The human body is organized into a hierarchy of different levels. From the simplest to the most complex, they are the chemical level, cellular level, tissue, organ, and system.

The Chemical Level

Just like all other matters on the Earth, the human body is made up of different chemical elements. Anelement is a pure substance that contains only one kind of atom—the smallest part of an element that can exist on its own. There are more than 90 elements occurring naturally on the Earth and about 25 of them can be found in the human body.

The four most important elements (constituting 96.2% of body weight) include:

• Oxygen (65%)
• Carbon (18.5%)
• Hydrogen (9.5%)
• Nitrogen (3.2%)

Less important elements (constituting 3.7% of body weight) include:

• Calcium (1.5%)
• Phosphate (1.0%)
• Potassium (0.4%)
• Sulphur (0.3%)
• Sodium (0.2%)
• Chlorine (0.2%)
• Magnesium (0.1%)

The remaining 0.1% belongs to the family of trace elements.

The chemical proportion of the human body will change as the age advances. For instance, the amount of calcium inside the human body progressively decreases as the age increases, which makes the bone become brittle and easier to break.

The Cellular Level

The human body is composed of trillions of cells closely functioning together. A cell is also the smallest living unit in the body.

Although there are many different kinds of cells in the human body, most of them contain a nucleus, a thick liquid called cytoplasm, and enclosed by a very thin layer of cell membrane.

1. Nucleus

The nucleus is the control center of a cell. It contains DNA (deoxyribonucleic acid) molecules, which house the cell’s chemical information. A DNA molecule is composed of millions of atoms, structured in a double helix shape. Before a cell divides, its DNA molecules replicate (i.e., copy themselves), to make sure that a complete set of information can be passed to each new cell.

The nuclear membrane separates the nucleus from the other components of the cell. However, the nuclear membrane contains numerous holes called nuclear pores, which allow chemical contact between the nucleus and the surroundings.

2. Cytoplasm

The cytoplasm is a transparent, jelly-like fluid that contains up to 90% water. It houses the organelles–some structures with special functions inside the cell. The number and types of organelles depend on the functions of the cell.

A typical cell usually includes the following organelles:

• Endoplasmic Reticulum — The endoplasmic reticulum is a highly folded system responsible for making and storing proteins and fats.
• Ribosomes — Although ribosomes are very small in size, they are usually numerous in number, and can scatter among the cytoplasm or attach to the endoplasmic reticulum. It is important for the synthesis of protein in the cell.
• Golgi Body — Golgi body is made up of membranes that store and release the substances produced by the cell. Besides, it also makes lysosome.
• Lysosome — Lysosome is a reservoir that contains digestive enzymes. These kind of enzymes can break down worn-out organelles and digest foreign substances. During the early stages of development, lysosomes even destroy the cell that surrounds them. This is called "self-digestion" or autolysis.
• Mitochondrion — Mitochondrion (plural: mitochondria) is an oval shape structure with a heavily folded inner membrane (i.e., cristae). This is where aerobic respiration takes place and energy is released. Mitochondria are often called the energy warehouse of the cell.

3. Cell Membrane

Cell membrane is formed by two layers of phospholipid molecules. They act as a protective shield, allowing some substances to pass through, but preventing the passage of others at the same time.

4. Cell Division

About 50 millions cells die in a second in the human body. At the same time, they are replaced by the same number of new cells. The life cycle of cells varies from one kind to another. For example, cells lining the skin and the alimentary canal have to experience a lot of wear and tear, and are almost replaced every 24 hours. On the contrary, highly specialized cells such as the neurons (i.e., nerve cells) do not divide at all once they have been formed.

Cell division is the process by which cells reproduce. There are two types of cell division: mitosis and meiosis.

• Mitosis is the division of a cell nucleus to produce two identical cells. It is the process that the body uses for growth and repair.
• Meiosis occurs in the reproductive system to produce sex cells. It actually involves two cell divisions, one after the other, to produce four new cells that are genetically unique.

