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Dayesha Rathuwaduge

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FUNCTIONAL ANATOMY OF THE NEPHRON (Logic-Based Note)

1. Nephron: the basic working unit

• A nephron consists of one renal tubule + its glomerulus.
• Kidney size and nephron number vary across species.
• Each human kidney contains ~1 million nephrons.
• All kidney functions (filtration, reabsorption, secretion, concentration) are executed through these units.

2. Glomerulus & Bowman’s capsule: where filtration begins

Structural layout

• The glomerulus is a tuft of capillaries (~200 µm diameter) that invaginates into the blind end of the nephron.
• This blind end forms Bowman’s capsule.
• Blood flow:
• Afferent arteriole → glomerular capillaries → efferent arteriole
• Afferent arteriole diameter > efferent arteriole diameter
• This size difference helps maintain high filtration pressure.

Filtration barrier: three coordinated components

Blood is separated from filtrate by two cellular layers plus a basement membrane:

1. Capillary endothelium

• Fenestrated endothelium
• Fenestrations are 70–90 nm wide.
• Allows plasma components to approach the filtration barrier.

2. Glomerular basement membrane (GBM)

• Continuous basal lamina
• No visible pores
• Provides size and charge selectivity.

3. Podocytes (visceral epithelium of Bowman’s capsule)

• Specialized epithelial cells covering the capillaries.
• Possess numerous interdigitating pseudopodia (foot processes).
• Spaces between foot processes form filtration slits (~25 nm wide).
• Each slit is bridged by a thin slit diaphragm.

Mesangial cells: internal support and regulation

• Stellate cells located between capillary endothelium and capillary basal lamina.
• Similar to pericytes elsewhere in the body.
• Especially abundant between adjacent capillaries where a shared basement membrane exists.
• Functions:
• Contractile → regulate glomerular filtration surface area.
• Secrete extracellular matrix.
• Phagocytose immune complexes.
• Participate in progression of glomerular disease.

Filtration selectivity

• Neutral molecules:
• Freely pass ≤ 4 nm
• Almost completely excluded ≥ 8 nm
• Charge matters as much as size (negatively charged molecules are restricted).
• Total filtration surface area of human glomerular capillaries ≈ 0.8 m².

3. Tubular epithelium: structure reflects function

Each nephron segment has distinct cell types aligned with its transport role.

4. Proximal convoluted tubule (PCT)

• Length: ~15 mm
• Diameter: ~55 µm
• Lined by single layer of tightly packed epithelial cells.
• Cells:
• Interdigitate laterally.
• Joined by apical tight junctions.
• Lateral intercellular spaces extend from extracellular space.
• Luminal surface has a dense brush border (microvilli) → massively increases surface area for reabsorption.

5. Loop of Henle: structural basis of concentration

Thin segments

• Descending limb + proximal ascending limb
• Made of thin, highly permeable cells

Thick ascending limb

• Composed of thick cells rich in mitochondria
• Specialized for active solute transport

Cortical vs juxtamedullary nephrons

• Cortical nephrons
• Glomeruli in outer cortex
• Short loops of Henle
• Juxtamedullary nephrons
• Glomeruli near corticomedullary junction
• Long loops extending deep into medulla
• Crucial for urine concentration
• Only ~15% of human nephrons are juxtamedullary.

6. Juxtaglomerular apparatus (JGA)

• The thick ascending limb returns to its parent glomerulus.
• It passes between:
• Afferent arteriole
• Efferent arteriole
• Specialized cells here form the macula densa.
• Components of JGA:
• Macula densa
• Lacis cells
• Renin-secreting granular cells (in afferent arteriole)
• This apparatus links tubular NaCl sensing to renal blood flow and renin release.

7. Distal convoluted tubule (DCT)

• Begins at the macula densa
• Length: ~5 mm
• Epithelium:
• Lower cells than PCT
• Few microvilli
• No brush border
• Designed for fine regulation rather than bulk reabsorption.

8. Collecting ducts: final urine adjustment

• Distal tubules merge to form collecting ducts (~20 mm long).
• Run through cortex and medulla → open into renal pelvis at apex of medullary pyramids.

Cell types

Principal (P) cells

• Predominant cell type
• Tall cells with few organelles
• Functions:
• Na⁺ reabsorption, aldosterone dependent
• Vasopressin-dependent water reabsorption

Intercalated (I) cells

• Fewer in number
• Present in DCT and collecting ducts
• Rich in:
• Microvilli
• Cytoplasmic vesicles
• Mitochondria
• Functions:
• Acid (H⁺) secretion
• HCO₃⁻ transport

Total nephron length

• Including collecting ducts: 45–65 mm

9. Renal medullary interstitial cells (RMICs)

• Specialized fibroblast-like cells in medullary interstitium.
• Contain lipid droplets.
• High expression of:
• COX-2
• Prostaglandin E synthase
• Produce PGE₂ (major renal prostanoid).
• PGE₂:
• Key paracrine regulator of salt and water balance.
• Sources of renal prostaglandins:
• RMICs
• Macula densa
• Collecting duct cells
• Arterioles and glomeruli (PGI₂ and others)

10. Renal blood vessels: a unique portal system

Basic flow pattern

• Interlobular artery → afferent arteriole → glomerular capillaries → efferent arteriole
• Efferent arteriole then forms:
• Peritubular capillaries
• ± Vasa recta (in juxtamedullary nephrons)
• Glomerular capillaries are the only capillaries in the body that drain into arterioles.
• Thus, renal circulation is technically a portal system.
• Efferent arterioles contain little smooth muscle.

Cortical vs juxtamedullary circulation

• Cortical nephrons
• Efferent arteriole → dense peritubular capillary network
• Juxtamedullary nephrons
• Efferent arteriole → peritubular capillaries + vasa recta
• Vasa recta form hairpin loops alongside loops of Henle.

