Extracellular fluids

Clinical Applications Perhaps the area in which ion-selective electrodes receive the widest use is in clinical analysis, where their selectivity for the analyte in a complex matrix provides a significant advantage over many other analytical methods. The most common analytes are electrolytes, such as Na+, K+, Ca +, H+, and Ch, and dissolved gases, such as CO2. For extracellular fluids, such as blood and urine, the analysis can be made in vitro with conventional electrodes, provided that sufficient sample is available. Some clinical analyzers place a series of ion-selective electrodes in a flow... 

The term hormone is used to denote a chemical substance, released from a cell into the extracellular fluid in low quantities, which acts on a target cell to produce a response. Hormones are classified on the basis of chemical stmcture most hormones are polypeptides, steroids (qv), or derived from single amino acids (qv) (Table 1). 

Sodium and Potassium. Whereas sodium ion is the most abundant cation in the extracellular fluid, potassium ion is the most abundant in the intracellular fluid. Small amounts of K" are requited in the extracellular fluid to maintain normal muscle activity. Some sodium ion is also present in intracellular fluid (see Fig. 5). Common food sources rich in potassium may be found in Table 7. Those rich in sodium are Hsted in Table 8. 

Active Transport. Maintenance of the appropriate concentrations of K" and Na" in the intra- and extracellular fluids involves active transport, ie, a process requiring energy (53). Sodium ion in the extracellular fluid (0.136—0.145 AfNa" ) diffuses passively and continuously into the intracellular fluid (<0.01 M Na" ) and must be removed. This sodium ion is pumped from the intracellular to the extracellular fluid, while K" is pumped from the extracellular (ca 0.004 M K" ) to the intracellular fluid (ca 0.14 M K" ) (53—55). The energy for these processes is provided by hydrolysis of adenosine triphosphate (ATP) and requires the enzyme Na" -K" ATPase, a membrane-bound enzyme which is widely distributed in the body. In some cells, eg, brain and kidney, 60—70 wt % of the ATP is used to maintain the required Na" -K" distribution. 

The volume of extracellular fluid is direcdy related to the Na" concentration which is closely controlled by the kidneys. Homeostatic control of Na" concentration depends on the hormone aldosterone. The kidney secretes a proteolytic enzyme, rennin, which is essential in the first of a series of reactions leading to aldosterone. In response to a decrease in plasma volume and Na" concentration, the secretion of rennin stimulates the production of aldosterone resulting in increased sodium retention and increased volume of extracellular fluid (51,55). 

AletabolicFunctions. The chlorides are essential in the homeostatic processes maintaining fluid volume, osmotic pressure, and acid—base equihbria (11). Most chloride is present in body fluids a Htde is in bone salts. Chloride is the principal anion accompanying Na" in the extracellular fluid. Less than 15 wt % of the CF is associated with K" in the intracellular fluid. Chloride passively and freely diffuses between intra- and extracellular fluids through the cell membrane. If chloride diffuses freely, but most CF remains in the extracellular fluid, it follows that there is some restriction on the diffusion of phosphate. As of this writing (ca 1994), the nature of this restriction has not been conclusively estabUshed. There may be a transport device (60), or cell membranes may not be very permeable to phosphate ions minimising the loss of HPO from intracellular fluid (61). 

Transport. Transcobalamin II dehvers the absorbed vitamin 3 2 to cells and is the primary plasma vitamin B22-binding transport protein. It is found in plasma, spinal fluid, semen, and extracellular fluid. Many cells, including the bone marrow, reticulocytes, and the placenta, contain surface receptor sites for the transcobalamin II—cobalamin complex. 

As stated above, calcium is an extremely important cellular ion for several cellular functions. The concentration of calcium in human extracellular fluid is about 2.5 mM, while the intracellular concentration is only 100-200 nM depending on the cell type. Thus, there is 10 000-20 000 fold concentration difference between the cell interior and exterior that has to be maintained by cellular pumping mechanisms. This requires a large amount of energy. " ... 

The integrity of mammalian kidneys is vital to body homeostasis, because the kidneys play the principal role in the excretion of metabolic wastes and the regulation of extracellular fluid volume, electrolyte balance, and acid-base... 

