Fluid, Electrolyte and Acid-Base
• The maintenance of normal volume and
normal composition of the extracellular
fluid is vital to life.
• Three types of homeostasis are involved in
this maintenance:fluid balance, electrolyte
balance, and acid-base balance.
• The ICF contains nearly2/3rd of total body
water;the ECF contains the rest.
• Exchange occurs between the ICF and ECF.
• Body fluids includes water and solutes.
• About 2/3rd of the body’s fluid is located within
cells and is called intracellular fluid (ICF).
• The other 1/3rd called extracellular fluid (ECF)
includes interstitial fluid, plasma and lymph, GI
tract fluids, synovial fluid, fluids of the eyes and
ears, CSF, pleural, pericardial and peritoneal fluids
and glomerular filtrate.
• The term fluid balance means that various body
compartments contain the normal amount of
• An inorganic substance that dissociates into ions is called
an electrolyte. Fluid balance and electrolyte balance are
• Water is the largest single constituent in the body-45-75%
of total body mass.
• Daily water gain and loss are about 2.5 L.
• sources of water gain are ingested liquids and foods, and
water produced by cellular respiration and dehydration
• Water is lost by the process of urination, evaporation from
the skin surface, exhalation of water vapor, and
defecation.in women also menstrual flow.
• The main way to regulate body water gain is by adjusting
the volume of water intake, mainly by drinking more or
less fluid. The thirst center in the hypothalamus governs
the urge to drink.
• Although increased amounts of water and solutes are lost
through sweating and exhalation during exercise, loss of
excess water or excess solutes depends mainly on
regulating excretion in the urine.
• The extent of urinary NaCl loss is the main determinant of
body fluid volume, whereas the extent of urinary water
loss is the main determinant of body fluid osmolarity.
• Angiotensin II and aldosterone reduce urinary loss of Na+
and Cl- and thereby increase the volume of body fluids.
• ANP promotes natriuresis, elevated excretion of Na+ (and
Cl- ), which decreases blood volume.
• The major hormone that regulates water loss and thus body
fluid osmolarity is ADH.
• An increase in the osmolarity of interstitial fluid draws
water out of cells and they shrink slightly. A decrease in
the osmolarity of interstitial fluid also causes cells to swell.
• When a person consumes water faster than the kidneys
excrete it or renal fn. Is poor-water intoxication,cells swell.
Electrolytes in Body Fluids
• Ions formed when electrolytes dissolve in body fluids
control the osmosis of water between fluid compartments,
help maintain acid-base balance, and carry electrical
• Plasma, interstitial fluid and ICF contain varying kinds and
amounts of ions.
• The conc. Of ions is expressed as mEq/L.
• sodium ions are the most abundant ions in ECF: involved
in impulse transmission, muscle contraction, and fluid and
electrolyte balance. The level of sodium is controlled by
aldosterone, ADH and ANP.
Electrolytes in Body Fluids
• Chloride ions are the major anions of the ECF. They play
a role in regulating osmotic pressure and forming HCl in
gastric juice. Cl- level is controlled indirectly by ADH and
by process that increase or decrease renal absorption of
• Potassium ions are the most abundant cations of ICF. They
play a key role in the resting membrane potential and
action potential of neurons and muscle fibers. Help
maintain ICF volume and contribute to regulation of pH.
K+ level is controlled by aldosterone.
• Bicarbonate ions (HCO3
- )are the second most abundant
anions in the ECF. Most important buffer in the plasma.
Electrolytes in Body Fluids
• Calcium is the most abundant mineral in the body.
Calcium salts are structural components of bones and teeth.
Ca+2 which are primarily extracellular cations, function in
blood clotting, neurotransmitter release, and contraction of
muscle. Ca+2 level is controlled by parathyroid and
• Phosphate ions are principally intracellular anions and their
salts are structural components of bones and teeth, also
required for the synthesis of nucleic acids and ATP and
participate in buffer reactions. Level is controlled by PTH
• Mg+2 ions -intracelluar cations that act as cofactors for
• The normal pH of systemic arterial blood is 7.35-7.45.
• pH is maintained by buffers. The important buffer systems
include proteins, carbonic acid-bicarbonate buffers and
• Homeostasis of pH is maintained by buffer systems, via
exhalation of carbon dioxide, and via kidney excretion of
H+ and reabsorption of HCO3
• The overall acid-base balance is maintained by controlling
the H+ conc. Of body fluids, especially ECF.
Mechanisms that maintain pH of
• Buffer systems-made of a weak acid or base and its salt.
Prevent drastic changes in body fluid pH.
• Proteins:the most abundant buffers in body and cells. His
and Cys are the two main acids and Hb in RBC’s.
• Carbonic acid-bicarbonate: important regulator of blood
pH. The most abundant buffers in the ECF.
• Phosphates:important buffer in ICF and in urine.
• Exhalation of CO2:increased exhalation-inc pH and
decreased exhalation dec. pH.
• Kidneys: renal tubules secrete H+ into the urine and
- , so it is not lost in the urine.
• An increase in exhalation of carbon dioxide results in inc.
blood pH and a decrease in exhalation causes dec. in blood
• The kidneys excrete H+ and reabsorb HCO3
• Acidosis is a systemic arterial blood pH of below 7.35; its
principal effect is depression of CNS. Alkalosis is a
systemic arterial blood pH over 7.45 and its principal
effect is overexcitability of CNS.
• Respiratory acidosis and alkalosis are disorders due to
changes in blood Pco2 ,(compensated by renal mechanisms)
whereas metabolic acidosis and alkalosis are disorders
associated with changes in blood HCO3