01 — Gross Anatomy
Outer Structure
The major anatomical regions visible in a coronal cross-section.
01.1
Fibrous Capsule
ProtectionA tough, fibrous outer coat of connective tissue that envelops the entire kidney. It protects the organ from trauma and infection, and can be stripped away surgically as a smooth, glistening layer. It is continuous with the outer coat of the ureter at the hilum.
01.2
Renal Cortex
Outer Zone · FiltrationThe outermost solid region of the kidney, roughly 1–1.5 cm thick, with a granular appearance due to the dense packing of glomeruli and proximal and distal tubules. It extends inward between the medullary pyramids as the renal columns of Bertin. Most filtration begins here.
01.3
Renal Medulla
Inner Zone · ConcentrationThe inner region of the kidney, divided into 8–18 conical renal pyramids. The medulla contains the loops of Henle and collecting ducts, which are responsible for concentrating urine by establishing an osmotic gradient from the corticomedullary junction to the papillary tip.
01.4
Renal Pyramids
Medullary StructureCone-shaped sections of the medulla with their base at the corticomedullary junction and their apex (the papilla) pointing toward the renal sinus. Each pyramid contains parallel arrays of collecting ducts and loops of Henle. A single pyramid together with its cortical cap forms one renal lobe.
01.5
Renal Columns
Cortical ExtensionsInward projections of cortical tissue that separate adjacent medullary pyramids. Also called the columns of Bertin, they carry interlobar blood vessels between the cortex and the renal sinus. They contain the same cortical tissue — glomeruli and convoluted tubules — as the outer cortex.
01.6
Renal Sinus
Central SpaceA fat-filled central cavity at the hilum that houses the renal pelvis, major and minor calyces, branches of the renal artery and vein, lymphatics, and nerves. The sinus provides a protected space for the kidney's plumbing to converge before exiting through the hilum.
01.7
Renal Pelvis
Urine CollectionA funnel-shaped expansion of the upper ureter that sits within the renal sinus. It receives urine from the major calyces and channels it into the ureter. Its walls contain smooth muscle that contracts in peristaltic waves to propel urine toward the bladder against gravity if needed.
01.8
Calyces
Urine FunnellingCup-shaped extensions that surround the tips of the renal papillae. Minor calyces (8–12 per kidney) each cup a single papilla and drain into 2–3 major calyces, which in turn drain into the renal pelvis. They are lined with transitional epithelium (urothelium) that can stretch as urine accumulates.
01.9
Renal Papilla
Drainage ApexThe pointed tip of each medullary pyramid, perforated by 10–25 collecting duct openings — the area cribrosa. Urine drips from these openings into the surrounding minor calyx. The papillae are vulnerable to ischaemic necrosis in certain conditions such as diabetes and analgesic overuse.
01.10
Hilum
Entry PointA concave notch on the medial border of the kidney through which the renal artery enters, the renal vein and ureter exit, and lymphatics and nerves pass. The order from front to back is typically: renal vein, renal artery, ureter — a useful anatomical mnemonic (VAU).
02 — The Functional Unit
The Nephron
Each kidney contains over one million nephrons — microscopic tubules that perform the actual work of filtration, reabsorption, and secretion.
02.1
Renal Corpuscle
Filtration UnitThe beginning of each nephron, located in the cortex. It consists of the glomerulus (a tuft of capillaries) enclosed within Bowman's capsule. Blood pressure forces small molecules from the blood into the capsular space, creating the glomerular filtrate — around 180 litres per day.
02.2
Glomerulus
Pressure FiltrationA dense knot of fenestrated capillaries fed by an afferent arteriole and drained by a narrower efferent arteriole. The size difference between the two arterioles creates elevated hydrostatic pressure within the glomerulus, driving filtration. The filtration membrane consists of three layers: fenestrated endothelium, the glomerular basement membrane, and podocyte foot processes.
