Medical Terminology: Fluids and Electrolytes

1. What are Fluids and Electrolytes?
Fluids in the body are found in three basic places: within the cells (intracellular), outside the cells (extracellular), and within the tissue spaces (interstitial space or third space). A balance should be maintained to keep concentrations of both fluids and electrolytes in the proper areas for normal function.
The cell walls are semipermeable to allow for movement (diffusion) of molecules. This helps to maintain osmotic pressure. Edema occurs when too much fluid enters the interstitial space. Peripheral edema usually collects in subcutaneous areas. The higher hydrostatic pressure in the vessel causes fluids to move into the inter-stitial areas that have lower pressure, allowing the fluid to build up.
Normal osmolarity of plasma is 270. to 300. mOsm/L. Isotonic or normotonic fluids have similar concentrations. This prevents fluids from shifting into spaces they do not belong. Hypertonic solutions have a concentration >300. mOsm/L and exert greater pressure, which pulls water from the isotonic area to the hypertonic solution in an attempt to equalize the osmolarity. Hypotonic solutions have a concentration of <270. mOsm/L and exert less pressure, which allows water to be pulled from the hypotonic area into the isotonic area.

2. Hormonal Regulation of Fluids and Electrolytes
Aldosterone is secreted by the adrenal cortex in response to sodium changes. Where sodium goes, water follows. Aldosterone signals the tubules within the nephrons in the kidneys to reabsorb sodium and therefore water. This increases blood osmolarity. Aldosterone also aids in control of potassium levels.
Renin is secreted by the kidneys in response to changes in sodium or fluid volume. In the circulation, renin acts on a plasma protein called renin substrate (also called angiotensinogen), converting it to angiotensin I. In the pulmonary circulation, angiotensin-converting enzyme converts angiotensin I to angiotensin II. This causes vascular constriction and aldosterone secretion.
Antidiuretic hormone (ADH) is produced in the brain and stored in the posterior pituitary. It is released when there is a change in the osmolarity of the blood. ADH acts on the renal tubules, causing them to reabsorb more water, which decreases blood osmolarity. When the osmolarity gets too low, the release of ADH is not needed and water is excreted in the urine.
Natriuretic peptides are secreted in response to increases in blood volume and blood pressure. When atrial natriuretic peptide (ANP) and brain natriuretic peptide (BNP) are secreted, kidney reabsorption of sodium is inhibited and the glomerular filtration rate is increased. Blood osmolarity is decreased and urine output is increased.

3. Acid-Base Balance
Maintaining acid-base balance keeps the pH level within the normal range of 7.35. to 7.45. The lungs and kidneys are integral in maintaining the normal acid-base balance. The body constantly monitors the pH level and makes adjustments in an attempt to correct any abnormalities.
Bicarbonate (HCO3) is regulated by the kidneys. Partial pressure of carbon dioxide (PCO2) is regulated by the lungs. If the patient develops acidosis, there will be a low pH and either a drop in HCO3. (metabolic) or a rise in PCO2. (respiratory). If the patient develops alkalosis there will be an increase in pH and either an increase in HCO3. (metabolic) or a drop in PCO2. (respiratory).
In an attempt to maintain as normal an internal environment as possible, the body attempts to compensate for the changes that are occurring. The lungs are able to correct much more rapidly than the kidneys.

4. Hyponatremia
Hyponatremia is an abnormally low amount of sodium in the blood. Low levels of sodium may be caused by loss of sodium from the body, movement of sodium from the blood to other spaces, or dilution of sodium concentration within the plasma.
Some causes include excretion of sodium, water imbalance, hormonal imbalance (such as excess ADH), ecstasy (methylenedioxymethamphetamine) use, hypothyroidism, renal failure, diuretics, diarrhea, vomiting, and wound drainage.

5. Hypernatremia
Hypernatremia is an abnormally high amount of sodium in the blood. Fluid volume may be altered as a result of changes in the levels of sodium. A mild rise in sodium levels causes tissue that is normally excitable to become more irritable (e.g., cardiac muscle).
The osmolarity of extracellular fluid also increases as the sodium level increases. This is an attempt to correct the sodium increase by bringing more fluid from the cells into the extracellular area. These dehydrated, more irritable cells have a decreased ability to respond to stimuli.
Causes may include insufficient water intake (patients who are NPO), insufficient sodium excretion because of hormone imbalance, renal failure, corticosteroids, increased sodium intake or increased water loss because of fever, hyperventilation, increased metabolism, and dehydration because of sweating, vomiting, or diarrhea.

