The cell is the fundamental functioning unit of the human body. Trillions of body cells work together to create an individual who when healthy is able to adapt to a constantly changing external environment while maintaining internal balance. Since approximately 50% to 60% of the human body is water internal balance involves the regulation of body fluids. The ability to maintain internal balance in the presence of external stressors is termed homeostasis.
Homeostatic balance is necessary for cells to perform their individual physiologic tasks. A steady delivery of nutrients (oxygen glucose fatty acids and amino acids) the continuous removal of metabolic wastes (carbon dioxide and other end products of cellular metabolism) and the maintenance of a stable physiochemical environment are essential to homeostatic balance and thus to normal cellular function. Disruption of any of these three factors results in homeostatic imbalance with cellular dysfunction and illness.
All organs and body structures are involved in the maintenance of homeostasis. The nervous system with its three major components (sensory integrative and motor) enables the individual to perceive and interact with the surrounding environment and coordinates and regulates the functions of the other organ systems. Without the musculoskeletal system the movements necessary to obtain nutrition would be impossible. The skin forms the first line of defense against a hostile external environment and prevents the loss of body fluids. The gastrointestinal tract lungs and liver are necessary for the acquisition conversion and storage of nutrients. The cardiovascular and hematologic systems deliver nutrients to the cells and carry wastes to the kidneys and lungs for excretion. In addition to the excretion of wastes the kidneys play an especially vital role in regulating the volume and composition of body fluids. The role of the kidneys in the regulation of fluid and electrolyte balance will be discussed in greater detail in Chapter 2. The endocrine system through the release of multiple hormones regulates the body’s metabolic functions and like the kidneys is involved in the regulation of the volume concentration and composition of body fluids.
Single-celled organisms depend on their surrounding environment for the delivery of nutrients and the removal of metabolic wastes. An amoeba living in a salty pond obtains its nutrients directly from the pond water and depends on the diluting effect of the pond for the dissipation of wastes. The amoeba however has little protection from changes in its environment. If the pond is flooded with rain water or dries up the amoeba’s environment is dramatically altered and its function is affected. Multicellular organisms on the other hand are protected from immediate changes in their external environment by the fluids that surround their cells. It is as if multicellular organisms carry their own “pond” around with them. The volume concentration and composition of the “pond” can be regulated in order to maintain a stable environment for the cells with sufficient nutrients and adequate removal of wastes. Thus homeostatic balance for the human body requires continuous regulation of the fluid that surrounds the cells. In this way the fluid within the cells is protected which ensures normal cellular function.