02.3
Bowman's Capsule
Filtrate CollectionA double-walled cup that surrounds the glomerulus. The visceral layer is made up of specialised cells called podocytes, whose interdigitating foot processes create filtration slits. The parietal layer forms the outer wall. Filtrate collects in the urinary space between the two layers before entering the proximal tubule.
02.4
Proximal Convoluted Tubule
Bulk ReabsorptionThe first and longest segment of the tubule, located in the cortex. Its cells are packed with mitochondria and lined with a brush border of microvilli to maximise surface area. Around 65–70% of the filtrate is reabsorbed here — including virtually all glucose and amino acids, most sodium, chloride, and water, and bicarbonate.
02.5
Loop of Henle
Concentration GradientA hairpin-shaped tubule that dips down into the medulla and returns to the cortex. The descending limb is permeable to water but not solutes; the ascending limb actively pumps out NaCl but is impermeable to water. This countercurrent mechanism creates the osmotic gradient in the medulla that drives urine concentration.
02.6
Distal Convoluted Tubule
Fine RegulationA shorter, coiled segment in the cortex that performs fine-tuning of filtrate composition. It responds to aldosterone (which promotes sodium reabsorption and potassium secretion) and parathyroid hormone (which regulates calcium). The DCT ends by connecting to a collecting duct.
02.7
Collecting Duct
Final ConcentrationMultiple nephrons drain into a single collecting duct, which runs through the medulla toward the papilla. Antidiuretic hormone (ADH/vasopressin) controls the permeability of collecting duct cells to water — when ADH is high, water is reabsorbed and urine becomes concentrated; when ADH is low, dilute urine is produced.
02.8
Juxtaglomerular Apparatus
Pressure SensingA specialised region where the distal tubule curves back to contact its own glomerulus. It comprises the macula densa (salt-sensing cells in the tubule wall), juxtaglomerular cells (modified smooth muscle that secrete renin), and extraglomerular mesangial cells. Together they regulate GFR and trigger the renin-angiotensin-aldosterone system.
03 — Vascular Architecture
Blood Supply
The kidney receives about 20–25% of cardiac output — a remarkable proportion for its size.
03.1
Renal Artery
Arterial InflowA direct branch of the abdominal aorta, the renal artery enters the kidney at the hilum and divides into anterior and posterior divisions supplying distinct vascular segments. The right renal artery is longer and passes posterior to the inferior vena cava. Segmental arteries are end-arteries — occlusion causes infarction of that segment.
03.2
Interlobar & Arcuate Arteries
Distributing VesselsSegmental arteries branch into interlobar arteries that run between renal pyramids within the renal columns. At the corticomedullary junction these arch over the pyramid bases as arcuate arteries, from which interlobular (cortical radiate) arteries ascend into the cortex to supply individual glomeruli via afferent arterioles.
03.3
Peritubular Capillaries & Vasa Recta
Post-filtration NetworkAfter leaving the glomerulus, efferent arterioles form two capillary networks. Around cortical nephrons they form peritubular capillaries that closely surround the tubules and reabsorb the substances recovered from the filtrate. Around juxtamedullary nephrons they form the vasa recta — long, straight loops that dip into the medulla and are essential for maintaining the osmotic gradient without washing it away.
03.4
Renal Vein
Venous DrainageBlood leaves the kidney via the renal vein, which exits at the hilum and drains into the inferior vena cava. The left renal vein is approximately three times longer than the right and crosses the aorta anteriorly — it also receives the left gonadal vein and left suprarenal vein, making it clinically relevant in cases of nutcracker syndrome.
04 — How It All Works
Key Renal Processes
Urine formation is a three-stage process of filtration, selective reabsorption, and secretion.
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Glomerular Filtration Blood pressure drives water, ions, glucose, amino acids, urea, and creatinine through the glomerular filtration membrane into Bowman's space. Large proteins and red blood cells are retained. This produces around 180 L of filtrate per day — 99% of which will be reclaimed downstream.
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Tubular Reabsorption As filtrate travels through the proximal tubule, loop of Henle, distal tubule, and collecting duct, the vast majority of water and useful solutes are transported back into the peritubular capillaries. Glucose and amino acids are completely reabsorbed; sodium, potassium, and bicarbonate are reabsorbed to varying degrees under hormonal control.