6. Hypocalcemia
Hypocalcemia is an abnormally low level of calcium in the blood. Decreased levels of calcium may be caused by inadequate intake or absorption (vitamin D deficiency, malabsorption), excess loss (associated with burns, renal disease, diuretics, or alcoholism), endocrine disorders (e.g., hypoparathyroidism), decreased serum albumin, hyperphosphatemia, or sepsis.

7. Hypercalcemia
Hypercalcemia is an abnormally high amount of calcium in the blood. Excess intake of calcium such as supplements or antacids or altered excretion of calcium as in patients with renal failure or those taking thiazide diuretics may cause hypercalcemia. Patients may also develop elevated calcium levels with prolonged immobility, glucocorticoid use, hyperthyroidism, hyperparathyroidism, lithium use, dehydration, or malignancies with metastasis to the bone.

8. Hypokalemia
Hypokalemia is a lower-than-normal level of potassium in the blood. A balance between the amount of potassium within the cell (intracellular) and outside the cell (extracellular) extracellular necessary. This allows the resting potential of the cell membrane to be maintained. When there are low potassium levels, a greater-than-normal stimulus is needed to depolarize the cell membrane.
Many cells become more sluggish, especially nerve cells. However, cardiac cells become more excitable. Fluid losses caused by diuretics or diarrhea, endocrine disorders such as hyperthyroidism or hyperaldosteronism, insufficient intake of potassium, and low magnesium levels can all contribute to low potassium levels. Dietary intake is the main source of potassium, so patients with poor nutritional intake or prolonged NPO status are also at risk for hypokalemia.

9. Hyperkalemia
Hyperkalemia is an elevated level of potassium in the blood. Dietary intake is the main source of potassium. Patients are at risk for hyperkalemia when there is excessive ingestion of potassium-rich foods or salt substitutes, they are on medications that cause potassium retention (ACE inhibitors, angiotensin receptor blockers, potassium-sparing diuretics such as amiloride or spironolactone, NSAIDs, trimethoprim, pentamidine), or there is excess release of potassium from the cells (hemolysis, acidosis, low insulin levels, beta blocker use, digoxin overdose, succinylcholine, or rhabdomyolysis).

10. Hypomagnesemia
Hypomagnesemia is a lower-than-normal magnesium level in the blood. Low serum levels of magnesium can be caused by lack of sufficient intake or absorption (malnutrition, vomiting, diarrhea, celiac disease, Crohn’s disease), excess excretion of magnesium (renal loss, chronic alcohol intake, diuretic use, aminoglycoside antibiotics, anti-neoplastics), or intracellular movement of magnesium (ascites, hyperglycemia, insulin administration). The cell membranes become more excitable in the setting of low magnesium levels. Patients may also have associated imbalances of potassium and calcium.

11. Hypermagnesemia
Hypermagnesemia is a greater-than-normal amount of magnesium in the blood. Patients with poor renal function or long-term abuse of magnesium-containing compounds have difficulty excreting magnesium.
The excess of magnesium in the blood causes the cell membranes to become less excitable than normal, requiring greater stimuli than would normally be needed to cause a required effect. As the magnesium level continues to rise, the cell membrane becomes more resistant to its natural stimuli.

12. Metabolic Acidosis
The acid-base balance of the blood is thrown off, causing it to become more acidic. There is an arterial pH of <7.35. There may be an overproduction of hydrogen ions (lactic acidosis in fever or seizures, diabetic ketoacidosis, starvation, alcohol or aspirin intake), deficient elimination of hydrogen ions (renal failure), deficient production of bicarbonate ions (renal failure, pancreatic insufficiency), or excess elimination of bicarbonate ions (diarrhea).

13. Metabolic Alkalosis
The acid-base balance of the blood is basic because of either a decrease in acidity or an increase in bicarbonate. Alkalosis is often associated with decreased levels of potassium or calcium. Metabolic alkalosis may be caused by excess intake of antacids, blood transfusions, long-term parenteral nutrition, prolonged vomiting or naso-gastric suctioning, Cushing’s disease, use of thiazide diuretics, or excess aldosterone.

14. Hypophosphatemia
Hypophosphatemia is a lower-than-normal amount of phosphorus in the blood. Chronic alcohol use, chronic obstructive pulmonary disease, asthma medications (loop diuretics, corticosteroids, adrenergic agonists, xanthine derivatives) are associated with low phosphate levels. Vitamin D is important in the intestinal absorption of phosphate. Parathyroid hormone stimulates the release of phosphate from the bone tissue. An overproduction can lead to hypophosphatemia.

15. Hyperphosphatemia
Hyperphosphatemia is a higher-than-normal amount of phosphorus in the blood. Patients may develop increased phosphate levels as a result of renal insufficiency, increase in phosphorus intake (supplements, laxatives, enemas, excess vitamin D), hypoparathyroidism, rhabdomyolysis, or as a result of cell destruction from chemotherapy. As phosphate levels increase, calcium levels decrease.

