Internal Structure

The internal structure of the kidney is organized into an outer renal cortex and an inner renal medulla. The medulla is divided into renal pyramids, while extensions of cortical tissue called renal columns pass between adjacent pyramids. The apex of each pyramid forms a renal papilla that projects into a minor calyx.

Urine produced by the nephrons passes into collecting ducts, through papillary ducts at the renal papillae, and then sequentially into the minor calyces, major calyces, renal pelvis, and ureter. This internal organization links the microscopic functional units of the kidney with its macroscopic collecting system.

General Organization

A section through the kidney reveals several major internal regions and structures.

• Renal cortex
• Renal medulla
• Renal pyramids
• Renal columns
• Renal papillae
• Minor calyces
• Major calyces
• Renal pelvis
• Renal sinus

Renal Cortex

The renal cortex forms the outer region of the renal parenchyma immediately deep to the fibrous capsule.

It contains renal corpuscles, proximal and distal convoluted tubules, cortical portions of collecting ducts, and numerous blood vessels.

Appearance of the Cortex

On gross examination, the cortex has a more granular appearance than the medulla.

This appearance reflects the large number of renal corpuscles and convoluted tubular profiles within the cortical tissue.

Cortical Labyrinth

The portions of the cortex between the medullary rays are called the cortical labyrinth.

The cortical labyrinth contains renal corpuscles and the convoluted portions of renal tubules.

Medullary Rays

Medullary rays are bundles of straight tubules and collecting ducts extending from the medulla outward into the cortex.

Despite their name, medullary rays are located within the renal cortex.

Renal Medulla

The renal medulla forms the deeper region of the kidney.

It is organized into multiple conical masses of tissue known as renal pyramids.

Renal Pyramids

The renal pyramids are cone-shaped or triangular masses forming the principal gross structural units of the medulla.

Each pyramid has a broad base directed toward the cortex and a narrower apex directed toward the renal sinus.

Base of a Renal Pyramid

The base of each renal pyramid lies near the corticomedullary junction.

Arcuate arteries and veins run near this boundary between the cortex and medulla.

Renal Papilla

The apex of a renal pyramid forms the renal papilla.

Each papilla projects into a minor calyx and represents the site where urine leaves the renal parenchyma and enters the gross collecting system.

Papillary Ducts

Collecting ducts converge within the renal pyramid to form larger papillary ducts, also called ducts of Bellini.

These ducts open on the surface of the renal papilla and discharge urine into a minor calyx.

Area Cribrosa

The tip of the renal papilla contains multiple openings of the papillary ducts, producing a perforated region called the area cribrosa.

Urine passes through these openings from the papillary ducts into the minor calyx.

Renal Columns

Renal columns, or columns of Bertin, are extensions of cortical tissue that project inward between neighboring renal pyramids.

They contain blood vessels, tubules, and cortical interstitial tissue.

Interlobar Vessels

The renal columns provide pathways for the interlobar arteries and veins.

Interlobar arteries travel between renal pyramids and give rise to arcuate arteries near the corticomedullary junction.

Corticomedullary Junction

The corticomedullary junction marks the boundary between the renal cortex and the bases of the medullary pyramids.

Arcuate vessels follow the contour of this junction and provide an important anatomical landmark.

Renal Lobe

A renal lobe consists of one renal pyramid together with the cortical tissue overlying its base and extending along its sides.

The lobar organization is particularly apparent during fetal development, when the external surface of the kidney is visibly lobulated.

Fetal Lobulation

The fetal kidney has external grooves corresponding to its underlying lobar organization.

These grooves normally become less prominent during growth, although persistent fetal lobulation may remain as a normal anatomical variant in adults.

Renal Lobule

A renal lobule is a smaller structural unit centered on a medullary ray and the surrounding cortical tissue draining toward the same collecting duct system.

Renal lobules are more readily appreciated histologically than on gross examination.

Renal Parenchyma

The term renal parenchyma refers to the functional tissue of the kidney, principally the cortex and medulla.

