The kidneys are paired retroperitoneal organs located on either side of the vertebral column extending between the 12th thoracic and the 3rd lumbar vertebral levels. They play an important role in the excretion of waste products, the regulation of extracellular fluid volume and osmolality, the maintenance of acid-base balance, hormone synthesis (e.g., erythropoietin), and gluconeogenesis. The kidneys receive their blood supply from the paired renal arteries and drain into the paired renal veins. The lymphatics drain into the paraaortic lymph nodes. The left renal vein passes between the abdominal aorta and the superior mesenteric artery. The kidneys receive sympathetic innervation via the sympathetic trunk and parasympathetic innervation via the vagus nerve. Anatomically, the kidneys are composed of the renal capsule, renal cortex, renal medulla, renal sinus, and renal hilum. The nephron is the functional unit of the kidney, and it is composed of a renal corpuscle and a renal tubule. The renal corpuscle consists of the glomerulus and the Bowman capsule, which are separated by the glomerular filtration barrier (GFB). The GFB, which is composed of the fenestrated glomerular capillary endothelium, the glomerular basement membrane, and the podocyte layer, is responsible for filtering blood plasma. The filtrate passes through the renal tubule, which is divided into the proximal convoluted tubule, the loop of Henle, the distal convoluted tubule, and the collecting duct. In the renal tubule, urine is formed from the filtrate via reabsorption, secretion, and excretion of substances. The kidneys originate embryologically from the mesoderm.
- Reddish-brown, bean-shaped, paired retroperitoneal organs lying on either side of the vertebral column at the level of T12–L3
- Each kidney weighs approximately 140–180 g
- Dimensions: 10–12 cm in length, 4–6 cm in width, and 3–4 cm in anteroposterior dimension
- Located at the superior pole: adrenal gland (within the perirenal fat)
- Both kidneys: diaphragm, psoas major, quadratus lumborum, the subcostal, iliohypogastric nerve, ilioinguinal nerve
- Left kidney: 11th and 12thribs
- Right kidney: 12thrib
- The outermost layer of the renal parenchyma (∼ 10 mm thick)
- Surrounds the renal medulla and extends inwards (as renal columns) dividing the medulla into renal pyramids
- Contains the glomeruli, proximal convoluted tubules, distal convoluted tubules, and cortical collecting ducts
- Consists of several renal medullary pyramids separated by the renal columns
- The base of each pyramid faces the outer cortex, the apex faces the renal sinus and forms a renal papilla, which drains into a minor calyx.
- Contains the loops of Henle and collecting ducts, which merge to form the papillary ducts at the renal papillae
Blood flow in the renal medulla is relatively low compared to that in the renal cortex. 
- Facilitates the development of an osmolality gradient that allows for effective urine concentration
- Causes medullary vulnerability to hypoxia when renal blood flow is decreased (renal ischemia)
- The inner portion of the kidney containing the renal calyces and renal pelvis
- The minor calyces draining from each renal papilla merge to form major calyces.
- The major calyces merge to form the renal pelvis, which represents the proximal portion of the ureter.
- Renal hilum: The medial fissure on each kidney where the renal pelvis, vessels, and nerves enter and exit
The kidneys receive 20–25% of the cardiac output. Despite this, the medulla is relatively hypoxic and vulnerable to ischemic damage.
Each kidney is encapsulated by layers of fascia and fat. These layers comprise (from the outside to the inside):
- Pararenal fat: the adipose tissue superficial to the renal fascia; protects the kidneys from trauma
- Definition: a single multilaminated connective tissue structure that envelops each kidney and adrenal gland along with a layer of perirenal fat
- Anatomical boundaries
- Anterior: The anterior perirenal fascia extends in front of the renal vessels and allows for communication between perirenal spaces.
- Posterior: The posterior perirenal fascia fuses with the fascia of psoas major.
- Superior: The superior perirenal space opens to the upper abdominal extraperitoneal space in continuity with the bare area of the liver.
- Lateral: The anterior perirenal fascia fuses with the coronary ligament (on the right) and gastrosplenic ligament (on the left).
