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Fluid, Electrolyte, and AcidBase Balance
Part I

Chapter 25 Objectives: Fluid, Electrolyte, and
Acid–Base Balance

Body Water Content

 Infants have low body fat, low bone mass, and are
73% or more water
 Total water content declines throughout life
 Males about 60% water; females are around 50%
 This difference reflects females’:
 Higher body fat
 Smaller amount of skeletal muscle

 In old age, only about 45% of body weight is water

Body Fluid Compartments

In lean adults, body fluids constitute 55% of female and 60% of male total body mass

Intracellular fluid (ICF) inside cells

About 2/3 of body fluid

Extracellular fluid (ECF) outside cells

Interstitial fluid between cell is 80% of ECF
Plasma in blood is 20% of ECF
Also includes lymph, cerebrospinal fluid, synovial fluid, aqueous humor, vitreous body, endolymph, perilymph, and pleural, pericardial, and peritoneal fluids
Copyright 2009, John Wiley &
Sons, Inc.

Body Fluid Compartments

Copyright 2009, John Wiley &
Sons, Inc.

Fluid Balance

2 barriers separate ICF, interstitial fluid and plasma

Plasma membrane separates ICF from surrounding interstitial fluid
Blood vessel wall divide interstitial fluid from plasma

Body is in fluid balance when required amounts of water and solutes are present and correctly proportioned among compartments
Water is by far the largest single component of the body making up 45-75% of total body mass
Process of filtration, reabsorption, diffusion, and osmosis all continual exchange of water and solutes among compartments
Copyright 2009, John Wiley &
Sons, Inc.

Sources of Body Water Gain and Loss

Fluid balance related to electrolyte balance

Body can gain water by

Intake of water and electrolytes rarely proportional
Kidneys excrete excess water through dilute urine or excess electrolytes through concentrated urine
Ingestion of liquids and moist foods (2300mL/day)
Metabolic synthesis of water during cellular respiration and dehydration synthesis (200mL/day)

Body loses water through

Kidneys (1500mL/day)
Evaporation from skin (600mL/day)
Exhalation from lungs (300mL/day)
Feces (100mL/day)
Copyright 2009, John Wiley &
Sons, Inc.

Daily Water Gain and Loss

Copyright 2009, John Wiley &
Sons, Inc.

Regulation of body water gain

Mainly by volume of water intake/ how much you drink
Dehydration – when water loss is greater than gain 

Decrease in volume, increase in osmolarity of body fluids
Stimulates thirst center in hypothalamus

Copyright 2009, John Wiley &
Sons, Inc.

Regulation of water and solute loss Elimination of excess body water through urine

Extent of urinary salt (NaCl) loss is the main factor that determines body fluid volume
Main factor that determines body fluid osmolarity is extent of urinary water loss
3 hormones regulate renal Na+ and Cl- reabsorption (or not)  1) Angiotensin II and 2) Aldosterone promote urinary
Na+ and Cl- reabsorption of (and water by osmosis) when dehydrated
 3) Atrial natriuretic peptide (ANP) promotes excretion of Na+ and Cl- followed by water excretion to decrease blood volume
Copyright 2009, John Wiley &
Sons, Inc.

Hormonal Regulation of Na+ and Cl

Copyright 2009, John Wiley &
Sons, Inc.

Major hormone regulating water loss is antidiuretic
 Also known(ADH) hormone as vasopressin

Produced by hypothalamus, stored & released from posterior pituitary
Permeability to water increases at kidney level
Produces concentrated urine

Copyright 2009, John Wiley &
Sons, Inc.

Movement of water between
 Normally, cells neither shrink or swell because compartments intracellular and interstitial fluids have the same osmolarity 

Increasing osmolarity of interstitial fluid draws water out of cells and cells shrink
Decreasing osmolarity of interstitial fluid causes cells to swell

Changes in osmolarity most often result from changes in Na+ concentration
Water intoxication – drinking water faster