The Kidneys
The kidneys are paired retroperitoneal organs that together receive roughly 20% of cardiac output and produce ~180 L of glomerular ultrafiltrate per day, of which ~1.5 L is excreted as urine.[3][4] For the reconstructive urologist, the kidney matters as a surgical territory (the fascial compartments, vascular pedicle, and collecting system that dictate access and reconstruction), a functional reserve (the ability to lose parenchyma to obstruction, infarction, or resection without decompensation), and a pressure-sensitive end-organ whose performance depends on unobstructed outflow and perfusion pressure. This article focuses on the gross, vascular, and collecting-system anatomy most relevant to operative planning, and on the physiology needed to interpret obstruction, ischemia, and loss of nephron mass — not a full nephrology text.
OpenStax Anatomy & Physiology — Internal structure of the kidney. (CC BY 4.0)
Surgical Anatomy
Each adult kidney is ~11 × 6 × 3 cm and 125–170 g. The right kidney sits 1–2 cm lower than the left because of the liver. Relations to know in the operating field:
| Aspect | Left kidney | Right kidney |
|---|---|---|
| Anterior | Stomach, pancreas (body/tail), spleen, splenic flexure, jejunum | Liver, duodenum (2nd portion), hepatic flexure |
| Posterior | Diaphragm, psoas, quadratus lumborum, transversus abdominis; 11th–12th ribs | Same; usually only 12th rib |
| Superior | Left adrenal | Right adrenal |
Upper pole relations explain the risk of pleural injury with posterior flank access above the 12th rib and splenic / hepatic capsule injury during mobilization. Pancreatic tail injury is a recognized pitfall on the left.
Fascial compartments
Understanding the retroperitoneal envelope is essential for urinoma tracking, abscess drainage, and correctly developing planes during nephrectomy, ureterolysis, and auto-transplantation.[2]
- Renal (fibrous) capsule — tightly adherent; stripped off during decapsulation during intraparenchymal dissection or partial nephrectomy.
- Perirenal fat — the immediate surround; harbors accessory polar vessels.
- Gerota's (renal) fascia — encloses kidney, perirenal fat, and adrenal. Fuses superolaterally and medially across the midline in only a limited band, and is open inferiorly, which is why urinomas and retroperitoneal hematomas preferentially track into the pelvis rather than across to the contralateral side.
- Pararenal fat — outside Gerota's; anterior and posterior compartments.
- Transversalis fascia — the lateral limit of the retroperitoneum.
Collecting system
The papillae drain through 7–14 minor calyces, which coalesce into 2–3 major calyces (typically upper, interpolar, lower), then the renal pelvis. Calyceal anatomy varies widely and is worth reviewing on preoperative imaging before endopyelotomy, UPJ reconstruction, or percutaneous access planning. The ureteropelvic junction (UPJ) lies on the medial aspect of the pelvis and is the most common site of crossing-vessel compression.
Innervation and referred pain
Visceral sensory fibers travel with sympathetic roots T10–L1. Pain from ureteral obstruction, pyelonephritis, or parenchymal distension classically refers to the flank and radiates to the ipsilateral groin or testicle/labium majus, reflecting shared innervation with the genitofemoral and ilioinguinal nerves.
Vascular Anatomy
The vascular pedicle is the single most consequential structure in any renal operation.
- A single renal artery enters each hilum in ~70–75% of kidneys. Accessory (polar) arteries are present in 20–30% and are almost always end-arteries with no collateral supply — ligation produces a segmental infarct.
- Each renal artery divides at the hilum into anterior and posterior divisions; the avascular Brödel's line lies posterolaterally at the boundary between them and is the classical plane for intersegmental incision in open surgery and for access in endoscopic surgery.
- Segmental arteries → interlobar → arcuate (at the corticomedullary junction) → interlobular (cortical radial) → afferent arterioles. Efferent arterioles emerging from juxtamedullary glomeruli give rise to the vasa recta that supply the medulla — a detail that matters because the medulla is essentially supplied by postglomerular blood and therefore more vulnerable to ischemia than the cortex.
