Such an action is called a reflex.
The three main parts of your brain are the cerebrum, cerebellum, and medulla. The cerebrum is the largest part of your brain. It has over 10 billion neurons. A deep groove separates the right half, or hemisphere, from the left. Both hemispheres of your cerebrum contain control centers for your senses. Each half also sends commands that allow you to control your skeletal muscles. Your cerebellum coordinates commands to your skeletal muscles and helps you keep your balance. The medulla works automatically to control your heart beat, your breathing, your blood pressure, and the muscles in your digestive system.
      

How Heredity Works

In this topic you will learn about how heredity is determined by a small, complex molecule.
Every cell is surrounded by a membrane. Inside the cell membrane is a fluid called cytoplasm. In the center of the cytoplasm is the cell nucleus, which contains chromosomes. Chromosomes contain the factors for hereditary traits.
Living organisms are made of two types of cells, body cells and sex cells. Most of the cells in any organism are body cells. Body cells contain a full set of chromosomes. A human body cell contains 46 chromosomes. Sex cells are produced inside the sex organs of an adult organism. Each sex cell contains only one-half the number of chromosomes of body cells. A human sex cell contains 23 chromosomes.
Cells grow. When they reach a certain size, they divide into two new cells. While a body cell is dividing into two identical cells, the nucleus divides by a process called mitosis. Mitosis is a gradual, continuous process. During mitosis a second set of chromosomes is formed inside the cell. When the cell splits and produces two new cells, each new cell has a full set of chromosomes and is identical to the original cell.
A sex cell has only half the number of chromosomes as a body cell. A sex cell is formed as a result of a process called meiosis. Meiosis is the division of a nucleus resulting in sex cells with half as many chromosomes as in other cells.
When an organism reproduces, a sperm cell and an egg cell unite to create a single, new cell. This new cell receives half of the needed chromosomes from the female's egg cell and half from the male's sperm. The new cell represents a new individual with a new combination of genetic material.
A portion of a chromosome that controls a particular inherited trait is called a gene. Genes contain the information needed for cells to function. Genes are arranged along the length of a chromosome. Organisms have two copies of each gene for a given trait. They get one copy from each parent. Both genes are located in the same place on a chromosome.
Genes are portions of a long, complex molecule called deoxyribonucleic acid, or DNA. DNA contains the codes that tell each cell how to operate. Each gene on a chromosome is a short section of the long DNA molecule that makes up the chromosome. The genetic characteristics of every living organism are contained in that organism's DNA. The DNA in each cell of a given organism is the same. The DNA of every organism is different from that of every other organism. The shape of the DNA molecule looks something like a spiral staircase or twisted ladder. This shape is called a double helix.