2.13b Blood Electrolytes & Body Water

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2.13b Updated 14 Dec. 2021. The following descending column of headings in order as each appears in text.

 Osmolality in Blood Plasma or Serum

Shrink & Swell Cells

“Tonicity”

Water Loss from Body

Polyuria too much urine

water loss from skin

Water loss from lungs

Water Loss from GI Tract

Dehydration

lost at sea and no drinking water

Potassium (K+) Electrolyte

Sodium (Na) Considered as Mineral 

Osmolality in Blood Plasma or Serum is the measure of dissolved particles per kilogram of body fluid while "osmolarity" is the particles per milliliter. The osmolality may be directly measured in blood plasma by an osmometer. It can also be estimated in a patient using the patient's blood tests as follows in S.I. units: ([Serum Na+] in mMoles/L x 2) + (Serum glucose in mMoles/L) + (serum urea mMoles/L). Or, in conventional units used in the U.S.A. as follows: (Serum Na⁺ in meq/L x 2) + (Serum glucose in mg/dL÷18) + (serum BUN blood urea nitrogen in mg/dL÷2.8)].

Substituting normal blood test numbers, the normal serum osmolality should be 270-290 milliOsmols/Liter).

This estimated value is useful to compare with a direct measure because in many poisonings an unusual amount of dissolved osmolal particles not accounted for in the above blood-test formula accumulates abnormally high in blood. In these cases the direct measurement is quite a bit higher than the calculated formula. This difference, called the osmolal gap, is a clue to unusual substances in a blood sample from poisoning or excess alcohol. The good thing about the osmolal gap test is that the blood test values are routinely done on every hospitalized patient. The gap is most useful for patients brought to emergency in mysterious overdose coma states.

An even simpler calculation, called the anion gap, is done by using the almost universally measured serum  Na+, Cl- and HCO3- in the following formula: The anion gap equals [Na+] minus [Cl-] minus [HCO3-] either in SI or conventional units.  The normal result (in conventional units used in USA and Japan) should be 8 to 12 mEq/L.This simple anion gap test will pick up poisonings from electrolyte excess like H+ acidosis by showing a larger than expected anion gap in the blood test sample. It does not need a direct osmometer test or complicated formula as the osmolal gap test does need. But it will miss certain types of non electrolyte osmolal poisonings. Thus, the procedure is always to first to determine if an anion gap exists. If it does, then determine if it includes an osmolal gap.

Shrink & Swell Cells: Extracellular particles like Na⁺ (Sodium ion of salt) do not normally pass through cell walls. They are effective osmoles compared to an ineffective osmole like urea, which passes quickly through the cell wall into and out of cells and exerts no osmotic effect. An effective osmole may accumulate in the extracellular body water (Serum sodium, Na+ from drinking salt water) and pull water molecules out of the cell causing the brain cell to shrink. Or it may get into a very low concentration in the extracellular body water (Low Na+ from only drinking water without salt) allowing excess water to be sucked into the cells causing the brain cell to swell. The anion gap test, if above 12 mEq/L will show the blood plasma is very hyperosmolal with risk of severe brain cell shrink. Or if below 8 mEq/L it shows the plasma is very hypo-osmolal and risks brain cell swell. By this method, one needs only simple blood test numbers to save life.

“Tonicity” is a relative term indicating greater or lesser osmolality of ECF across the cell membrane, and tells one what a solution of specific osmolality does to the cell. “Hypertonic” and “hypotonic” specify “greater than” or “less than” normal osmolality while isotonic means normal osmolality. 

Water Loss from Body: Your kidneys regulate water in body in response to anti-diuretic hormone (ADH). The ADH is made in the hypothalamus (in lowest part of Brain) and then secreted into nearby posterior pituitary gland and released into blood. It varies in response to the state of hydration or dehydration.

Effect of ADH is told by the name, which describes function (anti-diuresis, prevent peeing). When ADH is released, it opens pores in kidney microtubules, and water that has been filtered out of blood plasma into the millions of microscopic kidney units gets reabsorbed back into the blood. At maximal ADH secretion, urination stops. This is how the body conserves fluid and re-hydrates after fluid loss. Of the many stimuli to increase ADH, the most powerful is increasing the particle concentration (osmolality) of blood plasma. (Eat a bag of salty pretzels) Then your ECF becomes hypertonic to the cells of hypothalamus in brain, which are most sensitive to sodium concentration, and they shrink, and this is sensed by ADH cell-surface receptors, causing outpouring of ADH and reducing urine. A drop in blood pressure also stimulates ADH release in order to put fluid into blood circulation and restore BP to normal.

If too much ADH gets released suddenly, urination stops and the body gets overloaded with water. In extreme case, the body swells, the lungs fill with fluid stopping respiration, the blood pressure goes sky high, and hyponatremia (low Na) with brain swelling occurs. This can’t go on unopposed for long without death. 

