You are already familiar with the cell—the smallest structural and functional unit of a living organism. Each cell can perform all the basic activities required for life. Some organisms, like bacteria, are unicellular—an entire organism made of only one cell. These single-celled organisms rely on diffusion and osmosis to take in needed raw materials (like food and oxygen) and to release wastes (like carbon dioxide).

What happens when organisms become more complex? How do organisms built of billions, or even trillions of cells (like human beings), get the raw materials needed to each and every cell? The answer is body systems. Large, complex organisms need many levels of organization to ensure all cells get what they need to perform life functions.

Body system organization begins with the cell. Cells working together form tissues, tissues working together form organs, and organs working together form organ systems. For example, cardiac muscle cells group together to form cardiac muscle tissue, which in turn forms the heart. The heart pumps blood through the circulatory system, delivering needed materials (glucose, oxygen) and picking up waste (carbon dioxide) from cells all over the body.

Organs systems work together to efficiently and effectively provide all body cells with their basic needs to carry out life functions. You can think of the organs and systems as puzzle pieces. The whole animal is the picture on the puzzle. So, like anyone who assembles puzzles, let's look at the box to see the big picture first.

Directions: Watch Human Body Systems: The 11 Champions for a review of the human body systems and their functions.

Now that we have reviewed the human body systems, let's take a closer look at the major organs (structures) and functions associated with each system. 

Body systems do not function in isolation. All systems have to work together to carry out life functions. For example, let's say you are feeling hungry. The feeling of hunger is caused when the brain (nervous system) detects changes in the levels of nutrients (digestive system) and hormones (endocrine system) in the blood (circulatory system). If you respond to this feeling by choosing to get a snack, many other systems will be recruited. These systems will work collaboratively to help you eat and digest your food, distribute the nutrients to your body cells, and generate cellular energy from the nutrients. 

Directions: The chart below lists each body system on the left. Match each body system with a system listed on the right that it must interact with to carry out the function described. For example, the first body system, the circulatory system, works with the respiratory system to provide oxygen to cells and to eliminate carbon dioxide.

Now let's take a look at how multiple body systems work together to carry out life functions. Critical life functions include processes like regulation of internal environment (homeostasis), nutrient absorption, reproduction, and defense against injury or illness.

Directions: Three major processes/functions of human body systems are listed below. Brainstorm all of the systems that you think are involved in each of these processes. Click on each process to check your answer.

Interactions That Occur Among Systems that Perform the Function of Defense from Illness and Injury in Animals

English Version

In this video, Angela describes to her friend, Adam, the interactions that occur between human systems to prevent injury and illness.

Interacciones que se Producen Entre los Diferentes Sistemas para Realizar la Función de Defensa de Enfermedades y Lesion en los Animales

Spanish Version

En este vídeo, Alejandra le describe a su amigo Adam las interacciones que ocurren en los sistemas humanos para prevenir enfermedades.