- Inferior: The fascia surrounds the ureter and fuses with the iliac fascia.
- Perirenal fat: the adipose tissue that envelopes the kidneys; covered by renal fascia
- Renal capsule: a thin layer of collagen-rich connective tissue that forms a fibrous capsule around the kidney and adrenal gland
Vasculature, lymphatics, and innervation
|Sympathetic innervation|| |
|Parasympathetic innervation|| |
The left renal vein drains blood from the left kidney, the left suprarenal vein, and the left gonadal vein.
Blood Arrives through the Afferent arteriole and blood Exits through the Efferent arterioles.
- The functional unit of the kidney, composed of a renal corpuscle and a renal tubule
- There are approx. 1 million nephrons per kidney 
- The renal corpuscle filters fluid from the blood, producing an ultrafiltrate
- The renal tubule resorbs and secretes substances from the ultrafiltrate, producing urine
- See ”Physiology of the kidney.”
Two types of nephrons: cortical and juxtamedullary
- Distinguished by the location of the glomerulus
- Cortical nephrons (approx. 85%)
- Located close to the outer part of the renal cortex
- Short loop of Henle
- Mainly regulatory and excretory functions
- Juxtamedullary nephrons (approx. 15%)
- Glomerulus near the junction of the renal cortex and medulla
- Long loop of Henle
- Main function: concentration or dilution of urine
- Located in the renal cortex
- Formed of a tuft of capillaries (the glomerulus) enclosed by the Bowman capsule
- The glomerulus forms the vascular pole of the corpuscle
- The Bowman capsule forms the urinary pole of the corpuscle
- Within the Bowman capsule is the Bowman space, which is continuous with the renal tubule
- The Bowman capsule is separated from the capillaries of the glomerulus by the glomerular filtration barrier
Glomerulus: a tuft of capillaries within the Bowman capsule formed of three parts
- Afferent arterioles: the blood vessel that enters and supplies the glomerulus of the kidney for filtration
- Branch into anastomosing glomerular capillaries, between capillaries, lie intraglomerular mesangial cells (contractile cells capable of phagocytosis and lysosomal degradation)
- Efferent arterioles: the blood vessel that carries previously filtered blood away from the glomerulus (drains from the glomerulus)
Bowman capsule: cup-like sac that surrounds the glomerular capillaries
- Outer (parietal) surface: simple squamous epithelium
- Inner (visceral) surface: podocytes
- The space between the parietal and visceral surfaces, which is continuous with the proximal convoluted tubule, is referred to as the Bowman space.
- Glomerulus: a tuft of capillaries within the Bowman capsule formed of three parts
Function: filtration of blood across the glomerular filtration barrier, forming ultrafiltrate within the Bowman space
- The glomerular filtration barrier separates the blood within the glomerular capillaries from the ultrafiltrate in the Bowman space.
- Water, electrolytes, and other small molecules cross the filtration barrier to form the ultrafiltrate.
- Cells and large or negatively-charged molecules, e.g., proteins, cannot cross the filtration barrier and remain within the blood (size and charge selectivity).
- The glomerular filtration barrier is composed of three layers: the fenestrated glomerular capillary endothelium, the glomerular basement membrane, and the podocyte layer.
|Layers of the glomerular filtration barrier|
|Size barrier||Charge barrier|
|Fenestrated glomerular capillary endothelium|| || || |
|Glomerular basement membrane (GBM)|| || |
| || |
The glomerular filtration barrier ensures that large and/or negatively charged molecules in the blood cannot pass into the Bowman capsule.
