LECTURE 3 (FLUID & ELECTROLYTE BALANCE)

What percent of total body mass is fluid in a normal adult?

55–60%

What are the two major fluid compartments in the body?

Intracellular fluid (ICF) and extracellular fluid (ECF)

What percentage of fluid is intracellular fluid (ICF)?

About 2/3 of total body water

What percentage of fluid is extracellular fluid (ECF)?

About 1/3 of total body water

What makes up the extracellular fluid?

Plasma and interstitial fluid

What is interstitial fluid?

Fluid that surrounds the cells

What determines water movement between ICF and ECF?

Osmosis (osmotic gradients)

SECTION 2 — WATER GAIN & WATER LOSS

What are the sources of water gain?

Ingestion + metabolic water production

What are the major routes of water loss?

Urine, perspiration, lung exhalation, feces

Can the kidneys replace lost water?

No — they can only conserve volume

What happens when water loss exceeds gain?

Increased osmolarity, dehydration, ADH release

SECTION 3 — OSMOLARITY CHANGES THROUGH THE NEPHRON

What is the osmolarity of filtrate leaving the proximal tubule?

Isosmotic (300 mOsm)

What happens to filtrate in the descending loop of Henle?

Becomes more concentrated (water leaves)

Why is the descending limb permeable to water?

It has aquaporins, allowing osmosis

What happens in the ascending limb of Henle?

Solutes are reabsorbed, filtrate becomes dilute

Why is the ascending limb impermeable to water?

Lacks aquaporins

What creates the hyposmotic filtrate in the ascending limb?

Active reabsorption of ions

What determines final urine osmolarity?

Reabsorption in the collecting duct under hormonal control

SECTION 4 — ADH / VASOPRESSIN MECHANISM

What stimulates ADH release?

Increased plasma osmolarity or decreased blood volume

Where is ADH produced?

Hypothalamus

Where is ADH released from?

Posterior pituitary

What is the effect of ADH on the kidneys?

Increases water reabsorption in the collecting duct

How does ADH increase collecting duct permeability?

Inserts AQP2 water channels

What signaling pathway does ADH use?

cAMP second-messenger system

How does water move after ADH action?

By osmosis from filtrate → interstitium → blood

Does ADH increase or decrease urine osmolarity?

Increases

Does ADH increase or decrease urine volume?

Decreases

SECTION 5 — ALDOSTERONE MECHANISM

What stimulates aldosterone secretion?

Low blood pressure, high potassium, angiotensin II

Where is aldosterone produced?

Adrenal cortex

What does aldosterone do?

Increases Na⁺ reabsorption and K⁺ secretion

How does aldosterone work at the cellular level?

Binds cytoplasmic receptor → gene transcription → new pumps/channels

What transporter does aldosterone increase?

Na⁺/K⁺ ATPase + ENaC channels in collecting duct

Effect of aldosterone on blood volume?

Increases it (because water follows Na⁺)

SECTION 6 — ATRIAL NATRIURETIC PEPTIDE (ANP)

What stimulates ANP release?

Increased blood volume or atrial stretch

Where is ANP released from?

Atrial myocardial cells

What is the main function of ANP?

Increases Na⁺ excretion (natriuresis)

How does ANP affect water balance?

Increases water loss in urine (follows sodium)

Effect of ANP on blood pressure?

Decreases BP and blood volume

Does ANP inhibit or stimulate aldosterone?

Inhibits

Does ANP promote vasodilation or vasoconstriction?

Vasodilation

SECTION 7 — COUNTERCURRENT MULTIPLIER & EXCHANGER

What creates the medullary osmotic gradient?

Loop of Henle countercurrent multiplier

What preserves the gradient?

Vasa recta countercurrent exchanger

What increases medullary osmolarity besides ions?

Urea recycling

Why is the gradient important?

Allows urine to be concentrated when ADH is present

SECTION 8 — ELECTROLYTE FUNCTION SUMMARY

What is the most abundant ECF cation?

Sodium (Na⁺)

What ion determines ECF osmolarity?

Sodium

What is the most abundant ICF cation?

Potassium (K⁺)