Hoja de repaso: Kidney Structure and Function Fundamentals

📋 Course Outline

1. Kidney macroscopic structure and regions
2. Nephron anatomy and renal corpuscles
3. Juxtaglomerular apparatus and tubular segments
4. Renal circulation and blood supply
5. Kidney functions and urine formation overview
6. Glomerular filtration forces and GFR regulation
7. Renal autoregulation neural and hormonal control
8. Tubular reabsorption transport concepts
9. Renal clearance and inulin PAH methods
10. Glomerulotubular balance and urine concentration
11. Acid base regulation hydrogen ion buffering
12. Water balance and electrolyte regulation

📖 1. Kidney macroscopic structure and regions

🔑 Key Concepts & Definitions

• Retroperitoneal location : Kidneys are positioned retroperitoneally on both sides of the vertebral column.
• Renal sinus : The renal sinus is the shallow C-shaped space that receives the renal pelvis and calyces through the hilus.
• Hilus : The hilus is the longitudinal slit on the kidney where vessels and ducts enter or leave.
• Cortex : The cortex is the outer reddish-brown kidney zone containing mainly convoluted proximal and distal tubules.
• Medulla : The medulla is the inner paler, striated kidney zone formed by renal pyramids and straight ducts.

📝 Essential Points

• Kidneys are paired, flattened, bean-shaped organs with a multi-lobed appearance.
• Each kidney weighs about 300 g, which is roughly 0.4% of total body weight.
• The hilus mainly contains the renal pelvis and its major and minor calyces, where minor calyces join to form major calyces.
• A kidney section shows two zones: cortex and medulla, with their boundary called the corticomedullary junction.
• Structures entering at the hilus include the renal artery and nerves, while structures leaving include the renal vein, lymphatic vessels, and ureter.
• The medulla’s striated look comes from medullary pyramids made of straight ducts and cribiform plate sections, ending at the papilla.

💡 Memory Hook

Cortex = convoluted (outer brown); Medulla = pyramids/straight ducts (inner pale); Hilus = gateway to pelvis/calyces.

📖 2. Nephron anatomy and renal corpuscles

🔑 Key Concepts & Definitions

• Renal corpuscle : A renal corpuscle is the nephron’s filtration unit where blood is filtered to form the initial filtrate.
• Glomerulus : A glomerulus is a tuft of capillaries inside the renal corpuscle that performs filtration under pressure.
• Afferent arteriole : An afferent arteriole is the vessel that supplies the glomerular capillaries with blood for filtration.
• Efferent arteriole : An efferent arteriole is the smaller vessel that drains the glomerular capillaries after filtration.
• Glomerular filtration barrier : The glomerular filtration barrier is the three-layer membrane system that restricts what passes from blood into Bowman’s space.

📝 Essential Points

• The glomerulus sits in the internally dilated part of the nephron and is formed by invagination of a capillary tuft.
• Glomerular capillaries receive blood from afferent arterioles and drain into smaller efferent arterioles.
• The filtration barrier has three layers: fenestrated endothelium, basement membrane, and podocyte filtration slits.
• Glomerular endothelium is fenestrated with pores of about 70–90 nm diameter.
• Podocytes form filtration slits about 25 nm wide that limit passage into the filtrate.
• Mesangial cells lie between endothelium and basement lamina and are contractile.

💡 Memory Hook

Barrier layers go Endothelium (70–90 nm) → Basement membrane → Podocyte slits (≈25 nm).

📖 3. Juxtaglomerular apparatus and tubular segments

🔑 Key Concepts & Definitions

• Juxtaglomerular apparatus : A specialized kidney structure that links the glomerulus to signals about renal blood flow and helps regulate filtration.
• Afferent arteriole : A blood vessel that carries blood into the glomerulus and strongly influences glomerular hydrostatic pressure.
• Efferent arteriole : A blood vessel that carries blood away from the glomerulus and helps determine glomerular filtration pressure.
• Tubular reabsorption : A renal process that returns needed substances from the tubule back into the peritubular capillaries.
• Tubular secretion : A renal process that moves unwanted or excess substances from blood into the tubule for elimination.

📝 Essential Points

• Urine formation relies on three processes: glomerular filtration, tubular reabsorption, and tubular secretion.
• Glomerular filtration is driven by net hydrostatic pressure from the glomerular capillary against opposing oncotic pressure and tissue resistance.
• Substances with molecular size ≤ 68,000 are filtered, while plasma proteins are not permeable across the renal membrane.
• Glomerular filtrate is produced at about 125 mL/min (≈180 L/day), with the entire plasma volume filtered ~65 times/day.
• GFR changes when filtration pressure changes, such as increased renal blood flow or decreased plasma protein increasing GFR, while hemorrhage decreasing capillary BP decreases GFR.

