Body Fluids & Urinary System – Comprehensive Review

These question-and-answer flashcards review major concepts from the lecture notes: body-fluid compartments, hormonal and renal regulation, electrolyte balance, kidney anatomy and physiology, urine formation, counter-current mechanisms, acid-base control, and micturition.

What percentage of an average adult male’s body weight is water?

About 60 % (ranges 50-70 %, higher in males due to lower body-fat content).

Which two major fluid compartments contain all body water?

Intracellular fluid (ICF, ≈ 2⁄3 of total body water) and extracellular fluid (ECF, ≈ 1⁄3).

Name the two primary subdivisions of the extracellular fluid.

Interstitial fluid (≈ 80 % of ECF) and plasma (≈ 20 % of ECF).

What organ system is chiefly responsible for maintaining ECF volume and osmolarity?

The kidneys, working with hormonal, respiratory, and buffer systems.

Which hormone inserts aquaporin-2 channels into collecting-duct cells?

Antidiuretic hormone (ADH, vasopressin).

How much water is normally lost in urine each day in an average adult?

About 1.5 L of the 2.5 L total daily water loss.

List the four normal routes of daily water loss.

Urine (kidneys), evaporation through skin, exhaled water vapour (lungs), and faeces (GIT).

What triggers the thirst centre in the hypothalamus during dehydration?

Increased plasma osmolarity, decreased blood volume/pressure, dry mouth, and renin-angiotensin activation.

Which three hormones regulate renal Na⁺ and Cl⁻ handling?

Angiotensin II, aldosterone, and atrial natriuretic peptide (ANP).

How does ANP affect sodium balance?

It increases Na⁺ (and water) excretion by inhibiting renin, aldosterone, and Na⁺ reabsorption; it also vasodilates glomerular afferent arterioles to raise GFR.

Define hyponatraemia.

Plasma Na⁺ concentration below normal (< 136 mEq/L), often due to NaCl loss or water retention, leading to hypo-osmotic ECF.

What is water intoxication?

Excessive body water causing cellular swelling; in the brain this can produce confusion, seizures, coma, or death.

Which two ions dominate the osmolarity of ECF and ICF, respectively?

Na⁺ (with Cl⁻) dominates ECF; K⁺ (with phosphates and proteins) dominates ICF.

Name six clinically important electrolytes.

Sodium, potassium, chloride, bicarbonate, calcium, phosphate.

Which hormone raises blood Ca²⁺ by stimulating bone resorption and renal reabsorption?

Parathyroid hormone (PTH).

What is the normal plasma pH range compatible with life?

7.35 – 7.45 (arterial); pH < 6.8 or > 8.0 is life-threatening.

Identify the three main buffer systems of the body.

Protein buffer system, bicarbonate–carbonic-acid buffer, and phosphate buffer system.

How does the respiratory system correct acidosis?

By increasing ventilation to expel CO₂, lowering carbonic acid and raising pH.

What two processes allow the kidneys to control blood pH?

Secretion of H⁺ and reabsorption/generation of bicarbonate (HCO₃⁻).

Which specialised capillary network runs alongside juxtamedullary nephrons?

The vasa recta, acting as a counter-current exchanger.

State the three basic renal processes that form urine.

Glomerular filtration, tubular reabsorption, and tubular secretion.

What is the normal glomerular filtration rate (GFR) in adult males?

≈ 125 mL min⁻¹ (about 180 L/day).

Which arteriole has the larger diameter, afferent or efferent, and why is this important?

Afferent arteriole; its larger diameter maintains high glomerular hydrostatic pressure to drive filtration.

Give the equation for net filtration pressure (NFP) in the glomerulus.

NFP = GBHP – CHP – BCOP ≈ 55 – 15 – 30 = 10 mm Hg (favouring filtration).

Name two intrinsic (autoregulation) mechanisms that stabilise GFR.

Myogenic mechanism and tubuloglomerular feedback (macula densa control).

Which cells release renin and where are they located?

Juxtaglomerular (granular) cells in the afferent arteriole wall.

Describe the effect of angiotensin II on the kidneys.

Potent vasoconstriction (especially efferent arteriole), stimulates aldosterone & Na⁺ reabsorption, reduces GFR when systemic pressure is low.

What is tubular transport maximum (Tm)?

The maximum rate at which a substance can be reabsorbed because all its carrier proteins are saturated (e.g., glucose ≈ 375 mg min⁻¹).

Which nephron segment reabsorbs ~ 65 % of filtered Na⁺ and water?

The proximal convoluted tubule (PCT).

What transporter in PCT couples Na⁺ reabsorption with glucose uptake?

The Na⁺-glucose symporter (SGLT).

Which limb of the loop of Henle is water-permeable?

Descending limb (permeable to water, not to NaCl).

What is the main function of the thick ascending limb of Henle?

Active reabsorption of Na⁺, K⁺, and Cl⁻ (Na⁺/K⁺/2Cl⁻ symporter); impermeable to water – helps create the medullary osmotic gradient.

Explain counter-current multiplier.