Vasa recta specialization

• Descending vasa recta
• Nonfenestrated endothelium
• Have facilitated urea transporters
• Ascending vasa recta
• Fenestrated endothelium
• Structural differences support solute conservation and countercurrent exchange.

Quantitative facts

• Tubules and renal capillaries each have surface area ≈ 12 m².
• Blood volume within renal capillaries at any time: 30–40 mL.
• A single efferent arteriole supplies multiple nephrons, not just its parent nephron.

11. Renal lymphatics

• Kidneys have abundant lymphatic drainage.
• Lymph drains into the thoracic duct, then into venous circulation.

12. Renal capsule: mechanical constraint with clinical impact

• Capsule is thin but tough.
• Limits expansion during edema.
• Increased interstitial pressure:
• ↓ GFR
• May prolong anuria in acute kidney injury (AKI).

13. Innervation of renal vessels and tubules

Autonomic supply

• Renal nerves enter along renal blood vessels.
• Composition:
• Many postganglionic sympathetic efferents
• Few afferent fibers
• Possible vagal cholinergic input, function uncertain.

Sympathetic origin and distribution

• Preganglionic neurons:
• Lower thoracic + upper lumbar spinal cord
• Postganglionic cell bodies:
• Sympathetic chain
• Superior mesenteric ganglion
• Along renal artery
• Targets:
• Afferent and efferent arterioles
• Proximal and distal tubules
• Juxtaglomerular apparatus
• Dense noradrenergic innervation of thick ascending limb

Renal sensory pathways

• Pain fibers travel with sympathetic nerves.
• Enter spinal cord via thoracic and upper lumbar dorsal roots.
• Other afferents mediate renorenal reflex:
• ↑ ureteral pressure in one kidney
• ↓ sympathetic efferent activity to opposite kidney→ ↑ Na⁺ and water excretion in contralateral kidney

Final big-picture logic

• Structure determines function at every level:
• Filtration barrier → selectivity
• Tubular cell design → transport specificity
• Vascular arrangement → pressure control and solute conservation
• Innervation → rapid regulation of renal output

RENAL CIRCULATION — LOGIC-BASED NOTE (ZERO OMISSION)

1. Renal Blood Flow — Big Picture

• In a resting adult, kidneys receive 1.2–1.3 L of blood per minute.
• This equals just under 25% of cardiac output, which is very high relative to kidney size.
• Purpose: kidneys must filter large volumes of plasma continuously.

2. Measuring Renal Blood Flow & Plasma Flow

A. Direct methods

• Can be measured using electromagnetic or other flow meters (mainly experimental).

B. Indirect method — Fick principle

• Based on:
• Amount of a substance taken up or excreted per unit time
• Divided by the arteriovenous concentration difference across the kidney.
• Requirement for the substance:
• Not metabolized, stored, or produced by the kidney
• Does not affect renal blood flow
• Red cell concentration remains unchanged during renal passage

3. Renal Plasma Flow (RPF) — Concept

• Kidney filters plasma, not whole blood.
• Therefore:
• Renal plasma flow (RPF) is calculated instead of total blood flow.
• Formula logic:
• RPF = amount taken up per minute ÷ arteriovenous plasma concentration difference

4. Why PAH Is Used to Measure RPF

Properties of PAH (p-aminohippuric acid)

• Freely filtered at the glomerulus
• Actively secreted by proximal tubules
• Almost completely removed from plasma in one renal pass at low doses

Extraction ratio

• About 90% of arterial PAH is removed in a single pass.
• Extraction ratio =

(arterial PAH − renal venous PAH) ÷ arterial PAH

5. Effective Renal Plasma Flow (ERPF)

• In practice, renal venous PAH is not measured.
• Peripheral venous plasma PAH ≈ arterial PAH entering kidney.
• Therefore, calculated value is called:
• Effective Renal Plasma Flow (ERPF)

Formula (PAH clearance)

• ERPF = (Urine PAH × urine flow rate) ÷ Plasma PAH

Example calculation

• Urine PAH = 14 mg/mL
• Urine flow = 0.9 mL/min
• Plasma PAH = 0.02 mg/mL
• ERPF = (14 × 0.9) ÷ 0.02 = 630 mL/min

Normal value

• Average ERPF ≈ 625 mL/min

6. Converting ERPF to Actual RPF

• PAH extraction ratio ≈ 0.9
• Actual RPF = ERPF ÷ extraction ratio
• Example:
• 630 ÷ 0.9 ≈ 700 mL/min

7. Calculating Renal Blood Flow from RPF

• Plasma flow excludes red cells.
• To get renal blood flow, correct for hematocrit.

Formula

• Renal blood flow = RPF ÷ (1 − hematocrit)

Example

• Hematocrit = 45% (0.45)
• Renal blood flow = 700 ÷ 0.55 ≈ 1273 mL/min

8. Pressure Profile in Renal Vessels

• When mean arterial pressure = 100 mm Hg:

Pressures

• Glomerular capillary pressure: ~45 mm Hg
• Much lower than expected from indirect estimates
• Pressure drop across glomerulus: only 1–3 mm Hg
• Efferent arteriole: major pressure drop occurs here
• Peritubular capillary pressure: ~8 mm Hg
• Renal vein pressure: ~4 mm Hg

Key concept

• Glomerular capillary pressure ≈ 40% of systemic arterial pressure
• Similar pressure gradients seen in primates and humans

9. Chemical Regulation of Renal Blood Flow

Vasoconstrictors

• Norepinephrine (noradrenaline)
• Strong renal vasoconstrictor
• Greatest effect on:
• Interlobular arteries
• Afferent arterioles
• Angiotensin II
• Constricts both afferent and efferent (MORE) arterioles

Vasodilators

• Dopamine
• Synthesized in the kidney (PCT)
• Causes renal vasodilation
• Promotes natriuresis
• Prostaglandins
• Increase cortical blood flow
• Decrease medullary blood flow
• Acetylcholine
• Produces renal vasodilation

