Extracellular fluid description

Every cell possesses a plasma (or cell) membrane which isolates its contents from its surroundings. This membrane consists of a double layer of phospholipid molecules with proteins attached or dispersed within. The uneven distribution of proteins and their ability to move in the plane of the membrane led to the description of this structure as a fluid mosaic (Figure 1.2) (Chapter 5). Some of these proteins facilitate the transport of molecules and ions through the membrane, while others are receptors for extracellular molecules which provide information about conditions in adjacent cells, blood and elsewhere in the body. Physical or chemical damage to these membranes can render them leaky so that, for example, Na and Ca ions, the concentrations of which are much higher in the extracellular fluid, can enter the cell causing damage. On the outer surface of... 

Description. Calcium is the most common mineral in the human body. About 99% of the calcium in the body is found in bones and teeth, while the other 1% is found in the blood and soft tissue. Calcium levels in the blood and fluid surrounding the cells (extracellular fluid) must be maintained within a very narrow concentration range for normal physiological functioning. The physiological functions of calcium are so vital to survival that the body will danineraUze bone to maintain normal blood calcium levels when calcium intake is inadequate. Thus, adequate dietary calcium is a critical factor in maintaining a healthy skeleton. ... 

Epithelial cells comprise closely packed monolayers that separate different tissue compartments. One side of the epithelium faces the outside of the body, or the side occupied by the external environment. The other side faces the inside of the body, or the side containing the blood and extracellular fluids. The skin is made of epithelial cells, as are the lumens of the gastrointestinal tract, the kidneys, the lungs, and the urinary bladder (Putnam, 1995). Simple epithelium is one cell thick, and compound epithelium is several cells thick. The cells usually cover connective tissue, and are held together by a cementing substance to form a sheet (Hale et al., 1995). Their shapes give rise to descriptive names of columnar, cubical, and squamous (flat). 

Models of transport in environments as complex as the extracellular spaee of a tissue require a sophisticated description of the microstructure. In many tissues the extracellular space is stochastically arranged, consisting of an ensemble of individual spaces of random size that are distributed randomly. Useful models of transport in this situation can be based on percolation descriptions of porous media. Percolation descriptions have been used to describe fluid flow, electrical conduction, and phase transitions in random systems [108, 109]. 

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