External Anatomy

The kidneys are paired, reddish-brown, bean-shaped organs located on the posterior abdominal wall. They lie primarily in the upper lumbar region on either side of the vertebral column and are positioned retroperitoneally, behind the parietal peritoneum.

Externally, each kidney has a superior and inferior pole, anterior and posterior surfaces, and lateral and medial borders. The medial border contains the renal hilum, through which the renal vessels, renal pelvis, nerves, lymphatics, and associated structures enter or leave the kidney.

Location

The kidneys lie against the posterior abdominal wall on either side of the vertebral column.

They extend approximately from the level of T12 to L3, although their precise position varies with respiration, body position, body habitus, and individual anatomy.

Retroperitoneal Position

The kidneys are retroperitoneal organs.

They lie posterior to the parietal peritoneum rather than being suspended within the peritoneal cavity.

Right and Left Kidney Position

The right kidney normally lies slightly lower than the left kidney.

This asymmetry is related principally to the large right lobe of the liver, which occupies considerable space superior to the right kidney.

General Shape

Each kidney has a characteristic bean-shaped appearance.

The lateral border is convex, while the medial border is concave and interrupted centrally by the renal hilum.

Approximate Dimensions

Adult renal dimensions vary with sex, body size, age, and other factors. A typical adult kidney is approximately 10-12 cm long, 5-7 cm wide, and about 3 cm thick.

The left kidney is often slightly longer than the right.

Weight

Renal weight varies substantially among individuals.

Adult kidneys commonly weigh approximately 120-170 g each, with average values generally greater in males than females.

Long Axis

The long axis of each kidney is oriented obliquely.

The superior poles lie closer to the median plane than the inferior poles, so the kidneys diverge slightly as they extend inferiorly.

Superior Pole

The superior pole is the upper rounded extremity of the kidney.

It lies more medially and posteriorly than the inferior pole and is closely related to the corresponding suprarenal gland.

Inferior Pole

The inferior pole is the lower rounded extremity of the kidney.

It lies farther from the median plane than the superior pole because of the oblique orientation of the renal long axis.

Anterior Surface

The anterior surface of each kidney is convex and faces generally anterolaterally.

Its relationships differ substantially between the right and left kidneys because of differences in the surrounding abdominal organs.

Posterior Surface

The posterior surface is relatively flattened and faces posteromedially.

Both kidneys have broadly similar posterior relationships with the diaphragm, posterior abdominal wall muscles, ribs, vessels, and nerves.

Lateral Border

The lateral border of the kidney is smooth and convex.

It forms the outer curved margin of the characteristic bean-shaped renal outline.

Medial Border

The medial border is concave centrally.

Its central indentation contains the renal hilum, which opens into the renal sinus.

Renal Hilum

The renal hilum is a vertical cleft on the medial border of the kidney through which structures pass between the renal sinus and the surrounding retroperitoneum.

It provides passage for the renal vein, renal artery, renal pelvis and ureter, lymphatic vessels, autonomic nerves, and associated connective tissue.

Orientation of the Hilum

The renal hilum generally faces anteromedially rather than directly medially.

This orientation reflects the embryological rotation of the kidneys during their ascent from the pelvis.

Arrangement at the Renal Hilum

From anterior to posterior, the principal structures at the renal hilum are generally arranged as:

1. Renal vein
2. Renal artery
3. Renal pelvis

This relationship is often remembered as V-A-P, although smaller arterial branches, lymphatics, nerves, and anatomical variations can modify the detailed arrangement.

Renal Sinus

The hilum opens internally into the renal sinus.

The renal sinus is a cavity containing the renal pelvis and calyces, renal vessels and their branches, nerves, lymphatics, and adipose tissue.

External Coverings

The kidney is surrounded by several layers that protect and stabilize it within the retroperitoneum.

From the renal surface outward, these include the fibrous capsule, perirenal fat, renal fascia, and surrounding pararenal fat.

Fibrous Capsule

The fibrous renal capsule is a thin but tough connective tissue layer closely applied to the external surface of the kidney.

In a healthy kidney it can usually be separated relatively easily from the underlying renal parenchyma.

Perirenal Fat

Perirenal fat, also called perinephric fat, surrounds the kidney and suprarenal gland within the renal fascial compartment.

It cushions the kidney and contributes to its stabilization.

Renal Fascia

The renal fascia is a connective tissue envelope associated with the kidney, suprarenal gland, and perirenal fat.

Its anterior and posterior layers contribute to compartmental organization of the retroperitoneum.

Pararenal Fat

Pararenal fat lies external to the renal fascia, particularly posterior and posterolateral to the kidney.

It forms part of the retroperitoneal adipose tissue surrounding the renal compartment.

