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13.1: Organization of the Human Body

Difficulty Level: At Grade Created by: CK-12
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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?


  • The human body is organized at different levels, starting with the cell.
  • Cells are organized into tissues, and tissues form organs.
  • Organs are organized into organ systems such as the skeletal and muscular systems.

Practice I

Use these resources to answer the questions that follow.

  • http://www.hippocampus.org/Biology Biology for AP* Search: Animal Form and Function: Overview
  1. Define tissue and organ.
  • http://www.hippocampus.org/Biology Biology for AP* Search: The Structure of Animal Tissue
  1. How are tissues put together?
  2. What are connective tissues?
  3. What are epithelial tissues? Describe three types of epithelium.
  4. Describe the outer layers of skin.
  5. Describe the extracellular matrix.
  6. What are macrophages?
  7. What type of tissue is blood?
  8. List three types of muscle tissue.
  9. What is the role of nerve tissue?
  1. What is an organ system?
  2. Why are the reproductive and excretory systems grouped together?
  3. What is a hormone?
  4. How do the muscular and skeletal systems work together?
  5. How do the circulatory and respiratory systems work together?
  6. What are lymphocytes?
  7. Describe the role of the digestive system.

Practice II


1. What are the levels of organization of the human body?

2. Which type of tissue covers the surface of the body?

3. What are the functions of the skeletal system?

4. Which organ system supports the body and allows it to move?

5. Explain how form and function are related in human cells. Include examples.

6. Compare and contrast epithelial and muscle tissues.




Basic unit of structure and function of living things.
connective tissue

connective tissue

Tissue made up of cells that form the body’s structure, such as bone and cartilage.
epithelial tissue

epithelial tissue

Tissue made up of cells that line inner and outer body surfaces, such as skin.
muscle tissue

muscle tissue

Tissue made up of cells that can contract; includes smooth, skeletal, and cardiac muscle tissue.
nervous tissue

nervous tissue

Tissue made up of neurons, or nerve cells; carry electrical messages.


The structural and functional unit of the nervous system; a nerve cell.


Structure composed of more than one type of tissue that performs a particular function.
organ system

organ system

Group of organs that work together performing a specific function.


Group of cells of the same kind that perform a particular function in an organism.

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Difficulty Level:
At Grade
Date Created:
Feb 24, 2012
Last Modified:
Apr 28, 2016
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