Electrolytes And Acidbase Metabolism

Electrolytes are the ionic components of the body fluids, the most notable being sodium (Na+), potassium (K*), chloride (CI ), bicarbonate(HCO3 ), hydrogen (HO, calcium (Ca++), and phosphate (PO4 3). The precise concentrations of these electrolytes are important to body functioning, to maintain fluid osmolality and volume (sodium!), or to directly participate in body functions that require particular ions or precise pH.

Many diseases can change electrolyte concentrations. In order to understand this, one must first consider the factors that maintain normal electrolyte balance:

The kidney, unlike popular portrayals, is more than just a toilet that flushes out waste products. Its one million nephrons play a powerful role in maintaining the proper balance of numerous molecules vital to the body, such as sodium, potassium, bicarbonate, chloride, H+, glucose, etc. The kidney is also important in the excretion of many kinds of foreign molecules, including many drugs (the liver also eliminates many drugs). It does these things through filtration, reabsorption, secretion, and synthesis:

1) Filtration. Filtration of molecules occurs through the glomerular membrane.

Fig. 3-1. The glomerular membrane contains three layers:

a) the capillary endothelium of the glomerulus.

b) the inner wall of Bowman's capsule (sort of like the wall of a balloon that has been punched in by a fist and which is contacting the fingers). This layer contains "podocyte" cells.

c) a non-cellular glomerular basement membrane, which lies between the capillary endothelium and podocyte layers. (The glomerulus (Figs. 2-8, 2-9) also contains mesangial cells. These have contractile properties (partly stimulated by catecholamines and angiotensin II; inhibited by atrial natriuretic factor). When mesangial cells contract or relax, this respectively reduces or increases the surface area of the glomerulus, thereby altering the capacity for glomerular filtration.

The outer and inner layers of the glomerular membrane leak, since the cells do not tightly adhere to one

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SASsri«Kr erntin.tAL p©<rr fXOuuts (PoDocYnrs)

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