Kidney Anatomy, Function, and Filtration Mechanisms: A Comprehensive Review

Primary functions of the kidney

Filters blood, balances salts and water, helps regulate pH and blood pressure, conserves nutrients, eliminates metabolic wastes/toxins/drugs, acts as a sensory organ, and releases hormones.

Why is 25% of cardiac output to the kidney important?

It means the kidney is highly perfused and plays a major role in continuously filtering the blood.

Cortex

Outer kidney region; primary site of blood filtration.

Medulla

Inner kidney region; important for urine concentration and medullary osmotic gradients.

Renal hilum

Main entry/exit point for renal vessels, pelvis, and nerves.

Renal pelvis / calyx

Urine drainage pathway carrying urine away from the nephron/medullary collecting system.

Fundamental filtration unit of the kidney

The nephron.

How many nephrons are in one kidney?

About 1 million.

Two main nephron categories

Cortical nephrons and juxtamedullary nephrons.

Core nephron parts to know

Glomerulus, Bowman's capsule, proximal convoluted tubule (PCT), loop of Henle, distal convoluted tubule (DCT), macula densa, collecting duct.

Glomerulus

Capillary tuft where blood filtration begins.

Bowman's capsule

Receives filtrate from the glomerulus and feeds it into the tubule.

Three-layer glomerular filtration barrier

Fenestrated capillaries, basement membrane, and podocyte slit diaphragm.

Fenestrated capillaries

Leaky endothelial filtration layer that allows small molecules/fluid to move out.

Basement membrane

Size/charge barrier within the filtration system.

Podocyte slit diaphragm

Final cellular spacing/barrier layer contributing to filtration selectivity.

Glomerular filtration rate (GFR)

Measure of kidney filtration function.

Main determinants of GFR

Hydraulic pressure, osmotic/oncotic pressure, and hydraulic permeability.

Afferent constriction effect on GFR

Decreases GFR.

Efferent constriction effect on glomerular pressure/GFR

Can increase glomerular pressure and raise GFR.

How much fluid do kidneys filter and reabsorb per day?

About 180 L/day filtered and about 178.5 L/day reabsorbed (~99%).

Typical urine output per day

About 1.5 L/day.

Two main GFR autoregulatory mechanisms

Myogenic autoregulation and tubuloglomerular feedback.

Myogenic autoregulation

Pressure/stretch response in arterioles that helps stabilize GFR.

Tubuloglomerular feedback (TGF)

Macula densa senses NaCl/flow and adjusts afferent arteriolar tone to help stabilize GFR.

Macula densa location

Associated with the distal convoluted tubule near the nephron vascular pole.

What does the macula densa sense?

NaCl concentration and flow in the distal tubule.

High NaCl at the macula densa

ATP/adenosine signaling causes afferent vasoconstriction and decreases GFR.

Low NaCl at the macula densa

NO plus renin-support signaling leads to vasodilation and increased/restored GFR.

Overall purpose of macula densa feedback

To help keep filtration rate relatively constant.

Sympathetic stimulation effect on renal blood flow/GFR

Causes afferent vasoconstriction and decreases GFR.

Stress/norepinephrine effect on kidney filtration

Increases vasoconstriction and lowers GFR.

PCT major job

Bulk reclamation/reabsorption of sodium, bicarbonate, glucose, and water.

Descending limb major job

Water reclamation.

Ascending limb / TAL major job

Salt reclamation without major water reclamation.

DCT major job

Fine salt control.

Collecting duct major job

Final hormonally regulated Na+/K+/water handling.

Where does the highest sodium reabsorption occur?

In the proximal convoluted tubule (PCT).

By the time fluid reaches the DCT/macula densa, how much sodium has already been reabsorbed?

Most of it; roughly 80-90% has already been reclaimed.

Why is the distal nephron important even though less sodium is left?

Because it fine-tunes the final salt balance.

NHE3

Na+/H+ exchanger in the PCT; tied to bicarbonate handling.

Carbonic anhydrase location/concept

PCT bicarbonate-handling mechanism linked to sodium reclamation.