Osmotic pressure from high concentrations of dissolved solutes is a serious problem for cells. Bacterial and plant cells have strong, rigid cell walls to contain these pressures. In contrast, animal cells are bathed in extracellular fluids of comparable osmolarity, so no net osmotic gradient exists. Also, to minimize the osmotic pressure created by the contents of their cytosol, cells tend... 

As already noted, microtubules are also the fundamental building blocks of cilia and flagella. Cilia are short, cylindrical, hairlike projections on the surfaces of the cells of many animals and lower plants. The beating motion of cilia functions either to move cells from place to place or to facilitate the movement of extracellular fluid over the cell surface. Flagella are much longer structures found singly or a few at a time on certain cells (such as sperm cells). They pro-... 

The extracellular calcium Ca -sensing receptor plays a central role in maintaining a nearly constant level of extracellular calcium by sensing small changes in Ca and directly and/or indirectly altering the translocation of calcium ions into or out of the extracellular fluid so as to normalize CaQ+. Changes in the level of expression and/or function of the CaR reset the level of CaQ+. Recently developed activators (calcimimetics)... 

Hyperaldosteronism is a syndrome caused by excessive secretion of aldosterone. It is characterized by renal loss of potassium. Sodium reabsorption in the kidney is increased and accompanied by an increase in extracellular fluid. Clinically, an increased blood pressure (hypertension) is observed. Primary hyperaldosteronism is caused by aldosterone-producing, benign adrenal tumors (Conn s syndrome). Secondary hyperaldosteronism is caused by activation of the renin-angiotensin-aldosterone system. Various dtugs, in particular diuretics, cause or exaggerate secondary peadosteronism. 

Occurs when the volume of extracellular fluid is significantly diminished. Examples include hemorrhage, fluid loss caused by burns, diarrhea, vomiting, or excess diuresis Occurs when the heart is unable to deliver an adequate cardiac output to maintain perfusion to the vital organs. Examples include as the result of an acute myocardial infarction, ventricular arrhythmias, congestive heart failure (CHF), or severe cardiomyopathy. 

The presence and biological importance of oligosaccharide structures, usually as components of glycolipids and glycoproteins, in bacterial capsular and cell-wall polysaccharides, in mammalian cell membranes, in cytoplasm, and in extracellular fluids, are now well documented. They are important constituents in... 

This compartment contains about one-third of total body water and is distributed between the plasma and interstitial compartments. The extracellular fluid is a delivery system. It brings to the cells nutrients (eg, glucose, fatty acids, amino acids), oxygen, various ions and trace minerals, and a variety of regulatory molecules (hormones) that coordinate the functions of widely separated cells. Extracellular fluid removes COj, waste... 

The Ionic Compositions of Intracellular Extracellular Fluids Differ Greatly... 

The membranes of nerve cells contain well-studied ion channels that are responsible for the action potentials generated across the membrane. The activity of some of these channels is controlled by neurotransmitters hence, channel activity can be regulated. One ion can regulate the activity of the channel of another ion. For example, a decrease of Ca + concentration in the extracellular fluid increases membrane permeability and increases the diffusion of Na+. This depolarizes the membrane and triggers nerve discharge, which may explain the numbness, tinghng, and muscle cramps symptomatic of a low level of plasma Ca. ... 

Pinocytosis is a property of all cells and leads to the cellular uptake of fluid and fluid contents. There are two types. Fluid-phase pinocytosis is a nonselective process in which the uptake of a solute by formation of small vesicles is simply proportionate to its concentration in the surrounding extracellular fluid. The formation of these vesicles is an extremely active process. Fi-... 

Molecules released by exocytosis fall into three categories (1) They can attach to the cell surface and become peripheral proteins, eg, antigens. (2) They can become part of the extracellular matrix, eg, collagen and glycosaminoglycans. (3) They can enter extracellular fluid and signal other cells. Insulin, parathyroid hormone, and the catecholamines are all packaged in gran-... 

CICR). It is estimated that approximately 10% of the Ca involved in contraction enters the cytosol from the extracellular fluid and 90% from the sarcoplasmic reticulum. However, the former 10% is important, as the rate of increase of Ca in the myoplasm is important, and entry via the Ca channels contributes appreciably to this. 

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