6. Water & Electrolytes

What is the primary intracellular fluid electrolyte? What are the 2 primary extracellular fluid electrolytes?

ICF: Potassium

ECF: Sodium and Chloride

What is responsible for keeping K+ inside the cells and Na+ outside the cells?

The ATPase pump

What is the difference between blood plasma and interstitial fluid?

Blood plasma contains a way higher [ ] of proteins (particularly albumin) that is responsible for oncotic pressure (aka colloid osmotic pressure)

What are the definitions & differences between osmosis and diffusion?

osmosis: movement of water from low solute [ ] to high solute [ ]

diffusion: movement of solutes from high [ ] to low [ ]

What are the references ranges for the 4 primary electrolytes? (Na+, K+, Cl-, HCO₃-)

Na+: 135-145 mmol/L

K+: 3.5-5 mmol/L

Cl-: 98-105 mmol/L

HCO₃-: 23-29 mmol/L

What are the main renal processes of the 4 electrolytes & hydrogen?

Na+: reabsorbed through nephron, coupled with water

K+: reabsorbed through nephron then excreted in the distal tubules

Cl-: reabsorbed alongside sodium to maintain electroneutrality

HCO₃-: almost completely reabsorbed through nephron

H+: actively secreted in urine

What is a major role of sodium?

maintain osmotic pressure

What is the sodium “buffer system” that’s involved with lactate and after bicarbonate buffers a metabolic acid?

NaHCO₃ + H-Lactate → Na-Lactate (salt) + H₂CO₃ → CO₂ + H₂O

How does severe hyponatremia and hypernatremia affect the way water shifts in/out of brain cells?

hyponatremia: water shifts into brain cells (cerebral edema)

hypernatremia: water shifts out of brain cells (brain cell shrinkage)

Explain what hyponatremia with high serum osmolality may be due to?

Hyperglycemia

Because glucose draws water into the ECF space → lower measured Na+

Explain what hypernatremia with low urine osmolality may be due to?

diabetes insipidus

body cannot retain H₂O → increased urination of diluted urine. ECF loses water as well, concentrating ECF and increasing serum sodium

What is the major role of potassium?

maintain cardiac rhythm and contribute to neuromuscular conduction

What 2 symtpoms can imbalanced potassium levels such as hyperkalemia or hypokalemia cause?

cardiac arrhythmias & neuromuscular weakness

What can cause hyperkalemia and what is it a response to?

renal failure

electrolyte imbalance aka acidosis

Explain how acidosis can lead to hyperkalemia?

Excess H+ ions in the ECF will diffuse into the cells, creating an imbalance of + ions in the cell. To compensate, K+ will diffuse out, leading to increase in K+ → hyperkalemia

What are 3 illnesses that can cause hypokalemia?

vomiting, diarrhea, and hypderaldosteronism (excess excretion of K+)

Hyperchloremia and hypochloremia may be caused by what 2 conditions?

hyper: hypernatremia (since Cl- follows Na+ to maintain electroneutrality) or excess loss of HCO₃-

hypo: hyponatremia & conditions like vomiting (because you vomit HCl from stomach acid)

What is the chloride shift?

It is when bicarbonate is the cell diffuses into the plasma, so chloride shifts into the cell to maintain electriconeutrality. If bicarbonate diffuses into the cell, then chloride shifts out to the plasma

What is bicarbonate the major buffer for and where is it reabsorbed?

metabolically produced acids

PCt and DCT in kidneys

What conditions are associated with increased vs decreased bicarbonate levels?

increased: metabolic alkalosis

decreased: metabolic acidosis (b/c excess acids consume bicarbonate → decreased levels)

What does the anion gap represent & and what is the equation w/ the reference range?

It represents the unmeasured anions

[Na+] - [Cl- + HCO₃-] = anion gap (7-14 mmol/L)

What can the unmeasured anions be? (4)

albumin/proteins, alcohol, organic acids, phosphate, sulfate,

What are the 3 conditions that can lead to an increased anion gap?

1. metabolic acidosis (increase in acids → HCO₃- consumed → decrease HCO₃- in plasma → increase AG)

2. Decreased HCO₃-

   1. low albumin can decrease AG → mask actual increase in AG

3. abnormal Na+ increase (not matched by Cl- or HCO₃-)

Why does an increase K+ or chloride not usually affect the anion gap?

K+: patient would be dead at levels that would significantly alter the gap

Cl-: usually increased to replace HCO₃-

What does osmolality measure and what are the 4 major contributors to serum osmolality?

the number of dissolved solutes in a solution

Na+, Cl-, glucose, and urea

What are the normal osmolality ranges for plasma and urine?

plasma: 275-300 mOsm/kg

urine: 300-900 mOsm/kg

What is the calculation for serum osmolality?

2[Na+] + [Glucose]/20 + [BUN]/3

What is the osmol gap? What is the equation for it? What is the normal value of an osmol gap?

It is the value of osmolality that is affected by the unmeasured dissolved solutes (everything except Na+, glucose, and BUN)

Measured osmolality (from osmometer) - calculated osmolality = osmolality gap

9 mOsm/Kg

What are 2 responses to decrease osmolality in the case of increased osmolality?

being thirsty → increased water consumption dilutes ECF

secretion of ADH → retain water

What are the 2 hormones that control blood volume and what organs are they produced from?

Aldosterone: adrenal gland

ADH (antidiuretic hormone) or vasopression: posterior pituitary gland

What are the hormones that control blood pressure? Where is it released from?

epinephrine and norepinephrine

adrenal glands (adrenal medullar)

What is the stimulus for activating ADH and what is the action of ADH?

stimulus: plasma with increased osmolality circulates around posterior pituitary gland

action: ADH travels to collecting ducts of kidneys → more water reabsorption

What is the stimulus for activating aldosterone?

JGA(juxtaglomerulus-apparatus) in the nephrons (kidney cells) are stimulated by low blood pressure/volume to the kidneys

What are the 5 steps in the mechanism of aldosterone?

1. JGA secretes protease renin to convert plasma angiotensinogen → angiotensin 1

2. Angiotensin 1 travels to lungs where protease angiotensin-converting-enzyme (ACE) converts → angiotensin 2

3. Angiotensin 2 responsible for

   1. vasoconstriction → increased blood pressure

   2. circulating to adrenal cortex to stimulate aldosterone secretion

4. aldosterone circulates from adrenal → kidneys

5. Stimulates tubules to reabsorb Na+ and excrete K+

What class of compounds are epinephrine and norepinephrine called?

catecholamines (modified amines)

What is the stimulus and action of epinephrine and norepinephrine?

stimulus: physio/psychological threat → nerve impulse

action: increased heart rate & heart beat strength → increased blood pressure

What is pheochromacytoma? What does it cause increases in?

Rare tumor of the adrenal medulla

increased secretion of epinephrine & norepinephrine. increased production of metanephrine & normetanephrine → increased VMA

What metabolites are now used to analyze for pheochromacytoma?

plasma and urine metanephrines because they they offer higher sensitivity for testing

What is aldosterone induced hypertension?

Increase in secretion of aldosterone commonly due to adrenal adenoma (adrenal tumor)

What is the syndrome of inappropriate ADH (SIADH)?

Tumor that produces excess ADH → water retention → hyponatremia, low serum osmolality, and concentrated urine

What is diabetes insipidus? What does it NOT cause?

Lack of ADH secretion or kidney response to ADH → dilute urine excretion

abnormal blood pressure (because epinephrine & aldosterone maintain pressure despite water loss)