Exam 1 · Week 1 · BIO 280 Pathophysiology
M1 · Fluid & Electrolytes · Acid–Base
Where the water sits, the three hormones that move it, and how to read a blood gas in four steps.
▸M1Fluid & Electrolytes · Acid–BaseWeek 1
💡 The one idea
Water follows salt, and salt follows the hormones. Every fluid problem in this module is one of three questions: how much water, how much sodium, and which hormone is out of balance.
Sodium tells you where the water went. Potassium tells you what the heart will do about it.
💦 Where the water is
60% of body weight is water.
- Intracellular — two thirds. Inside the cells. This is the big tank.
- Extracellular — one third. Outside the cells, and split three ways: intravascular (in the vessels, ~1/4 of the ECF), interstitial (between the cells), transcellular (CSF, joint, pleural, GI — the small "third space").
Why it matters: only the intravascular part carries blood pressure. Fluid can be in the body and useless if it has moved into the interstitium.
⚖️ The three regulators
| System | Triggered by | Does | Net effect |
|---|---|---|---|
| RAAS | Low renal perfusion / low BP | Renin (juxtaglomerular cells) → angiotensinogen → angiotensin I; ACE (lungs) converts it to angiotensin II → vasoconstriction + aldosterone | ↑ volume, ↑ BP, ↑ vascular tone, ↑ CO |
| ADH (vasopressin) | ↑ serum osmolality, low volume | Tells the collecting ducts to reabsorb water — water only, not salt | ↑ plasma volume, ↑ BP, concentrated urine |
| ANP / BNP | Atrial and ventricular stretch | The brakes: excretes sodium and water, vasodilates, opposes RAAS | ↓ plasma volume, ↓ BP |
Aldosterone regulates sodium reabsorption — and it trades: sodium in, potassium out. That trade is why aldosterone problems always show up as a potassium problem too.
Wherever sodium goes, water follows.
⭐ Edema — pushing vs pulling
Edema is fluid leaving the vessel for the interstitial space. Only three mechanisms do it:
- ↑ Hydrostatic pressure — hydrostatic is a pushing force. Too much pressure inside pushes fluid out. Heart failure, venous obstruction, fluid overload, pregnancy.
- ↓ Colloidal osmotic (oncotic) pressure — oncotic is a pulling force, and albumin is what pulls. Lose the protein and nothing holds fluid in. Liver failure, nephrotic syndrome, malnutrition, burns.
- ↑ Vascular permeability — the wall itself leaks. Inflammation, sepsis, anaphylaxis, burns.
💧 SIADH vs Diabetes Insipidus — too much vs none
Both are ADH problems and they are exact opposites. SIADH = Soaked. DI = Dry.
| SIADH (too much ADH) | DI (no ADH) | |
|---|---|---|
| Water | Reabsorbed — retained | Lost — huge urine volumes |
| Plasma volume | ↑ (overload) | ↓ |
| Serum sodium | Low (dilutional) | High (concentrated) |
| Serum osmolality | ↓ | ↑ |
| Urine | Small, dark, high specific gravity & osmolality | Huge, pale, low specific gravity & osmolality |
| Shows as | Irritability, confusion, headache, cramps, twitching, pulmonary congestion, ↑ BP | Thirst, dehydration, hypotension, tachycardia |
In DI the plasma volume falls while urine volume rises, so plasma osmolality rises and urine osmolality falls.
🚨 Potassium is the one that stops the heart
Both ends of the potassium range are lethal, and both cause dysrhythmias. Never treat a potassium result as a number to recheck later.
Hypokalemia <3.5 | Hyperkalemia >5.0 | |
|---|---|---|
| Muscle | Weak, flaccid, cramps, ↓ reflexes | Weak, then flaccid paralysis |
| Gut | Slows — ileus, constipation | Speeds — cramping, diarrhea |
| ECG | Flat T, U wave, ST depression | Peaked T, wide QRS, then sine wave |
| Causes | Loop/thiazide diuretics, vomiting, diarrhea, insulin, alkalosis | Renal failure, K-sparing diuretics, ACE inhibitors, acidosis, cell breakdown (crush, burns, tumor lysis) |
Acidosis drives potassium out of the cells (serum K rises), alkalosis drives it in (serum K falls). The total body potassium may not have changed at all.
🧪 The other electrolytes, in one line each
- Sodium
135–145— a water problem far more often than a salt problem. Low = swollen cells = brain symptoms (confusion, seizures). - Calcium
8.5–10.5— low = irritable (tetany, Chvostek, Trousseau, laryngospasm); high = sluggish (stones, bones, groans, moans). - Magnesium
1.5–2.5— behaves like calcium's twin. Low = irritable and arrhythmic; high = depressed reflexes then respirations. - Phosphate — moves opposite to calcium. One up, the other down.
🧘 Acid–base in four steps
Normals: pH 7.35–7.45 · PaCO₂ 35–45
· HCO₃⁻ 22–26
- pH — below 7.35 acidosis, above 7.45 alkalosis.
- CO₂ — the lung number. It moves opposite the pH in a respiratory problem.
- HCO₃⁻ — the kidney number. It moves with the pH in a metabolic problem.
- Compensation — if the other system has moved to help: partial if the pH is still abnormal, full if the pH is back in range with both numbers still off.
ROME — Respiratory Opposite, Metabolic Equal.
| Disorder | pH | CO₂ | HCO₃⁻ | Typical cause |
|---|---|---|---|---|
| Respiratory acidosis | ↓ | ↑ | — | Hypoventilation: COPD, opioids, chest trauma, sedation |
| Respiratory alkalosis | ↑ | ↓ | — | Hyperventilation: anxiety, pain, fever, early PE, high altitude |
| Metabolic acidosis | ↓ | — | ↓ | DKA, lactic acidosis/shock, renal failure, diarrhea |
| Metabolic alkalosis | ↑ | — | ↑ | Vomiting/NG suction, antacids, diuretics |
The lungs compensate in minutes; the kidneys take hours to days. That timing is the exam's favorite distinguishing detail.
✅ Vomiting and diarrhea go opposite ways
Both lose fluid, but they lose different things:
- Vomiting loses gastric acid → metabolic alkalosis (and loses potassium).
- Diarrhea loses intestinal bicarbonate → metabolic acidosis (and loses potassium too).
Same electrolyte lost, opposite acid–base result. If a question gives you one of these and asks for the gas, that is the whole question.
🎯 Module quiz
Questions for this module.
Nothing here yet — drop it in when you have it