In opposite case when osmolality of blood plasma drops, the hypothalamus cells swell and stop ADH secretion and large amounts of dilute urine are peed out. In a defect of ADH secretion, diabetes insipidus (Not the usual diabetes which is diabetes mellitus), this can happen without drinking much water and then too much body fluid gets lost in peeing and the vascular system becomes contracted and the person goes into shock and dies despite drinking water.

Normally there is a balance back and forth between slight hypernatremia (high Na+) and slight hyponatremia (low Na+) resulting in pulses in your body fluids of more or less ADH. This is in response to water or salt in diet. For example, by drinking a quart of water quickly, a person temporarily shuts off his ADH and urinates heavily to get rid of the excess water. To turn on ADH, one can go without water for hours or else eat a lot of salt at once. Either will raise the osmolality of the blood plasma and stimulate ADH release, but eating salt is more dangerous because salt stimulates thirst, and the rise in ADH with high water intake can lead to overfilling of the vascular system and high blood pressure and stressing the heart; so, excess salt in diet is a No Go for health or longevity. A good rule is: No added salt, no obviously salted food and no excess salt-processed food (canned, frozen).

With normal heart and normal kidneys, your body has a huge tolerance for water loading. So my advice to drink a lot of water every day is OK. But if one has problem with heart or kidneys failure or severe hypertension, then a doctor should be consulted before trying to force fluid.

Polyuria too much urine: Normal volume of urine in 24 hrs ranges from 0.75 to 2.5 Liters depending on one’s daily fluid intake. Urination in excess of 2.5 L a day is polyuria and one cause is excess solute in blood that does not enter cell and thus must be excreted by the kidneys taking excess water with it. This is osmotic diuresis because it occurs with increasing osmolality of plasma. A common excess is glucose in an untreated diabetes mellitus (New machine Coke addict!) . Non-osmotic polyuria is from decreased release of ADH due to damage in hypothalamus or pituitary gland. Most commonly, polyuria is due to just drinking too much water. In that case it is a washout or flushing effect. 

A defective thirst mechanism may make polyuria by causing un-satiated drinking. Such a person continues to thirsty despite heavy water intake. He drinks water copiously in public. In this group, mental illness is common.

In water loss from skin, as visible sweating or insensible perspiration, the amount lost depends on body heat. Sweat contains 50 mEq/L of Na⁺ (140 mEq/L in blood plasma, so sweat is hypotonic). Water loss from skin at normal room temperature is 30 ml per each 100 calories of body-generated heat. It rises with fever, increasing metabolic rate as in hyperthyroidism, external overheating as in heat wave, and burn.

Water loss from lungs is due to the water content of exhaled air being higher than inhaled air; the difference being respiratory water loss, normally 13 ml per 100 calories generated body heat (About 300 ml per day with normal breathing; but note this depends also on humidity of the air and can be prevented by humidifier)

Water Loss from GI Tract: About 100 ml water is lost daily from GI tract normally. Since fluid of intestine is isotonic with (same osmolality as) blood plasma, the intestinal loss from diarrhea causes equal loss of particles and water from body fluid. In case of intestinal obstruction, a large amount of isotonic fluid passes from blood plasma into obstructed intestine and this leads to drop in blood volume and low blood pressure, rapid pulse, shock and death.

Fluid loss from specific part of GI tract causes electrolyte problem in blood. Because vomit from stomach is hi-K⁺, hi-Cl^-, acid and hypertonic, its prolongation causes body fluid to become low in K⁺, and alkaline (hi-pH, hypokalemic alkalosis) and makes the patient dehydrated.

Dehydration: means abnormal loss of body water without replacement. In Isotonic Dehydration, the net salt and water loss is equal, leaving no change in tonicity but decrease in volume of extracellular body fluid (ECF) and most frequently is seen with loss of intestinal fluid as in severe or prolonged diarrhea. Intestinal fluid drains the ECF volume and the salt and water is equal. The worst effect of excess loss is reduction of blood plasma from the diarrhea GI fluid, causing drop in blood pressure with collapse. The famous disease that kills in this way is Cholera because of tremendous isotonic diarrhea. If this dehydration is treated by drinking water with insufficient salt, as is frequent, the body fluid inside and outside cell becomes hypotonic and blood test will reveal low Na⁺ (hyponatremia). If it is prolonged, the brain cells swell, leading to headache, mental confusion, and death. That is why treatment of severe diarrhea from cholera should include re-hydration with isotonic fluid (soda, Gatorade, and other sport drink) and not water alone. Severe vomiting requires treatment with intravenous (IV) isotonic saline or special electrolyte mixture in IV.

Hypertonic Dehydration is loss of water alone or water in excess of salt, leaving higher salt concentration in Extracellular Fluid. This is what develops when denied access to water (lost in desert) or unable to drink (damage to throat). Since water is lost involuntarily, daily, mostly in urine but also by sweat, in breathing, and from GI tract, the body fluid becomes hypertonic and blood test shows high Na⁺ (hypernatremia). In these cases if lack of water is prolonged especially if by uncontrolled diabetes mellitus or too much alcohol, the brain cell shrinks, leading to coma, brain damage and death. Good treatment is re-hydration with pure water.