💡 Memory Hook

Net filtration pressure = 60 − 25 − 10 = 25 mmHg (pressure minus oncotic minus tissue resistance).

📖 4. Renal circulation and blood supply

🔑 Key Concepts & Definitions

• Glomerular filtration rate : Glomerular filtration rate is the volume of plasma filtered by the glomeruli per unit time and it depends on filtration pressure and resistance.
• Renal blood flow : Renal blood flow is the amount of blood delivered to kidney capillaries per unit time and it is linked to capillary pressure and flow resistance.
• Renal autoregulation : Renal autoregulation is the kidney’s ability to keep renal blood flow and GFR relatively constant despite changes in arterial pressure within a set range.
• Tubuloglomerular feedback : Tubuloglomerular feedback is a salt-sensing mechanism where macula densa adjusts arteriolar tone to stabilize GFR.
• Neural regulation of GFR : Neural regulation of GFR is control of GFR via sympathetic innervation of afferent and efferent arterioles.

📝 Essential Points

• Renal filtration rate is commonly expressed as about 125 mL/min, which corresponds to roughly 180 L/day.
• GFR changes when filtration pressure changes, so factors that alter renal blood flow or capillary pressure can shift GFR.
• Renal blood flow follows the relation Blood Flow = Capillary Pressure / Flow resistance.
• Kidney autoregulation maintains relatively constant RBF and GFR when arterial pressure is in the 80–180 mmHg range.
• Autoregulation limits urine changes because GFR stays stable and glomerulotubular balance increases reabsorption when GFR rises.
• Sympathetic stimulation can increase during hemorrhage and exercise, causing vasoconstriction to conserve blood volume or redirect flow to other organs.

💡 Memory Hook

Autoregulation: 80–180 mmHg keeps RBF & GFR steady—then tubuloglomerular feedback fine-tunes salt delivery.

📖 5. Kidney functions and urine formation overview

🔑 Key Concepts & Definitions

• Renal tubular reabsorption : Renal tubular reabsorption is the process by which filtered water and solutes are returned from tubular fluid back into the blood.
• Passive transport : Passive transport is solute or water movement down an electrochemical or concentration gradient without direct ATP use.
• Paracellular transport : Paracellular transport is movement of solutes and water between cells through the spaces between them.
• Transcellular transport : Transcellular transport is movement of solutes and water through cells via transporters or channels across cell membranes.
• Glucose transport maximum : Glucose transport maximum (TmG) is the highest rate at which glucose can be reabsorbed before glucose begins to appear in urine.

📝 Essential Points

• In the proximal tubule, Na+ reabsorption is isosmotic so water follows osmotically and tubular fluid osmolality stays similar to plasma.
• About 65% of water and sodium reabsorption occurs in the proximal tubule, and Cl− follows passively along the electrochemical gradient created by Na+ movement.
• In the proximal tubule, ~90% of bicarbonate, calcium, and K+ are reabsorbed, and ~99% of glucose and amino acids are reclaimed.
• Descending thin limb is water-permeable but Na−Cl-impermeable, reabsorbing ~25% of filtered H2O.
• Ascending limb is water-impermeable: thin ascending reabsorbs Na−Cl passively, while thick ascending uses Na−K−2Cl cotransport and is impermeable to H2O.

💡 Memory Hook

Proximal tubule = “bulk isosmotic reclaim”; loop of Henle = “water down, salt up” (descending water-permeable, ascending water-impermeable).

📖 6. Glomerular filtration forces and GFR regulation

🔑 Key Concepts & Definitions

• Electrochemical gradient : A driving force where ions move along their combined electrical and concentration differences created by transport activity.
• Proximal convoluted tubule : A nephron segment that reabsorbs most filtered solutes and water by high permeability and active transport.
• Renal threshold for glucose : A concentration limit above which filtered glucose cannot be fully reabsorbed and begins to appear in urine.
• Counter-current mechanism : A kidney system that uses opposing fluid flows and medullary osmotic gradients to generate concentrated urine.
• Vasa recta counter-current exchange : A blood-flow process that exchanges solutes with the medulla while preserving the medullary osmotic gradient.