It contains the nephrons, collecting ducts, blood vessels, interstitial tissue, and associated supporting structures.

Nephron

The nephron is the basic functional unit responsible for filtration and modification of tubular fluid.

Each nephron consists of a renal corpuscle and a renal tubule that ultimately connects with the collecting duct system.

Renal Corpuscle

The renal corpuscle consists of a glomerulus surrounded by Bowman's capsule.

Renal corpuscles are found only within the cortex and are absent from the renal medulla.

Glomerulus

The glomerulus is a tuft of specialized capillaries supplied by an afferent arteriole and drained by an efferent arteriole.

Filtration of plasma across the glomerular filtration barrier produces fluid that enters Bowman's space.

Proximal Convoluted Tubule

The proximal convoluted tubule begins at the urinary pole of the renal corpuscle and lies predominantly within the cortical labyrinth.

It performs extensive reabsorption of filtered water, electrolytes, nutrients, and other substances.

Loop of Henle

The nephron continues into the loop of Henle, whose straight segments extend toward or into the renal medulla.

The length of the loop varies between cortical and juxtamedullary nephrons.

Distal Convoluted Tubule

The distal convoluted tubule returns to the cortex and lies near its parent renal corpuscle before connecting through subsequent tubular segments with the collecting system.

It participates in regulated electrolyte and acid-base handling.

Cortical Nephrons

Most nephrons are cortical nephrons, with renal corpuscles located in the outer portions of the cortex and relatively short loops of Henle.

Their loops extend only a limited distance into the medulla.

Juxtamedullary Nephrons

Juxtamedullary nephrons have renal corpuscles close to the corticomedullary junction and long loops of Henle that descend deeply into the medulla.

They are particularly important in establishing the medullary osmotic gradient required for production of concentrated urine.

Collecting Duct System

Collecting ducts receive tubular fluid from multiple nephrons.

They pass from the cortex through the medulla and progressively converge toward the renal papillae.

Medullary Collecting Ducts

Within the medulla, collecting ducts become progressively larger as they receive fluid from upstream collecting pathways.

Near the papilla, they converge to form papillary ducts.

Minor Calyces

A minor calyx is a cup-shaped component of the collecting system that surrounds one or sometimes more renal papillae.

Urine entering through papillary duct openings collects within the minor calyx.

Major Calyces

Several minor calyces unite to form a major calyx.

The number and arrangement of major calyces vary among individuals, but they ultimately converge toward the renal pelvis.

Renal Pelvis

The renal pelvis is the expanded proximal portion of the ureter within or near the renal sinus.

It receives urine from the major calyces and narrows as it continues into the ureter.

Urine Flow Through the Kidney

1. Collecting ducts receive tubular fluid from nephrons.
2. Collecting ducts converge toward the renal papilla.
3. Papillary ducts discharge urine through the area cribrosa.
4. Urine enters a minor calyx.
5. Minor calyces join to form major calyces.
6. Major calyces drain into the renal pelvis.
7. The renal pelvis continues into the ureter.

Renal Sinus

The renal sinus is a cavity extending into the kidney from the renal hilum.

It contains the renal pelvis, major and minor calyces, renal vessels and their branches, nerves, lymphatic vessels, and adipose tissue.

Relationship Between Hilum and Sinus

The renal hilum is the external opening on the medial border of the kidney, while the renal sinus is the internal cavity into which the hilum opens.

Structures entering through the hilum branch or expand within the renal sinus before reaching the renal parenchyma.

Renal Pelvis and Sinus

The renal pelvis occupies part of the renal sinus and receives urine from the major calyces.

Its shape and degree of extension within or outside the sinus vary among individuals.

Renal Arterial Organization

The internal structure of the kidney is closely related to its arterial branching pattern.

Renal arteries divide into segmental arteries, which ultimately give rise to interlobar, arcuate, and cortical radiate arteries.