Dietary influence

• High-protein diet
• Raises glomerular capillary pressure
• Increases renal blood flow

10. Functions of the Renal Nerves

Sympathetic effects (via norepinephrine)

1. Renin secretion
• Direct β1-adrenergic stimulation of juxtaglomerular cells
2. Increased Na⁺ reabsorption
• Likely direct tubular action
• Receptors involved: α and/or β (not fully settled)

Innervation density

• Rich innervation of:
• Proximal tubule
• Distal tubule
• Thick ascending limb of loop of Henle

11. Graded Effects of Renal Nerve Stimulation

As stimulation increases:

1. Increased sensitivity of granular (JG) cells
2. Increased renin secretion
3. Increased sodium reabsorption
4. At highest levels:
• Renal vasoconstriction
• Decreased GFR
• Decreased renal blood flow

12. Physiologic Role of Renal Nerves

• Role in Na⁺ homeostasis is not fully settled
• Reason:
• Transplanted kidneys (initially denervated) function nearly normally
• Functional innervation returns slowly over time

13. Sympathetic Tone & Reflex Control

• Strong sympathetic stimulation → marked fall in renal blood flow
• Mediated mainly by:
• α1-adrenergic receptors
• Lesser role of postsynaptic α2 receptors
• There is tonic sympathetic discharge at rest.
• During hypotension:
• Reduced baroreceptor firing
• Reflex renal vasoconstriction
• Renal blood flow decreases:
• During exercise
• When standing up from supine position

14. Autoregulation of Renal Blood Flow

Pressure range

• Renal blood flow remains constant between 90–220 mm Hg perfusion pressure.

Mechanisms

• Present in:
• Denervated kidneys
• Isolated perfused kidneys
• Abolished when vascular smooth muscle is paralyzed

Key contributors

• Myogenic response
• Stretch of afferent arteriole → smooth muscle contraction
• Nitric oxide (NO) involvement
• Angiotensin II at low pressures
• Preferential efferent arteriole constriction
• Helps maintain GFR

Clinical link

• ACE inhibitors can precipitate renal failure in low-perfusion states by removing this compensatory efferent constriction.

15. Regional Renal Blood Flow & Oxygen Use

Renal Cortex

• Function: filtration
• Blood flow: ~5 mL/g/min
• Oxygen extraction: low
• Arteriovenous O₂ difference: 14 mL/L
• PO₂ ≈ 50 mm Hg

Renal Medulla

• Function: maintain osmotic gradient
• Blood flow:
• Outer medulla: ~2.5 mL/g/min
• Inner medulla: ~0.6 mL/g/min
• High metabolic activity:
• Especially Na⁺ reabsorption in thick ascending limb
• PO₂ ≈ 15 mm Hg
• Highly vulnerable to hypoxia

Protective mediators

• NO
• Prostaglandins
• Cardiovascular peptides
• Act locally (paracrine) to balance low flow with metabolic demand

GLOMERULAR FILTRATION — LOGIC-BASED NOTE

1️⃣ What GFR Actually Means (Start Here)

• Glomerular filtration rate (GFR) is the volume of plasma ultrafiltrate formed per minute by all functioning nephrons.
• It reflects how well the kidneys are filtering blood, not how much urine is produced.

2️⃣ Principle Behind Measuring GFR (Core Logic)

To measure GFR accurately, we need a substance that behaves in a very specific way in the kidney.

An ideal GFR marker must:

• Be freely filtered at the glomerulus
• Be neither reabsorbed nor secreted by renal tubules
• Be not metabolized
• Be non-toxic

If all these are true, then:

• Amount filtered per minute = amount excreted per minute

3️⃣ Inulin — The Gold Standard

• Inulin is a fructose polymer (MW ≈ 5200).
• It satisfies all requirements of an ideal GFR marker in humans and most animals.
• Therefore:
• Filtered load = excreted load
• Its clearance equals true GFR

4️⃣ Clearance Concept (Essential Exam Logic)

Renal plasma clearance:

• Defined as the volume of plasma completely cleared of a substance per unit time
• Expressed in mL/min

Clearance formula (for any substance X):

• Clearance of X (CX) =(Urine concentration of X × Urine flow rate) / Plasma concentration of X

So for GFR measurement:

• GFR = CX

5️⃣ Practical Measurement of Inulin Clearance

Procedure:

1. Loading dose of inulin IV
2. Continuous infusion to maintain constant plasma level
3. Allow time for equilibration
4. Collect:
• Timed urine sample
• Plasma sample halfway through collection
5. Measure inulin concentration in urine and plasma

Example calculation:

• U_inulin = 35 mg/mL
• V̇ = 0.9 mL/min
• P_inulin = 0.25 mg/mL
• Clearance = (35 × 0.9) / 0.25
• GFR = 126 mL/min

6️⃣ Creatinine Clearance — Clinical Substitute

• Creatinine is:
• Freely filtered
• Slightly secreted by tubules
• Therefore:
• Creatinine clearance slightly overestimates GFR
• Despite this:
• Endogenous creatinine clearance is a reasonable clinical estimate
• Most commonly used clinical marker:
• Plasma creatinine (PCr)
• Normal ≈ 1 mg/dL

7️⃣ Normal GFR — Quantitative Perspective

• Average healthy adult:
• ≈ 125 mL/min
• After surface area correction:
• Women ≈ 10% lower than men
• Daily filtration:
• 125 mL/min = 180 L/day
• Normal urine output:
• ≈ 1 L/day
• Therefore:
• >99% of filtrate is reabsorbed

Scale comparison:

• Kidneys filter per day:
• 4 × total body water
• 15 × ECF volume
• 60 × plasma volume

8️⃣ Determinants of GFR — Starling Forces Applied

GFR follows the same principles as capillary filtration elsewhere.