When cell division becomes uncontrolled, it may lead to growths called tumors, which often spread out rapidly, and even interfere with normal body functioning. Scientists believe that certain carcinogens, accidental rearrangement of DNA molecules in a cell, and virus infection can all be factors that trigger the action of cancers.

Tissue

A tissue contains cells of similar structure and functions, and also other substances, working together to carry out specific tasks. There are four main categories of tissue in the human body:

1. Epithelial tissue is formed by special cells that cover the surface of the body. It also forms the internal surfaces of many organs and glands. It has the functions of protecting tissues, helping secretion, and disposing substances.
2. Connective tissue can be found all over the body such as bones, cartilages, ligaments, blood, and fats. It holds the different parts of the body together, forming the human body structure, and transport substances to different parts of the body.
3. Muscular tissue consists of fiber-shaped cells. It has the ability to contract, and is responsible for the movements of different body parts.
4. Nervous tissue consists of neurons, forming a network to transmit messages inside the body. It is responsible for carrying and controlling electrical signals between the brain and different body parts.

When a tissue is damaged (e.g., tearing a muscle), its cells will divide to repair the damaged area. However, the ability to regenerate varies from one kind of tissue to another.

Organ

An organ is a structure made of two or more tissue that carries out a particular range of tasks. The heart, lungs, kidneys, stomach, liver, muscles, etc. are some of the important organs in the human body.

System

When a collection of organs that work together to perform specific tasks, they form a body system. The human body can be distinguished into 10 systems, and each of them is essential for life.

1. The skeletal system consists of 206 bones. Its functions include:

• Strengthen and support the body.
• Protect important internal organs.
• Provide attachments and levers for muscular contractions.
• Produce blood cells.
• Site to storage and liberate minerals (e.g., calcium, phosphate, etc.).

2. The muscular system contains three different kinds of muscle, which vary in shapes and functions.

• Skeletal muscles are attached to the skeleton, and can be moved according to the mind (i.e., voluntary).
• Smooth muscles is found on the wall of hollow organs inside the body, and cannot be controlled by the mind (i.e., involuntary).
• Cardiac muscles can only be found in the heart. Its strong and powerful contractions are alsoinvoluntary.

Functions of the muscular system include:

• Constitute different kinds of body movements through muscular contractions (e.g. heart beat and breathing).
• Maintain posture, stabilize joints, and produce energy.

3. The nervous system is composed of the brain, spinal cord, and millions of nerve cells (i.e., neurons), which constitute a huge and complicated network. The major function of the nervous system is to pass information and nerve impulses from one part of the body to another.

4. The respiratory system consists of the lungs and the many air tracts of different sizes. Cells and tissues inside the human body must be continuously supplied with oxygen to produce energy for survival. The main function of the respiratory system is to provide passages and room for gaseous exchange.

5. The circulatory system is composed of the heart and a complicated network of blood vessels. Its main function is to transport nutrients and oxygen to the body tissues, and remove metabolic wastes from the cells at the same time.

6. The lymphatic system is made up of a network of lymph vessels that runs along the veins and arteries. It is a system to drain excess fluid from the spaces between cells. Functions of the lymphatic system include:

• Drain excess fluid from the spaces between cells (i.e., lymph) back to the bloodstream.
• The lymph node along the lymph vessels contains cells that can destroy bacteria, and helps the immune response.

7. The digestive system contains the esophagus and certain organs responsible for digestion (e.g., stomach, intestines, liver, and gall bladder). It main function is to break down food into smaller substances for better absorption.

8. The urinary system is made up of two horsebean-shaped kidneys, two ureters, urinary bladder, and the urethra. Major functions of the urinary system include:

• Discharge metabolic wastes outside the body through urination.
• Maintain internal balance of water and salt.
• Adjust pH of blood.

9. The endocrine system is made up of special glands called the endocrine glands. Its major functions is to secrete hormones to regulate metabolism of the body, growth, development and reproduction.

10. The reproductive system differs very much in male and female. The major function of the reproductive system is for  the production of offspring.

Each of the above systems is responsible for the processes that are essential for life. In physical education and sports, we are particularly interested in the skeletal system, muscular system, nervous system, respiratory system and circulatory system.