16.13 Physical human systems  
  People sometimes call the human body a machine--the most wonderful one ever built. Of course, the human body is not a machine. But it can be compared to one in many ways. Like a machine, the body is made up of many parts. Each part of the body, like each part of a machine, does special jobs. But all the parts work together and so make the body or the machine run smoothly. Also like a machine, the body needs energy to work. In such a machine as a car, the energy comes from petrol. In the body, it comes from food and oxygen.  
  Although the human body can be compared to a machine, it is far more amazing than any machine. It can do things that no machine can do. For example, the body can grow. The body starts out as one cell. In time, this tiny cell develops into a body consisting of trillions of cells. The human body can also replace certain worn-out parts. Each day, about 2 billion of the body's cells wear out and are replaced. Thus, the body is always rebuilding itself. Every 15 to 30 days, for instance, the human body replaces the outermost layer of skin.  
  The human body can defend itself against hundreds of diseases. The body can also repair itself after most small injuries. Many body parts, such as the heart and kidneys, work continuously. The heart of a 70-year-old person, for example, has pumped at least 174 million litres of blood during that person's life. In addition, the person's kidneys have removed wastes from more than 3.8 million litres of blood.  
  By using its senses, the body can detect changes in its surroundings, such as changes in temperature, light, or sounds. It can adjust to these changes immediately. The body's senses are truly incredible. For instance, people can learn to identify thousands of odour's, yet smell is one of the least developed senses in human beings. The human body can also detect changes that occur within itself, such as changes in body temperature. The various parts of the body continuously adjust their activities to keep the "inside" environment normal. Such adjustments rely on a system of nerves that carries messages from one part of the body to another. The messages travel at speeds of up to 90 metres per second.  
  The most remarkable part of the human body is the brain. The human brain is so highly developed that it makes people different from all other living things. Their magnificent brain makes people able to think. They can compose silly rhymes or beautiful poetry. They can imagine a dream world or study the mysteries of the atom. No animal--no matter how cunning--and no computer--no matter how powerful--can think like a human being.  
  The brain and the exquisitely complex nervous system, which ramifies throughout the body, work in close collaboration with the blood-borne hormones. These are the body's inbuilt control systems, orchestrating all that we are and we do.  
16.5.1 What the body is made of- chemical elements and molecules  
  Like all things--living and nonliving--the human body consists of atoms of chemical elements. The most common chemical elements in the body are carbon, hydrogen, nitrogen, and oxygen. The body also contains smaller amounts of many other elements, including calcium, iron, phosphorus, potassium, and sodium. The most common molecule in the human body is water. A molecule of water consists of two atoms of hydrogen and one atom of oxygen. Water makes up about 65 per cent of the body. Most of the chemical reactions that occur in the body require water.  
  Except for water, all of the chief molecules in the body contain the element carbon. The most important carbon-containing molecules are large, complicated structures called macromolecules. There are four main kinds of macromolecules in the body: carbohydrates, lipids, proteins, and nucleic acids. Carbohydrates provide energy that powers all the body's activities. Lipids have several jobs. Some lipids, particularly the fats, store extra fuel. Other lipids serve as one of the building materials for the cells that make up the body. Proteins also have various duties. Many proteins serve as building blocks for cells. Other proteins, called enzymes, speed up the chemical reactions within the body. Nucleic acids carry instructions that tell each cell how to perform its particular jobs. For more information on macromolecules, see the article LIFE (The chemical basis of life).  
16.5.2 Organs and organ systems  
  Cells and tissues. The cell is the basic unit of all living things. The cells of the human body consist chiefly of molecules of water, proteins, and nucleic acids. The molecules that make up the cells are not alive, but the cells themselves are living things. Each of the body's cells is able to take in food, get rid of wastes, and grow. Most of the cells can also reproduce. A thin covering consisting of lipid molecules encloses each cell. This lipid envelope permits only certain substances to enter or leave the cell.  
  The body has many basic kinds of cells, such as blood cells, muscle cells, and nerve cells. Each kind of cell has special features and jobs. Cells of the same type form tissues. The body has four chief kinds of tissues.  
 
(1) Connective tissue helps support and join together various parts of the body. Most connective tissue is strong and elastic
(2) Epithelial tissue covers the body surface and so forms the skin. It also lines such body openings as the mouth and throat. Epithelial tissue prevents harmful substances from entering the body.
(3) Muscle tissue consists of threadlike fibres that can contract (shorten). Muscle tissue makes it possible for the body to move
(4) Nervous tissue carries signals. Its system of nerve cells permits various parts of the body to communicate with one another.
 
16.5.3 Specialized cell colonies  
  An organ consists of two or more kinds of tissues joined into one structure that has a certain task. The heart, for example, is an organ whose job is to pump blood throughout the body. Connective tissue, muscle tissue, and nervous tissue make up the heart.  
  Groups of organs form organ systems. Each organ system carries out a major activity in the body. For example, the digestive system consists of various organs that enable the body to use food. Similarly, the nervous system is made up of organs that carry messages from one part of the body to another. The remainder of this article discusses the main organ systems of the human body. For more detailed descriptions of the major organs and organ systems, see the articles listed in the Related Articles at the end of this article.  
16.5.4 Organ Systems  
  Given the detailed analysis of the previous chapters we can view the human body as a series of cohesive self contained but interdependent systems:  
 
SYSTEM PURPOSE
Skin The outer membrane and filter system
Skeletal System The skeletal system
Neural System The data processing and pattern recognition system
Strong Chemical Fission System The extraction of atomic and ergon particles plus the reaction to produce internal movement and a source of energis
Weak Chemical Fission System The extraction of polymer and monomer molecular structures for building, maintenance and good health
Central Control System The endorcrine system of glands producing hormones for the transmission of actions throughout the body upon receiving neuro transmitters.
Muscular System The movement support systems
Circulatory System The transport mechanism for polymer and molecular structures
Waste Systems The waste disposal system of fission reactions
Immune System The bodies internal defence network
Reproductive System The sexual reproductive organs
Sensory Systems The information gathering systems of the body
 
 

 

  
 
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