Thursday, September 23, 2010

2.13a The Water of Our Body - Internal Milieu

Physician's Notebooks 2  - - See Homepage

2.13a: Body Water and Electrolytes - Update 06 December 2017. Note that this chapter is in a, b and c sections. The below section introduces body water, electrolytes and osmolality.

The human body is mostly water
Estimating Volume of Body Fluids
Extra-cellular Fluid Composition 
Intracellular Fluid Composition 

The human body is mostly water in which is dissolved electrolyte and non electrolyte particles. The body fluid's main electrolytes are Na+, K+; Ca++, Mg2+, NH4+ (ammonium), Cl, (PO4)3- (phosphate) and HCO3 (bicarbonate). Also to be kept in mind is that organic molecular fragments (chemical radicals like acetate and citrate that contain C carbon, H hydrogen and O oxygen in combination) carry negative electric charges (are anions) as do protein fragments inside the cells. These organic ions balance off the positive-charge electrolytes (the cations) and are important because they are inside the cell and get constantly used up producing body energy which needs to be balanced by an intracellular decrease in positive inorganic ions, mostly Na+.
   The total body fluid is the water that contains the dissolved electrolytes and the dissolved non-electrolytes (the latter, electrically neutral organic molecules like glucose and cholesterol). It has 2 compartments: 1) the outside-of-cells extra-cellular fluid (ECF) that includes the blood plasma (the fluid of blood); and 2) the cell, or intracellular  fluid (ICF).
Estimating Volume of Body Fluids (For medical students before a physiology test or quiz): Body weight in pounds is easy to measure and it can estimate the volume of the body fluid. Total body water (TBW in liters)= Body weight in pounds divided by four. If you weigh 160 lb ("lb", libra, latin for the pound weight unit; 1 lb = 0.45 kg), your TBW is 40 Liters (L).  The ECF is 40% of the TBW. Thus, a 160 lb person with TBW 40 Liters (L) has 16L ECF and 24L ICF. Blood plasma is 8% of TBW, so a 160 lb man with 40 L TBW, 16L of which are ECF, can be estimated to have 3.2 L blood plasma. And since plasma normally is 60% of total blood volume (TBV), it calculates to TBV 5 Liters. 
   Since water passes easily and rapidly between ICF and ECF across the cell membranes, it maintains what is called osmotic equilibrium by going into or out of cells in response to unequal concentrations of the dissolved particles like excess Na+ inside a cell or like the glucose molecules outside the cells in a diabetic without insulin. Each particle in fluid needs to be surrounded by a unit of water molecules. Since particles are constantly being added to the ECF by eating and drinking, and leaving the ECF in the urine and loss in feces and sweat and breathed air; and since some particles (organic electrolytes like acetate and protein ion) are disappearing from the ICF due to being used up by the cells' metabolism, the water in cells is constantly shifting back and forth across cell membrane, causing a shrink and swell of the cell. The body control is always attempting to limit cell swell or cell shrink.
Extra-cellular Fluid Composition: Most electrolyte particles dissolved in the ECF are Na+, Cl, and HCO3(bicarbonate) each of whose normal value in blood plasma is well known in milli-equivalent per liter. On routine blood testing, the Na+, Cl, and HCO3 are normally reported respectively as around 140, 104, and 24 meq/L (milliequivalents per liter).

Intracellular Fluid Composition: Inside our cells, the main electrolyte particles are K+, Mg++, PO43– (phosphate), , and the negative ions of proteins and organic radicals like acetate. This difference between ICF and ECF is kept stable by the cell membranes controlling the entry and exit of these ions from the cell and by “pumps” one of which pushes excess Na+ out of cells and pulls balancing K+ into cells from the surrounding ECF. The Na+ and the K+ are the most important in keeping an electrical and osmotic balance between the inside (ICF) and outside (ECF) of the cells.
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  • 2.13b Blood Electrolytes & Body Water

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