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The human body

The human body is a complex system made up of many different types of cells. Cells are the smallest units and the basic building blocks of the body's structures. A baby is born with all the cells needed to develop into a full-grown adult. Even though they are present in a child's body, some cells may not become fully mature until certain stages in the child's development (for example, puberty). The human body is made up of trillions of cells. Each cell has its own structure and specific function. Cells of the same type are grouped together to form tissues (such as the lining of the intestine or the surface of the skin). Several types of tissue together form an organ (such as the lungs, liver, bladder, heart and bones). A single organ can contain billions of cells. Even though these cells may not be identical, they work together to perform a specific function.

Examples of different types of cells are:

  • epithelial cells – cover or line organs and glands and make up the outer layer of the skin
    • endothelial cells – line the blood vessels, lymph vessels and cavities of the heart
    • mesothelial cells – line other cavities, including the pleura and pericardium
  • muscle cells
  • nerve cells
  • connective tissue cells – function mainly to support organs or other body tissues and hold them together
    • bone cells
    • blood cells
    • cartilage cells
    • fat cells


Human cells vary in size, but they are all very small and can only be seen with a microscope. All cells have the same basic structure. Cells have a membrane that holds the contents together and regulates what goes in and out of a cell. The inside of a cell contains many small parts called organelles (including the nucleus and mitochondria) and fluid that surrounds the organelles.

The nucleus is the organelle that controls the function of the cell. It contains 46 chromosomes (23 different pairs), which are rod-like structures made up of DNA and protein. Each DNA molecule is made up of many genes (the basic unit of heredity), which instruct the cell to work, grow, divide and die.

The cytoplasm is the term for inner part of a cell. It contains the:

  • fluid part of the cell called the cytosol, which surrounds the organelles
  • organelles that perform the cell's functions


Each of the body’s organs performs a specific function. Organs can also work as part of a group (organ system).

Main organ systems
SystemExamples of organs



spinal cord









blood vessels




intestines (bowels)



lymph (lymphatic)

lymph vessels

lymph fluid




adrenal glands

pituitary gland

thyroid gland





male reproductive

prostate gland

testicles (testes)

female reproductive







connective tissue



Communication between cells and organs

Communication between organ systems allows the body to adjust the function of each organ to meet the needs of the whole body. Through this communication, the body keeps itself in balance. Communication occurs mainly through the nervous system or hormones, or directly between cells.

  • The nervous system sends messages from the brain to the organs. For example, the heart pumps blood throughout the body at different rates and pressures depending on the signals it receives from the brain.
  • Hormones are chemical messengers that allow an organ or tissue to communicate with other organs or tissues through the bloodstream. For example, certain cells in the pancreas produce hormones that regulate various functions in the body, such as stimulating the liver, muscles and fatty tissues to absorb or store blood sugar.
  • Cells communicate with each other through a network of chemical signals or messenger molecules they send to each other. For example, some signals or molecules tell cancer cells to grow and divide uncontrollably (some newer targeted therapies take advantage of this process by blocking this communication).


The thin layer of tissue that covers the lungs and lines the chest cavity. It protects and cushions the lungs and produces a fluid that acts like a lubricant so the lungs can move smoothly in the chest cavity.


The double-layered sac that surrounds the heart. It protects the heart and produces a fluid that acts like a lubricant so the heart can move normally in the chest.

deoxyribonucleic acid (DNA)

The molecules inside the cell that program genetic information. DNA determines the structure, function and behaviour of a cell.


The basic biological unit of heredity passed from parents to a child. Genes are pieces of DNA and determine a particular characteristic of an individual.


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