Fluid, Electrolyte, and Acid-Base Balance
Distribution of body fluids:
The distribution of body fluids, or total body water (TBW), involves the presence of
intracellular (ICF) and extracellular (ECF) fluids. Water is the major constitutuent of body
tissues, and the TBW in an individual range from 40% to 60% of total body weight.
The ICF refers to the fluid contained within the cells and constitues about 2/3 of the total
body fluids in adults. ICF contains solutes such as Oxygen, electrolytes and glucose. ICF
provides a medium in which metabolic processes of the cell take place. It contain electrolytes as
K, Mg, HPO, SO as primary electrolytes (principal electrolytes).
ECF is the fluid outside the cells and account for 1/3 of the total body fluids. ECF is the
transport system that carries nutrients to and waste products from the cells. It contains
electrolytes such as sodium, chloride, bicarbonate that represents the principal electrolytes in this
The ECF is further broken down into several components: intravascular (contained
within the blood vessels). It contains electrolytes and a protein-rich fluid (plasma) and large
amount of albumin. Interstitial (surrounding the cell and the location of most ECF, 3/4 of ECF),
it contains little or no protein. And transcellular (contained within specialized body cavities such
as cerebrospinal, synovial, pleural, and so on). In the newborn about 50% of the body fluid is
contained within the ECF, whereas 30% of the toddler’s body fluid is contained within the ECF.
The importance of body water to body function is related not only to its abundance but
also to the fact that it is the medium in which body solutes are dissolved and all metabolic
reactions take place. In addition, it is a transporter of nutrients, waste products and other
substance, a lubricant, an insulator and shock absorber, and one’s mean of regulating and
maintaining body temperature.
Composition of Body Fluids
ICF and ECF contain oxygen, dissolved nutrients, excretory products as CO2 and charged
particles called ions. Ions come from dissolved salts (eg. NaCl; sodium cholride breaks into Na+
(ion) and Cl- (ion) these charged particles are called electrolytes because they can conduct
electricity). The positive charge ion called cation ( eg of cations Na+, K+, Mg+2) and the
negative charge ion called anion( eg. Cl-, HCO3-, HPO4-).
Electrolytes are measured in milliequivalents per liter of water ( mEq/L). Clinically the mEq
system is commonly used but not with all electrolytes, because the laboratory tests are usually
performed using the blood plasma (ECF) which reflect what is happening in the ECF, not what
happens within the cells.
* Other body fluids such as gastric and intestinal secretions also contain electrolytes
Mechanisms of fluid movement:
Water is retained in the body in a relatively constant amount and, with few exceptions, is
freely exchangeable among all body fluid compartments. The proximity of the extravascular
compartment to the cells allows for continual change in volume and distribution of fluids, largely
determined by solutes (especially sodium) and physical forces. Transport mechanisms are the
basis for all activity within the cells, and since they have limited ability to store materials,
movement in and out of cells must be rapid.
Internal control mechanisms (such as thirst, antidiuretic hormone (ADH), and
aldosterone(which enhances sodium reabsorption)) are responsible for distribution and
maintenance of fluid balance.
1- Diffusion: molecules move from a solution of higher concentration to a solution of lower
concentration. This motion affected by temperature, size of molecules and the concentration of
2- Hydrostatic pressure/ Filtration: movement of fluid and solutes from an area of higher
hydrostatic pressure to one of lower pressure. The pressure created by the weight of fluids
(increased pressure in the arteries forces fluids through the capillary walls into the interstitial
3- Active Transport: movement of substance across the cell membrane from a less
concentration solution to higher concentrated by active transport (a carrier).
4- Osmotic pressure: water movement across the cell membranes from low solute concentration
(low osmotic pressure) to higher concentration of solute (high osmotic pressure). Osmolality is
the concentration of solutes in the body fluids, usually reported as milliosmols per kilogram
(mOsm/kg). Sodium is the greatest determinant of serum osmolality. Example in hypernatremia
where the serum Na results in water movement from ICS to ECS, and the hyponatremia is the
Tonicity refers to the osmolality of solution:
1- Isotonic solution has the same osmolality as body fluids ( eg Normal saline 0.9%).
2- Hypertonic solution has a higher osmolality than body fluids ( eg Normal saline 3%).
3- Hypotonic solution has a lower osmolality than body fluids ( eg Normal saline 0.45%)
It occurs when body fluids are lost in excess of fluid gain. The great majority of disturbances in
hydration and electrolytes balance occur secondary to vomiting and diarrhea
Causes of dehydration are:
I) Lack of oral intake.
