Renal physiology mbbs 1st yr

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Renal Physiology - MBBS 1st Year Complete Notes

Sources: Guyton & Hall Textbook of Medical Physiology, Ganong's Review of Medical Physiology (26th Ed.)

1. FUNCTIONS OF THE KIDNEYS

The kidneys serve multiple homeostatic roles beyond just excretion:

Function	Details
Excretion of waste	Urea (amino acid metabolism), creatinine (muscle creatine), uric acid (nucleic acids), bilirubin metabolites
Fluid & electrolyte balance	Precisely matches excretion to intake over time
Osmolality regulation	Produces hypertonic or hypotonic urine as needed
Acid-base balance	Excretes H⁺, reabsorbs HCO₃⁻
Blood pressure regulation	Renin-angiotensin-aldosterone system (RAAS), pressure natriuresis
Erythropoiesis	Secretes erythropoietin (EPO) in response to hypoxia
Hormone metabolism	Activates Vitamin D (1,25-DHCC); metabolises/excretes hormone metabolites
Gluconeogenesis	Especially during prolonged fasting

Guyton & Hall, p. 3125-3148

2. FUNCTIONAL ANATOMY OF THE NEPHRON

Gross Structure

Each kidney has approximately 1 million nephrons. The nephron is the functional unit, consisting of a renal corpuscle (glomerulus + Bowman's capsule) and a renal tubule.

Types of Nephrons

Type	Location	Loop of Henle	Function
Cortical nephrons (~85%)	Cortex	Short loop, stays in cortex	General filtration
Juxtamedullary nephrons (~15%)	Cortex-medulla junction	Long loop, dips into medulla	Urine concentration

Nephron Segments (in order)

Glomerulus - filtration
Bowman's capsule - collects filtrate
Proximal convoluted tubule (PCT) - bulk reabsorption
Loop of Henle - descending (thin) + ascending (thin + thick)
Distal convoluted tubule (DCT) - regulated reabsorption
Collecting duct (CD) - final concentration/dilution

Blood Supply

Afferent arteriole → glomerular capillaries → efferent arteriole → peritubular capillaries (cortical nephrons) or vasa recta (juxtamedullary nephrons)
Unique feature: two capillary beds in series - glomerular capillaries (high pressure, for filtration) and peritubular capillaries (low pressure, for reabsorption)
Ganong's, p. 671-672

3. GLOMERULAR FILTRATION

What is GFR?

GFR = volume of plasma ultrafiltrate formed per minute. Normal = 125 mL/min (180 L/day). Of this, ~178.5 L is reabsorbed; only ~1.5 L/day is excreted as urine.

Filtration Barrier (Glomerular Membrane)

Three layers prevent passage of large proteins and cells:

Fenestrated endothelium - prevents passage of RBCs and platelets
Glomerular basement membrane (GBM) - negatively charged; repels anionic proteins (e.g., albumin)
Podocyte foot processes with slit diaphragms - final filtration barrier

Freely filtered: water, electrolytes, glucose, urea, creatinine, small molecules Not filtered: plasma proteins, blood cells

Starling Forces Governing GFR

GFR = Kf × (PG - PB - πG + πB)

Starling forces at the glomerular capillary (Guyton & Hall, Fig. 27.5)

Force	Value (mm Hg)	Direction
Glomerular hydrostatic pressure (PG)	60	Favors filtration
Bowman's capsule hydrostatic pressure (PB)	18	Opposes filtration
Glomerular colloid osmotic pressure (πG)	32	Opposes filtration
Bowman's capsule COP (πB)	~0	Favors filtration
Net filtration pressure	+10

Kf (filtration coefficient) = ~12.5 mL/min/mm Hg (400x higher than other capillaries)
Guyton & Hall, p. 3567-3596

Factors Increasing/Decreasing GFR

Increase GFR	Decrease GFR
↑ Afferent arteriole dilation	↑ Afferent arteriole constriction
↓ Efferent arteriole constriction (mild)	↑ Efferent arteriole dilation
↑ Blood pressure (within limits)	↑ Bowman's capsule pressure (obstruction)
↑ Kf (↑ surface area)	↑ Plasma proteins (↑ πG)

4. RENAL AUTOREGULATION

The kidneys maintain relatively constant GFR and renal blood flow despite changes in arterial pressure from ~70 to 210 mm Hg.

