The cell is a fundamental component of all the living creatures. It carries nucleic acids, which determine the genome. More cells that have the same structure and function compose a tissue. There are four types of tissues in our body – muscular, connective, nerve and epithelial. Several types of tissue connect into an organ. More organs make an organ system. There are twelve systems in a human body – nervous, digestive, cardiovascular, muscular, skeletal, endocrine, urinary, immune, integumentary, respiratory, excretory and reproductive.

A homeostasis is the ability of an organism to preserve the ideal conditions in its internal environment. All of our organ systems work in a way to maintain homeostasis. For example, our cells use oxygen which they metabolize into carbon dioxide. The respiratory system then eliminates carbon dioxide and provides more oxygen for new metabolic processes. Thus, the balance is maintained.

After a meal, the glucose blood level rises, which stimulates the endocrine pancreas to secrete insulin. This hormone facilitates the transport of glucose into the cells. Inside the cell, glucose molecules are phosphorylated and can be instantly used to generate energy or it is being stored as glycogen. Liver and muscle cells have a major role in the process of glycogenesis, and in maintaining the concentration of glucose in a normal range after a meal.

Most of the processes that are performed in our body are regulated by feedback inhibition. This is the way in maintaining homeostasis. For example, thyroid hormones are very important in regulating metabolism. When their level in the blood is low, that affect pituitary gland to secrete more TSH, which stimulates thyrocytes to produce more thyroxine and triiodothyronine. Their high blood level inhibits the secretion of TSH, which eventually lowers the concentration of the thyroid hormones. This represents an example of feedback inhibition, where one stimulus, high or low, lead to a series of changes that can bring its values to a normal range.

A teeth brushing is an action that begins in the central nervous system, as the taught and peripheral nervous system which induces muscle movement. In order to use a toothbrush, we need an involvement of the muscular and skeletal system as well as digestive system, because the mouth with its structures is a part of it.