Per nephron equation:

GFR =Kf × [(PGC − PT) − (πGC − πT)]

Where:

• Kf = Ultrafiltration coefficient
• Permeability × surface area
• PGC = Glomerular capillary hydrostatic pressure
• PT = Hydrostatic pressure in Bowman’s space
• πGC = Plasma oncotic pressure
• πT = Oncotic pressure in Bowman’s space (normally negligible)

9️⃣ Glomerular Permeability — Size & Charge Selectivity

Size selectivity:

• < 4 nm → freely filtered
• 4–8 nm → progressively reduced filtration
• 8 nm → almost no filtration

Charge selectivity:

• Glomerular basement membrane contains negatively charged sialoproteins
• Consequences:
• Anionic molecules filtered less than neutral ones
• Cationic molecules filtered more than neutral ones

Albumin example:

• Diameter ≈ 7 nm
• Despite size, filtration is only 0.2% of plasma level
• Reason: negative charge repulsion

🔟 Protein Handling & Albuminuria

• Normal urine protein:
• <100 mg/day
• Mostly from shed tubular cells
• Albuminuria = abnormal albumin in urine
• In nephritis:
• Loss of negative charges in GBM
• Albuminuria can occur without pore enlargement

1️⃣1️⃣ Size of Capillary Bed — Role of Mesangial Cells

• Mesangial cell contraction ↓ Kf
• Mechanisms:
• Reduced filtration surface area
• Distortion and narrowing of capillary loops
• Flow diverted away from some capillary loops

Important regulator:

• Angiotensin II
• Acts directly on mesangial cells
• Receptors present in glomeruli
• Mesangial cells may also produce renin

1️⃣2️⃣ Hydrostatic & Oncotic Pressures — Why GFR Is High

• PGC is high because:
• Afferent arteriole is short and straight
• Efferent arteriole has high resistance
• Opposing forces:
• PT (Bowman’s space pressure)
• Plasma oncotic pressure (πGC)

Along the capillary:

• Fluid filtration ↑ plasma protein concentration
• πGC rises progressively
• Net filtration pressure:
• Starts ≈ 15 mmHg
• Falls to zero before efferent end
• Result:
• Filtration equilibrium reached
• Filtration becomes flow-limited, not diffusion-limited

Effect of renal plasma flow:

• ↑ RPF → slower rise in πGC→ filtration continues over a longer capillary length→ GFR increases

1️⃣4️⃣ Changes in GFR — Predictable Effects

• Autoregulation stabilizes GFR within normal BP range
• Below autoregulatory range:
• GFR falls sharply
• Efferent constriction > afferent constriction:
• Helps maintain GFR
• But reduces tubular blood flow

1️⃣5️⃣ Filtration Fraction (FF)

• FF = GFR / RPF
• Normal:
• 0.16–0.20
• Key concept:
• GFR varies less than RPF
• In hypotension:
• RPF ↓ more than GFR
• Due to efferent constriction
• → FF increases

1️⃣6️⃣ Mesangial Cell Modulators (Complete List)

Cause contraction:

• Endothelins
• Angiotensin II
• Vasopressin
• Norepinephrine
• Platelet-activating factor
• Platelet-derived growth factor
• Thromboxane A₂
• PGF₂
• Leukotrienes C₄ & D₄
• Histamine

Cause relaxation:

• ANP
• Dopamine
• PGE₂
• cAMP

1️⃣7️⃣ Factors Affecting GFR — Full List

• Renal blood flow changes
• Glomerular capillary hydrostatic pressure
• Systemic blood pressure
• Afferent or efferent arteriolar constriction
• Bowman’s capsule pressure
• Ureteric obstruction
• Renal edema inside tight capsule
• Plasma protein concentration
• Dehydration
• Hypoproteinemia (minor effect)
• Changes in Kf
• Permeability
• Effective filtration surface area

🔒 FINAL EXAM LOCK

GFR is determined by filtration pressure and Kf; it is measured by clearance of a freely filtered, non-secreted, non-reabsorbed substance (inulin), normally ~125 mL/min, tightly regulated by arteriolar tone, mesangial cells, and plasma oncotic forces.

TUBULAR FUNCTION — LOGIC-BASED MASTER NOTE

1️⃣ Core Accounting Principle of Tubular Handling

For any substance X:

• Filtered load = GFR × Plasma concentration of X
• After filtration, the tubules can:
• Add X to filtrate → tubular secretion
• Remove X from filtrate → tubular reabsorption
• Do both

Excretion rate of X:

• = Filtered amount + Net tubular transfer (TX)

Clearance logic

• Clearance = GFR → no net secretion or reabsorption
• Clearance > GFR → net tubular secretion
• Clearance < GFR → net tubular reabsorption

This framework explains why substances like inulin, glucose, and PAH behave differently.

2️⃣ How Tubular Transport Is Studied (Evidence Base)

Understanding tubular function comes from:

• Micropuncture of live nephrons
• In vivo tubule perfusion
• Isolated perfused tubule segments
• Tubular cell culture
• Molecular cloning of transporters

These methods revealed segment-specific transport mechanisms.

3️⃣ Mechanisms of Tubular Reabsorption & Secretion

Transport modes

• Endocytosis → small proteins & peptide hormones (proximal tubule)
• Passive diffusion (paracellular & transcellular)
• Facilitated diffusion
• Active transport (primary & secondary)

Transport machinery

• Channels
• Exchangers
• Cotransporters
• Pumps

Polarity is critical

• Luminal membrane transporters ≠ basolateral transporters
• This polarized distribution allows net vectorial transport
• Same principle as intestinal epithelium

4️⃣ Transport Maximum (Tm) Concept

• Active transport systems have a maximum rate (Tm)
• Below Tm → transport ∝ filtered load
• Above Tm → saturation, no further increase
• Some systems have very high Tm → hard to saturate

This explains threshold phenomena (e.g., glucose).