- Venous drainage: interlobular → arcuate → interlobar → segmental → main renal vein. The left renal vein receives the left gonadal vein, left adrenal vein, and lumbar branches, and passes anterior to the aorta between the SMA and aorta (vulnerable to nutcracker physiology). The right renal vein is short, drains directly, and usually has no tributaries.
- Lymphatics: right kidney → right paracaval/interaortocaval; left kidney → left paraaortic/preaortic. Cross-drainage is common on the right.
Accessory renal arteries are the commonest cause of surgical catastrophe at the renal hilum. On any CT urogram obtained for renal/UPJ reconstruction, count arteries and veins on both sides before proceeding, note lower-pole vessels crossing the UPJ, and plan for ligation only when the accessory artery supplies a dispensable segment that has been confirmed with intraoperative clamping and Doppler.
Nephron Overview (Clinically Relevant Level)
Each kidney contains ~1 million nephrons. Function is the sum of individual nephron performance, and this is why the kidney tolerates progressive parenchymal loss so well up to a point: single-nephron GFR rises in the surviving nephrons to offset mass loss — hyperfiltration — which buys time at the cost of accelerating later decline.
Glomerular filtration depends on three things:
- The ultrafiltration coefficient (area × hydraulic permeability of the barrier) — falls in glomerulosclerosis, diabetic nephropathy, and with nephron loss.
- The hydrostatic pressure gradient between glomerular capillary and Bowman's space — falls with ureteral obstruction (rising intra-Bowman pressure) and with renal vein obstruction or renal tamponade (rising venous pressure).
- The oncotic pressure opposing filtration — modestly altered by plasma protein concentration.
Tubular function can be collapsed, for operative purposes, into three jobs:
- Bulk reabsorption (proximal tubule) — recovers 60–70% of filtered Na⁺ and water, ~80% of filtered bicarbonate, and nearly all filtered glucose, amino acids, and low-molecular-weight proteins. Injury here manifests as low-molecular-weight proteinuria, glucosuria despite euglycemia, aminoaciduria, and phosphaturia — the Fanconi phenotype. This is the earliest laboratory signature of nephrotoxic or ischemic tubular injury after contrast, aminoglycosides, or prolonged warm ischemia.
- Concentration (loop of Henle + collecting duct) — generates and exploits the medullary osmotic gradient under vasopressin control. Loop-of-Henle injury (as in chronic obstruction or medullary ischemia) produces impaired concentration and a fixed, dilute urine output — a common finding after relief of long-standing obstruction (post-obstructive diuresis).
- Fine tuning (distal nephron) — regulated final Na⁺, K⁺, H⁺, and water handling under aldosterone and vasopressin. Relevant in interpreting post-operative electrolyte and acid-base derangements.
The outer medulla normally operates at a PₒO₂ of only 10–20 mmHg — a fraction of cortical tension — because countercurrent exchange in the vasa recta shunts oxygen from descending to ascending limbs before it ever reaches the deep medulla, while the thick ascending limb is simultaneously the most oxygen-hungry segment in the nephron. This narrow margin is why the medulla is the first territory to fail in ischemia, contrast nephropathy, aminoglycoside injury, and prolonged partial-nephrectomy clamp times.[1]
Functional reference values
| Parameter | Typical adult value |
|---|---|
| Renal blood flow | ~1 L/min (20–25% of cardiac output) |
| GFR | ~100–120 mL/min/1.73 m² |
| Filtration fraction | ~20% |
| Daily ultrafiltrate | ~180 L |
| Daily urine output | ~1.0–2.0 L |
| Minimum urine osmolality | ~50 mOsm/kg (max dilution) |
| Maximum urine osmolality | ~1,200 mOsm/kg (max concentration) |
Endocrine Output
Three hormonal roles matter operatively:
- Renin (juxtaglomerular cells) — triggered by reduced afferent-arteriolar perfusion pressure, sympathetic input, and reduced distal NaCl delivery. Clinically important in renovascular hypertension, post-clamp ischemic hypertension, and the hemodynamic swings after nephrectomy/revascularization.