Layers Around the Kidney

Layer	Relationship
Fibrous capsule	Closely covers renal surface
Perirenal fat	Surrounds kidney within renal fascia
Renal fascia	Encloses the perirenal compartment
Pararenal fat	Lies external to renal fascia, especially posteriorly

Suprarenal Glands

Each suprarenal gland lies adjacent to the superior aspect of the corresponding kidney.

The suprarenal glands are separate endocrine organs and are separated from the kidneys by connective tissue within the perirenal compartment.

Posterior Relationships

The posterior relationships of the kidneys are broadly similar on both sides.

They include the diaphragm superiorly and muscles of the posterior abdominal wall inferiorly.

Diaphragm

The superior part of the posterior surface of each kidney is related to the diaphragm.

The diaphragm separates the kidneys from structures within the thorax and from the pleural recesses superiorly.

Posterior Abdominal Wall Muscles

Each kidney is related posteriorly to several muscles.

• Psoas major medially
• Quadratus lumborum centrally
• Transversus abdominis laterally

These muscles form an important muscular bed for the kidneys.

Relationship to the Ribs

The left kidney is related posteriorly to the 11th and 12th ribs.

The right kidney, because of its lower position, is generally related posteriorly only to the 12th rib.

Subcostal Structures

The subcostal vessels and subcostal nerve cross posterior to the kidneys.

They form part of the neurovascular relationships that are important during posterior surgical approaches to the kidney.

Iliohypogastric and Ilioinguinal Nerves

The iliohypogastric and ilioinguinal nerves descend across the posterior abdominal wall in relation to the lower posterior aspects of the kidneys.

Their position is clinically relevant during retroperitoneal surgical procedures.

Anterior Relations of the Right Kidney

The anterior surface of the right kidney is related to several abdominal organs.

• Right suprarenal gland
• Liver
• Second part of the duodenum
• Right colic flexure
• Small intestine

The exact areas of contact vary among individuals.

Liver

A large portion of the anterior surface of the right kidney is related to the right lobe of the liver.

This relationship contributes to the lower position of the right kidney compared with the left.

Duodenum

The medial anterior portion of the right kidney is related to the second part of the duodenum.

This relationship is retroperitoneal and is important during surgical mobilization of the duodenum.

Right Colic Flexure

The inferior anterior region of the right kidney is related to the right colic flexure and adjacent colon.

These relationships are important when approaching the kidney through the abdominal cavity.

Anterior Relations of the Left Kidney

The anterior surface of the left kidney has a different set of visceral relationships.

• Left suprarenal gland
• Stomach
• Spleen
• Pancreas
• Left colic flexure and descending colon
• Jejunum

Stomach

The upper anterior region of the left kidney is related to the stomach.

The peritoneal cavity and associated peritoneal structures intervene over portions of this relationship.

Spleen

The superolateral portion of the left kidney is related to the spleen.

This close anatomical relationship is important during surgery and trauma involving the left upper abdomen.

Pancreas

The pancreas crosses anterior to the left kidney, particularly in relation to its central region.

Pancreatic and renal anatomy are therefore closely related within the upper retroperitoneum.

Left Colon

The lower lateral region of the left kidney is related to the left colic flexure and descending colon.

These structures may need to be mobilized during anterior approaches to the left kidney.

Jejunum

Loops of jejunum may lie anterior to the lower medial region of the left kidney.

This relationship varies with the position and filling of the small intestine.

Comparison of Right and Left Kidneys

Feature	Right Kidney	Left Kidney
Relative position	Slightly lower	Slightly higher
Posterior ribs	Mainly 12th rib	11th and 12th ribs
Major anterior upper relation	Liver	Stomach and spleen
Duodenal relation	Second part of duodenum	No comparable major relation
Pancreatic relation	Limited	Pancreas crosses anterior surface

Renal Mobility

The kidneys are not completely immobile.

They undergo a small degree of vertical movement with respiration as the diaphragm moves, and their position can also vary with posture.

Respiratory Movement

During inspiration, diaphragmatic descent can move the kidneys slightly inferiorly.

This mobility is relevant during physical examination and imaging of the kidneys.

Renal Fixation

The kidneys are maintained in position by a combination of renal fascia, surrounding fat, vascular attachments, intra-abdominal pressure, and relationships with neighboring structures.

No single structure acts as an isolated suspensory mechanism.

Nephroptosis

Nephroptosis refers to excessive downward mobility of a kidney when a person moves from a supine to an upright position.

It should be distinguished from an ectopic kidney, which occupies an abnormal position because of altered embryological ascent.

Palpation of the Kidney

Normal adult kidneys are often difficult to palpate because of their deep retroperitoneal position.