Autoregulation of renal blood flow and GFR (Ganong's, Fig. 37-4)

Mechanisms:

Myogenic mechanism - stretch of afferent arteriole smooth muscle → vasoconstriction
Tubuloglomerular feedback (TGF) - macula densa senses ↑ NaCl delivery → adenosine release → afferent arteriole vasoconstriction → ↓ GFR

Juxtaglomerular Apparatus (JGA)

Macula densa: specialized cells of thick ascending limb; NaCl sensor
Juxtaglomerular (JG) cells: in afferent arteriole wall; secrete renin
Extraglomerular mesangial cells (Lacis cells): communication between macula densa and JG cells
Ganong's, p. 1644-1665

5. MEASUREMENT OF GFR - RENAL CLEARANCE

Clearance formula:

C = (U × V) / P

Where: C = clearance (mL/min), U = urine concentration, V = urine flow rate (mL/min), P = plasma concentration

Ideal Marker for GFR Measurement

Must be:

Freely filtered (not bound to protein)
NOT reabsorbed
NOT secreted
Non-toxic, not metabolized

Inulin Clearance (Gold standard)

Polymer of fructose (MW ~5,200)
Cinulin = GFR = ~125 mL/min
Requires IV infusion; not practical clinically

Creatinine Clearance (Clinical use)

Endogenous product of muscle creatine metabolism
Freely filtered, not reabsorbed, slightly secreted → slightly overestimates GFR (~10%)
Normal: ~125 mL/min (males), ~110 mL/min (females)
Used to estimate GFR in clinical practice

Para-aminohippuric acid (PAH) Clearance

Measures Effective Renal Plasma Flow (ERPF)
Almost completely cleared in one pass (filtered + secreted)
CPAH ≈ ERPF = ~625 mL/min
Renal blood flow (RBF) = ERPF / (1 - hematocrit) ≈ 1,200 mL/min
Filtration fraction (FF) = GFR/ERPF = 125/625 = ~0.2 (20%)
Ganong's, p. 1404-1641

6. TUBULAR REABSORPTION AND SECRETION

Overview

Segment	Key Function
PCT	Bulk reabsorption (65-70% of Na⁺, water, HCO₃⁻, glucose, amino acids)
Thin descending Loop of Henle	Water reabsorption (osmotic)
Thick ascending Loop of Henle	Na⁺-K⁺-2Cl⁻ cotransport (impermeable to water - "diluting segment")
DCT	Na⁺ reabsorption (via NCC cotransporter); regulated
Collecting duct	Final adjustment of Na⁺, K⁺, water; ADH and aldosterone action

Proximal Convoluted Tubule (PCT)

Reabsorbs ~65% of filtered Na⁺, Cl⁻, water, K⁺, HCO₃⁻
100% glucose and amino acids reabsorbed (by secondary active transport - Na⁺-glucose cotransporter, SGLT)
Tm (transport maximum): maximum rate of tubular reabsorption
- Glucose Tm = ~375 mg/min; renal threshold = ~180-200 mg/dL plasma glucose
- Above threshold → glucosuria
Isoosmotic reabsorption (fluid remains isotonic)
Na⁺/H⁺ exchanger at apical membrane; Na⁺-K⁺-ATPase at basolateral membrane

Loop of Henle

Thin descending limb: permeable to water, NOT to solutes → fluid becomes hyperosmotic
Thick ascending limb: Na⁺-K⁺-2Cl⁻ (NKCC2) cotransporter; impermeable to water → dilutes the tubular fluid
Counter-current multiplier creates hyperosmotic medullary interstitium (up to 1200 mOsm/kg)
Loop diuretics (furosemide) block NKCC2 → prevent concentration, cause diuresis

Distal Convoluted Tubule (DCT)

