Water – Electrolyte Balance

Ricardo R. Santos, M.D.

Normal daily intake and output of water (in ml/day)

• Intake

Fluid ingested 2100

From metabolism 200

Total intake 2300

• Output

Insensible – skin 350

Insensible – lungs 350

Sweat 100

Feces 100

Urine 1400

Total output 2300

Sources of water

1. Preformed water

*Water found in food and drink

2. Metabolic water

*Water produced through the catabolic breakdown of nutrients during cellular

respiration

Body Fluid Compartments

• Extracellular Fluid - 14.0 L (20%)

Plasma - 3.0 L

Interstitial fluid – 11.0 L

• Intracellular fluid – 28.0 L (40%)

• Transcellular fluid – 1 to 2 L

Other fluids

1. Lymph

2. Transcellular fluids

- Cerebrospinal fluid

- Gastrointestinal fluids (digestive juices)

- Synovial fluid

- Eye fluids (aqueous & vitreous humors)

- Ear fluids (perilymph & endolymph)

- Pleural, pericardial and peritoneal fluids

General Considerations

• In a 70-kg man, total body water is 60% of body weight or 42 L.

• As person grows older %age of body fluid gradually decreases.

• Increase in fat decreases body water.

• Women have less %age of body water than men

Obligatory H20 reabsorption

Facultative H20 reabsorption

Site in the nephron

PCT

DCT and CD

Hormone involved

none

ADH (vasopressin)

High plasma osmolarity

no effect

stimulatory

Urine volume depends decreased

Types of Water Reabsorption in the Kidney

The mechanisms for the regulation of body fluids are centered in the hypothalamus.

The regulation of body fluid volume and extracellular osmolarity is under the control of ADH and aldosterone.

Primary factors that trigger release of ADH:

1.Osmoreceptors

2.Baroreceptors (pressure receptors)

Secondary factors: stress, pain, hypoxia, K

Thirst response

Connected to the response of the osmoreceptors.

- increased plasma osmolarity stimulates osmosreceptors which in turn stimulates sensation of thirst

Thirst center is in the hypothalamus.

Other factors involved:

1. degree of dryness of mucosal linings of mouth and pharynx

2. stretch receptors in the GIT

Major Electrolytes

Extracellular Intracellular

(mEq/L) (mEq/L)

Cations sodium potassium

calcium magnesium

Anions chloride phosphate

bicarbonate proteins

Sodium

• Normal plasma level = 135 – 145 mEq/L

• Predominant cation in the ECF

• Plays a crucial role in the excitability of muscles & neurones

• Important in regulating fluid balance

• Sodium regulation at the cellular level is controlled by the Na-K pump

• Body levels of Na (retention/excretion) are controlled by aldosterone

• Aldosterone is controlled by renin-angiotensin system

Secretion, Transport and Metabolism of Aldosterone

Renin-angiotensin system - involved in regulation of blood pressure and electrolyte metabolism - primary substance in this process is angiotensin II

Angiotensinogen Angiotensin I renin converting enzyme

Angiotensin II

aminopeptidase

Angiotensin III

angiotensinase

Degradation products

Potassium

• Normal plasma level = 3.5 – 5.5 mEq/L

• Critical for electrical conduction of nerve impulses - particularly cardiac electrical conduction

• Major cation in the ICF

• Potassium balance at the cellular level is maintained by the Na-K pump

• Kidney can excrete K and in exchange for Na – controlled by aldosterone

• Body is much more sensitive to small changes in serum K levels than to small changes in other serum electrolytes

Calcium

• Normal level: 4.0 – 5.5 mEq/L or 8.5 – 10 mg/dl

• Required for normal skeletal, cardiac and smooth muscle contraction

• Needed for blood clotting

• Intestinal absorption of dietary calcium requires vitamin D

• Calcium metabolism is regulated by parathyroid hormone and vitamin D

• Calcitonin from the thyroid gland causes ECF levels of calcium levels to decrease by inhibition of bone resorption, inhibits vitamin D absorption and increases renal excretion of calcium.

Magnesium

• Normal value: 1.5 – 2.5 mEq/L

• Needed to prevent overexcitability of muscles

• Has a sedative effect on neuromuscular junction, inhibits acetylcholine release, and diminishes muscle cell excitability

• Acts as a cofactor in enzyme reactions

• Participates in bone and teeth production

Ionic composition of plasma and interstitial fluid

• Ionic compositions are similar.

• Protein concentration is higher in the plasma.

• Because of Donnans effect, concentrations of cations is slightly greater in the plasma by 2%.

• Proteins are negatively charge more proteins in the plasma bind more cations.

• Interstitial fluids have more anions than plasma.

Basic principles osmosis and osmotic pressure

• Osmosis is the net diffusion of water across a semipermeable membrane from a region of high water conc. to one that has a lower water conc.

• In osmosis, water diffuses from a region of low solute conc. to one that has a high solute concentration.

• Cell membrane is relatively impermeable to most solutes but highly permeable to water.

Concentration terms

• Important in order to describe the concentration of solute particles.

• Total number of particles in a solution is measured in terms of osmoles.

• One osmole (osm) is equal to 1 mole (mol) (6.02 x 1023) of solute particles.

