Levels of Organization
Living things have different levels of organization.    The simplest level of organization is that of the cell.   A group of cells with a similar function is called a tissue.   Groups of tissues working together to perform a common function are called organs.  An example of this would include the nervous, muscle, and other tissues which make up the heart.   Groups of organs working together to perform a common function are referred to as a system or organ system.   The blood vessels, blood, and the heart are organs which work together to form the circulatory system.   Many different systems function together to allow a complex organism to function.

Homeostasis
All the components of the living things, from the cells and the organelles within them to the organ systems of complex organisms must interact to maintain a balanced internal environment within the organism.   Organisms possess many control mechanisms to detect internal and external changes and make changes to correct any deviations.    This maintenance of a stable internal environment by an organism is called homeostasis.   Homeostasis in an organism is constantly threatened.  Failure to respond effectively can result in disease or death.