Water Balance

Total body water (TBW)

– Intracellular fluid

– Extracellular fluid:

o Interstitial fluid

o Intravascular fluid

o Lymph, synovial, intestinal, CSF, sweat,

urine, pleural, peritoneal, pericardial,

and intraocular fluids

Age-Related Considerations

Children

– More susceptible to changes in body fluids

– Greater body surface area

Elderly

– Decreased % total body water

– Increased adipose tissue and decreased muscle mass

– Diminished thirst perception

– Decline in renal function

Water Balance- Regulation

Mechanisms

– ADH

o Increases when person is dehydrated leading to water retention

– Thirst sensation

– Osmoreceptors (hypothalamus)

o Stimulated by hyperosmolality and plasma volume depletion

– Baroreceptors (blood vessels):

o Stimulated by high blood pressure (hypervolemia)

Water Movement Between Fluid Compartments

Forces favoring filtration:

– Capillary hydrostatic pressure (blood pressure)

– Interstitial oncotic pressure (water-pulling)

Forces favoring reabsorption:

– Plasma oncotic pressure (water-pulling)

– Interstitial hydrostatic pressure

Starling hypothesis

– Net filtration = [Forces favoring filtration] – [Forces opposing filtration]

Edema

Accumulation of fluid within the interstitial spaces

May be generalized OR organ specific

Causes:

Increase in capillary hydrostatic pressure

Decrease in plasma oncotic pressure

Increase in capillary permeability

Lymph obstruction (lymphedema)

Types

Pitting:

– Pregnancy

– Heart failure

Non pitting edema:

– Lymphedema (due to lymphatic obstruction)

– Note: Pretibial myxedema is not edema!

Sodium and Chloride Balance

Sodium

Primary ECF cation

Regulates osmotic forces, thus water

Functions:

– Neuromuscular excitability

– Acid-base balance

– Cellular chemical reactions

– Membrane transport

Chloride

– Primary ECF anion

– Functions

o Electroneutrality

Regulation:

– Renin-angiotensin-aldosterone system (RAAS)

o Aldosterone leads to:

• Sodium and water reabsorption

• Potassium and hydrogen secretion (loss in urine)

– Natriuretic peptides

Hyponatremia

Serum sodium level <135 mEq/L

Cause plasma hypo-osmolality and cellular swelling

– Decreases the ECF osmotic pressure, so water moves into the cell via osmosis

Causes:

Pure sodium loss

– Occurs through body processes such as sweat

Low intake

Dilutional hyponatremia

Effects:

Lethargy, confusion, decreased reflexes, seizures, and coma

Hypovolemia, hypotension, tachycardia

Dilutional hyponatremia is associated with weight gain, edema, ascites, and jugular vein distention

Hypernatremia

Serum sodium >147 mmol/L

Related to sodium gain or water loss

Causes:

Water movement from the ICF to the ECF

Administration of hyperosmolar solutions

Effects:

Intracellular dehydration, convulsions

Pulmonary edema

Hypertension

Hypochloremia

Usually the result of hyponatremia or elevated bicarbonate concentration

Develops as a result of vomiting and the loss of HCl

Observed in cystic fibrosis

Alters acid-base balance

Hypokalemia

Potassium level <3.5 mmol/L

Causes:

Shift of K+ from ECF into ICF:

– Insulin

o E.g. Hyperglycemic patients receiving Insulin require fluid containing potassium to accommodate for the shift that occurs as potassium moves into cells

– B2 agonists

Increased renal excretion:

– E.g. hyperaldosteronism

Effects:

Skeletal muscle weakness

Smooth muscle atony

ECG changes

Hyperkalemia

Potassium level >5.5 mmol/L

Rare because of efficient renal excretion

Causes:

Shift of K+ from ICF into ECF:

– Insulin deficiency

o E.g. diabetic ketoacidosis

– Acidosis

– Massive cell destruction (tumor lysis syndrome)

– Decreased renal excretion

Tissue trauma, crush injury

Potassium-sparing diuretics

Effects:

Dysrhythmias

Cardiac arrest during diastole

Calcium & Phosphate

Calcium:

99% of stores are located in bone and 10% in serum

– Of the 10% in serum, 50% is ionized and 50% is complexed with albumin (40%) or inorganic elements (10%), such as phosphate

Functions:

– Bone & teeth integrity

– Blood clotting

– Hormone secretion

– Cell receptor function

– Plasma membrane stability

– Transmission of nerve impulses

– Muscle contraction

Phosphate:

Similar to calcium, most phosphate is also located in the bone

Functions:

– Bone integrity

– High-energy bonds located in ATP

– Creatine phosphate

– Acts as an anion buffer

Regulation

Calcium and phosphate concentrations are rigidly controlled

– Ca+2 x HPO4– = K(constant)

