Kidney
Ren (Greek: nephros)
Also known as: kidneys, renal, nephros
Paired, bean-shaped retroperitoneal organs on the posterior abdominal wall (≈T12–L3) that filter the blood and produce urine, regulating fluid, electrolyte and acid–base balance and secreting erythropoietin, renin and calcitriol.
Overview
Each kidney is ≈11–12 cm long, 5–7 cm wide and ≈3 cm thick, weighing ≈125–170 g in men and slightly less in women. It has superior and inferior poles, convex lateral and concave medial borders, and anterior and posterior surfaces. The medial border is indented at the renal hilum, which opens into a central cavity, the renal sinus, containing the renal pelvis, calyces, branches of the renal vessels, nerves, lymphatics and fat. On section the parenchyma has an outer pale cortex and an inner darker medulla of 8–18 renal pyramids separated by renal columns; each pyramid with its overlying cortex forms a renal lobe.
The kidneys lie retroperitoneally in the paravertebral gutters, their long axes parallel to the lateral border of psoas major, with the hila facing anteromedially. The left kidney lies at ≈T11–L2/L3 and the right kidney ≈1–2 cm lower (T12–L3) because of the liver. The hila lie near the transpyloric plane (L1), the left slightly above and the right slightly below it. The kidneys move 2–3 cm vertically with respiration because they are attached to the diaphragm through fascia.
Each kidney is wrapped, from inside out, by the fibrous renal capsule, perirenal fat, renal (Gerota) fascia and pararenal fat. Approximately one million nephrons per kidney carry out filtration and tubular processing, receiving ≈20–25% of the cardiac output through the renal artery.
- Location
Retroperitoneal, posterior abdominal wall either side of the vertebral column (≈T12–L3); right kidney lower than left.
Parts & subdivisions
- Fibrous capsule, perirenal fat, renal fascia and pararenal fat — support, cushion and isolate the kidney.
- Medial slit transmitting renal vein, renal artery and renal pelvis (anterior → posterior) into the renal sinus.
- Outer granular zone containing renal corpuscles, convoluted tubules and cortical collecting ducts.
- Inner zone of renal pyramids containing loops of Henle, vasa recta and collecting ducts.
- Urine passes from renal papillae into 7–13 minor calyces, 2–3 major calyces and the funnel-shaped renal pelvis.
- ≈1 million functional units per kidney: renal corpuscle plus tubule draining into collecting ducts.
Anatomy
- Innervation
Renal plexus (from the coeliac, aorticorenal and lumbar splanchnic nerves): sympathetic fibres (T10–L1) cause vasoconstriction, increase renin release (β1) and Na⁺ reabsorption. Visceral pain afferents follow sympathetic fibres to T10–L1 → loin pain referred to the flank. Parasympathetic fibres from the vagus have little established function.
- Arterial supply
Renal artery (from the abdominal aorta at ≈L1–L2, just below the superior mesenteric artery); the longer right renal artery passes posterior to the inferior vena cava. Within the sinus it divides into ≈5 segmental (end) arteries → interlobar → arcuate → cortical radiate (interlobular) arteries → afferent arterioles → glomeruli → efferent arterioles → peritubular capillaries / vasa recta. Accessory (polar) renal arteries occur in ≈25–30% of people.
- Venous drainage
Peritubular capillaries and vasa recta → cortical radiate → arcuate → interlobar veins → renal vein → inferior vena cava. The left renal vein is longer, crosses anterior to the aorta just below the superior mesenteric artery and receives the left gonadal, left suprarenal and often left inferior phrenic veins; the right renal vein is short and drains directly into the IVC.
- Lymphatic drainage
Lateral aortic (lumbar/para-aortic) lymph nodes around the origin of the renal arteries.
- Relations
Right kidney — anterior: right suprarenal gland (superior pole), liver (separated by the hepatorenal recess/Morison pouch), descending duodenum (near the hilum), right colic flexure and small intestine. Left kidney — anterior: left suprarenal gland, stomach, spleen, tail of pancreas and splenic vessels, left colic flexure/descending colon and jejunum. Posterior (both): diaphragm (separating the upper pole from the costodiaphragmatic recess of the pleura — at risk in posterior surgical approaches), psoas major medially, quadratus lumborum, transversus abdominis aponeurosis, and the subcostal (T12), iliohypogastric and ilioinguinal (L1) nerves. The left kidney is related to the 11th and 12th ribs, the right to the 12th rib.