NaCl cotransporter (NCC) reabsorbs Na⁺ and Cl⁻
Thiazide diuretics block NCC
Ca²⁺ reabsorption stimulated by PTH

Collecting Duct

Two cell types:
- Principal cells: Na⁺ reabsorption via ENaC (aldosterone-sensitive), K⁺ secretion
- Intercalated cells (Type A): H⁺ secretion; HCO₃⁻ reabsorption (acid excretion)
ADH (vasopressin): inserts aquaporin-2 (AQP2) into apical membrane → water reabsorption → concentrated urine
Ganong's, p. 1885-1912

7. REGULATION OF WATER EXCRETION - CONCENTRATION/DILUTION OF URINE

Countercurrent Mechanism

The medullary osmotic gradient (300-1200 mOsm/kg, cortex to papilla) is essential for producing concentrated urine.

Countercurrent Multiplier (Loop of Henle):

Thick ascending limb actively pumps NaCl into interstitium (without water)
Creates progressively higher osmolality in medulla
Thin descending limb equilibrates with hyperosmotic interstitium (loses water)

Countercurrent Exchanger (Vasa Recta):

Hairpin capillaries of juxtamedullary nephrons run parallel to Loop of Henle
Preserve medullary osmotic gradient by equilibrating without washing it out

Role of ADH (Vasopressin)

Released from posterior pituitary when plasma osmolality rises (>285 mOsm/kg) or when volume depletes
Binds V2 receptor → cAMP → PKA → inserts AQP2 water channels into collecting duct apical membrane
Water moves from tubular lumen into hyperosmotic interstitium → concentrated urine (max ~1200 mOsm/kg)
Without ADH: dilute urine (~50-100 mOsm/kg)

State	ADH	Urine
Dehydration	↑↑	Hyperosmotic (max ~1200 mOsm)
Water overload	↓↓	Hypo-osmotic (min ~50 mOsm)
Diabetes insipidus	Absent/ineffective	Large volumes, dilute urine

Ganong's, p. 1896-1908

8. REGULATION OF Na⁺ AND EXTRACELLULAR FLUID VOLUME

Renin-Angiotensin-Aldosterone System (RAAS)

↓ BP / ↓ NaCl at macula densa / ↑ sympathetics → Renin release from JG cells
Renin cleaves angiotensinogen → Angiotensin I
ACE (lung) converts Ang I → Angiotensin II
Ang II effects:
- Stimulates aldosterone from adrenal cortex → Na⁺ retention, K⁺ excretion (collecting duct)
- Efferent arteriole constriction → maintains GFR at low BP
- ADH release → water retention
- Thirst stimulation
- Na⁺/H⁺ exchange in PCT

Aldosterone

Steroid hormone from zona glomerulosa
Acts on collecting duct principal cells
Activates sgk (serum/glucocorticoid kinase) → ↑ ENaC (epithelial Na channels) at apical membrane
Net effect: ↑ Na⁺ reabsorption, ↑ K⁺ secretion, ↑ H⁺ secretion

Atrial Natriuretic Peptide (ANP)

Released from atria when volume/pressure increases
Causes natriuresis (↑ Na⁺ excretion) and diuresis
Inhibits RAAS; dilates afferent arteriole, constricts efferent → ↑ GFR
Ganong's, p. 1888-1895

9. POTASSIUM REGULATION

Most filtered K⁺ is reabsorbed in PCT (65-70%) and thick ascending limb
K⁺ is secreted by principal cells of collecting duct
Factors ↑ K⁺ secretion: aldosterone, ↑ tubular flow, metabolic alkalosis, ↑ plasma K⁺
Factors ↓ K⁺ secretion: acidosis (H⁺ competes with K⁺)
Ganong's, p. 1909-1912

10. ACID-BASE REGULATION BY KIDNEYS

Mechanism	Site	Details
HCO₃⁻ reabsorption	PCT (80-85%)	H⁺ secreted via Na⁺/H⁺ exchanger; H⁺ + HCO₃⁻ → H₂CO₃ → CO₂ + H₂O (carbonic anhydrase)
Titratable acid excretion	Distal tubule/collecting duct	H⁺ + HPO₄²⁻ → H₂PO₄⁻ (buffers H⁺ in urine)
Ammonium excretion	PCT + collecting duct	NH₃ + H⁺ → NH₄⁺; main buffer for H⁺ in acidosis