|Overview of the renal tubules|
|Proximal convoluted tubule (PCT)|| || |
|Loop of Henle||Thin descending loop of Henle|| |
|Thick ascending loop of Henle|| |
|Distal convoluted tubule|| |
|Connecting tubule and collecting duct|| |
- A specialized structure within the nephron
- Located between the distal convoluted tubule (DCT) and the afferent arteriole of the nephron
- Maintains the GFR by regulating the renin-angiotensin-aldosterone system (RAAS)
- Modified smooth muscle cells located in the afferent arterioles
- Function: renin synthesis
- Composed of tall cuboidal cells located at the distal end of the thick ascending loop of Henle
Monitors the NaCl concentration within the lumen of the DCT
- Hypoosmolar urine; triggers the release of renin → vasoconstriction of the efferent arteriole → increase in GFR
- Hyperosmolar urine; triggers the release of adenosine → vasoconstriction of the afferent arteriole → decrease in GFR
Extraglomerular mesangial cells 
- Play a role in autoregulation of blood flow to the kidney (exact functioning is not entirely understood)
- Form a network of cells, connecting the sensory cells of the macula densa with juxtaglomerular effector cells
- May also signal to contractile glomerular mesangial cells and, thereby, directly affect vasoconstriction
- Juxtaglomerular cells
Production of urine
- Excretion of metabolic waste and end-products of metabolism (e.g., urea, drugs)
- Regulation of extracellular fluid volume and osmolality
- Maintenance of acid-base balance
- Maintenance of electrolyte concentrations
- Regulation of blood pressure and blood volume
- Participation in gluconeogenesis (glutamine and glutamate) and ketogenesis
- Hormone synthesis
- See also “Physiology of the kidney.”
- Origin: intermediate mesoderm
- Development begins at the 4th week of embryonic development.
- Precursor cells migrate cranial to caudal.
- The urogenital ridge gives rise to the pronephros, mesonephros, and metanephros.
Pronephros: the first embryonic excretory organ (rudimentary in human embryos)
- Arises in the cervical region of the embryo during the 3rd–4th week of embryonic development
- Migrates caudally to connect with the pronephric duct
- Composed of nephrotomes and nephric tubules → temporary excretory function
- Degenerates with the development of the mesonephros
Mesonephros: the second embryonic excretory organ, developing caudally to the pronephros and degenerating with the development of the metanephros
- Arises in the 4th week of embryonic development
Cranial end: functions as interim kidney during the 1st trimester
- Forms Bowman capsule
- Functions until the end of the 2nd month before it degenerates
Caudal end: contributes to male genital system
- Mesonephric tubules contribute to the mesonephric duct.
- The mesonephric duct forms the epididymis, ductus deferens, seminal vesicle, and the ejaculatory duct in male embryos.
Metanephros: the third embryonic excretory organ, developing caudally to the mesonephros and persisting as the permanent kidney
- Arises during the 5th week of embryonic development
- Canalization is complete by the 10th week of embryonic development.
- Maturing of the kidneys continues until week 35–36 of embryonic development.
Ureteric bud (metanephric diverticulum)
- Originates from the caudal end of the mesonephric duct
- Differentiates into the collecting duct system: ureter, renal pelvis, major calyces, minor calyces, and collecting ducts
Ureteropelvic junction: junction between ureter and renal pelvis
- Canalizes last and represents the most common site of prenatal urinary tract obstruction
- Obstruction of the ureteropelvic junction may cause prenatal hydronephrosis, which can be discovered during prenatal ultrasound screenings.
Metanephric blastema (metanephric mesenchyme)
- Differentiation stimulated by ureteric bud signals
- Differentiates into nephrons: glomerulus, proximal convoluted tubule, loop of Henle, and distal convoluted tubule
- Ureteric bud (metanephric diverticulum)
- Congenital anomalies of the kidneys can result from, e.g., due to abnormal interaction between the ureteric bud and the metanephritic blastema.
The ureteropelvic junction canalizes last and is the most common site of obstruction.
If the inferior poles of both kidneys fuse during fetal development, they form a horseshoe kidney and become trapped underneath the inferior mesenteric artery.
- Congenital anomalies of the kidneys
- Renal cyst disorders
- Acute kidney injury
- Chronic kidney disease
- Tubulointerstitial nephritis
- Renal tubular acidosis
- Nephrotic syndrome
- Nephritic syndrome
- Renal cell carcinoma
- Renal oncocytoma
- Transitional cell carcinoma