- Histology
Cortex: renal corpuscles, proximal and distal convoluted tubules, cortical collecting ducts and medullary rays (straight tubules and collecting ducts extending into the cortex; a medullary ray with its surrounding cortex forms a lobule). Medulla: loops of Henle, collecting ducts, vasa recta and sparse interstitium. Excretory passages (calyces, pelvis) are lined by urothelium.
- Development
Metanephros (from week 5): the ureteric bud forms the collecting system, the metanephric mesenchyme the nephrons. The kidney ascends from the pelvis to the lumbar region and rotates medially (weeks 6–9). Anomalies: pelvic kidney, horseshoe kidney (arrested by the inferior mesenteric artery), renal agenesis, duplex systems.
Function
- Filter ≈180 L of plasma per day and excrete metabolic wastes (urea, creatinine, uric acid) and drugs
- Regulate ECF volume, blood pressure and plasma osmolality
- Regulate Na⁺, K⁺, Ca²⁺, Mg²⁺, phosphate and acid–base balance
- Secrete erythropoietin, renin and calcitriol
- Gluconeogenesis and peptide-hormone degradation
- Physiology
Renal blood flow ≈1.1–1.2 L/min; renal plasma flow ≈600–650 mL/min; GFR ≈125 mL/min (≈180 L/day); filtration fraction ≈20%; >99% of filtered water and Na⁺ reabsorbed; urine output ≈1–2 L/day. RBF and GFR are autoregulated over MAP ≈80–180 mmHg.
Clinical significance
Renal angle tenderness and surface anatomy
The kidneys are palpated bimanually in thin subjects (the right lower pole is more often felt). The renal angle — between the 12th rib and the lateral border of erector spinae — is tender in pyelonephritis and obstruction. Percutaneous renal biopsy is taken from the lower pole through the posterior approach.
Horseshoe and ectopic kidneys
A horseshoe kidney (≈1 in 400–500) has fused lower poles joined across the midline by an isthmus that lies anterior to the great vessels, arrested beneath the inferior mesenteric artery. The ureters pass anterior to the isthmus, predisposing to obstruction, stones and infection.
Renal cell carcinoma
The commonest primary renal malignancy in adults (clear-cell type from PCT epithelium, VHL gene). Classic (but now uncommon) triad: haematuria, flank pain, palpable mass. It may invade the renal vein and IVC and cause paraneoplastic polycythaemia (EPO), hypercalcaemia (PTHrP) or hypertension. A left-sided tumour obstructing the left gonadal vein can cause a left varicocele that does not empty on lying down.
Mnemonics
Hilum order
VAP — Vein, Artery, Pelvis from anterior to posterior.
Test yourself
At the renal hilum, what is the usual order of the main structures from anterior to posterior?
- Renal artery, renal vein, renal pelvis
- Renal vein, renal artery, renal pelvis
- Renal pelvis, renal artery, renal vein
- Renal vein, renal pelvis, renal artery
Answer: Renal vein, renal artery, renal pelvis
VAP: the renal vein is most anterior, the renal artery is intermediate and the renal pelvis (continuing as the ureter) is most posterior.
PAH clearance approximates effective renal plasma flow because PAH is:
- Freely filtered and fully reabsorbed
- Filtered and almost completely secreted in a single pass
- Not filtered but completely reabsorbed
- Bound to albumin and excluded from the filtrate
Answer: Filtered and almost completely secreted in a single pass
PAH is filtered and then actively secreted by organic anion transporters of the PCT, so ≈90% of the plasma reaching the kidney is cleared of it in one pass. C_PAH ≈ effective RPF ≈600–650 mL/min.
A man with bilateral renal artery stenosis develops acute kidney injury after starting lisinopril. What is the mechanism?
- Afferent arteriolar constriction by bradykinin
- Direct tubular toxicity of the ACE inhibitor
- Hyperkalaemia-induced vasoconstriction
- Loss of angiotensin II-mediated efferent arteriolar constriction, lowering glomerular capillary pressure
Answer: Loss of angiotensin II-mediated efferent arteriolar constriction, lowering glomerular capillary pressure
With low renal perfusion pressure, GFR is maintained by angiotensin II constricting the efferent arteriole. Blocking angiotensin II formation dilates the efferent arteriole, glomerular capillary pressure falls and GFR collapses.
Compiled from standard anatomy and physiology references — see the Urinary System sources. For education only; not medical advice.