SGLT2 location/concept

PCT sodium-glucose cotransporter involved in glucose and sodium reabsorption.

GLUT transporters in kidney study context

Return reabsorbed glucose from tubular cells toward blood after SGLT2-mediated uptake.

NKCC2

Na+-K+-2Cl− symporter in the thick ascending limb; loop diuretic target.

NCC

Na+-Cl− cotransporter in the distal convoluted tubule; thiazide target.

ENaC

Epithelial sodium channel in the collecting duct; potassium-sparing diuretic target.

Na+/K+ ATPase overall kidney concept

Basolateral energy-dependent engine that powers many tubular transport processes.

Aquaporin overall kidney concept

Water channel mediating water reclamation; know concept more than specific subtype mapping.

Aldosterone overall nephron site

Acts in the collecting duct to increase sodium reabsorption and affect potassium handling.

Where is most water reclaimed?

PCT and descending limb.

Why does water follow sodium?

Because water moves osmotically where sodium goes if water permeability allows it.

Why do natriuretics often become diuretics?

Blocking sodium reclamation keeps sodium in the tubule, and water follows osmotically.

Aquaporins high-yield point

They mediate water reclamation; do not over-focus on memorizing every subtype location.

What should each drug flashcard include?

Class, where it acts, mechanism of action, prototype drug, and broad indication/use.

What is a diuretic?

A drug that increases urine formation.

What is a natriuretic?

A drug/effect that increases sodium excretion.

What is an aquaretic?

A drug/effect that increases water excretion.

Best diuretic summary study slide concept

Know where each class acts, how it works there, and what solute/water process it changes.

Carbonic anhydrase inhibitors

Class: Carbonic anhydrase inhibitors | Where it acts: PCT | MOA: Inhibits carbonic anhydrase, decreasing NaHCO3 reabsorption and causing diuresis | Prototype: Acetazolamide | Broad use: Diuresis / bicarbonate-handling concept | High-yield note: think metabolic acidosis concept.

SGLT2 inhibitors

Class: SGLT2 inhibitors | Where it acts: PCT | MOA: Blocks SGLT2, decreasing glucose reabsorption and paired sodium reabsorption | Prototype/class identifier: Canagliflozin / gliflozins | Broad use: Type 2 diabetes with mild diuresis | High-yield note: focus on generics, not brand names.

Loop diuretics

Class: Loop diuretics | Where it acts: Thick ascending limb (TAL) | MOA: Blocks NKCC2, decreasing major NaCl reabsorption and causing strong diuresis | Prototype: Furosemide | Broad use: Edema, especially acute pulmonary edema | High-yield note: one of the most effective diuretic classes.

Thiazides

Class: Thiazides | Where it acts: DCT | MOA: Blocks NCC, decreasing NaCl reabsorption | Prototype: Hydrochlorothiazide (HCTZ) | Broad use: Diuresis / hypertension | High-yield note: hyperglycemia is one side effect worth recognizing.

Potassium-sparing diuretics — ENaC blockers

Class: Potassium-sparing diuretics (ENaC blockers) | Where it acts: Collecting duct | MOA: Blocks ENaC, decreasing sodium reabsorption while relatively sparing potassium | Prototype: Amiloride | Broad use: Often combined with thiazides or loop diuretics to offset potassium loss.

Potassium-sparing diuretics — aldosterone antagonists

Class: Potassium-sparing diuretics (aldosterone antagonists) | Where it acts: Collecting duct | MOA: Blocks aldosterone/mineralocorticoid signaling, decreasing sodium reabsorption and altering potassium handling | Prototype: Spironolactone | Broad use: Opposes aldosterone effects and helps limit potassium loss with other diuretics.

Osmotic diuretics

Class: Osmotic diuretics | Where it acts: Thin descending limb (major) and PCT (minor) | MOA: Osmotic effect keeps water in the tubule and promotes water loss | Prototype: Mannitol | Broad use: Water diuresis / water-handling concept.

RAAS

Renin-angiotensin-aldosterone system; regulates arterial blood pressure, vasoconstriction, renal sodium reabsorption, and fluid balance.