📝 Essential Points

• Na+ reabsorption in the proximal tubule creates an electrochemical gradient that passively drives downstream ion and water movement.
• About 7/8 of filtered water is passively reabsorbed in the proximal tubule along with Na+ movement.
• K+ reabsorption in the proximal tubule is ~90% active, while glucose is actively reabsorbed up to the renal threshold.
• HCO3− is totally reabsorbed in the proximal tubule, and urea is passively reabsorbed there.
• In the proximal tubule, PO4− is actively reabsorbed in a Tm-dependent way, while SO4, amino acids, and trace proteins are actively reabsorbed.
• Counter-current concentration depends on loop of Henle anatomy, vasa recta, and ADH, with medullary osmotic pressure rising from ~300 mosm superficially to ~1200 mosm near the papilla.

💡 Memory Hook

PCT = “passive + active” (Na+ gradient pulls water; glucose up to threshold).

📖 7. Renal autoregulation neural and hormonal control

🔑 Key Concepts & Definitions

• Renal clearance : Renal clearance is the plasma volume cleared of a substance per unit time, reflecting excretion and (for some substances) kidney filtration and flow.
• Clearance of substance x : Clearance of substance x is computed from urine concentration, urine flow rate, and plasma concentration to quantify how much plasma is cleared per minute.
• Inulin clearance : Inulin clearance is the renal clearance of inulin, used as an experimental measure of glomerular filtration rate (GFR) because inulin is freely filtered.
• PAH clearance : PAH clearance is the renal clearance of para-aminohippurate, used to estimate renal plasma flow (RPF) because PAH is cleared efficiently by the kidneys.
• Glomerulotubular balance : Glomerulotubular balance is the tendency for tubular reabsorption to adjust in proportion to the filtered load delivered to the tubules.

📝 Essential Points

• Clearance is defined as the volume of plasma that would contain the amount of a substance excreted in urine per minute.
• Clearance equation: $C_x=\dfrac{U_x\,V}{P_x}$ where $U_x$ is urine concentration, $V$ is urine flow rate, and $P_x$ is plasma concentration.
• For GFR measurement, the test substance must be freely filtered and neither reabsorbed nor secreted, and it should not bind plasma proteins, be metabolized, stored, or alter renal blood flow.
• Inulin is the standard substance for GFR because it is freely filtered with no net reabsorption or secretion, so $C_{in}=\text{GFR}$.
• Interpretation rule for clearance vs 125 ml/min (inulin reference): $C<125$ ml/min implies reabsorption, $C=0$ implies complete reabsorption or no filtration, $C=125$ ml/min implies no net reabsorption or secretion, and
• PAH is used for renal plasma flow determination, with $C_{PAH}=\dfrac{U_{PAH}\,V}{P_{PAH}}$ and $C_{PAH}=\text{RPF}$ (renal plasma flow).

💡 Memory Hook

Clearance = “urine concentration × urine flow ÷ plasma concentration” (C = U·V/P).

📖 8. Tubular reabsorption transport concepts

🔑 Key Concepts & Definitions

• PAH clearance : A renal transport concept where para-aminohippurate is used to estimate effective renal plasma flow from its clearance.
• Effective RBF : A renal plasma-flow measure that accounts for the fraction of plasma that actually perfuses functioning nephrons.
• Glomerulotubular balance : A tubular control property where tubules raise reabsorption when tubular load or inflow increases, often alongside changes in GFR.
• Hydrogen ion regulation : The body’s acid–base control process that keeps H+ concentration near normal using buffers plus respiratory and renal mechanisms.
• Chemical buffer systems : First-line acid–base defenses that neutralize added H+ or OH− with minimal immediate pH change.

📝 Essential Points

• If clearance $C>125\,\text{ml/min}$, the substance is treated as secreted rather than only filtered.
• PAH is infused like insulin in the described protocol to measure renal handling of PAH.
• Effective RBF is computed as $\text{RPF}\times 100\,\text{ml/min}\,(100-\text{Hct})$ using hematocrit to correct plasma fraction.
• Total RBF uses a correction factor because about 10% of blood containing PAH does not pass through nephrons.
• Glomerulotubular balance occurs mainly in the PCT (and also in the loop of Henle) and is mainly for Na reabsorption.
• Chemical buffers act within seconds, respiratory buffering within 1–3 minutes, and renal buffering over hours to days.

💡 Memory Hook

PAH→RPF→(1−Hct) gives flow; then GFR↑ triggers Na reabsorption via glomerulotubular balance.

📖 9. Renal clearance and inulin PAH methods

🔑 Key Concepts & Definitions

• Renal clearance : Renal clearance is the volume of plasma cleared of a substance per unit time by the kidneys.
• Inulin clearance : Inulin clearance is the clearance of inulin used to estimate the glomerular filtration rate (GFR) under ideal handling conditions.
• PAH clearance : PAH clearance is the clearance of para-aminohippurate used to assess effective renal plasma flow (ERPF) when extraction is high.
• GFR : GFR is the rate at which the kidneys filter plasma at the glomeruli, expressed as a volume per time.
• Effective renal plasma flow : Effective renal plasma flow is the plasma flow that actually gets cleared of a marker substance across the kidneys.