II) GI ; vomiting, diarrhea, malabsorption
V) Diabetes mellitus
VI) Tachypnea as in bronchiolitis
Types of dehydration
I. Isotonic /Isonatremic dehydration: occurs when fluids and electrolytes losses are in
same proportion as they exist in the body. Fluid osmolarity is not affected. And there is deficit of
TBW. This can be seen in 70% of children with diarrhea (plasma Na+ remains normal 130-150
II. Hypertonic dehydration/hypernatremia: When the water intake decreases and Na
increases. Proportionally greater loss of water than Na. It can occur when insensible loss of water
from skin and respiration tract is high. In this case the Na increase the osmotic pressure in the
blood vessels that shifts the fluids from the IC to the ECS (plasma Na+ > 150 mEq/L) .
10-20% of children with dehydration have this type. The causes can be
i. Administration of hypertonic IV fluids
ii. Increase of Na intake
iii. Failure of ADH
iv. Increase of insensible loss of water as in burn, fever, respiratory infections.
The defense mechanisms for this case are:
a. stimulation of thirst
b. stimulation of ADH.
III. Hypotonic/ Hyponatremia dehydration: This type refer to decrease in Na and
retention of water. It can be caused by excessive plain water intake and defect in renal water
excretion and failure in SIADH ( syndrome of inappropriate antidiuretic hormone).. it can be
seen in cystic fibrosis due to excessive lose of Na via sweat. The water shifts from ECS to ICS
causing circulatory collapse. This type is occur in 10% of children with dehydration.
Infants prone for water loss due to greater volume of extracellular fluid, which can be lost by
illness or environmental conditions. The infants and children are more vulnerable to fluid and
electrolyte imbalances due to
I) greater surface area to body mass (more evaporation)
II) higher metabolic rate
III) Immature kidneys function.
Edema ( increase the interstitial fluid volume)
excess interstitial fluid caused by
I) Fluid Volume Excess that increase the capillary pressures, pushing fluid into the
interstitial tissues by filtration. Which can be seen in heart failure and renal failure.
II) Low levels of plasma proteins which will reduce the oncotic pressure so that fluid is
not drawn into the capillaries from interstitial tissues. As in nephritic syndrome and malnutrition.
III) allergic reaction capillaries become more permeable allowing the fluid to escape
into interstitial tissues. The albumin can move easily from the capillaries membrane pulling with
it the fluid. This can be seen in burns, truma and allergic reactions
IV) increase in interstitial oncotic pressure: the protein enter the interstitial fluid more
than they leave causing increase in interstitial oncotic pressure that in turn pull the fluid into
tissue as in tumors and hypothyroidism.
V) Obstructed lymph flow impairs the movement of fluid from interstitial tissues back
into the vascular compartment.
Homeostasis: a balance of fluids, electrolytes and acids and bases in the body; that reflects a
an acid is the substance releases hydrogen ions (H)
Base (alkalis) is the substance that can accept hydrogen ions.
pH is the relative acidity or alkalinity of a solution: higher hydrogen ions more acidic which is
low pH. And less hydrogen ions more alkaline which is high pH.
- Body fluids are slightly alkaline
- normal pH of arterial blood is 7.35-7.45
- several body systems including buffers, the respiratory system, and the renal system are
maintaining the narrow pH
- a drop in pH is called acidosis
- a rises in pH is called alkalosis
Regulating acid-base balance
1- Buffers: major buffers system in ECF is the bicarbonate ( HCO-3) and carbonic acid (
H2CO3). Besides bicarbonate and carbonic acid buffers, plasma proteins, hemoglobin and
phosphates also function as buffers in body fluids.
2- Respiratory Regulation: regulating acid-base balance by eliminating or retaining
carbon dioxide (CO2) by altering the rate and depth of respirations.
- if the blood level of carbonic acid increase the rate and depth of respirations increase to excrete
carbon dioxide to fall the level of carbonic acid
- if the blood level of bicarbonate increase the rate and depth of respirations decrease to retained
the carbon dioxide and rise the level of carbonic acid.
- PCO2 refer to pressure of carbon dioxide in venous blood
-PaCO2 refer to pressure of carbon dioxide in arterial blood. Normal PaCO2 is 38-40 mmHg
3- Renal Regulation: kidneys maintain acid-base balance by excreting or conserving
bicarbonate and hydrogen ions
- if acidity increased the kidneys reabsorb and regenerate bicarbonate and excrete H
- in the case of alkalosis excess bicarbonate is excreted and H ion is retained
- normal serum bicarbonate level is 22-26 mEq/L
Factors Affecting Body Fluid, Electrolytes and Acid-Base Balance
1- age: infant has immature kidneys, rapid respiration and more body surface area than
adult which make the infant losses the fluid rapidly. In elderly people the thirst response often is
blunted and kidney become less able to conserve water that will affect the fluid balance.
2- Gender and Body Size: people with a higher percentage of body fat have less fluid.