• A soln. containing 1 mole of glucose in each liter has a conc. of 1 osm/L.

Concentration terms

• A soln with 1 mole of NaCl per liter will have an osmolar conc of 2 osm/L

• Osmole is too large a unit for expressing osmotic activity of solutes in the body.

• Therefore, the term milliosmole (mOsm), which equals 1/1000 osmole, is commonly used.

Concentration terms

• Osmolal concentration of soln is called:

1. Osmolality - when conc is expressed as

osmoles per kg of water.

2. Osmolarity – when conc is expressed as

osmoles per liter of solution

• The two terms can be used synonymously

• It is easier to express body fluid quantities in liters of fluid than in kg of water.

Concentration Terms

• Osmotic pressure – the precise amount of pressure required to prevent the osmosis.

• It is an indirect measurement of the water and solute concentrations of a solution.

• The higher the osmotic conc of a solution, the lower the water conc but the higher the solute conc.

• Osmotic pressure of a solution is directly proportional to the concentration of osmotically active particles in that solution.

Concentration Terms

• Total osmolarity of the body fluid compartments is 300 mOsm/L

• 80% of total osmolarity of the ECF is due to sodium and chloride.

• Corrected osmolar activity – 282 mOsm/L

• Total osmotic pressure – 5443 mm Hg

• Impermeant solute – one that will not permeate the cell membrane

Concentrations Terms

• The terms isotonic, hypotonic and hypertonic refer to whether solutions will cause a change in cell volume.

• Tonicity of solutions depends on the conc of impermeant solutes.

• Isosmotic – solutions with an osmolarity the same as the cell.

• Hyperosmotic and hypo-osmotic refers to solutions that have a higher osmolarity or lower osmolarity, respectively compared with normal ECF.

Concentrations Terms

• Isotonic solutions – 0.9% soln of NaCl or a 5% glucose solution.

- have osmolarity of 282 mOsm/L

- cell will not change in volume

• Hypotonic solution – < than 0.9% NaCl

- osmolarity less than normal

- water will diffuse into the cell (swell)

• Hypertonic solution - > than 0.9% NaCl

- osmolarity more than normal

- water will flow out of the cell (shrink)

Clinical problems with fluid balance

• Hypotonic dehydration

• Isotonic dehydration

• Hypertonic dehydration

• Hypotonic overhydration

• Isotonic overhydration

• Hypertonic overhydration

Hypotonic dehydration

• Hypotonic contraction of ECF

• Fluid has fewer solutes than normal plasma

• Relatively uncommon – loss of more solute (usually Na) than water

• Causes fluid to shift from the blood stream into the cells, leading to decreased vascular volume and eventual shock

• Increased cellular swelling

• Cerebral edema causes increased intracranial pressure, headache and confusion

• Seen in heat stroke or exhaustion

Isotonic dehydration

• Isotonic contraction of ECF

• Fluid has the same osmolarity as normal plasma

• Most common form of dehydration

• Occurs when fluids and electrolytes are lost in even amounts

• There are no intercellular shifts

Isotonic dehydration

• Causes:

- diuretic therapy

- excessive vomiting

- excessive urine loss

- hemorrhage

- decreased fluid intake

Hypertonic dehydration

• Hypertonic contraction of the ECF

• Fluid has more solutes than normal plasma

• Second most common type of dehydration

• Occurs when water loss from ECF is greater than solute loss

• Causes fluid to be pulled from the cells into the blood stream, leading to cellular shrinkage or dehydration.

Hypertonic dehydration

• Causes: - Excessive insensible fluid loss: hyperventilation and pure

water loss with high fevers

- Watery diarrhea

- Diabetic ketoacidosis

- Diabetes insipidus

- Iatrogenic: prolonged NPO, tube feedings with inadequate amounts of water

Hypotonic overhydration

• Hypotonic expansion of the ECF

• Decreased serum osmolarity leads to fluid shifting from the blood stream into the cells

• Causes interstitial edema, cellular swelling and electrolyte dilution

Hypotonic overhydration

• Causes:

- Too much IV D5W: the body metabolizes the glucose rapidly, leaving plain hypotonic fluid in the blood stream

- Keeping patient NPO with ice chips over long periods of time

- Tap water enemas

- Psychogenic cause: excessive drinking of plain water

Isotonic overhydration

• Isotonic expansion of the ECF

• Hypervolemia

• Fluid equilibrates between blood and interstitial fluid compartments

• Edema

• Rarely happens in persons with normal heart and kidneys

Isotonic overhydration

• Causes:

- Over administration of IV isotonic fluids

- Excessive saline enemas

- Increased sodium intake resulting in compensatory water retention

Hypertonic overhydraton

• Hypertonic expansion of the ECF

• Increased serum osmolarity leads to shifting fluids from the cells into the blood stream

• Causes cell shrinkage and fluid volume overload

• Leads to increased blood pressure and increased cardiac workload

• Eventually lead to decrease cardiac output and congestive heart failure

Hypertonic overhydration

• Causes:

- Over administration of hypertonic IV fluids

- Over use of hypertonic enemas

- Hypertonic tube feedings

- Ingestion of sea water