– If the concentration of one increases the other decreases;

o I.e. If phosphate level increases, calcium absorption from gut decreases

Serum Ca+2 : 8.8-10.5 mg/dL

Serum PO4– : 2.5 to 4.5 mg/dL

Hypercalcemia

Causes

Hyperparathyroidism

Malignancy

Sarcoidosis

Excess vitamin D

Effects

Nonspecific symptoms including:

– Fatigue

– Weakness

– Lethargy

– Anorexia

– Nausea and/or vomiting

Renal stones, polyuria, renal failure

Bone pain

Dysrhythmias, with potential for cardiac arrest during systole

Hypocalcemia

Causes

PTH deficiency

Vitamin D deficiency

Inadequate intestinal absorption

Bone/ tissue deposition of Ca

Effects

Increased neuromuscular excitability

Tingling and/or muscle spasm occurring in the hands, feet, and/or facial muscles

– Muscle spasms may also manifest as intestinal cramping, hyperactive bowel

sounds

Tetany:

– Carpopedal spams

Acid-Base Imbalances

Normal arterial blood pH

– 7.35 to 7.45

– Obtained by arterial blood gas (ABG) sampling

o Venous blood gas (VBG) sampling is also used, however, the reference ranges differ

o VBGs are often used for initial testing to rule out respiratory conditions and complications (e.g. in the emergency departments), as both registered nurses and laboratory technicians can collect venous blood samples when a Physician is unavailable

Acidosis

– Systemic increase in H+ concentration or decrease in bicarbonate

Alkalosis

– Systemic decrease in H+ concentration or increase in bicarbonate

Acid-Base Balance: Basic facts

Acid-base balance: Careful regulation to maintain a normal pH

pH: inverse logarithm of the H+ concentration

– When H+ is high in number, pH is low (acidic);

– When H+ is low, pH is high (alkaline)

pH scale: Logarithmic scale ranging from 0 -14 whereby 0 is very acidic, 14 is very alkaline

– Each number represents a factor of 10

– I.e. If a solution moves from a pH of 6 to a pH of 5, the number H+ have increased 10 times (the solution is 10 times more acidic)

Sources of acids

– End products of protein, carbohydrate, and fat metabolism

To maintain normal pH (7.35-7.45), excess H+ is neutralized or excreted

Two major organs are involved in the regulation of acid and base balance:

1. Lungs

2. Kidneys

Body acids exist in two forms:

1. Volatile

o H2CO3 (can be eliminated as CO2 gas)

2. Nonvolatile

o Sulfuric, phosphoric, and other organic acids

o Eliminated by the renal tubules with the regulation of HCO3–

Buffering Systems

A buffer is a chemical that can bind excessive H+ or OH– without a significant change in pH

A buffering pair consists of a weak acid and its conjugate base

The most important plasma buffering system is the carbonic acid-bicarbonate system

– CO2+ H2O ↔ H2CO3 ↔ H+ + HCO3-

Carbonic Acid-Bicarbonate System

Operates in lungs and kidneys

The greater the PCO2, the more H2CO3 is formed

– At pH of 7.4 (normal), the ratio of HCO3 to H2CO3 is 20:1

– HCO3 and H2CO3 can increase or decrease, but ratio must be maintained to maintain normal pH required for body function

For example, if HCO3 decreases, pH decreases →

(acidosis)

– pH can be returned to normal if carbonic acid also decreases

– This type of pH adjustment is referred to as compensation

Respiratory system compensates by increasing ventilation to expire CO2 or by decreasing ventilation to retain CO2

Renal system compensates by producing acidic or alkaline urine

Other Buffering Systems

Protein buffering (hemoglobin)

Proteins have negative charges, so can serve as buffers for H+

Renal buffering

Secretion of H+ in the urine and reabsorption of HCO3–

Ion exchange (between ICF and ECF)

Exchange of K+ for H+ (through Na/K balance, Na/H balance) in acidosis and alkalosis

Acidosis and Alkalosis

Four categories of acid-base imbalances:

1. Respiratory acidosis

o Elevation of pCO2 as a result of hypoventilation

2. Respiratory alkalosis

o Depression of pCO2 as a result of hyperventilation

3. Metabolic acidosis

o Depression of HCO3– or an increase in non-carbonic acids

4. Metabolic alkalosis

o Elevation of HCO3– usually caused by an excessive loss of metabolic acids

Anion Gap

DifferenceCalculated measure of the difference between cations and anions in plasma or urine

A calculated parameter

Used cautiously to distinguish different types of metabolic acidosis

Normal anion gap = <12 mmol/L

– By rule, the concentration of anions (–) should equal the concentration of cations (+), but not all anions are routinely measured

The anion gap increases in acidosis

– Acidotic conditions can be remembered by the acronym MUD PILES CAT

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