Introduction

Body Compartments

Body cell mass may be defined as oxygen-requiring, carbon-dioxide producing, glucose-burning cellular mass.[28] This mass includes skeletal muscle, nervous system tissues, and organs, such as liver, lung, intestines, pancreas, spleen, endocrine and paracrine glands, bone marrow, and lymphatic system. Anabolic and catabolic processes of cells occur in this compartment.

Approximately 80% of BCM is intracellular water (ICW) and the remaining 20% includes cell membrane lipids, electrolytes, and carbohydrates. Overall, lean tissue hydration reflects an ECW to ICW of 0.5-0.7.

The other sub-compartment of lean tissues includes extracellular fluids, bone, and collagen structures.

Extracellular fluids include 95% water and are approximately 22-28% of body weight contained in vascular, lymphatic, and interstitial spaces.

Tissues providing transport for plasma proteins, ions, gases, carbohydrates, lipids, and various metabolites. Plasma proteins account for 4% of body weight and include red blood cells, granulocytes, lymphocytes, and platelets. The remainder of the extracellular compartment reflects bone tissue (approximately 15% of body weight) and collagen tissues (non-vital tendons, ligaments, connective tissues, and epidermis [non-vital layers]).

Bone composition varies according to type. In cortical bone 80% of the weight is bone structure and in trabecular bone is 20% bone structure. The remainder of bone tissue weight is in marrow and fat.

Though there are metabolic processes associated with bone tissues, the calorie use is low (except during growth or injury).

The third compartment in a three-compartment body composition model is fat tissues. These tissues account for 12-25% of healthy weight and is the most variable of body compartments. This compartment depends on caloric balance, decreasing with deficits and increasing with excesses. Fat hydration is much lower than other tissues with the ECW:ICW of 11:4 compared to 1:2 in lean tissues. Fat tissues can be divided into neutral or stored lipids accounting for approximately 85% of fat weight, adipocytes (3% of fat weight), and extracellular water (11% of fat weight). Adipose tissues function to provide a storage place for calories, and thermal insulation. Electrical measures tend to lump another category of fat into this compartment: essential adipose tissues. These are the tissues that surround cell membranes and are associated with collagen tissue playing important metabolic and calorie-using functions.

Homeostasis of body composition is maintained by renal, central nervous system, hormonal, and neurogenic influences. An example of homeostatic mechanisms is the hydration of fat-free mass, which is maintained at approximately 0.73 liters of water per kilogram. Alterations in hydration trigger the central nervous system thirst mechanisms. Blood pressure and fluid preservation are regulated by the kidneys.

Changes in homeostasis occur during disease. How the body composition profile changes is dependent on the contributing factors, including stress related to disease, malnutrition, condition duration, and treatments applied. Hormonal alterations produced by stress responses, tumor production, and other disease-related conditions initially influence fluid shifts and maintenance of BCM. A typical response includes the reduction of potassium in intracellular spaces and an elevation in levels of extracellular water. The reduction of intracellular potassium leads to a reduction in cell membrane potential, an increase in intracellular sodium, and diminished electrical charge-holding capability. Hydration of lean tissues may alter from a normal 0.5-0.7 upward to 1.0 or greater. These edematous changes occur long before the serious clinical signs that would be found on physical exam. Obvious bone density changes may or may not occur, depending on weight loss and hormonal balance. If malnutrition comes into play, nutritional treatment is essential to reverse these detrimental changes

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