The Cell and Fluid Homeostasis

Cell Structure and Organization

Organisms evolved from single cells floating in the primordial sea ( Fig. 1.1 ). A key to appreciating how multicellular organisms exist is through understanding how the single cells maintained their internal fluid environment when exposed directly to the outside environment, with the only barrier being a semipermeable membrane. Nutrients from the “sea” entered the cell, diffusing down their concentration gradients through channels or pores, and waste was transported out through exocytosis. In this simple system, if the external environment changed (e.g., if salinity increased due to excess heat and evaporation of sea water or if the water temperature changed), the cell adapted or perished. To evolve to multicellular organisms, cells developed additional barriers to the outside environment to allow better regulation of the intracellular environment.

In multicellular organisms, cells undergo differentiation, developing discrete intracellular proteins, metabolic systems, and products. The cells with similar properties aggregate and become tissues and organ systems (cells → tissues → organs → systems).

Various tissues serve to support and produce movement (muscle tissue), initiate and conduct electrical impulses (nervous tissue), secrete and absorb substances (epithelial tissue), and join other cells together (connective tissue). These tissues combine and support organ systems that control other cells (nervous and endocrine systems), provide nutrient input and continual excretion of waste (respiratory and gastrointestinal systems), circulate the nutrients (cardiovascular system), filter and monitor fluid and electrolyte needs and rid the body of waste (renal system), provide structural support (skeletal system), and provide a barrier to protect the entire structure (integumentary system [skin]) ( Fig. 1.2 ).

The Cell Membrane

The human body is composed of eukaryotic cells (those that have a true nucleus) containing various organelles (e.g., mitochondria, smooth and rough endoplasmic reticulum, Golgi apparatus) that perform specific functions. The cell, with its nucleus and organelles, is surrounded by a plasma membrane consisting of a lipid bilayer primarily made of phospholipids, with varying amounts of glycolipids, cholesterol, and proteins. The lipid bilayer is positioned with the hydrophobic fatty acid tails of phospholipids oriented toward the middle of the membrane and the hydrophilic polar head groups oriented toward the extracellular or intracellular space. The fluidity of the membrane is maintained in large part by the amount of short-chain and unsaturated fatty acids incorporated within the phospholipids; incorporation of cholesterol into the lipid bilayer reduces fluidity ( Fig. 1.3 ). The oily, hydrophobic interior region makes the bilayer an effective barrier to fluid (on either side), with permeability only to some small hydrophobic solutes, such as ethanol, that can diffuse through the lipids.

To accommodate multiple cellular functions, the membranes are actually semipermeable because of a variety of proteins inserted in the lipid bilayer. These proteins are in the form of ion channels, ligand receptors, adhesion molecules, and cell recognition markers. Transport across the membrane can involve passive or active mechanisms and is dictated by the membrane composition, concentration gradient of the solute, and availability of transport proteins (see Chapter 2 ). If the integrity of the membrane is disrupted by changing fluidity, protein concentration, or thickness, transport processes will be impaired.