Renal System
The kidneys maintain ~60% of body weight as water, divided into compartments: Intracellular fluid (ICF): 2/3 of total body water. Extracellular fluid (ECF): 1/3 (includes plasma + interstitial fluid). Maintaining the right volume and composition of these fluids is crucial for normal cellular function, blood pressure, and metabolism — and the kidneys are the key players.
⚙️ 1️⃣ Body Fluid Compartments
| Compartment | % of Body Weight | Major Cation | Major Anion |
|---|---|---|---|
| ICF | ~40% | K⁺ | Phosphate, proteins |
| ECF | ~20% | Na⁺ | Cl⁻, HCO₃⁻ |
🧠 High-yield fact: Water moves freely between compartments — driven by osmotic gradients, not active transport.
💧 2️⃣ Regulation of Body Fluid Volume
Fluid volume is mainly determined by total body sodium (Na⁺), because Na⁺ is the major ECF osmotic solute. Thus: Where Na⁺ goes, water follows. The body regulates ECF volume by controlling Na⁺ balance, and regulates osmolality by controlling water balance.
A. Neural Regulation — Baroreceptor Reflex
- Sensors: Atrial stretch receptors, carotid sinus, aortic arch
- Low volume / BP → activates sympathetic nervous system (SNS)
- Constricts renal arterioles → ↓ GFR (conserve water)
- Stimulates renin release
- ↑ Na⁺ reabsorption (via proximal tubules)
💡 Rapid response (seconds–minutes)
B. Hormonal Regulation — RAAS System
The Renin–Angiotensin–Aldosterone System (RAAS) is the body’s main long-term regulator of volume and BP.
- ↓ BP or ↓ Na⁺ → Juxtaglomerular cells release renin
- Renin converts angiotensinogen → angiotensin I
- ACE (lungs) converts angiotensin I → angiotensin II
- Angiotensin II: Constricts arterioles → ↑ BP. Stimulates aldosterone from adrenal cortex → ↑ Na⁺ and water reabsorption. Stimulates thirst and ADH release.
🧠 Key effects: ↑ Na⁺, ↑ water, ↑ BP
💡 Clinical pearl: ACE inhibitors block this system → ↓ BP, ↓ Na⁺ retention.
C. Aldosterone — The Sodium Saver
- Secreted by adrenal cortex (zona glomerulosa)
- Acts on distal tubule & collecting duct
- ↑ Na⁺ reabsorption and K⁺ secretion
🧠 Summary: Aldosterone saves salt and squeezes blood vessels → restores volume.
💡 Deficiency (Addison’s disease): Na⁺ loss, dehydration, hypotension. Excess (Conn’s syndrome): Na⁺ retention, hypertension, hypokalemia.
D. Antidiuretic Hormone (ADH / Vasopressin) — The Water Keeper
- Secreted by posterior pituitary
- Stimulus: ↑ plasma osmolality (detected by hypothalamic osmoreceptors) or ↓ blood volume
- Action: Inserts aquaporin-2 channels in collecting ducts → ↑ water reabsorption
💡 High ADH: concentrated urine (dehydration). Low ADH: dilute urine (overhydration).
🧠 Clinical note: Diabetes insipidus: ↓ ADH or renal insensitivity → excessive dilute urine. SIADH: ↑ ADH → water retention, hyponatremia.
E. Atrial Natriuretic Peptide (ANP) — The Volume Reliever
- Released from atria when they stretch due to high blood volume
- Effects: ↓ Renin, ↓ aldosterone. ↑ Na⁺ and water excretion (natriuresis). Vasodilation → ↓ BP.
💡 Mnemonic: ANP = “A Nice Pee.” 😄
⚡ 3️⃣ Regulation of Major Electrolytes
| Electrolyte | Normal Plasma Range | Regulated by | Main Function |
|---|---|---|---|
| Na⁺ | 135–145 mEq/L | Aldosterone, ANP | ECF volume, nerve function |
| K⁺ | 3.5–5.0 mEq/L | Aldosterone, pH, insulin | Nerve & muscle excitability |
| Ca²⁺ | 8.5–10.5 mg/dL | PTH, calcitonin, vitamin D | Muscle contraction, coagulation |
| Cl⁻ | 95–105 mEq/L | Follows Na⁺ | Osmotic balance |
| HCO₃⁻ | 22–28 mEq/L | Kidneys | Buffer for acid-base balance |
🧠 Potassium Regulation (High Yield)
- Aldosterone: ↑ K⁺ secretion → ↓ plasma K⁺
- Acidosis: K⁺ moves out of cells → hyperkalemia
- Alkalosis: K⁺ moves into cells → hypokalemia
- Insulin & catecholamines: shift K⁺ into cells (via Na⁺/K⁺ ATPase)
💡 Clinical tip: K⁺ imbalance → arrhythmias, muscle weakness → always check K⁺ in renal and cardiac patients!
💧 Calcium and Phosphate Balance
- Parathyroid hormone (PTH): ↑ Ca²⁺ reabsorption (kidney). ↓ phosphate reabsorption. Stimulates vitamin D activation → ↑ intestinal Ca²⁺ absorption.
- Calcitonin: lowers plasma Ca²⁺
- Vitamin D (calcitriol): enhances Ca²⁺ and phosphate absorption in the gut
🧠 Clinical tie: Hypocalcemia → tetany, cramps. Hypercalcemia → stones, bones, groans, moans.
🩸 4️⃣ Integration — Water and Salt Homeostasis
| Condition | Response |
|---|---|
| Dehydration | ↑ ADH, ↑ RAAS, ↓ ANP → water retention |
| Overhydration | ↓ ADH, ↓ RAAS, ↑ ANP → water excretion |
| Na⁺ depletion | ↑ Aldosterone, ↑ thirst |
| Na⁺ excess | ↑ ANP → natriuresis |
💡 Ultimate goal: maintain ECF volume + osmolality + BP within narrow limits.
🧠 Summary Table
| Regulator | Main Stimulus | Primary Effect | Net Result |
|---|---|---|---|
| RAAS | ↓ BP / ↓ Na⁺ | ↑ Na⁺ & water retention | ↑ Volume, ↑ BP |
| Aldosterone | Angiotensin II, ↑ K⁺ | ↑ Na⁺ reabsorption, ↑ K⁺ excretion | ↑ Volume |
| ADH | ↑ Osmolality / ↓ Volume | ↑ Water reabsorption | ↓ Osmolality |
| ANP | ↑ Atrial stretch | ↑ Na⁺ excretion | ↓ Volume, ↓ BP |