INTRODUCTION TO THE BODY SYSTEMS

The Circulatory System

The circulatory system is the body's transport system. It consists of a group of organs that transport blood throughout the body. The heart pumps the blood and the arteries and veins transport it. Oxygen-rich blood 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 smallest blood 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 goes back 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.

The 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. The first major muscle movement is swallowing food or liquid. The start of swallowing is voluntary, but once it begins, the process becomes involuntary and continues under the control of the nerves. The esophagus, which connects the throat above with the stomach below, is the first organ into which the swallowed food goes. Where the esophagus and stomach join, there is a ring like valve that closes the passage between the two organs. When food nears the closed ring, the surrounding muscles relax and allow the food to pass into the stomach, and then it closes again. The food then enters the stomach, which completes three mechanical tasks: stores, mixes, and empties. First, the stomach stores the swallowed food and liquid, which requires the muscle of the upper part of the stomach to relax and accept large volumes of swallowed material.

Then, the lower part of the stomach mixes up the food, liquid, and digestive juices produced by the stomach by muscle action. And then the stomach empties the contents into the small intestine. The food is digested in the small intestine and dissolved by the juices from the pancreas, liver, and intestine, and the contents of the intestine are mixed and pushed forward to allow further digestion. Last, the digested nutrients are absorbed through the intestinal walls. The waste products, including undigested parts of the food, known as fiber, and older cells that have been shed from the mucosa, move into the colon. Waste products usually in the colon remain for a day or two until the feces are expelled by a bowel movement.

The Endocrine System

The endocrine system is made up of glands that produce and secrete hormones. These hormones regulate the body's growth, metabolism (the physical and chemical processes of the body), and sexual development and function. The hormones are released into the bloodstream and may affect one or several organs throughout the body.

Hormones are chemical messengers created by the body. They transfer information from one set of cells to another to coordinate the functions of different parts of the body.

The major glands of the endocrine system are the hypothalamus, pituitary, thyroid, parathyroids, adrenals, pineal body, and the reproductive organs (ovaries and testes). The pancreas is also a part of this system; it has a role in hormone production as well as in digestion.

The 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 body is 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.

The Lymphatic System

Closely connected with the blood and circulatory system, the lymphatic system is an extensive drainage system that returns water and proteins from various tissues back to the bloodstream. It is comprised of a network of ducts, called lymph vessels or lymphatics, and carries lymph, a clear, watery fluid that resembles the plasma of blood. Some scientists consider this system to be part of the blood and circulatory system because lymph comes from blood and returns to blood, and because its vessels are very similar to the veins and capillaries of the blood system. Throughout the body, wherever there are blood vessels, there are lymph vessels, and the two systems work together. 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 fluids and 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.

The 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 nerve impulses tell your muscles and organs what to do and how to respond to the environment. There are two parts of your nervous system that work together: the central nervous system and the peripheral nervous system. The central nervous system (CNS) acts as the command center of the body. It interprets incoming sensory information, and then sends out instructions on how the body should react. The CNS consists of two major parts: the brain and the spinal cord.

The peripheral nervous system (PNS) is the part of the nervous system outside of the CNS. It consists mainly of nerves that extend from the brain and spinal cord to areas in the rest of the body. Cranial nerves carry impulses to and from the brain while spinal nerves carry impulses to and from the spinal cord. The PNS can be divided into two systems: the somatic nervous system and the autonomic nervous system. The somatic nervous system controls the voluntary movements of the skeletal muscles. The autonomic nervous system control activities in the body that are involuntary or automatic. These include the actions of the heart, glands, and digestive organs and associated parts.

The autonomic nervous system can be divided further into two subdivisions: the parasympathetic and sympathetic nervous systems. These two subdivisions work against each other. The parasympathetic nervous system regulates involuntary activities that keep the body running smoothly under normal, everyday conditions. The sympathetic nervous system controls involuntary activities that help the body respond to stressful situations.

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 nerve impulses 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 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 bronchial tubes, 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 blood vessels and enters the blood stream. At the same time, carbon dioxide passes into the lungs and is exhaled.

The 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 nine months.

The 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 bladder is full, urine is discharged through the urethra. The Skeletal System Ø The Muscular System