RAAS sequence

Angiotensinogen → renin → angiotensin I → ACE → angiotensin II.

Where is angiotensinogen made?

In the liver.

Where does renin come from in this course framing?

From the kidney.

What converts angiotensin I to angiotensin II?

ACE.

AT1 receptor

Main RAAS receptor to associate with vasoconstriction.

AT2 receptor

Counter-regulatory receptor concept associated with vasodilation in the course framing.

Three major renin-regulation pathways

Macula densa pathway, intrarenal baroreceptor pathway, and beta-adrenergic pathway.

Macula densa effect on renin

High NaCl decreases renin release; low NaCl increases renin release.

RAAS inhibitors — big three classes

Direct renin inhibitors, ACE inhibitors, and ARBs.

ACE inhibitors

Class: ACE inhibitors | Where it acts: ACE step in RAAS (Ang I → Ang II) | MOA: Blocks conversion of angiotensin I to angiotensin II and increases bradykinin by preventing its breakdown | Prototype: Enalapril | Broad use: Hypertension and cardiovascular/renal protection | High-yield adverse effects: hypotension, cough, hyperkalemia.

ACE inhibitor high-yield prodrug concept

Enalapril is the key ACE inhibitor to know; conceptually it is a prodrug used to improve bioavailability before conversion to the active form.

ARBs

Class: Angiotensin receptor blockers (ARBs) | Where it acts: AT1 receptor in RAAS | MOA: Blocks AT1-mediated vasoconstrictive signaling | Prototype/class identifier: Sartans | Broad use: Hypertension and renal/cardiovascular protective contexts.

Direct renin inhibitors

Class: Direct renin inhibitors (DRIs) | Where it acts: First RAAS step at renin's action on angiotensinogen | MOA: Directly inhibits renin, blocking formation of angiotensin I | Prototype: Aliskiren | Broad use: Hypertension.

Two big kidney drug-disposition mechanisms

Metabolism and transport/excretion.

Glomerulus in drug handling

Major site of filtration.

PCT in drug handling

Major site of active secretion/transport and bulk reclamation concepts.

Loop/countercurrent concept in drug handling

Countercurrent mechanisms can contribute to concentration concepts within the tubular lumen.

DME in kidney zones

Drug-metabolizing enzymes.

High-yield CYP kidney concept

The kidney can contribute to CYP-mediated drug metabolism; examples noted as class exemplars include CYP3A5 and CYP2B6.

What did he say about busy slides in the condensed review deck?

He tried to remove really busy slides he would not ask questions from anyway.

What did he say about the review deck and the exam?

The condensed review deck contains what he is going to use and put on the exam.

What did he say about exam style?

It should be straightforward if you know your core concepts.

What did he say about prototype drugs in red?

They are representative class drugs he wants you to know.

What did he say about brand names?

Do not focus on them; focus on generic names/class patterns.

What did he say not to over-study?

Do not over-study giant side-effect lists, detailed PK trivia, or every busy speaker-note detail.

What should you prioritize from the review transcript?

Nephron structure, three-layer filtration barrier, macula densa/TGF, sodium and water handling by region, transporter-location matches, diuretic sites/MOA/prototypes, and RAAS basics.

If a question asks where the highest sodium reabsorption occurs, what is the answer and why?

Proximal convoluted tubule (PCT), because it is the major bulk reclamation site for sodium and other solutes.

If a question asks which nephron region is associated with water reclamation via aquaporin concept, what is the core answer?

Descending limb (and broadly the water-reclamation portions of the nephron).

If a question asks where fine salt control happens, what is the core answer?

Distal convoluted tubule.

If a question asks where aldosterone acts, what is the core answer?

Collecting duct.

If a question asks which diuretic class works at NKCC2, what is the answer?

Loop diuretics.

If a question asks which diuretic class works at NCC, what is the answer?

Thiazides.

If a question asks which diuretic class works at ENaC, what is the answer?

Potassium-sparing diuretics (ENaC blockers, e.g., amiloride).