16. Dehydration
Dehydration is a state of having less-than-normal body fluids, caused by an excess loss of fluids or an inadequate intake of fluids. Dehydration may be actual or relative. A relative dehydration exists when the amount of fluid and electrolytes in the body is correct, but the placement is not correct.
If fluid shifting has occurred and the fluid is now in the interstitial areas rather than in the circulating blood volume, the patient may actually be experiencing a relative dehydration. Even though there is adequate fluid within the body, it cannot be used at this time. More commonly, dehydration is actual and caused by loss of fluid from the body or lack of adequate hydration.

Basic Questions
Fluids and Electrolytes

1. What is the interstitial space?
The interstitial space is the fluid within the tissue spaces.

2. What is edema?
Edema occurs when too much fluid enters the interstitial space.

3. What is peripheral edema?
Peripheral edema usually collects in the subcutaneous layer of skin.

4. What is normal osmolarity of plasma?
Normal osmolarity of plasma is 270. to 300. mOsm/L. Isotonic or normotonic fluids have similar concentrations. This prevents fluids from shifting into spaces they do not belong.

5. What are hypertonic solutions?
Hypertonic solutions have a concentration >300. mOsm/L and exert greater pressure, which pulls water from the isotonic area to the hypertonic solution in an attempt to equalize the osmolarity.

6. What are hypotonic solutions?
Hypotonic solutions have a concentration of <270. mOsm/L and exert less pressure, which allows water to be pulled from the hypotonic area into the isotonic area.

7. What is aldosterone?
Aldosterone is secreted by the adrenal cortex in response to sodium changes. Where sodium goes, water follows. Aldosterone signals the tubules within the nephrons in the kidneys to reabsorb sodium and therefore water. This increases blood osmolarity. Aldosterone also aids in control of potassium levels.

8. What is renin?
Renin is secreted by the kidneys in response to changes in sodium or fluid volume. In the circulation, renin acts on a plasma protein called renin substrate (also called angiotensinogen), converting it to angiotensin I. In the pulmonary circulation, angiotensin-converting enzyme converts angiotensin I to angiotensin II. This causes vascular constriction and aldosterone secretion.

9. What is antidiuretic hormone (ADH)?
ADH is produced in the brain and stored in the posterior pituitary. It is released when there is a change in the osmolarity of the blood. ADH acts on the renal tubules, causing them to reabsorb more water, which decreases blood osmolarity. When the osmolarity gets too low, the release of ADH is not needed and water is excreted in the urine.

10. What are natriuretic peptides?
Natriuretic peptides are secreted in response to increases in blood volume and blood pressure. When atrial natriuretic peptide (ANP) and brain natriuretic peptide (BNP) are secreted, kidney reabsorption of sodium is inhibited and the glomerular filtration rate is increased. Blood osmolarity is decreased and urine output is increased.

11. What regulates bicarbonate?
The kidneys regulate bicarbonate (HCO3).

12. What regulates partial pressure of carbon dioxide?
The lungs regulate partial pressure of carbon dioxide (PCO2).

13. What is alkalosis?
If the patient develops alkalosis, there is an increase in pH and either an increase in HCO3. (metabolic) or a drop in PCO2. (respiratory).

14. What is hyponatremia?
Hyponatremia is an abnormally low amount of sodium in the blood.

15. What is hypernatremia?
Hypernatremia is an abnormally high amount of sodium in the blood.

16. What is hypocalcemia?
Hypocalcemia is an abnormally low level of calcium in the blood.

17. What is hypercalcemia?
Hypercalcemia is an abnormally high amount of calcium in the blood.

18. What is hypokalemia?
Hypokalemia is a lower-than-normal level of potassium in the blood.

19. What is hyperkalemia?
Hyperkalemia is an elevated level of potassium in the blood.

20. What is hypomagnesemia?
Hypomagnesemia is a lower-than-normal magnesium level in the blood.

21. What is hypermagnesemia?
Hypermagnesemia is a greater-than-normal amount of magnesium in the blood.

22. What is hypophosphatemia?
Hypophosphatemia is a lower-than-normal amount of phosphorus in the blood.

23. What is hyperphosphatemia?
Hyperphosphatemia is a higher-than-normal amount of phosphorus in the blood.

24. What is dehydration?
Dehydration is a state of having less-than-normal body fluids, because of an excess loss of fluids or an inadequate intake of fluids.

25. Why is an electronystagmongram administered?
An electronystagmongram is administered to assess the underlying cause of loss of balance and vertigo.