Kidney and Urinary Tract Anatomy: Nephron Structure, Renal Blood Supply, and Clinical Correlations

The kidneys are retroperitoneal organs at T12-L3 that filter approximately 180 liters of plasma per day through roughly 1 million nephrons per kidney. Each nephron consists of a glomerulus (filtration), proximal convoluted tubule (reabsorption of 65% of filtered sodium, water, and all glucose), loop of Henle (concentration gradient in the medulla), distal convoluted tubule (fine-tuning of electrolytes), and collecting duct (final water reabsorption under ADH control). The renal blood supply is unique — it has two capillary beds in series (glomerular and peritubular), receiving about 25% of cardiac output despite the kidneys being less than 1% of body weight. The ureters have three points of constriction where kidney stones most commonly lodge: the ureteropelvic junction, the crossing of the iliac vessels, and the ureterovesical junction.

The kidneys sit against the posterior abdominal wall in the retroperitoneal space, embedded in perirenal fat and enclosed in the renal fascia (Gerota's fascia). The right kidney sits slightly lower than the left because the liver pushes it down — a useful fact for ultrasound localization and for exams that show you a cross-sectional image and ask you to identify the side. Each kidney has a hilum on its medial surface where the renal artery enters, the renal vein and ureter exit, and lymphatics and nerves pass. The structures at the hilum from anterior to posterior are: renal vein, renal artery, renal pelvis (VAP — vein, artery, pelvis from front to back). This order matters for surgical approaches — the vein is encountered first during an anterior nephrectomy. Internally, the kidney is divided into an outer cortex and an inner medulla. The cortex contains the glomeruli (the filtration units) and the convoluted tubules. The medulla contains the loops of Henle and collecting ducts, organized into cone-shaped renal pyramids. The tip of each pyramid (the renal papilla) projects into a minor calyx, which drains into a major calyx, which drains into the renal pelvis, which becomes the ureter. The urine pathway: nephron → collecting duct → papilla → minor calyx → major calyx → renal pelvis → ureter → bladder. The renal columns (of Bertin) are extensions of cortical tissue that project inward between the pyramids. These are sometimes mistaken for masses on imaging — a common radiology pitfall called a column of Bertin hypertrophy.

Each kidney contains approximately 1 million nephrons, and you cannot grow new ones — the nephrons you are born with are all you will ever have. This is why chronic kidney damage is cumulative and irreversible. The glomerulus is a tuft of capillaries enclosed in Bowman's capsule. Blood enters through the afferent arteriole (wider) and exits through the efferent arteriole (narrower). The diameter difference creates high pressure within the glomerular capillaries — about 55 mmHg — which drives filtration. The filtrate (essentially plasma minus large proteins) enters Bowman's space and flows into the proximal convoluted tubule. The proximal convoluted tubule (PCT) is the workhorse of reabsorption. It reclaims approximately 65% of filtered sodium and water, all filtered glucose and amino acids, and most filtered bicarbonate. The cells are packed with mitochondria (for active transport energy) and have a brush border of microvilli that massively increases surface area. When the PCT is damaged (by toxins, ischemia, or medications like aminoglycosides), the kidney loses its ability to reclaim these substances — resulting in glucosuria (glucose in urine) and aminoaciduria even with normal blood levels. The loop of Henle dips into the medulla and creates the concentration gradient that allows the kidney to produce concentrated urine. The descending limb is permeable to water (water leaves, making the filtrate more concentrated). The ascending limb is impermeable to water but actively pumps out sodium and chloride (the thick ascending limb contains the Na-K-2Cl cotransporter — the target of loop diuretics like furosemide). This countercurrent multiplication system is what allows humans to produce urine 4 times more concentrated than plasma. The distal convoluted tubule (DCT) fine-tunes sodium and potassium balance under aldosterone control, and reclaims calcium under parathyroid hormone control. The collecting duct performs final water reabsorption controlled by ADH (antidiuretic hormone) — when ADH is present, aquaporin-2 channels insert into the collecting duct membrane and water follows the concentration gradient back into the medullary interstitium. Without ADH (as in diabetes insipidus), the collecting duct is impermeable to water and you produce massive amounts of dilute urine. AnatomyIQ has interactive nephron diagrams that let you trace filtrate from glomerulus to collecting duct and see which substances are reabsorbed or secreted at each segment.

The kidneys receive 20-25% of cardiac output — approximately 1.2 liters per minute — despite comprising less than 1% of body weight. This disproportionate blood flow exists because the kidneys are filtering organs, not just metabolic ones. They need to process a large volume of blood to effectively regulate fluid balance, electrolytes, and waste removal. The blood supply follows a unique pattern with two capillary beds in series. Blood arrives via the renal artery, which branches into segmental arteries, then interlobar arteries (running between the pyramids), then arcuate arteries (arching along the corticomedullary junction), then interlobular arteries (radiating outward through the cortex), then afferent arterioles (one per glomerulus). The first capillary bed is the glomerulus — a high-pressure filtration bed. Blood exits via the efferent arteriole (not a venule — this is the key). The efferent arteriole then feeds into the second capillary bed: the peritubular capillaries that surround the proximal and distal tubules in the cortex. These low-pressure capillaries reabsorb the water and solutes that the tubules reclaim from the filtrate. For juxtamedullary nephrons (the 15% of nephrons with long loops dipping deep into the medulla), the efferent arterioles form vasa recta — long, straight vessels that parallel the loop of Henle and play a critical role in maintaining the medullary concentration gradient. This two-bed system is elegant: high pressure in the glomerulus drives filtration, then low pressure in the peritubular capillaries drives reabsorption. If pressure changes in either bed (from dehydration, heart failure, or NSAIDs constricting the afferent arteriole), filtration and reabsorption both shift — which is why kidney function is so sensitive to blood pressure and medication effects. This content is for educational purposes only and does not constitute medical advice.

The ureters are 25-30 cm muscular tubes that carry urine from the renal pelvis to the bladder by peristalsis. They run retroperitoneally, crossing the psoas major and entering the bladder at the trigone. The ureter passes under the uterine artery in females (water under the bridge — the ureter runs below the artery) and under the vas deferens in males. These crossing relationships matter surgically — the ureter is at risk of injury during hysterectomy and pelvic surgery. The three points of ureteral constriction are where kidney stones most commonly get stuck: (1) the ureteropelvic junction (UPJ) — where the renal pelvis narrows to become the ureter, (2) the pelvic brim — where the ureter crosses the bifurcation of the common iliac artery, and (3) the ureterovesical junction (UVJ) — where the ureter enters the bladder wall. The UVJ is the narrowest point and the most common site for stone impaction. Kidney stone pain (renal colic) is classically described as severe, colicky flank pain that radiates to the groin and inner thigh. The pain follows the dermatome pattern of the ureteral innervation (T11-L2). A stone in the upper ureter causes flank pain. A stone in the mid ureter causes pain radiating to the lower abdomen. A stone at the UVJ causes pain radiating to the groin, and the patient may also experience urinary frequency and urgency because the inflamed distal ureter irritates the bladder. The shifting location of pain as the stone moves distally is a classic exam scenario.