5️⃣ Paracellular Transport & “Leaky” Epithelium

• Renal tubules (especially proximal tubule) are leaky
• Tight junctions allow water & electrolytes through
• Contribution of paracellular transport is significant

Clinical proof:

• Paracellin-1 (tight junction protein) → Mg²⁺ reabsorption
• Loss-of-function mutation → Mg²⁺ + Ca²⁺ wasting

6️⃣ Sodium Reabsorption — Central Organizer

Why Na⁺ matters

• Determines ECF volume, osmolarity, and water balance
• Drives cotransport of:
• H⁺
• Glucose
• Amino acids
• Phosphate
• Organic acids

Universal principle

• Na⁺ enters cells down electrochemical gradient
• Na⁺ exits cells via Na⁺/K⁺-ATPase (basolateral membrane)
• Pumps 3 Na⁺ out / 2 K⁺ in

7️⃣ Segment-Wise Na⁺ Handling (Must-Know Percentages)

• Aldosterone regulates only the final 3%
• Thin limb of loop of Henle → no active Na⁺ pumping

8️⃣ Glucose Reabsorption — Model Secondary Active Transport

Basic facts

• Filtered ≈ 100 mg/min
• Normally almost 100% reabsorbed
• Urinary glucose negligible

Transport maximum

• TmG:
• Men ≈ 375 mg/min
• Women ≈ 300 mg/min

Renal threshold paradox

• Predicted threshold ≈ 300 mg/dL
• Actual threshold ≈ 200 mg/dL arterial
• Cause → splay

Splay explained

• Nephrons have variable Tm
• Transporters differ in affinity
• Earlier glucose loss in some nephrons

9️⃣ Glucose Transport Mechanism

Apical membrane:

• SGLT-2 (Na⁺-glucose cotransporter)
• Transports D-glucose only

Basolateral membrane:

• GLUT-2 → facilitated diffusion

Additional:

• SGLT-1 & GLUT-1 (minor role)
• Phlorhizin competitively inhibits SGLT

🔟 Other Secondary Active Transport Examples

Amino acids

• Proximal tubule
• Apical Na⁺-dependent cotransport
• Basolateral Na⁺-independent exit

Chloride

• Reabsorbed with Na⁺ & K⁺ in TAL
• Cl channels:
• ClC-Kb
• Mutations → Dent disease
• Causes hypercalciuria + kidney stones

1️⃣1️⃣ PAH Transport — Prototype of Secretion

• PAH is:
• Filtered
• Actively secreted
• Secretion reaches TmPAH
• As plasma PAH ↑:
• Clearance initially high
• Falls toward inulin clearance

Used clinically to estimate ERPF

1️⃣2️⃣ Tubuloglomerular Feedback (TGF)

Purpose

• Stabilizes distal NaCl delivery

Sensor

• Macula densa

Mechanism

1. ↑ Flow → ↑ NaCl at macula densa
2. Na–K–2Cl entry ↑
3. Na⁺/K⁺-ATPase ↑ → ATP breakdown
4. ↑ Adenosine release
5. Adenosine → A1 receptors
6. ↑ Ca²⁺ in afferent arteriole
7. Afferent vasoconstriction
8. ↓ GFR

Also suppresses renin release (mechanism incompletely defined)

1️⃣3️⃣ Glomerulotubular Balance (GTB)

• ↑ GFR → ↑ proximal reabsorption
• % reabsorbed stays constant
• Especially prominent for Na⁺

Mechanism

• ↑ Oncotic pressure in peritubular capillaries
• Promotes Na⁺ & water reabsorption
• Occurs within seconds (intrarenal)

1️⃣4️⃣ Water Handling — Big Picture

• Filtered/day ≈ 180 L
• Urine/day ≈ 1 L
• ≥87% water reabsorbed always
• Final urine volume varies without altering solute excretion
• Vasopressin is key regulator

1️⃣5️⃣ Aquaporins

Renal aquaporins:

• AQP-1 → proximal tubule, descending limb
• AQP-2 → collecting duct (vasopressin-regulated)
• AQP-3, AQP-4 → basolateral CD

1️⃣6️⃣ Proximal Tubule Water Handling

• Fluid remains iso-osmotic
• Due to AQP-1 on both membranes
• By end of PT:
• 60–70% solute reabsorbed
• 60–70% water reabsorbed
• TF/P inulin ≈ 2.5–3.3

AQP-1 knockout:

• ↓ water permeability by 80%
• Impaired urine concentration

1️⃣7️⃣ Loop of Henle — Dilution & Concentration

Descending limb

• Water permeable (AQP-1)
• Solute impermeable
• Fluid becomes hypertonic

Ascending limb

• Water impermeable
• Na–K–2Cl active transport from apical membrane to inside cell
• Fluid becomes hypotonic

By distal tubule:

• TF/P inulin ≈ 5
• ~20% filtered water remains

1️⃣8️⃣ Distal Tubule

• Extension of thick ascending limb
• Water impermeable
• Further dilution of tubular fluid

1️⃣9️⃣ Collecting Ducts

With vasopressin

• AQP-2 inserted into apical membrane
• Via V2 receptor → cAMP → PKA
• Cytoskeleton dependent
• Up to 99.7% water reabsorbed
• Urine osmolality up to 1400 mOsm/kg

Without vasopressin

• CD water impermeable
• Urine osmolality as low as 30 mOsm/kg
• Urine flow up to 15 mL/min

2️⃣0️⃣ Countercurrent Mechanism

Countercurrent multiplication

• Loop of Henle
• Generates medullary gradient

Countercurrent exchange

• Vasa recta
• Preserves gradient

Longer loops → higher concentrating ability

2️⃣1️⃣ Role of Urea

• Contributes significantly to medullary hypertonicity
• Transporters:
• UT-A1, UT-A3 (CD, vasopressin-regulated)
• UT-B (RBCs, vasa recta)

