2.13a The Water of Our Body - Internal Milieu

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2.13a: Body Water and Electrolytes - Update 13 Dec. 2021 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 sodium, or natrium Na⁺, potassium, or kalium K⁺; calcium Ca⁺⁺, magnesium Mg²⁺, ammonium NH₄⁺, chloride Cl^–, phosphate (PO₄)^3- and bicarbonate HCO₃^– . 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 in body metabolism which needs to be balanced by an intracellular decrease in positive inorganic ions, mostly sodium ion, Na+ which is constantly being supplied in food and drink.

   The total body fluid solvent is the water that contains the dissolved electrolytes and 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 inside of the cells, or intracellular fluid (ICF).

Estimating Volume of Body Fluids: Body weight in avoirdupois pounds (lbs) or in metric kilograms (kgm; 1 kgm = 2.2 lbs) is easy to measure and it can estimate the volume of the body fluid. Total body water volume (TBV in liters) = Body weight in lbs divided by 4. If you weigh 160 lbs, your TBV is 40 Liters (L).  The ECF is 40% of the TBV. Thus, a 160 lb person with TBV 40 Liters (L) has 16L ECF and 24L ICF. Blood plasma is 8% of TBV, so a 160 lb man with 40 L TBV, 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 shrink and as well as 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 is well known in U.S.A., in milli-equivalent per liter (in Europe by SI in mMole per liter). On routine blood testing, the Na⁺, Cl^–, and HCO₃^– are normally reported respectively as around 140, 104, and 24 mMole or mEq/L (milliequivalents per liter).

Intracellular Fluid Composition: Inside our cells, the main electrolyte particles are K⁺, Mg⁺⁺, PO₄^3– (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