- Erythropoietin (peritubular cortical fibroblasts) — released in response to hypoxia. CKD-associated anemia follows parenchymal loss; an abrupt hemoglobin drop is expected after bilateral renal mass loss.
- Active vitamin D (proximal-tubule 1α-hydroxylase) — converts 25(OH)D to 1,25(OH)₂D. Failure drives the secondary hyperparathyroidism and bone disease of advanced CKD.
Clinical Correlations for the Reconstructive Urologist
- Medullary ischemic injury and warm ischemia time. The outer medulla's chronic hypoxia is why partial nephrectomy technique targets <25 min warm ischemia and why hypothermic or zero-ischemia techniques are preferred for complex or solitary-kidney tumors.[1]
- End-arterial segmental supply. Accessory and polar renal arteries have no collaterals; inadvertent ligation during partial nephrectomy, living donor procurement, transplant, or UPJ reconstruction infarcts the corresponding wedge.
- Obstruction and backpressure nephropathy. The rigid capsule and encapsulating Gerota's fascia allow transmitted pyelonephric pressure to compress the parenchyma. Chronic partial obstruction atrophies the cortex first; decompression must be weighed against duration of obstruction, split-function, and the risk of post-obstructive diuresis.[2]
- Renal tamponade in volume overload. Elevated central venous pressure, tense ascites, or abdominal compartment syndrome collapse the pressure gradient across the encapsulated kidney and produce functional decline without any intrinsic parenchymal disease.[2]
- Split renal function assessment. MAG3 or DMSA renography quantifies the contribution of each kidney — essential before any nephrectomy in a patient with bilateral disease, before UPJ repair in a poorly-functioning unit, and in assessing recovery after obstruction relief.
- Hyperfiltration after nephrectomy. Loss of ~50% of nephron mass raises single-nephron GFR in the remaining kidney, producing a measured GFR decline of only ~30% rather than 50%. Over years, hyperfiltration can drive proteinuria and glomerulosclerosis in the remaining kidney — the reason donor nephrectomy follow-up targets blood pressure and proteinuria control.
- Low-molecular-weight proteinuria as a tubular-injury marker. Cystatin C, β₂-microglobulin, and retinol-binding protein are normally reabsorbed by the proximal tubule; their appearance in the urine signals proximal tubular injury rather than glomerular disease.[5]
- Gerota's fascia, urinoma, and abscess. Urinomas from calyceal tears, pyeloplasty leak, or percutaneous-tract extravasation are contained within Gerota's and preferentially track inferiorly toward the pelvis; abscesses can cross into the pararenal spaces once Gerota's is violated. Imaging interpretation after any upper-tract reconstruction depends on knowing these compartments.
Videos
References
1. Brezis M, Rosen S. "Hypoxia of the Renal Medulla — Its Implications for Disease." N Engl J Med. 1995;332(10):647–655. doi:10.1056/NEJM199503093321006
2. Boorsma EM, Ter Maaten JM, Voors AA, van Veldhuisen DJ. "Renal Compression in Heart Failure: The Renal Tamponade Hypothesis." JACC Heart Fail. 2022;10(3):175–183. doi:10.1016/j.jchf.2021.12.005
3. Eckardt KU, Coresh J, Devuyst O, et al. "Evolving Importance of Kidney Disease: From Subspecialty to Global Health Burden." Lancet. 2013;382(9887):158–169. doi:10.1016/S0140-6736(13)60439-0
4. Benzing T, Salant D. "Insights into Glomerular Filtration and Albuminuria." N Engl J Med. 2021;384(15):1437–1446. doi:10.1056/NEJMra1808786
5. Hoenig MP, Brooks CR, Hoorn EJ, Hall AM. "Biology of the Proximal Tubule in Body Homeostasis and Kidney Disease." Nephrol Dial Transplant. 2025;40(2):234–243. doi:10.1093/ndt/gfae177