📝 Essential Points

• Inulin is treated as freely filtered with no net tubular reabsorption or secretion, so its clearance tracks GFR.
• PAH is handled by the tubules with net secretion, so its clearance reflects renal plasma flow rather than filtration alone.
• A marker’s clearance depends on filtration plus net tubular handling (reabsorption and secretion), so different markers give different physiological readouts.
• When a substance is extracted almost completely during one pass, its clearance approaches the effective renal plasma flow.
• Comparing inulin clearance (filtration marker) with PAH clearance (secretion-extraction marker) helps separate filtration from plasma flow effects.

💡 Memory Hook

Inulin = “In” filtration only; PAH = “PAH” secreted out → clearance tracks flow.

📖 10. Glomerulotubular balance and urine concentration

🔑 Key Concepts & Definitions

• Glomerulotubular balance : A kidney control mechanism that keeps tubular reabsorption matched to the glomerular filtration rate (GFR) so tubular fluid composition stays relatively stable.
• Macula densa : A specialized group of cells in the distal tubule that senses Na+ concentration in the tubular filtrate and signals the juxtaglomerular apparatus.
• Juxtaglomerular apparatus : A kidney structure that releases renin and helps regulate blood pressure and electrolyte balance through the renin–angiotensin–aldosterone system.
• Natriuretic hormone : A hormone that promotes salt excretion and water loss by opposing aldosterone’s effects on renal handling of electrolytes.
• Micturition reflex : A spinal autonomic reflex that empties the bladder and produces the urge to urinate when bladder filling reaches a triggering level.

📝 Essential Points

• When Na+ concentration delivered to the macula densa increases, renin secretion from juxtaglomerular cells is inhibited.
• Higher Na+ reaching the macula densa corresponds to a faster rise in Na+ concentration in the distal tubule filtrate.
• Increased macula densa Na+ leads to decreased aldosterone secretion and decreased Na+ reabsorption, increasing urinary Na+ excretion.
• A fall in NaCl, extracellular fluid volume, and arterial blood pressure activates the renin–angiotensin–aldosterone pathway.
• ANP increases glomerular filtration rate and promotes salt excretion with effects opposite to aldosterone.
• Urination is a complex act using autonomic and somatic pathways plus reflexes modulated by higher brain centers.

💡 Memory Hook

Macula densa reads Na+: high Na+ → less renin → less aldosterone → more Na+ in urine.

📖 11. Acid base regulation hydrogen ion buffering

🔑 Key Concepts & Definitions

• Hydrogen ion buffering : A buffering system is a chemical mechanism that limits changes in hydrogen ion concentration when acids or bases are added.
• Intravesical pressure : Intravesical pressure is the pressure inside the bladder measured as it fills and stretches.
• Cystometrogram : A cystometrogram is a graph showing intravesical pressure changes as bladder volume increases.
• Law of Laplace : Law of Laplace relates pressure in a spherical-walled structure to wall tension and radius.

📝 Essential Points

• A buffer works by resisting shifts in hydrogen ion concentration rather than eliminating the added acid or base.
• Cystometrogram plots intravesical pressure versus intravesical volume.
• Ia shows an initial slight rise in pressure, followed by Ib, a long nearly flat segment.
• When bladder wall tension increases, the radius also increases, so pressure rises less during filling.
• II shows a sudden sharp pressure rise when the micturition reflex is triggered.

💡 Memory Hook

Buffer = “brake on H+ change”; Laplace = “P rises with tension, but radius can expand to blunt P.”

📖 12. Water balance and electrolyte regulation

🔑 Key Concepts & Definitions

• Micturition reflex : A spinal reflex that triggers urination when the bladder is sufficiently distended, without requiring conscious control.
• Cortical control of micturition : A higher-level brain control that gradually replaces reflex-only voiding, reaching complete control around age 3 years.
• Diabetes insipidus : A condition with reduced ADH that impairs water reabsorption, leading to excessive urine output.
• Atonic bladder : A bladder state with loss of tone where damaged bladder afferent pathways abolish reflex contractions, causing overflow dribbling.
• Automatic bladder : A bladder state caused by spinal cord damage above the sacral region where voluntary control is lost but sacral reflex centers remain intact.

