Gross Anatomy of the Kidney

Core Concepts

Position and supportive layers

Each kidney lies retroperitoneal at about T12–L3, with the right kidney slightly lower than the left because the liver sits above it. Three layers surround each kidney, from inside out. The renal capsule is a thin, tough fibrous coat directly on the kidney surface; it protects the organ and helps contain infection. Next is the adipose capsule (perirenal fat), a cushion that protects the kidney and helps hold it in place. Outermost is the renal fascia, connective tissue that anchors the kidney to the abdominal wall. Rapid weight loss can shrink the fat cushion; when support weakens, a kidney may move more than usual (nephroptosis), though its significance is debated.

The hilum and renal sinus

The hilum is the medial indentation where structures enter and leave. Passing through it, anterior to posterior, are the renal vein, the renal artery, and the renal pelvis (which narrows into the ureter) — "vein, artery, ureter." The hilum opens into the renal sinus, a fat-filled cavity inside the kidney that houses the renal pelvis, calyces, and branches of the renal vessels. Because the sinus is enclosed by kidney tissue, pressure can rise when drainage is blocked — contributing to the pain of an obstructed kidney (educational point).

Internal gross structure: cortex, medulla, and the drainage funnel

A coronal section shows two regions. The renal cortex is the outer layer (about 1 cm thick) that receives most of the blood flow and contains the filtering units. The renal medulla lies deep and is organized into 8–15 cone-shaped renal pyramids, whose bases face the cortex and whose tips (papillae) point inward. Cortical tissue extends between pyramids as renal columns (columns of Bertin). Urine flows from each papilla into a minor calyx; minor calyces merge into 2–3 major calyces, which join to form the renal pelvis, which narrows into the ureter. The drainage route is therefore: papilla → minor calyx → major calyx → renal pelvis → ureter.

Blood supply: two capillary beds in series

The renal artery (from the abdominal aorta) enters the hilum and divides into segmental arteries, then interlobar (between pyramids), arcuate (at the cortex–medulla junction), and interlobular (cortical radiate) arteries. Each interlobular artery gives off afferent arterioles that feed the glomerular capillary tufts; blood leaves each tuft through an efferent arteriole and forms a second capillary network — peritubular capillaries in the cortex, or long straight vessels called vasa recta beside the medullary loops. Blood then collects into veins and returns to the inferior vena cava. Two features stand out: the kidneys receive about 20–25% of cardiac output (commonly taught) because they filter the plasma volume many times a day; and the glomerular and peritubular beds are two capillary networks in series — filtration under high pressure, reabsorption under low.

Innervation and lymphatics

Sympathetic fibers (T12–L2 splanchnic nerves) innervate the renal vessels and juxtaglomerular cells; strong activation constricts the renal arteries and reduces flow. Parasympathetic (vagal) fibers reach the kidney but play a minor role. Lymphatics drain to lumbar lymph nodes.

How It Works / Step-by-Step Process

Blood path: aorta → renal artery → segmental → interlobar → arcuate → interlobular → afferent arteriole → glomerulus (filtration) → efferent arteriole → peritubular capillaries / vasa recta (reabsorption) → veins → renal vein → inferior vena cava.

Urine path: glomerular filtrate → tubules in the cortex → loops into the medulla and back → collecting ducts → papilla → minor calyx → major calyx → renal pelvis → ureter → bladder.

Common Confusions