Interlobar Arteries

Interlobar arteries travel through renal columns between adjacent renal pyramids.

At the corticomedullary junction they give rise to arcuate arteries.

Arcuate Arteries

Arcuate arteries curve along the bases of the renal pyramids near the corticomedullary junction.

They give rise to cortical radiate arteries that extend outward into the cortex.

Cortical Radiate Arteries

Cortical radiate arteries, traditionally called interlobular arteries, extend through the renal cortex.

They give rise to afferent arterioles supplying the glomeruli.

Vascular Organization

Vessel	Characteristic Location
Interlobar arteries	Renal columns between pyramids
Arcuate arteries	Corticomedullary junction
Cortical radiate arteries	Renal cortex
Afferent arterioles	Supply individual glomeruli
Vasa recta	Extend into renal medulla

Vasa Recta

The vasa recta are straight vessels that descend into and ascend from the medulla, particularly in association with juxtamedullary nephrons.

Their arrangement supports countercurrent exchange and helps preserve the osmotic gradient of the medullary interstitium.

Outer and Inner Medulla

The renal medulla can be subdivided into outer and inner medullary regions based on differences in tubular composition and organization.

The outer medulla can be further divided into outer and inner stripes, while the inner medulla extends toward the papilla.

Outer Medulla

The outer medulla contains straight portions of proximal and distal nephron segments, thick limbs of loops of Henle, collecting ducts, and associated vascular structures.

Its organization differs from both the cortex and deeper inner medulla.

Inner Medulla

The inner medulla contains thin limbs of loops of Henle, collecting ducts, vasa recta, and interstitial tissue.

Collecting ducts become progressively larger as they approach the papillary tip.

Cortex and Medulla Comparison

Feature	Renal Cortex	Renal Medulla
Position	Outer renal parenchyma	Inner renal parenchyma
Renal corpuscles	Present	Absent
Convoluted tubules	Prominent	Absent as convoluted portions
Loops of Henle	Some portions present	Prominent straight portions
Collecting ducts	Present	Prominent
Gross appearance	Granular	Striated

Striated Appearance of the Medulla

The medulla has a characteristically striated appearance because of the parallel arrangement of loops of Henle, collecting ducts, and blood vessels.

These structures generally run toward the renal papilla.

Granular Appearance of the Cortex

The cortex appears granular because of the presence of numerous renal corpuscles and convoluted tubules.

Medullary rays interrupt this granular pattern with more radially arranged straight tubular structures.

Renal Papilla and Calyx Relationship

The renal papilla projects into the lumen of a minor calyx.

This close relationship creates the transition from urine transport within the renal parenchyma to transport through the urothelial-lined collecting system.

Urothelium

The calyces and renal pelvis are lined by urothelium, also called transitional epithelium.

This specialized epithelium continues into the ureter and accommodates changes in luminal volume while maintaining a barrier to urine.

Renal Papillary Fornix

The margin of a minor calyx surrounds the projecting renal papilla and forms a fornix around it.

This region represents an important anatomical interface between renal parenchyma and the collecting system.

Renal Lobe and Lobule Comparison

Feature	Renal Lobe	Renal Lobule
Central structure	Renal pyramid	Medullary ray
Associated tissue	Overlying and adjacent cortex	Surrounding cortical tissue
Scale	Gross anatomical unit	Smaller histological unit
Visibility	Apparent in gross sections	Best appreciated microscopically

Functional Organization

The internal architecture of the kidney reflects its major physiological functions.

Filtration occurs in cortical renal corpuscles, extensive tubular modification occurs throughout cortical and medullary nephron segments, and the medullary arrangement permits concentration or dilution of urine before it enters the calyceal system.

Countercurrent Organization

The long loops of Henle of juxtamedullary nephrons and the accompanying vasa recta are arranged in parallel within the medulla.

This organization contributes to the countercurrent mechanisms that establish and preserve the medullary osmotic gradient.

