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Organization of the Human Body

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Organization of the Human Body


  • Name the levels of structural organization of the human body and explain how each level is related.
  • Identify the various organ systems of the body and briefly explain the purposes and functions of each.
  • Classify all organs discussed into their respective organ systems.
  • Identify the organs shown on a diagram and identify their purpose.

How is the human body similar to a well-tuned machine?

Many people have compared the human body to a machine. Think about some common machines, such as drills and washing machines. Each machine consists of many parts, and each part does a specific job, yet all the parts work together to perform an overall function. The human body is like a machine in all these ways. In fact, it may be the most fantastic machine on Earth.

As a preview of the human machine, the Emmy award-winning video, Inside The Living Body, at this link is highly recommended: http://www.youtube.com/watch?v=chqwSh4ii84&feature=related .

Levels of Organization

The human machine is organized at different levels, starting with the cell and ending with the entire organism (see Figure below ). At each higher level of organization, there is a greater degree of complexity.

The human organism has several levels of organization.


The most basic parts of the human machine are cells—an amazing 100 trillion of them by the time the average person reaches adulthood! Cells are the basic units of structure and function in the human body, as they are in all living things. Each cell carries out basic life processes that allow the body to survive. Many human cells are specialized in form and function, as shown in Figure below . Each type of cell in the figure plays a specific role. For example, nerve cells have long projections that help them carry electrical messages to other cells. Muscle cells have many mitochondria that provide the energy they need to move the body.

You can watch a video about some of the specialized cells of the human body and how they function at this link: http://www.youtube.com/watch?v=I8uXewS9dJU&feature=related

Different types of cells in the human body are specialized for specific jobs. Do you know the functions of any of the cell types shown here?


After the cell, the tissue is the next level of organization in the human body. A tissue is a group of connected cells that have a similar function. There are four basic types of human tissues: epithelial, muscle, nervous, and connective tissues. These four tissue types, which are shown in Figure below , make up all the organs of the human body.

The human body consists of these four tissue types.

  • Connective tissue is made up of cells that form the body’s structure. Examples include bone and cartilage.
  • Epithelial tissue is made up of cells that line inner and outer body surfaces, such as the skin and the lining of the digestive tract. Epithelial tissue protects the body and its internal organs, secretes substances such as hormones, and absorbs substances such as nutrients.
  • Muscle tissue is made up of cells that have the unique ability to contract, or become shorter. Muscles attached to bones enable the body to move.
  • Nervous tissue is made up of neurons , or nerve cells, that carry electrical messages. Nervous tissue makes up the brain and the nerves that connect the brain to all parts of the body.

Organs and Organ Systems

After tissues, organs are the next level of organization of the human body. An organ is a structure that consists of two or more types of tissues that work together to do the same job. Examples of human organs include the brain, heart, lungs, skin, and kidneys. Human organs are organized into organ systems, many of which are shown in Figure below . An organ system is a group of organs that work together to carry out a complex overall function. Each organ of the system does part of the larger job.

You can watch overviews of the human organ systems and their functions at the links below.

Many of the organ systems that make up the human body are represented here. What is the overall function of each organ system?

Organ System Overview

  • Integumentary System (skin):  Forms the bidy's external, protective outer covering. Cushions and protects deep tissue from injury. Waterproofs the body's external surface. Regulates body temperature. Removes urea and salts with perspiration using sweat glands. Oil glands secrete substances that moisten and soften the skin. Contains temperature, pain and pressure receptors.  Synthesis of Vitamin D takes place in the skin.
  • Skeletal System:  Provides and supportive and protective framework to the body. Allows muscles to provide movement. Stores minerals such as calcium. Is the location of hematopoesis, or blood cell formation.
  • Muscular Sytem:  The contraction of muscles allow for movement of the body. Manipulation of muscles by the body's nervous system allows control of bodily function. Produces body heat needed to maintain homeostasis
  • Nervous System: Control system of the body. Has the ability to detect changes in the environment using sensory receptors. Allows response to internal and external stimuli by activiting glands and muscles.
  • Endocrine System:  Chemical control system of the body. Contains glands that secrete hormones that regulate bodily processes such as reproduction, growth, and metabolism.
  • Circulatory System:  The body's transport and delivery system. The powerful heart is a pump that transports oxygen, hormones and nutrients to body cells and carbon dioxide and waste products away from body cells. 
  • Lymphatic System: Cleanses the blood by removing debris in lymphatic fluid. Houses white blood cells used to fight infection. Returns fluid leaked from blood back into the blood stream.
  • Respiratory System: Supplies the body with oxygen needed for metabolism. Removes carbon dioxide gas produced as a waste product of metabolism from the body.
  • Digestive System:  Breaks down food into absorbable units and delivers nutrients from the food to body cells. Collects indigestible materials and removes them from the body as fecal matter.
  • Urinary System: Removes nitrogenous wastes (urea and uric acid) from the body. Regulates the body's water and salt balances in the blood. Also regulates acid-base balance in the blood.
  • Reproductive System: Responsible for the production of offspring. In males, sperm and male sex hormones are produced. Male sex glands and ducts help deliver sperm to the femail reproductive tract. In females, ovaries produce eggs and female sex hormones.  The organs of the female reproductive tract provide a location in which fertilization occurs. The uterus provides a safe, developmental site for the fetus. Mammary glands secrete milk to nourish the infant.


  • cell : Basic unit of structure and function of living things.
  • connective tissue : Tissue made up of cells that form the body’s structure, such as bone and cartilage.
  • epithelial tissue : Tissue made up of cells that line inner and outer body surfaces, such as skin.
  • muscle tissue : Tissue made up of cells that can contract; includes smooth, skeletal, and cardiac muscle tissue.
  • nervous tissue : Tissue made up of neurons, or nerve cells; carry electrical messages.
  • neuron : The structural and functional unit of the nervous system; a nerve cell.
  • organ : Structure composed of more than one type of tissue that performs a particular function.
  • organ system : Group of organs that work together performing a specific function.
  • tissue : Group of cells of the same kind that perform a particular function in an organism.



Use these resources to answer the questions that follow.

  1. Define tissue and organ .
  2. How are tissues put together?
  3. Name and define the purposes of the four types of tissue discussed.
  4. What is an organ system?
  5. How do the muscular and skeletal systems work together?
  6. List all 11 organ systems and briefly define the function of each.
  7. List two to three important organs for each of the 11 organ systems discussed.

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