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NOTES - Fluids, electrolytes, pH

Body Fluids – water accounts for nearly the entire volume of body fluids. Normal amount of water in
human body ▪ Adult female = 55%, male = 60% ▪ More adipose, less water
Distribution Of Water
Intracellular fluid (ICF) – Inside cells (25 L), 2/3 of body fluids, 40% body weight
Extracellular fluid (ECF) – Outside cells (15 L), Interstitial (tissue) fluid, plasma, lymph, etc.
Link between intra and
extracellular environments
Water is continually exchanged
through capillary walls and plasma
membranes by Osmosis
Water is universal solvent for 2
types of solutes:
o Electrolytes – Chemicals that
dissociate into free ions
(charged) e.g., Na+ (cations), Cl- (anions)
o Non-electrolytes – Chemical bonds prevent dissociation, no charge (glucose, lipids,
creatinine, urea etc)
Electrolyte Pattern Distinct
ECF – Na+ (sodium) – main cation (dominant electrolyte
to plasma osmolarity)
ICF – K+ (potassium) – main cation (dominant electrolyte
to intracellular osmolarity)
Na+ – K+ pump – maintains these concentrations using
active transport (ATP, thus oxygen required)
Despite these electrolyte differences the osmolarity of ECF and ICF is the same
Therefore, so is water concentration
Water Balance
Water intake (2.5 L) = water output (2.5 L)
Sources for water intake: Foods and fluids, Metabolic processes (200 mL)
Thirst is driving force – osmoreceptors in hypothalamus detect increase in blood osmolarity
(↓saliva) > dry mouth, increase thirst
Requires action of angiotensin II and antidiuretic hormone (ADH) to accentuate thirst and
reduce urinary loss of water
Sources For Water Output
Expired air from lungs
Cutaneous transpiration
Urine production
Kidneys major regulators of body fluid composition
Tubular reabsorption of water and electrolytes regulated by
hormones – ADH, Aldosterone, Angiotensin, ANP
Acid-Base Balance of Blood, pH
Relative concentration of hydrogen ions [H+]
Blood pH = 7.35 to 7.45 (enzymes are pH sensitive)
H+ originates from metabolism (lactic acid, carbonic acid) and ingested nutrients (fatty acids,
amino acids)
pH Maintained by:
1. Buffers
2. Lungs (CO2 and H2O)
3. Kidneys (H+ and H2CO3-)
Buffers (Handbag) – chemicals that can regulate pH
Prevent dramatic changes in [H+]
o Act instantaneously
o Bind to H+ when pH drops
H+ + B- = HB
o Release H+ when pH rises
HB = H+ + BComposed of a weak acid (H+ donor) and a weak base (H+ acceptor)
Major Chemical Buffers
1. Protein buffer system (ICF)
• Amino acids can act as weak acid or base
o release H+ (acid group) or bind to H+ (amine group)
o R group most buffering
2. Bicarbonate buffer system (ECF: plasma)
• Mix of carbonic acid H2CO3 (acid=H+ donor) and bicarbonate HCO3- (base=H+ acceptor)
• Carbonic acid concentration is regulated by respiration (CO2 and H2O)
• Bicarbonate ion concentration is regulated by kidneys (H+ and HCO3-)
• Bicarbonate is how we transport CO2 around body
• We use this equation to buffer our blood
3. Phosphate buffer system (CSF and urine)
Chemical Buffer Systems – can tie up excess acids or bases but only LUNGS and KIDNEYS can
eliminate them from body
Reversible Chemical Reactions
Symbolized with double-headed arrows
CO2 + H2O ↔ H2CO3 ↔ H + + HCO3 - (H+ donor) (H+ acceptor)
Law of mass action determines direction
o proceeds from side with greater quantity to less
Equilibrium exists reactions when ratio of reactants to products is stable (equal)
Respiratory Mechanisms
In healthy people, CO2 is expelled from lungs at same rate it is produced at tissues
Thus, H+ does not accumulate and has no effect on blood pH
However, if there is a pH change, Chemoreceptor’s in medulla oblongata (brain), adjust your
breathing to restore homeostasis within minutes
With deeper and more rapid breathing MORE CO2 and H+ is expelled, pH rises (alkaline)
With shallow and slower breathing LESS CO2 and H+ is expelled, pH decreases (acid)
Renal Mechanisms
Kidneys ultimate pH regulatory organs
o Remove all METABOLIC acids (non-CO2)
o Sole regulators of alkaline substances (HCO3-)
Tubular secretion or reabsorption of bicarbonate ions (HCO3-), or generate new bicarbonate
ions if necessary
Takes hours to days to restore pH
When blood pH rises (alkaline)
Hydrogen ions retained by tubular reabsorption
Bicarbonate ions excreted (tubular secretion)
When blood pH falls (acid)
Hydrogen ions excreted, or neutralised by HCO3Bicarbonate ions reabsorbed
Thus, urine pH varies (4.5 to 8.0)
Compensatory Mechanisms – lungs and kidneys work together to correct pH imbalances