PART 1
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
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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.
Plasma
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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
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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)
Electrolytes
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Despite these electrolyte differences the osmolarity of ECF and ICF is the same
Therefore, so is water concentration
Water Balance
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Water intake (2.5 L) = water output (2.5 L)
Sources for water intake: Foods and fluids, Metabolic processes (200 mL)
Dehydration
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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
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Faeces
Expired air from lungs
Cutaneous transpiration
Sweat
Urine production
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Kidneys major regulators of body fluid composition
Tubular reabsorption of water and electrolytes regulated by
hormones – ADH, Aldosterone, Angiotensin, ANP
PART 2
Acid-Base Balance of Blood, pH
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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
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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
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Symbolized with double-headed arrows
CO2 + H2O ↔ H2CO3 ↔ H + + HCO3 - (H+ donor) (H+ acceptor)
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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)
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DEMONSTRATION
Respiratory Mechanisms
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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
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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
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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)
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Hydrogen ions retained by tubular reabsorption
Bicarbonate ions excreted (tubular secretion)
When blood pH falls (acid)
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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