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Tubular Secretion Cells lining the tubules actively secrete certain substances from the peritubular capillaries into the tubular fluid — including hydrogen ions, potassium ions, ammonia, drugs, and toxins. This is a vital mechanism for eliminating substances that were not filtered at the glomerulus and for fine-tuning blood pH.
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Urine Concentration (Countercurrent Mechanism) The loop of Henle and vasa recta work together as a countercurrent multiplier and exchanger. By actively pumping NaCl from the ascending limb, the medullary interstitium becomes progressively hypertonic toward the papilla. ADH then opens aquaporin channels in the collecting duct, allowing water to flow out — concentrating urine up to 1200 mOsm/kg.
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Acid–Base Regulation The kidneys maintain blood pH between 7.35 and 7.45 by excreting hydrogen ions and regenerating bicarbonate. In acidosis, tubular cells secrete more H⁺ and produce ammonium (NH₄⁺) as a buffer. In alkalosis, bicarbonate is excreted. This renal compensation is slower than respiratory compensation but more complete.
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Renin–Angiotensin–Aldosterone System (RAAS) When blood pressure falls or sodium is low, juxtaglomerular cells release renin. Renin cleaves angiotensinogen to angiotensin I, which is converted to angiotensin II in the lungs. Angiotensin II constricts efferent arterioles (raising GFR), stimulates aldosterone secretion (increasing sodium and water retention), and promotes thirst — restoring blood pressure.
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Erythropoietin Secretion Peritubular fibroblasts in the cortex and outer medulla sense low oxygen tension and secrete erythropoietin (EPO), a hormone that stimulates red blood cell production in the bone marrow. Chronic kidney disease reduces EPO output, causing the normocytic anaemia characteristic of renal failure.
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Vitamin D Activation The kidney performs the final, rate-limiting step in vitamin D activation. The enzyme 1α-hydroxylase converts 25-hydroxyvitamin D (produced in the liver) to active 1,25-dihydroxyvitamin D (calcitriol). Calcitriol promotes intestinal calcium absorption and bone mineralisation; its production is stimulated by PTH and low serum calcium.
05 — Did You Know
Notable Kidney Facts
Remarkable Reserve
A person can live a normal, healthy life with a single kidney. The remaining kidney undergoes compensatory hypertrophy — it grows larger and increases its filtration rate to around 75% of the original two-kidney capacity within weeks. This is why living kidney donation is medically feasible.
Filtration Scale
The kidneys filter the entire blood volume approximately 40 times per day. Of the 180 litres of filtrate produced, only 1–1.5 litres leaves the body as urine. The rest — including virtually all glucose, amino acids, and most water — is selectively reclaimed by the tubules. A momentary failure to reabsorb glucose causes glycosuria, the hallmark of uncontrolled diabetes.
Nephron Loss
Humans are born with their full complement of nephrons — roughly 700,000 to over 1 million per kidney — and cannot generate new ones after birth. Nephron number declines with age; after 40, the kidneys lose about 1% of their filtering capacity per year. Low birth weight is associated with fewer nephrons and higher lifelong risk of hypertension and chronic kidney disease.
Kidney Stones
Renal calculi (kidney stones) form when urine becomes supersaturated with stone-forming salts — most commonly calcium oxalate. Stones smaller than 5 mm usually pass spontaneously; larger ones may require lithotripsy (shockwave fragmentation) or surgical removal. The pain caused by a stone moving through the ureter — renal colic — is frequently described as among the most severe pain a human can experience.
Blood Pressure & the Kidney
The relationship between the kidney and blood pressure is bidirectional: high blood pressure damages renal vessels and reduces GFR, while damaged kidneys drive blood pressure higher through RAAS activation and fluid retention — a vicious cycle. Approximately 80–85% of cases of secondary hypertension have a renal cause. ACE inhibitors and angiotensin receptor blockers are first-line therapies specifically because they break this cycle.