Download Ch 24 Notes

Saladin Ch. 24 - FLUID, ELECTROLYTE & ACID-BASE BALANCE
I. WATER BALANCE
FLUID COMPARTMENTS - BODY FLUIDS [55-60% of body weight]
1 – Intracellular fluid [ICF] = 65% of body fluids
2 – Extracellular fluid [ECF] = 35%
Interstitial fluid [25% of ECF]
Plasma [8%]
Transcellular fluid [2%]
3 – Other fluids – lymph, CSF, GI fluids, synovial fluid, ocular humors, pleural,
pericardial and peritoneal fluids, glomerular filtrate.
FLUID MOVEMENTS -BALANCE – maintaining proper distributions of fluids and solutes
Water moves by osmosis which is determined by electrolyte movement & balance
WATER GAIN & LOSS
Total ingested water [2500 mL/day]
Metabolic [from aerobic resp. and dehydration synthesis] [200mL/day]
Food & drink – [2300 mL]
Average outputs – increase with activity, environment, etc.
Total output [2500mL]
Kidneys – urine [1500mL/day]
Skin & sweat – 500mL/day
Lungs – esp. [300mL/day]
GI tract – feces [200mL/day]
REGULATION OF WATER UPTAKE
Regulated by adjusting water ingestion – thirst
Local responses – decreased saliva  dry mouth
Increased blood osmotic pressure  hypothalamus  thirst [may get ADH]
Decreased blood volume  release of renin  angiotensin II  thirst
REGULATION OF WATER OUTPUT
Regulated by urinary water and NaCl loss – kidneys can’t replace loss, just reduce further loss.
ADH increases water reabsorption in renal collecting ducts  aquaporins inserted
into principal cell membranes  water reabsorption.
DISORDERS OF WATER BALANCE
Fluid deficiency
Hypovolemia without hyperosmolarity – loss through bleeding, burns, vomiting, etc.
Dehydration – get increased osmolarity; Diabetes mellitus & insipidus, overuse of
diuretics, diarrhea; [can lead to shock] use isotonic salts solutions for replacement of
extreme fluid loss – prevents hypotonic swelling.
Fluid excess
Water intoxication – take in faster than can remove – hypotonic ECF
[hyponatremia]  cell swelling [and cerebral edema].
Fluid sequestration
Edema – accumulation of interstitial fluid. [Increases blood pressure, may
involve vessel blockage, congestive heart failure] /
Saladin Ch. 24
1 of 4
II. ELECTROLYTE BALANCE
Electrolyte = substance that dissociates into + [cations] and – [anions] in water solution and
conducts electricity in solution.
Functions
Essential minerals – enzyme cofactors
Control osmosis of water
Help maintain acid base balance
Conduct electrical current
SODIUM [normal plasma levels 142 mEq/L]
Functions:
90% of ECF cations
important in impulse transmission
contributes to osmotic pressure – most important solute in determining water distribution
Adult need 0.5 g/day – in America always have EXCESS – problem is getting rid of it
Concentrations are maintained by maintaining water levels – “water follows salt”
65% of Na+ in renal filtrate is automatically reabsorbed.
Levels regulated by
aldosterone – renin-angiotensin system - [increases reabsorption of Na+ and excretion of
K+], ascending loop, DCT & collecting ducts
ADH [increases water reabsorption in response to increases in Na],
ANP [increases Na+ excretion by kidneys by inhibing ADH & aldosterone]
Estrogens – enhances Na reabsortion and water retention
Progesterone may decrease Na+ reabsorption by blocking aldosterone.
Glucocorticoids – can cause edema
Imbalances
Hypernatremia  water retention, hypertension & edema
Hyponatremai hypotonic hydration if not corrected
POTASSIUM
Functions:
Most abundant intracellular cation
Functions in impulse conduction, OP regulation, protein synthesis, Na/K pump and pH
Levels regulated by the kidney –
aldosterone [hi K+ increases aldosterone which stimulates principal cells to secrete K+]
Imbalances
Hyperkalmia – get different responses if fast or slow onset. Fast - The heart is really
sensitive to too much [can cause membrane depolarization - hyperexcitability]; Slow
– cells become LESS excitable
Hypokalmia - too little [causes hyperpolarization and non-responsiveness].
