Kidney

Ren

For medical students2 min readUpdated 2026-10-10

Kidneys (renes) are paired urine-producing organs that form the foundation of the human urinary system. Their primary function is urine formation through the complex structural interplay of the vascular network and epithelial tubular system.

FunctionPrimary urine-producing organ of the urinary system

CoveringsSurrounded by renal fascia, adipose capsule, and fibrous capsule

VasculatureFeatures two consecutive capillary beds

ParenchymaDivided into an outer cortex and inner medulla

General Organization of the Urinary System

The urinary system is functionally and anatomically divided into two main groups of organs. The first group comprises the urine-producing organs, represented directly by the kidneys (renes). The second group includes the urinary tract: the ureters, urinary bladder, and urethra.

The anatomy of the urethra shows prominent sex differences. In males, the urethra is long, closely linked to the reproductive system, and passes through the prostate gland, urogenital diaphragm, and spongy body of the penis. In females, the urethra is much simpler: significantly shorter, passing exclusively through the urogenital diaphragm.

Renal Coverings

Externally, the kidney is protected by three layers arranged from periphery to center:

Renal fascia — the outermost layer.
Adipose capsule — a layer of fat tissue lying beneath the fascia.
Fibrous capsule (capsula fibrosa) — a thin, smooth membrane tightly adherent to the organ's parenchyma.

A notable histological feature of the fibrous capsule is the presence of smooth muscle elements (myocytes) alongside connective tissue, making it resemble the splenic capsule. Contraction of these myocytes is believed to help maintain intrarenal pressure for efficient plasma filtration and to assist in propelling formed urine out of the kidney.

Structure of the Parenchyma

The renal tissue located beneath the fibrous capsule includes the urine-forming structures, which are visually and topographically divided into two zones:

Renal cortex (cortex). Occupies the peripheral, subcapsular position and appears dark red on a fresh cut surface. Extensions of the cortex project deep into the medullary structures, forming renal columns (columnae renales).
Renal medulla (medulla). Located deeper than the cortex and has a lighter color. It is organized into 8–12 renal pyramids. Fine extensions called medullary rays project from the medulla into the cortex.

Intrarenal Pathways and Microcirculation

The urine-collecting cavity system (collecting system) begins within the organ. The apices of the renal pyramids—the renal papillae—numbering from one to three, open into minor calyces. There are typically 8–9 minor calyces in a kidney. By fusing, they form 2–3 major calyces, which then empty into the renal pelvis (pelvis renalis). Topographically, only the upper part of the pelvis lies within the kidney sinus, while its lower part exits the renal hilum and continues as the ureter.

At the microscopic level, renal function is based on the interaction between the vascular system and epithelial tubules. The key hemodynamic principle here is the passage of blood through two capillary beds. The first bed forms the capillary glomeruli enclosed within Bowman's capsules, and the second bed densely surrounds the epithelial tubular system.

Mnemonic

To remember the layers of the kidney from outside to inside, use the mnemonic: "Fatty Fibers Surround" (Renal Fascia, Adipose capsule, Fibrous capsule).

Frequently asked questions

What segments make up the nephron?

The tubular portion of the nephron consists of five sequentially continuous segments.

Proximal convoluted tubule — begins at Bowman's capsule and forms loops in the renal cortex near the renal corpuscle.
Proximal straight tubule — a short segment following the convoluted tubule.
Thin limb — part of the loop of Henle; the descending limb is represented by the thin tubule, and the ascending limb begins as its continuation.
Distal straight tubule — part of the loop of Henle following the initial segment of the ascending limb.
Distal convoluted tubule — a continuation of the ascending limb of the loop of Henle, located in the cortex, draining into the collecting duct.

What epithelium lines the minor and major renal calyces?

The minor and major renal calyces, serving as intrarenal urinary pathways, are lined by transitional epithelium (urothelium).

This specialized type of stratified epithelium is adapted for stretching and consists of three layers:

Basal layer — small cells with oval nuclei resting on the basement membrane.
Intermediate layer — polygonal cells.
Superficial layer — very large (often binucleated) cells whose shape changes from dome-shaped to flattened when the organ is distended.

What cell types comprise the juxtaglomerular apparatus of the kidney?

The juxtaglomerular apparatus (JGA) of the kidney consists of three elements:

Macula densa — a specialized region of the distal convoluted tubule wall.
Juxtaglomerular cells — located in the tunica media of the arterioles, primarily the afferent arteriole.
Extraglomerular mesangial cells (polkissen or Goormaghtigh cells) — located in the space between the two arterioles and the macula densa.

The main function of the JGA is the synthesis of the enzyme renin, playing a key role in blood pressure regulation.

How does the fibrous capsule of the kidney differ from other connective tissue capsules?

Its histological composition includes smooth muscle myocytes. Their function is to maintain intrarenal pressure for plasma filtration and to assist in urine excretion.

What is the unique feature of the renal blood supply?

Blood in the kidney passes sequentially through two capillary networks. The first network is located in the glomeruli, and the second surrounds the system of epithelial tubules.

How is the renal pelvis formed?

The renal pelvis is formed by the merging of 2–3 major calyces, which in turn are formed by 8–9 minor calyces collecting urine from the renal papillae.

Go deeper

Histological structure of the epithelial tubules surrounded by the secondary capillary network.
Organization of the first capillary network: structure of the renal corpuscle and its capsule.
Microscopic structure of the renal papillae and their contact with minor calyces.
Topography of the renal hilum and the exit of structures from the organ.
Cellular composition of the fibrous capsule and the distribution of myocytes.