High protein diet → ↑ urea → ↑ concentrating ability

2️⃣2️⃣ Osmotic Diuresis

Cause:

• Non-reabsorbed solutes retain water

Effects:

• ↓ Proximal Na⁺ reabsorption
• ↓ Medullary gradient
• ↑ Urine volume
• ↑ Na⁺ loss

Examples:

• Mannitol
• Glucosuria (diabetes mellitus)
• Excess NaCl or urea infusion

2️⃣3️⃣ Osmotic vs Water Diuresis

2️⃣4️⃣ GFR & Urine Concentration

• ↓ GFR → ↓ loop flow
• ↑ Medullary gradient
• ↑ Urine concentration
• Can concentrate urine without vasopressin

2️⃣5️⃣ Free Water Clearance (CH₂O)

CH₂O = Urine flow − Osmolar clearance(how much plasma is needed to account for the amount of solute excreted in urine each minute.)

• Negative → concentrated urine
• Positive → dilute urine

Values:

• Max antidiuresis: –1.3 mL/min
• No vasopressin: +14.5 mL/min

2️⃣6️⃣ Tubular Secretion — Clinical Relevance

Secreted substances:

• PAH derivatives
• Penicillin
• Iodinated dyes
• Steroid & glucuronide conjugates
• 5-HIAA

2️⃣7️⃣ Diuretics & Tubular Secretion

• Loop diuretics & thiazides are organic anions
• Secreted by proximal tubule
• Must reach lumen to act
• Thiazides act by inhibiting Na–Cl cotransport in the distal tubule

2️⃣8️⃣ Genetic Transporter Disorders

Bartter syndrome

• Defect in TAL transport
• Causes:
• Na⁺ wasting
• Hypovolemia
• ↑ Renin & aldosterone
• No hypertension
• Metabolic alkalosis

Genes involved:

• Na–K–2Cl cotransporter
• ROMK-put K+ to lumen
• ClC-Kb
• Barttin

Barttin mutation → deafness

Polycystic Kidney Disease

• PKD-1 & PKD-2 mutations
• Abnormal Ca²⁺ signaling
• Progressive cyst formation → renal failure

REGULATION OF Na⁺, WATER, AND K⁺ EXCRETION — LOGIC NOTE

I. REGULATION OF Na⁺ EXCRETION

1. Core physiological logic

• Sodium is freely filtered at the glomerulus.
• ~99% of filtered Na⁺ is reabsorbed along the nephron.
• Na⁺ is actively reabsorbed in all segments except the thin descending limb.
• Because Na⁺ is:
• The most abundant extracellular cation
• Responsible for >90% of plasma and interstitial osmotic pressure
• → Total body Na⁺ determines ECF volume.

📌 Therefore: Na⁺ balance = ECF volume control.

2. Functional outcome

• The kidney matches Na⁺ excretion to Na⁺ intake over a very wide range.
• Outcomes:
• High Na⁺ intake / saline infusion → natriuresis
• ECF depletion (vomiting, diarrhea) → Na⁺ retention
• Urinary Na⁺ output range:
• <1 mEq/day (low-salt diet)
• >400 mEq/day (high-salt intake)

3. Mechanisms controlling Na⁺ excretion

Na⁺ excretion varies by altering:

A. Glomerular filtration

• Changes in GFR
• Includes effects of tubuloglomerular feedback

B. Tubular Na⁺ reabsorption

• Most regulation occurs in the final ~3% of Na⁺ reaching the collecting ducts
• This small fraction is where fine control happens

II. EFFECTS OF ADRENOCORTICAL STEROIDS (ALDOSTERONE)

1. Primary actions

• Mineralocorticoids (especially aldosterone) cause:
• ↑ Na⁺ reabsorption
• ↑ K⁺ secretion
• ↑ H⁺ secretion
• Na⁺ reabsorption often coupled with Cl⁻

2. Time course of action

• Latency of 10–30 minutes
• Reason:
• Aldosterone acts via nuclear receptors
• Alters DNA transcription → protein synthesis
• Rapid membrane effects may exist but do not significantly affect whole-animal Na⁺ excretion

3. Site and molecular mechanism

• Major site: collecting ducts
• Mechanism:
• ↑ number and activity of ENaC channels, ROMK @ principal cell
• Enhances Na⁺ entry from tubular lumen, K + secretion

4. Pathological correlation — Liddle syndrome

• Mutations in β (most common) or γ ENaC subunits
• Channels become constitutively active
• Results in:
• Excess Na⁺ retention
• Hypertension
• Low aldosterone levels (feedback suppression)

III. OTHER HUMORAL FACTORS AFFECTING Na⁺ EXCRETION

1. Aldosterone modulation by diet

• Low salt intake → ↑ aldosterone
• Produces marked but slow Na⁺ retention

2. Natriuretic substances

Prostaglandin E₂ (PGE₂)

• Causes natriuresis
• Mechanisms:
• Inhibits Na⁺/K⁺-ATPase
• ↑ intracellular Ca²⁺ → inhibits ENaC

Endothelin & IL-1

• Cause natriuresis
• Likely via ↑ PGE₂ production

Atrial Natriuretic Peptide (ANP)

• ↑ intracellular cGMP
• cGMP inhibits ENaC-mediated Na⁺ transport

Endogenous ouabain-like substance

• Inhibits Na⁺/K⁺-ATPase
• Increases Na⁺ and HCO₃⁻ excretion
• Acts mainly on proximal tubules

3. Renin–angiotensin system contribution

• Kidneys:
• Contain significant ACE
• Convert ~20% of circulating angiotensin I → angiotensin II
• Also produce angiotensin I locally

4. Aldosterone escape phenomenon(protective)

• Chronic mineralocorticoid excess:
• Does NOT cause edema in normal individuals
• Reason:
• Kidneys escape aldosterone effects
• Likely mediated by ANP
• Escape is impaired or absent in:
• Nephrotic syndrome
• Cirrhosis
• Heart failure
• These patients → persistent Na⁺ retention and edema

