Although your body is separated into many different systems, each individual system works with each other to maintain  homeostasis.  Homeostasis is the process of maintaining a constant internal environment even when there are changes in the external environment.

Normal body temperature is 98.6 degrees.  Your body temperature is constant whether or not the outside temperature is 20, 70 or 110 degrees.  When it is 20 degrees (cold) your body will shiver and you get goosebumps.  Both of these actions help produce heat which will keep your internal temperature at 98.6 degrees.  When it is 110 (hot) your body starts to sweat.  Sweat evaporates from your skin (which is a cooling process) which will keep your temperature at 98.6 degrees.

In the example above, your nervous system notices that your body is hot/cold and tells your muscles (muscular system) to contract.  Your circulatory system is also working with your immune system to bring fluids up to your skin (to sweat) and to circulate your blood to help keep you warm when it is cold.

Breathing is another example of your body systems working together.  When you breathe, you think of the respiratory system.  However, think about what the respiratory system does.  It delivers oxygen (nutrients) and removes carbon dioxide (waste).  However, the respiratory system only exchanges these gases through the lungs.  These gases need to travel throughout the entire body.  The circulatory system allows the gases (and blood) to travel everywhere.  The skeletal and muscular systems are also involved because they help push the blood from one end of the body to the other.

A third example is when a pathogen enters the body.  Your immune system is responsible for keeping invaders out.  When you breath in a pathogen, it can sometimes cause you to cough.  Your immune system recognizes the invader and communicates to your body that it is bad.  Then your skeletal and muscular system is responsible for creating the contractions to create the cough. 

All of your body systems are dependent on each other.  Any task that you complete as a human requires several body systems, including sleeping. 

1. Your nervous system is involved with all aspects of human function.  Circulation, digestion, endocrine and excretory systems are controlled by the autonomic nervous system.  The autonomic nervous function is controlled by your body with out conscious thought.  Imagine having to regulate your heart rate, rate of digestion, liver, and hormones with your mind.  The brain and spinal cord work together to carry out these tasks. 

2&3.  To think about this in a more advanced fashion suppose you were to touch a hot stove.  First skin (integumentary system) is filled with sensory neurons (nervous system) that transmit an impulse from your skin to your central nervous system (brain and spinal cord) signalling pain, the brain then sends the message back to the motor neurons which control the muscles (musculoskeletal system) which are attached to the bones.  The motor neurons cause the muscles to contract and move away from the heat source.  In response to the burn the blood vessels dilate...