📝 Essential Points

• Micturition is purely reflex in young children and occurs when bladder distension reaches a sufficient level.
• Myelination is not complete early in life, so reflex control dominates until cortical takeover begins at about 2½ years.
• Complete voluntary control is achieved at about 3 years when the cortex takes over micturition control.
• Cerebral lesions (tumors, Parkinson’s, vascular accidents) can alter bladder sensation perception and cause voiding dysfunction such as loss of control or dribbling.
• In diabetes insipidus, reduced ADH leads to impaired water conservation and increased urine production.

💡 Memory Hook

Distension → spinal reflex; cortex takes over by ~3 years.

📊 Synthesis Tables

Nephron types and cortical location

Nephron type	Location/origin	Proportion
Cortical	More superficially located; originates from outer 2/3rd of cortex	More numerous; forms 85% of total nephron
Juxta-medullary	Located deeper; originates from inner 1/3rd of cortex; longer LH	Forms 15% of total nephron

⚠️ Common Pitfalls & Confusions

1. Mixing up hilus contents: hilus is mainly renal pelvis and major/minor calyces, while renal artery/nerves enter and renal vein/lymphatic vessels/urethra leave.
2. Confusing glomerular filtration barrier layers: endothelium is fenestrated (~70–90 nm), basement membrane is between layers, and podocyte filtration slits are ~25 nm.
3. Using the wrong filtration force: net filtration pressure is (60 − 25 − 10) mmHg = 25 mmHg, not simply hydrostatic pressure alone.
4. Thinking autoregulation changes urine directly: urine changes are limited because GFR stays stable and glomerulotubular balance increases reabsorption when GFR rises.
5. Swapping loop of Henle permeability: descending limb is water-permeable but Na–Cl-impermeable, while ascending limb is water-impermeable (thin ascending Na–Cl passively; thick ascending Na–K–2Cl with H2O impermeable).
6. Misapplying clearance interpretation: C<125 ml/min implies reabsorption, C=125 ml/min implies no net reabsorption or secretion, and C>125 ml/min implies secretion.
7. For micturition, confusing reflex initiation with conscious control: the micturition reflex is spinal and can be inhibited/facilitated by higher brain centers, and young children void purely reflexly.

✅ Exam Checklist

1. State kidney macroscopic location (retroperitoneal), shape (paired, flattened, bean-shaped, multi-lobed), weight (~300 g), and define hilus and renal sinus.
2. List hilus contents entering vs leaving the kidney (renal artery and nerves enter; renal vein, lymphatic vessels, and urethra leave) and describe cortex vs medulla and corticomedullary junction.
3. Describe nephron types (cortical vs juxta-medullary), their cortical origin, and their proportions (85% vs 15%).
4. Define renal corpuscles (glomerulus + Bowman’s capsule) and explain how glomerular capillaries are supplied (afferent) and drained (efferent).
5. Reproduce the three-layer glomerular filtration barrier and the key pore/slit sizes (~70–90 nm endothelium; ~25 nm podocyte filtration slits) plus the role of mesangial cells.
6. Explain the three basic renal processes (GF, TR, TS) and the four excretion scenarios using the course’s relationships (e.g., E = GF − TR when TR but no TS).
7. Compute/recall net glomerular filtration pressure from the given forces (60 mmHg hydrostatic, 25 mmHg oncotic, 10 mmHg tissue resistance) and state the filtered molecular size limit (≤68,000) and protein impermeability.
8. State normal glomerular filtrate production rate (125 ml/min or 180 L/day) and entire plasma volume filtered (~65 times/day), and list the factors that alter filtration pressure/GFR.
9. Describe renal blood flow relation (Blood Flow = Capillary Pressure / Flow resistance) and the autoregulation pressure range (80–180 mmHg) plus why urine changes little.
10. Explain tubuloglomerular feedback: macula densa sensing near afferent/efferent arterioles, how Na+ delivery/conc affects JG renin secretion, and the link to stabilizing GFR.
11. Describe neural regulation of GFR via sympathetic innervation of afferent and efferent arterioles and the effect during hemorrhage/exercise.
12. Describe hormonal regulation of GFR including vasoconstrictors (sympathetic, angiotensin II, endothelin) and vasodilators (ANP, NO, bradykinin, prostaglandin) with the course’s stated effects.
13. Summarize urine formation transport logic: proximal tubule is isosmotic with ~65% water/Na reabsorption, key proximal reabsorption percentages (e.g., HCO3− totally; glucose/amino acids ~99%), and loop of Henle segmental:
14. State descending limb vs thin ascending vs thick ascending permeability and transport (water-permeable vs water-impermeable; Na–Cl passive vs Na–K–2Cl cotransport; H2O impermeable in ascending).