Collecting Duct Function

The collecting ducts provide the final tubular pathway through the renal parenchyma.

Their permeability to water and handling of electrolytes are hormonally regulated, allowing final urine concentration to be adjusted before urine enters the papillary ducts.

Internal Structure on Imaging

Ultrasonography, computed tomography, and magnetic resonance imaging can demonstrate aspects of the internal renal architecture.

Imaging can distinguish the renal parenchyma from the renal sinus and can demonstrate the collecting system particularly when it is filled with contrast material or abnormally dilated.

Renal Sinus Fat

Adipose tissue normally occupies spaces within the renal sinus around the vessels and collecting system.

Its appearance on cross-sectional imaging helps delineate the renal pelvis, calyces, and hilar vascular structures.

Hydronephrosis

Hydronephrosis is dilation of the renal pelvis and calyces caused by impaired urinary drainage.

Persistent or severe obstruction can progressively compress and thin the surrounding renal parenchyma.

Renal Calculi

Urinary calculi may form within or pass through the calyces and renal pelvis.

The branching anatomy of the collecting system influences the location of stones and their relationship to urinary obstruction.

Staghorn Calculus

A large staghorn calculus can occupy the renal pelvis and extend into multiple calyces, producing a branching configuration that reflects the shape of the collecting system.

Such calculi can impair drainage and are associated with significant renal complications.

Papillary Necrosis

Renal papillary necrosis involves ischemic or destructive injury to renal papillae.

Because the papillae form the terminal portion of the pyramids and project directly into the calyces, damaged papillary tissue may detach and enter the urinary collecting system.

Cortical Scarring

Renal disease can produce localized or diffuse loss of cortical tissue and external surface irregularity.

Recognition of the normal relationship between cortex, medulla, pyramids, and calyces assists in interpreting such structural changes.

Internal Structural Pathway

1. The fibrous capsule covers the renal parenchyma.
2. The cortex forms the outer region beneath the capsule.
3. Renal columns extend from the cortex between the pyramids.
4. Renal pyramids form the medulla.
5. The apex of each pyramid forms a renal papilla.
6. Papillary ducts open at the area cribrosa.
7. Urine enters the minor calyces.
8. Minor calyces unite into major calyces.
9. Major calyces drain into the renal pelvis.
10. The renal pelvis continues as the ureter.

Key Anatomical Features

• The renal parenchyma is divided into cortex and medulla.
• The renal cortex lies immediately deep to the fibrous capsule.
• Renal corpuscles are located within the cortex.
• Medullary rays extend into the cortex and contain straight tubules and collecting ducts.
• The renal medulla is organized into renal pyramids.
• Renal columns are extensions of cortex between adjacent pyramids.
• The base of each pyramid faces the cortex.
• The apex of each pyramid forms a renal papilla.
• Papillary ducts open at the area cribrosa.
• Renal papillae project into minor calyces.
• Minor calyces unite to form major calyces.
• Major calyces drain into the renal pelvis.
• The renal pelvis continues into the ureter.
• A renal lobe consists of a pyramid and its associated cortical tissue.
• The renal sinus contains the collecting system, vessels, nerves, lymphatics, and fat.

Clinical Importance

The internal structure of the kidney provides the anatomical framework for understanding renal filtration, tubular processing, urine concentration, and urinary drainage. The cortex contains the renal corpuscles and convoluted tubules, while the medulla contains the highly organized straight tubular and vascular structures required for concentrating urine.

The transition from renal parenchyma to the collecting system occurs at the papillae. Urine passes from papillary ducts into minor calyces, then through major calyces and the renal pelvis before entering the ureter. Obstruction anywhere along this pathway can dilate upstream portions of the collecting system and eventually damage renal parenchyma.

Understanding internal renal anatomy provides the foundation for studying renal cortex, renal medulla, renal pyramids, renal columns, nephrons, collecting ducts, renal papillae, calyces, renal pelvis, renal blood vessels, hydronephrosis, renal calculi, and renal imaging.