11. MICTURITION (URINATION) REFLEX

Anatomy of the Bladder

Detrusor muscle: smooth muscle; contracts to expel urine (40-60 mm Hg)
Trigone: triangular area between 2 ureteral orifices and internal urethral orifice
Internal sphincter: smooth muscle (involuntary) - tonic contraction keeps bladder closed
External sphincter: skeletal muscle (voluntary) - conscious control of urination

Innervation

Nerve	Spinal Level	Function
Pelvic nerves (parasympathetic)	S2-S4	Detrusor contraction, urination
Pudendal nerve (somatic)	S2-S4	External sphincter control
Hypogastric nerve (sympathetic)	L1-L2	Relaxes detrusor, contracts internal sphincter → storage

Micturition Reflex

Bladder fills → stretch receptors activate
Afferent signals via pelvic nerves → sacral micturition center (S2-S4)
Efferent parasympathetic signals → detrusor contraction + internal sphincter relaxation
Voluntary relaxation of external sphincter → voiding
Higher centers (pontine micturition center, cortex) can inhibit or facilitate the reflex

Cystometrogram

Bladder pressure vs. volume graph
First urge at ~150 mL; strong desire at ~300-400 mL; max capacity ~600 mL
Intravesical pressure remains low (5-15 cm H₂O) until near capacity due to detrusor plasticity
Guyton & Hall, p. 3324-3373

12. DIURETICS (Overview for 1st Year)

Class	Site of Action	Mechanism	Example
Osmotic	PCT, descending loop	↑ Osmolarity → ↓ water reabsorption	Mannitol
Carbonic anhydrase inhibitors	PCT	Blocks CA → ↓ H⁺ secretion → ↓ HCO₃⁻ reabsorption → ↑ NaHCO₃ excretion	Acetazolamide
Loop diuretics	Thick ascending limb	Block NKCC2 → ↓ NaCl reabsorption, ↓ medullary gradient	Furosemide
Thiazides	DCT	Block NCC cotransporter	Hydrochlorothiazide
K⁺-sparing	Collecting duct	Block ENaC (amiloride) or block aldosterone receptor (spironolactone)	Spironolactone, Amiloride

13. QUICK REVISION TABLE - KEY VALUES

Parameter	Normal Value
GFR	125 mL/min (180 L/day)
Urine output	~1-1.5 L/day
Renal blood flow (RBF)	~1,200 mL/min (25% of CO)
Renal plasma flow (RPF/ERPF)	~625 mL/min
Filtration fraction (FF)	~0.2 (20%)
Glucose Tm	~375 mg/min
Renal threshold for glucose	~180-200 mg/dL
Urine osmolality range	50-1200 mOsm/kg
Creatinine clearance (male)	~125 mL/min
Plasma osmolality	285-295 mOsm/kg

HIGH-YIELD EXAM POINTS FOR MBBS

Inulin is gold standard for GFR; PAH measures ERPF
Filtration fraction = GFR/RPF = 0.2
Net filtration pressure = 60 - 18 - 32 = +10 mm Hg
Thick ascending limb is impermeable to water (the "diluting segment") - site of loop diuretics
ADH acts on V2 receptors → AQP2 water channels → concentrated urine
Aldosterone → sgk → ↑ ENaC → ↑ Na⁺ reabsorption, ↑ K⁺ secretion
PCT reabsorbs glucose (100%) by secondary active transport via SGLT cotransporters
Glucosuria occurs when plasma glucose exceeds ~180-200 mg/dL (renal threshold)
JGA = macula densa + JG cells + lacis cells → controls renin secretion + TGF
Micturition reflex: sacral (S2-S4) parasympathetic → detrusor contraction; voluntary control via external sphincter (pudendal nerve)

Ganong's Review of Medical Physiology, 26th Ed. | Guyton and Hall Textbook of Medical Physiology

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