Person lost at sea and no drinking water also gets hypertonic dehydration. And, as sometime happens, if he tries to survive by drinking seawater, he will kill self quicker because seawater is more hypertonic than his hypertonic dehydrated body fluid, so it only makes the hypertonicity of his blood worse, sucks water from cell and shrinks brain more quickly causing rapid delirium and death. But if he has access to isotonic fluid, drinking it can prolong survival because kidneys will help by excreting excess Na⁺ and conserving the water for the body, causing gradual return of body fluid to normal. So if marooned in ocean on lifeboat and your water runs out, catch fish and drink its squeezed fresh flesh fluid, (but don’t eat the flesh because the raw fish flesh is too salty). By the way, in the Pacific, flying fish are frequent and easy to catch from a life raft.

Hypotonic Dehydration is where salt loss is more than water loss, leaving lower salt concentration in body fluid. This may be seen in patient on strict low-salt diet, one given a Na-losing diuretic, and in the kidney disease, “salt-losing nephritis.” Also it is typical of inappropriate excessive secretion of ADH from disease in the brain. Hypotonic dehydration leads to brain swelling and is treated in hospital by limiting water intake and carefully administering salt.

Potassium (K⁺) Electrolyte Disorder: Sodium (Na⁺) and K⁺ are the two most important electrolytes and they have an opposite relationship to each other in ECF and ICF. The K⁺ is mostly inside cell, in the ICF, and the Na⁺ mostly outside, in the ECF.  Normal cell metabolism requires this, and a “pump” is continuously working to pump excess intracellular Na⁺ out of the cell and extracellular or plasma K⁺ into the cell.

Our bodies are sensitive to blood plasma K⁺ concentration. If K⁺ ions go below 4 mEq/L a body state of “hypokalemia” (low blood plasma potassium ion concentration) exists, leading to muscle weakness and slowing heart. Worse is high plasma K⁺ (hyperkalemia). If K⁺ goes high, one feels weak and sick, and the heart may go irregularly fast and then stop.

Potassium deficiency or hypokalemia is most often from severe dehydration of vomiting or diarrhea in which water only is used to re-hydrate; it can be also caused by diuretic pill (a thiazide) for high blood pressure, which pushes potassium into urine. That is another reason dehydration from vomiting or severe diarrhea should be re-hydrated by drinking mixed electrolyte sport drink. Also if you take chlorthiazide or like pills for high Blood Pressure be sure to get test for K+.

Potassium in food is best obtained from fruit (banana has a lot).

High K+ state of blood (hyperkalemia) is not usually due to eating excess K in food because normal kidneys excrete excess K⁺. Whenever a first blood test comes back with high K+ result, first think of false high K+ due to blood sample standing too long or trauma to red blood cell from too tight tourniquet. The test should be repeated avoiding these sources of error. True high K+ is seen in heart failure patient who takes potassium pill or a diuretic spirinolactone pill to prevent low K+. It may be a sign of hypoadrenalism (low adrenal corticoid production, Addison disease). Also it is part of end-stage kidneys failure; in those cases the dietary advice would be to avoid high-K food and especially avoid the potassium-containing (KCl) salt supplement that some take to give salty taste to food. High K+ is only rarely discovered from symptom, but once it rises very high it causes rapid irregular heart that may terminate in cardiac arrest sudden death. It is picked up on electrocardiogram or blood test. Serum K+ above 6 meq/L must be treated medically as emergency. Medicine causing high K⁺needs to have the serum K⁺ monitored by frequent blood testing.

Sodium (Na) Considered as Mineral and Electrolyte (Na+): The sodium in table salt as NaCl, in solution becomes Na⁺ and Cl^-, both important electrolytes. Na⁺ is the main cation electrolyte of the ExtraCellularFluid (ECF). In blood plasma its usual normal range concentration is 136-145 meq/L. Its plasma concentration normally determines the osmolality of blood, and a body sensor reacts to changes in plasma [Na⁺] (e.g., hypothalamus secretes anti-diuretic hormone – ADH – as [Na⁺] rises above 140).

Hyponatremia is diagnosed by blood test that shows plasma [Na⁺] below 135 mEQ or mMole per liter. It is due to prolonged diarrhea, which causes loss of Na⁺ that is not replaced due to drinking fluid without Na⁺ or is due to excessive ADH effect with inappropriate thirst that makes one drink too much water with very little urination as in mental illnesses, or due to kidneys disease sodium loss or excessive use of Na-losing diuretic pill in heart disease. Hyponatremia causes fatigue and mental dullness. If allowed to progress it leads to brain swelling convulsion and death.

Hypernatremia is high plasma [Na⁺]. Significant high begins at 150 mEq/L or mMole/L. Symptom develops because of shrinkage of cell in brain and is mental confusion leading to coma and death. Main cause is decreased water intake.

This Chapter continues in the next section.  Click

2.13c Acidity and Alkalinity of the Body