A pair of fist-sized organs seated against the back of the abdominal wall, the kidneys filter around 180 litres of blood per day — silently regulating the chemistry of life. This guide maps every key structure and what it does.
| 180L | Blood filtered daily |
| 1M+ | Nephrons per kidney |
| 1.5L | Urine produced daily |
| ~150g | Average kidney weight |
The major anatomical regions visible in a coronal cross-section.
A 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.
The 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.
The 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.
Cone-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.
Inward 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.
A 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.
A 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.
Cup-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.
The 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.
A 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).
Each kidney contains over one million nephrons — microscopic tubules that perform the actual work of filtration, reabsorption, and secretion.
The 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.
A 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.
A 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.
The 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.
A 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.
A 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.
Multiple 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.
A 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.
The kidney receives about 20–25% of cardiac output — a remarkable proportion for its size.
A 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.
Segmental 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.
After 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.
Blood 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.
Urine formation is a three-stage process of filtration, selective reabsorption, and secretion.
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.
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.
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.
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.
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.
Human Kidney Anatomy — A Field Guide — Educational reference · Nephrology & Human Anatomy