IV. REGULATION OF WATER EXCRETION

A. WATER DIURESIS

1. Control mechanism

• Governed by vasopressin (ADH)
• ADH secretion:
• ↑ with ↑ plasma osmolality
• ↓ with ↓ plasma osmolality

2. Timeline after water ingestion

• Begins: ~15 minutes
• Peaks: ~40 minutes
• Mechanisms:
• Small early inhibition from oropharyngeal reflex
• Major inhibition after water absorption lowers plasma osmolality

B. WATER INTOXICATION

1. Physiological limit

• Maximum water excretion during diuresis:
• ~16 mL/min
• Intake beyond this → cellular swelling

2. Clinical consequences

• Brain cell swelling →
• Convulsions
• Coma
• Death (rare)

3. Situations causing water intoxication

• Excess water intake with:
• Exogenous vasopressin
• Non-osmotic ADH release (surgery, trauma)
• Oxytocin administration postpartum (if water intake not monitored)

V. REGULATION OF K⁺ EXCRETION

1. Segmental handling

• Proximal tubule:
• Active K⁺ reabsorption
• Distal tubule & collecting duct:
• K⁺ secretion

2. Determinants of K⁺ secretion

• Proportional to tubular flow rate
• High flow:
• Prevents luminal K⁺ accumulation
• Sustains secretion due to washout by highflow

3. Electrical and ionic coupling

• Na⁺ reabsorption →↓ intracellular negativity
• Favors K⁺ movement into lumen
• Less Na⁺ delivery distally →↓ K⁺ excretion decreased delivery

4. Interaction with H⁺ secretion

• ↑ H⁺ secretion →↓ K⁺ secretion
• Mechanism:
• K⁺ reabsorbed in exchange for H⁺ via H⁺/K⁺-ATPase

VI. DIURETICS — MECHANISM-BASED OVERVIEW

1. General principle

• Diuretics modify:
• Na⁺ transport
• Water handling
• K⁺ balance
• Studying them explains renal regulation logic

2. Key diuretic classes and actions

Carbonic anhydrase inhibitors (e.g., acetazolamide)

• ↓ H⁺ secretion
• ↓ Na⁺ reabsorption
• ↓ HCO₃⁻ reabsorption
• ↑ K⁺ excretion (due to ↓ H⁺ competition)

Loop diuretics (furosemide, bumetanide, ethacrynic acid)

• Inhibit Na⁺-K⁺-2Cl⁻ cotransporter
• Site: thick ascending limb
• Cause:
• Profound natriuresis
• Kaliuresis

Thiazides

• Inhibit Na⁺-Cl⁻ cotransporter
• Site: early distal tubule
• Less potent than loop diuretics
• Increase Na⁺ delivery to collecting ducts → ↑ K⁺ loss

K⁺-sparing diuretics

• Act in collecting ducts
• Mechanisms:
• Block aldosterone (spironolactone)
• Block ENaC (amiloride, triamterene)
• Prevent K⁺ loss

VII. EFFECTS OF DISORDERED KIDNEY FUNCTION

A. Loss of concentrating & diluting ability

• Early disease:
• Polyuria
• Nocturia
• Advanced disease:
• Urine osmolality fixed near plasma
• Causes:
• Loss of nephrons
• Disruption of countercurrent mechanism
• Remaining nephrons:
• Hyperfilter → osmotic diuresis
• Progressive damage → nephron loss → renal failure

B. Uremia

• Accumulation of toxic metabolites
• Symptoms:
• Lethargy, anorexia
• Nausea, vomiting
• Confusion, seizures, coma
• BUN & creatinine:
• Severity markers, not primary toxins
• Treatment:
• Dialysis
• Kidney transplantation (definitive)

C. Acidosis

• Due to:
• Reduced total H⁺ excretion
• ↓ NH₄⁺ production
• In most CKD:
• Urine maximally acidified
• Renal tubular acidosis:
• Specific defect in acidification

D. Abnormal Na⁺ handling & edema

• Causes:
1. ↓ GFR (acute glomerulonephritis)
2. ↑ Aldosterone (nephrotic syndrome)
3. Heart failure (often secondary to renal disease)

VIII. PROTEINURIA — CLINICAL LOGIC

• Due to ↑ glomerular permeability
• Mainly albumin
• Severe in nephrosis:
• Protein loss > synthesis
• Hypoproteinemia
• ↓ oncotic pressure
• ↓ plasma volume + edema

Benign condition

• Orthostatic albuminuria
• Proteinuria only when standing
• Normal urine when supine

✅ Final take-home logic

• Na⁺ → ECF volume → BP
• Water → plasma osmolality → ADH
• K⁺ → flow, Na⁺ delivery, acid–base balance
• Kidney disease disrupts all three axes progressively.

Bladder Physiology – Filling, Emptying & Neural Control

1️⃣ BLADDER FILLING (Storage Phase)

Ureteral Transport of Urine

• Ureter walls contain smooth muscle bundles arranged spirally, longitudinally, and circularly, but no distinct muscle layers.
• Peristaltic waves (1–5 per minute) propel urine from the renal pelvis to the bladder.
• Urine enters the bladder in spurts, synchronized with each peristaltic contraction.

Anti-reflux Mechanism

• Ureters pass obliquely through the bladder wall.
• There are no true ureteric sphincters, but the oblique intramural course:
• Keeps ureters collapsed during bladder filling
• Opens only during peristaltic waves
• Prevents vesicoureteral reflux

2️⃣ BLADDER EMPTYING (Micturition Phase)

Bladder Muscle Arrangement

• Bladder smooth muscle fibers are arranged:
• Spiral
• Longitudinal
• Circular
• The circular muscle layer forms the detrusor muscle, which is the main force for bladder emptying.