Process in juxtamedullary nephron loops in which opposing fluid flows and active NaCl transport in ascending limbs generate a vertical osmotic gradient in the renal medulla (300 → 1200 mOsm).

How does ADH concentrate urine?

Makes late DCT and collecting-duct cells water-permeable via aquaporin-2 insertion, allowing water reabsorption into hyperosmotic medulla, reducing urine volume and raising osmolarity.

What role does urea play in concentrating urine?

It passively diffuses from medullary collecting ducts into interstitium, contributing ~40 % of medullary osmolarity and enhancing water reabsorption.

Define plasma clearance.

Volume of plasma completely cleared of a substance by the kidneys per unit time (mL min⁻¹).

Which substance’s clearance equals GFR because it is filtered but neither reabsorbed nor secreted?

Inulin (creatinine is used clinically as an approximation).

What muscle forms the wall of the urinary bladder?

Detrusor muscle (smooth muscle).

Differentiate internal and external urethral sphincters.

Internal sphincter: smooth muscle, involuntary; external sphincter: skeletal muscle in urogenital diaphragm, voluntary.

Describe the micturition reflex origin.

Stretch receptors in bladder wall (when ~200-400 mL urine) send signals to sacral spinal cord (S2-S3), activating parasympathetic outflow that contracts detrusor and relaxes internal sphincter.

Which branch of the autonomic nervous system chiefly promotes bladder emptying?

Parasympathetic nervous system.

What is obligatory versus facultative water reabsorption?

Obligatory: water follows solutes (mainly Na⁺) in PCT and descending loop; facultative: ADH-regulated water reabsorption in late DCT/collecting ducts.

How does aldosterone affect K⁺ balance?

Stimulates principal cells of distal nephron to reabsorb Na⁺ and secrete K⁺, lowering plasma K⁺.

What percentage of filtered water remains as urine under normal conditions?

About 1 % (≈ 1–2 L/day).

Which specialized cells sense tubular NaCl and regulate afferent arteriole tone?

Macula densa cells (part of juxtaglomerular apparatus) – tubuloglomerular feedback.

Why does alcohol increase urine output?

It inhibits ADH secretion, reducing water reabsorption in distal nephron.

Name two factors that stimulate ADH release besides osmolarity changes.

Low blood volume/pressure (via baroreceptors) and non-osmotic stimuli such as pain, nausea, or stress.

What is counter-current exchange and where does it occur?

Passive exchange of water and solutes between descending and ascending limbs of vasa recta, preserving medullary osmotic gradient.

Which buffer system provides about two-thirds of intracellular buffering power?

Protein buffer system (including hemoglobin in RBCs).

How does hemoglobin buffer acids in systemic capillaries?

It binds H⁺ produced when CO₂ is converted to bicarbonate, preventing large pH drops.

What is the Henderson–Hasselbalch ratio for bicarbonate buffer at normal pH?

[HCO₃⁻] : [H₂CO₃] ≈ 20 : 1.

Describe renal handling of HCO₃⁻ in PCT.

Filtered HCO₃⁻ combines with secreted H⁺, forms H₂CO₃, converts to CO₂ + H₂O (via carbonic anhydrase), CO₂ diffuses into cells, is reconverted to HCO₃⁻ which is transported into blood.

What is the effect of hyperventilation on blood pH?

Excess CO₂ loss lowers carbonic acid, causing respiratory alkalosis (blood pH rises).

What defines isotonic, hypotonic, and hypertonic solutions relative to plasma?

Isotonic: 300 mOsm (e.g., 0.9 % NaCl); hypotonic: < 300 mOsm (cells swell); hypertonic: > 300 mOsm (cells shrink).

Why is 0.3 M urea isosmotic but not isotonic to plasma?

Urea crosses membranes rapidly, so although osmolarity equals plasma, it enters cells, raising intracellular osmolarity and causing water influx → cell swelling (hypotonic behaviour).

Give two clinical causes of hypernatraemia.

Excessive water loss (e.g., diabetes insipidus, profuse sweating) or excessive Na⁺ intake/aldosterone excess.

Name three specialised body fluids other than plasma and interstitial fluid.

Cerebrospinal fluid (CSF), lymph, and amniotic fluid (others: milk, aqueous humour, sweat, tears).

What is the primary function of erythropoietin released by kidneys?

Stimulates red blood cell production in bone marrow.

Which vitamin is activated by kidneys, and what is its active form?

Vitamin D₃ is converted to 1,25-dihydroxycholecalciferol (calcitriol).

Define osmolarity.

Total number of dissolved particles (osmoles) per litre of solution; human body fluids ≈ 300 mOsm L⁻¹.

Which equation relates osmotic pressure to solute concentration?

Van’t Hoff’s law: π = C R T (π osmotic pressure, C osmolarity, R gas constant, T absolute temperature).

What determines whether a substance’s renal clearance is greater, equal, or less than GFR?

If it is secreted (clearance > GFR), neither reabsorbed nor secreted (clearance = GFR), or reabsorbed (clearance < GFR).

Why is creatinine clearance only an approximation of GFR?

Creatinine is freely filtered but a small amount is secreted, so clearance slightly overestimates true GFR.