Fluid, Electrolyte, and Acid Base Balance

Fluid compartment in body?

Water ; 60-80% of body weight

ICF (within cells) : 2/3 of body fluids (40% of body weight)

ECF (outside cells) : 1/3 or 20% of body weight

ECF makeup (Interstial fluid : 80%, Plasma 20%

Composition of Body Fluids

Water

Solute : Non-electrolytes (can’t conduct electricity) like organic molecules, glucose, lipids, urea, créatine

Electrolytes : dissociate into ions (has a charge postive or negative) —> like salts, acids, gases and some proteins (usually negative charged)

Function of Electrolytes in Body Fluid?

Control movement of water between fluid circulation comportements

Maintain acid-base balance

Carry electric current and produce Nerve impulses

Act as co-factors for enzymes

ECF electrolyte compartment

higher protein in plasma (than IF)

Major cation : Na+

Major anion : Cl-

ICF electrolyte compartment

Contains more soluble proteins than plasma

Low Na+ and Cl-

Major cation : K+

Major anion : HPO42- (phosphate due to it being in DNA and RNA as a component of nucleotides)

Where is Na+ high in?

ECF

Where is Cl- higher in?

ECF

Where is K+ higher in?

ICF

Where is Ca2+ higher in?

ECF (still low amount) but there is relatively zero in cell)

Where is HCO3- higher in? (Bicarbonate)

ECF

Where is HPO42- higher in? (Hydrogen phosphate)

ICF

Where is SO42- higher in? (Sulfate)

ICF

Where are protein anions higher in?

ICF (virtually none if IF due to lymphatic system!)

Average water intake?

~2500 mL/day

10% metabolism (product of cellular respiration)

Food : 30%

Beverages : 60%

Average Water Output

~2500 ml/day

4% feces

8% sweat

28% insesinbke loss via skin and lungs

60% urine

ECF osmolality

Maintained around 280-300 mOms

(Total ion concentration in fluid)

Hypothalamic osmoreceptors detect ICF osmolality

Rise in osmolality?

Dehydration

Stimulates thirst centers

Cause ADH to release

Decrease in osmolality?

over hydration

Cause thirst inhibition

Causes ADH inhibition

Where are granular cells found?

In afférent artériole

Where are mineralcorticid produced??

adrenal cortex

(Like adlisterone)

increase ECF osmolaliy?

Omsoreceptors in hypothalamus detect change along with decrease saliva, dry mouth → hypothalamic thirst center gets stimulated → Sensation of thirst (drink) → Water moistens mouth, throat, stretches stomach and intestine → water absorbed from GI tract

Decreased plasma volume?

Decreased BP → Granular cells in afférent artériole detect change in kidney → R.A.A mechanism → increase angiotensin II → hypothalamic thirst center → drink → Water moistens, mouth throat and fills stomach+ intestine → water absorbed from GI tract → decreased ECF osmolality and increased Plasma volume

Influence of Antidiuretic hormone (ADH)

Water réabsorption in collecting duct is proportional to ADH release → influences if it’s diluted or concentrated urine

What can trigger ADH release?

Hypothalamic osmoreceptors → detects blood osmolality (if high, ADH release, if low ADH inhibition)

Large changes in blood volume or pressure (Decreased Bp causes increased ADH release due to vessel baroreceptors and R.A.A mechanism

Dehydration

Can be due to bleeding, severe burns, prolonged vomiting/diarrhea, profuse sweating, water deprivation, diuretic abuse and endocrine disturbances

Signs : cottony oral mucosa, thirst, dry flushed skin (oliguria)

May lead to weight loss, fever, mental confusion, hypovelmic shock and loss of electrolytes

Hypotonic hydration (water intoxication)

Low solute level (too much water)

Occurs with renal insufficiency or rapid water intake

ECF osmolality decreases causing hypoatremia (low Na+ in blood so water leaks out) → swelling of cells

Severe metabolic disturbances, nausea, vomiting, muscle cramp, cerebral edema and possible death

Treatment : fluid restriction (slowly bring back electrolyte level) in severe cases hypertonic saline could be used

Edema

Results in tissue swelling (not cell swelling)

Only volume of IF is increase which affects exchange (increased distance for diffusion of O2 and nutrients from blood into cells)

Role of Sodium

Most abundant cation in ECF

only cation exerting significant osmotic pressure

Water follows salt → total body content determined EVF volume and therefore BP

Concentration of it : influences excitability of neurons and muscles

Importance of Potassium?

Affects resting membrane potential in neurons and muscles

Increased ECF [K+] → hyperkalemia (high K+ in blood causing depolarization) → decreased RMP → depolarization→ reduced excitability

Decreased ECF [K+] → hypokalemia (low K+ in blood) → hyperpolaeization → nonresponsivness

What can disruption in K+ (either hyper- or hypokalemia) lead to?

In heart it can interfere with electrical conduction and can lead to sudden death

Aldosterone

Plays biggest role in regulation of Na+ by kidneys

Stimulates Na+ reabsoprtion and K+ secretion

When aldosterone is high?

Na+ is actively reabsorbed in DCT and CT (so water follows and ECF volume increases) > K+ is secreted in exchange

Normal Na+ absorption?

65% is reaborfbed in proximal tubules and 25% is reclaimed in Nephron loops

What does renin do?

Catalyze production of angiotensin I (ACE converts it to angiotensin II) → prompts aldosterone release from adrenal cortex → Na+ reabsoprtion by kidney tubules

What can trigger release of aldosterone

deceased Na+ content (triggers renin release and increased angiotensin I and then aldosterone release)

Increased K+ concentration in the ECF

Aldosterone effects are slow (hours to days)