CHLORIDE
Functions:
Major extracellular anion
Functions in OP regulation, HCl formation[GI]
Levels regulated by Na+ movements [follows Na+]
CALCIUM
Functions:
Ca Most abundant mineral in body
Saladin Ch. 24
2 of 4
Abundant extracellular cation
Functions: bone, teeth, blood clotting, impulse conduction, muscle, contraction
Levels controlled by
PTH - stimulates osteoclast release of Ca from bone, stimulates formation of
calcitrol from Vit. D – enhances intestinal uptake, stimulates kidney reabsorption.
Calcitonin – stimulates Ca incorporation into bone [mostly in children].
Imbalances:
Hypercalcemia – from alkalosis or hypothyroidism – inhibits depolarization  weakness,
arrhythmia
Hypocalcemia – Vit. D deficiency, lactation, pregnancy, etc. Too little can increase
excitability & may lead to tetanus;
PHOSPHATE
Functions:
Nucleic acids, ATP, phospholipids, bone, etc.
Control:
Kidneys reabsorb if levels drop,
Parathyroid hormone stimulates excretion
III. ACID/BASE BALANCE – critical to metabolic processes
Blood pH 7.35-7.45 normal
3 mechanisms to maintain this
buffer systems
changes in respiration
excretion by kidney
ACIDS, BASES & BUFFERS
Acid = proton [H+] donor;.
Most H+ originates as metabolic products.
Strong dissociates completely – gives lot of H’s
Weak acid only dissociates a little – few H’s
Base = proton acceptor
Strong – binds a lot, weak binds less
BUFFER SYSTEMS
Buffers resist changes in pH when strong acid or strong base are added.
Buffer system “physiological buffers” – remove or add H or base as needed – urinary
system [slow] & respiratory system [fast]
Chemical buffer - Weak acid and salt of that acid
Work by converting strong acids/bases to weak ones that don’t dissociate as much
to give operative ions [H+, OH-]
Bicarbonate/Carbonic Acid System – only important ECF buffer.
Important in blood
Carbonic acid and sodium bicarbonate - H+ + HCO3- ⇌ H2CO3 ⇌H2O + CO2
Phosphate Buffer System
Important in cytosol and kidneys
OH- + H2PO4- ⇌ H2O + HPO42H+ + HPO42- ⇌ H2PO4-
Saladin Ch. 24
3 of 4
Protein Buffer System – in cells and plasma [most important buffer – does ¾ of all]
Proteins are amphoteric – change charge with pH
The acid group [COOH] can give up and H+ to neutralize a base [lowers pH]
The amine group can accept an H+, raising pH
RESPIRATORY CONTROL OF pH
Exhalation of CO2 lowers H+ by lowering carbonic acid
With alkalosis – get shallow slow breathing to help ACCUMULATE CO2
RENAL CONTROL OF pH
Kidneys can expel H’s by secretion – neutralize it in tubules with bicarb, phosphate, etc.
Only way to remove non-carbonic acid H+ [phosphoric uric, lactic acid, ketone bodies]
Also only way to regulate alkaline substances – can break down bicarb to CO2 and
reabsorb that.
pH DISORDERS – see Table 24.32, p. 947
COMPENSATION = physiologic changes that occur to bring pH back to normal
Normal Blood Values
pH 7.35 – 7.45
PCO2 35 – 45 mm Hg
HCO3- 22 – 26 mEq/L
Blood Acidosis
pH below 7.35
depression of CNS  coma
Blood Alkalosis
pH above 7.45
Over excitability of CNS and PNS  nervousness  spasms  convulsions
Respiratory Acidosis
Elevated pCO2, low pH
Caused by hypoventilation, decreased external respiration [ex. Pulmonary edema,
emphysema]
Compensation – renal H+ secretion, increased bicarbonate reabsorption
Treatment – respiratory and IV bicarbonate
Respiratory Alkalosis
Low pCO2, high pH
Caused by hyperventilation [e.g. From oxygen deficiency]
Compensation – renal – decrease H+ secretion, bicarbonate elimination
Treatment – breathe into a bag
Metabolic Acidosis
Low systemic and arterial bicarbonate, low pH
Caused by loss of bicarbonate, accumulation of another acid, ketosis, or kidney failure
Compensation – respiratory hyperventilation [increased rate and depth]. PCO2 falls.
Treatment – NaHCO3 IV
Metabolic Alkalosis
High bicarbonate, high pH
Caused by loss of acid or intake of alkaline drugs [vomiting is the most common cause]
Compensation – respiratory – hypoventilation [slow, shallow]. PCO2 rises.
Treatment – electrolyte/fluid therapy
Saladin Ch. 24
4 of 4