3- Environmental Temperature: both salt and water are lost through sweating in hot
4- Lifestyle: diet, exercise, stress and alcohol consumption all affect the fluid and
Disturbances in Fluid Volume, Electrolyte, and Acid-Base Balances
Many factors affect the fluid and electrolyte balance such as illness, surgery, medications, burns,
vomiting, diarrhea and nasogastric suction. The majority of childhood illnesses that caused
imbalances they occur secondary to vomiting and diarrhea.
The imbalances can be:
I) total body deficit or excess of fluid and electrolyte and the osmolality of the body is not
II) when relationship between fluid and electrolyte has been altered and the osmolality is altered.
III) or both a and b.
2- Electrolyte Imbalances
1- Potassium ( 95% of K of body in IC fluid)
Hypokalemia: K < 3.5 mEq/L caused by vomiting, diarrhea and gastric suction diuretics,
alkalosis. The K shifts from EC to IC space and also insulin promotes K to enter skeletal muscles
and hepatic cells. CM are; cardiac arrhythemia, muscle weakness, shallow breathing, polyuria
Hyperkalemia : K > 5.0 : most commonly occur in children as a result of too rapid administration
of IV potassium chloride, and caused by renal failure, shift of K from IC to EC by tissue damage,
and metabolic acidosis. CM are malaise, muscle weakness, oliguria to anuria, abnormal cardiac
function and diarrhea
2- Calcium ( required for activation of numerous enzymes, cardiac and neural and muscular
hypocalcemia : Ca < 4.0 mEq/L, caused by hypoparathyroidism, Vit D deficiency,burns,
infections diarrhea, renal failure. CM tetany, when giving cow’s milk early in infancy so that the
milk is high conc of phosphate which drops the Ca level, laryngospasm, numbness and seizures.
hypercalcemia : Ca > 5.5 mEq/L, caused by increase administration of Vit Aand D, prolonged
immobilization and hyperparathyroidism. CM; nausea, vomiting, constipation and flank pain
The abnormalities in PCO2 increase or decrease is called respiratory alkalosis or acidosis
because PCO2 regulated by respiration
i. Increase in PCO2 —————- respiratory acidosis
ii. Decrease in PCO2—————-respiratory alkalosis.
The abnormalities of plasma bicarbonate concentration refer to metabolic process
I) Increase in HCO3———————metabolic alkalosis
II) Decrease in HCO3——————- metabolic acidosis
1- Respiratory Acidosis
Hypoventilation and CO2 retention cause carbonic acid level to increase which will drop the pH
level below 7.35. This can be casued by
II) central nervous system depression
III) anesthesia, alcohol ,
IV) aspiration of foreign body.
When respiratory acidosis occur the kidneys will retain bicarbonate to restore the normal ratio of
bicarbonate:carbonic acid (20:1) in order to restore the normal pH.
II) blurred vision
2- Respiratory Alkalosis
Hyperventilation the CO2 is exhaled causing the carbonic acid to fall and rise the pH above 7.45.
This can be caused by
IV) respiratory infection.
With respiratory alkalosis the kidneys will excrete bicarbonate to return normal pH.
I) increase irritability of central and peripheral nervous system.
II) Light headache
III) Altered consciouness
IV) Paresthesia of extremities
3-Metabolic Acidosis (Diarrhoea)
when bicarbonate is low in relation to the carbonic acid in the body , causing the pH to fall. This
can be caused by
I) renal failure
II) inability of the kidneys to excrete H ions.
III) Increase of anaerobic metabolism
IV) Decrease in blood volume causing the kidney to function less effectively
Metabolic acidosis will stimulate the respiratory center causing the rate and depth of respiration
to increase ( in which the CO2 is eliminated and the carbonic acid is fall).
I) Increase depth of respiration
IV) Impaired growth (rickets)
V) Wt loss
VII) Muscle weakness and listlessness.
4- Metabolic Alkalosis (vomiting)
When the amount of bicarbonate in the body exceeds the normal 20:1 ratio. This can be caused
with ingestion of antacid, vomiting which causing losing in H.
I) Muscles hypertonic
III) nasogastric suctioning
IV) diuretics; decrease the ECF leaving HCO3 uncharged
V) Hypokalemia The metabolic alkalosis will stimulate the respiratory center to slow and
shallow the breathing (causing to retain CO2 which will increase the carbonic acid level).
VI) HCO3 retention may result from, massive blood transfusion, excessive administration of
ii. Muscle cramp
glucose facilitates the absorption of sodium (and hence water) on a 1:1 molar basis in
the small intestine;
�sodium and potassium are needed to replace the body losses of these essential ions
during diarrhoea (and vomiting);
�citrate corrects the acidosis that occurs as a result of diarrhoea and dehydration.