Urethral Sphincters

• Smooth muscle bundles pass on either side of the urethra:
• Sometimes called the internal urethral sphincter
• Do not form a complete ring
• Not essential for micturition
• Further down is the external urethral sphincter:
• Made of skeletal muscle
• Located at the membranous urethra
• Under voluntary control

Additional Anatomical Points

• Bladder epithelium:
• Superficial flat cells
• Deep cuboidal cells
• Bladder innervation is summarized in standard physiology diagrams (e.g., pelvic nerve pathways).

3️⃣ BASIC PRINCIPLES OF MICTURITION

• Micturition is fundamentally a spinal reflex.
• It is:
• Facilitated or inhibited by higher brain centers
• Voluntarily modulated, similar to defecation

Pressure–Volume Relationship

• Urine enters the bladder with minimal rise in intravesical pressure until the bladder is well filled.
• This is due to bladder plasticity:
• When stretched, initial tension falls back instead of being maintained.

4️⃣ CYSTOMETRY & CYSTOMETROGRAM

Cystometry

• Bladder is emptied via catheter.
• Refilled in 50 mL increments.
• Intravesical pressure is recorded at each step.

Cystometrogram

• Plot of bladder pressure vs bladder volume
• Shows three distinct segments:

Segment Ia

• Initial slight pressure rise with early filling.

Segment Ib

• Long, nearly flat phase despite increasing volume.
• Explained by Laplace’s law:
• Pressure = 2 × wall tension / radius
• As bladder fills:
• Wall tension ↑
• Radius ↑
• Pressure remains low

Segment II

• Sudden sharp pressure rise
• Represents activation of the micturition reflex

Sensations of Filling

• First urge to void: ~150 mL
• Strong fullness: ~400 mL

5️⃣ EVENTS DURING MICTURITION

• External urethral sphincter relaxes
• Perineal muscles relax
• Detrusor muscle contracts
• Urine flows through urethra

Role of Smooth Muscle Bands Near Urethra

• These bands:
• Do not contribute to voiding
• In males, their key role is preventing retrograde ejaculation into the bladder

6️⃣ VOLUNTARY CONTROL OF URINATION

• Exact initiating mechanism is not fully understood.
• Likely initial step:
• Relaxation of pelvic floor muscles
• Causes downward pull on bladder → triggers detrusor contraction

Voluntary Actions

• External sphincter and perineal muscles can be:
• Contracted to prevent urine flow
• Contracted to interrupt urination
• Learned ability to maintain sphincter contraction allows delayed voiding in adults

Post-void Differences

• Female urethra: empties by gravity
• Male urethra: residual urine expelled by bulbocavernosus muscle contractions

7️⃣ REFLEX CONTROL OF MICTURITION

Intrinsic vs Reflex Activity

• Bladder smooth muscle has intrinsic contractility.
• However, stretch-induced reflex contraction has a lower threshold when nerves are intact.

Neural Pathway

• Afferent limb: pelvic nerves (from stretch receptors)
• Efferent limb: parasympathetic fibers to bladder
• Integration center: sacral spinal cord

Threshold

• Normal reflex contraction begins at 300–400 mL bladder volume in adults.

Role of Sympathetic Nerves

• No role in micturition
• In males:
• Cause bladder neck contraction
• Prevent semen from entering bladder during ejaculation

8️⃣ BRAINSTEM & CORTICAL MODULATION

Brainstem Centers

• Pontine facilitatory center
• Midbrain inhibitory center
• Posterior hypothalamus also facilitates micturition

Effects of Lesions

• Transection above pons:
• Lowers reflex threshold
• Less filling triggers voiding
• Transection at top of midbrain:
• Reflex threshold remains normal

Cortical Influence

• Lesions of superior frontal gyrus:
• Reduced urge to urinate(chu bara enne nan)
• Difficulty stopping micturition once started
• Other cortical areas also modulate voiding.

Voluntary Initiation

• Bladder can be made to contract voluntarily even with very small urine volumes.
• Abdominal muscle contraction:
• Increases intra-abdominal pressure
• Aids voiding but is not essential

9️⃣ EFFECTS OF DEAFFERENTATION (Afferent Loss Only)

• Occurs when sacral dorsal roots are cut or damaged (e.g., tabes dorsalis).
• Consequences:
• Reflex contractions abolished
• Bladder becomes:
• Distended
• Thin-walled
• Hypotonic
• Some contractions still occur due to intrinsic smooth muscle stretch response

🔟 EFFECTS OF DENERVATION (Afferent + Efferent Loss)

• Seen with tumors of cauda equina or filum terminale.
• Initial phase:
• Flaccid, distended bladder
• Later phase:
• Development of spontaneous contraction waves
• Dribbling of urine
• Bladder becomes:
• Small
• Shrunken
• Hypertrophied

Mechanism

• Likely due to denervation hypersensitivity
• Occurs even though nerves lost are preganglionic
• Explains difference from pure afferent loss (mechanism not fully understood)

1️⃣1️⃣ EFFECTS OF SPINAL CORD TRANSECTION

During Spinal Shock

• Bladder:
• Flaccid
• Unresponsive
• Overfills
• Leads to overflow incontinence

After Spinal Shock

• Voiding reflex returns
• No voluntary control
• No higher center modulation

Clinical Variants

• Some patients trigger voiding via mass reflexes (e.g., thigh stimulation)
• Reflex may become hyperactive:
• Reduced bladder capacity
• Hypertrophied bladder wall
• Called spastic neurogenic bladder
• Infection worsens reflex hyperactivity

1️⃣2️⃣ CLINICAL SUMMARY – ABNORMAL MICTURITION

Three major neurogenic bladder types:

1. Afferent nerve interruption
2. Afferent + efferent interruption
3. Loss of descending brain control

Common feature in all:

• Bladder contracts
• Incomplete emptying
• Residual urine remains