The lower pole of the right kidney may be more accessible to examination in some individuals because the right kidney lies slightly lower.

Renal Enlargement

An enlarged kidney may extend inferiorly and become more readily palpable.

Knowledge of its normal retroperitoneal position and surrounding visceral relationships assists in distinguishing renal enlargement from masses arising in neighboring organs.

Costovertebral Region

The posterior location of the kidneys places them near the angle between the 12th rib and the vertebral column.

Clinical tenderness in this region may occur in renal or upper urinary tract disease, although such tenderness is not specific to a single diagnosis.

Posterior Surgical Access

The posterior and posterolateral surfaces of the kidneys can be approached through the retroperitoneum without intentionally entering the peritoneal cavity.

Knowledge of the 12th rib, pleura, diaphragm, posterior abdominal wall muscles, and subcostal neurovascular structures is important during such approaches.

Pleural Relationship

The upper poles of the kidneys lie close to the diaphragm and the inferior limits of the pleural cavities.

This relationship is particularly important during procedures involving the upper posterior kidney because an excessively superior approach may risk pleural injury.

Renal Hilum in Surgery

The renal hilum is clinically important because it contains the major vascular inflow, venous drainage, and urinary outflow structures of the kidney.

Identification and control of hilar structures are central components of many renal surgical procedures.

Hilar Arrangement

Position	Principal Structure
Anterior	Renal vein
Intermediate	Renal artery
Posterior	Renal pelvis

Renal Surface and Internal Architecture

The smooth external surface of the adult kidney overlies a highly organized internal arrangement of cortex, medulla, pyramids, columns, papillae, and the collecting system.

The external hilum provides the gateway between this internal architecture and the vessels and ureter outside the kidney.

Fetal Lobulation

The fetal kidney has a distinctly lobulated external surface.

These lobulations usually become much less conspicuous as the kidney grows, producing the relatively smooth external surface typical of the adult kidney.

Persistent Fetal Lobulation

Shallow grooves may persist between renal lobes in some adults, producing persistent fetal lobulation.

This is usually a normal anatomical variant and should not automatically be interpreted as renal scarring.

Dromedary Hump

A dromedary hump is a normal contour variation that may occur along the lateral border of the left kidney, often related to impression by the spleen.

Recognition of this variant on imaging can prevent confusion with a renal mass.

Imaging of External Renal Anatomy

Ultrasonography, computed tomography, and magnetic resonance imaging can demonstrate renal size, position, contour, orientation, and relationships with adjacent structures.

Cross-sectional imaging is particularly useful for evaluating the renal hilum and retroperitoneal relationships.

External Anatomical Features

Feature	Description
Shape	Bean-shaped
Position	Retroperitoneal on posterior abdominal wall
Poles	Superior and inferior
Surfaces	Anterior and posterior
Borders	Convex lateral and concave medial
Hilum	Cleft on medial border
Right kidney	Usually slightly lower than left

Key Anatomical Features

• The kidneys are paired retroperitoneal organs.
• They lie on the posterior abdominal wall on either side of the vertebral column.
• The right kidney usually lies slightly lower than the left.
• Each kidney has superior and inferior poles.
• Each kidney has anterior and posterior surfaces.
• The lateral border is convex.
• The medial border is concave centrally.
• The renal hilum lies on the medial border.
• The hilum opens into the renal sinus.
• The principal hilar structures are generally arranged renal vein, renal artery, and renal pelvis from anterior to posterior.
• The superior poles are related to the suprarenal glands.
• The kidneys are related posteriorly to the diaphragm, psoas major, quadratus lumborum, and transversus abdominis.
• The left kidney is related to the 11th and 12th ribs.
• The right kidney is generally related to the 12th rib.
• The kidneys are surrounded by a fibrous capsule, perirenal fat, renal fascia, and pararenal fat.

Clinical Importance

The external anatomy of the kidney is important for understanding physical examination, radiological imaging, renal trauma, biopsy, and surgical approaches. Its retroperitoneal position determines many of its relationships and permits selected posterior approaches without deliberate entry into the peritoneal cavity.

The differences between the right and left kidneys are particularly important clinically. The liver lowers the position of the right kidney, while the left kidney has close relationships with the stomach, spleen, pancreas, and left colon. Posteriorly, the relationship of the upper kidney to the diaphragm, ribs, and pleura must be considered during invasive procedures.

Understanding external renal anatomy provides the foundation for studying the renal hilum, renal sinus, renal coverings, internal kidney anatomy, renal vessels, ureter, retroperitoneal spaces, renal imaging, kidney surgery, and anatomical relationships of the upper urinary tract.