Kidneys

Last updated: January 31, 2023

Summary

The kidneys are paired retroperitoneal organs located on either side of the vertebral column extending between the 12^th thoracic and the 3^rd 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.

Organ fact sheet: kidneys

Gross anatomy

Overview

Characteristics
- 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
Relations
- Located at the superior pole: adrenal gland (within the perirenal fat)
- Anteriorly
  - Left kidney: spleen, stomach, pancreas, splenic flexure, jejunum
  - Right kidney: right lobe of the liver, descending duodenum, hepatic flexure
- Posteriorly
  - Both kidneys: diaphragm, psoas major, quadratus lumborum, the subcostal, iliohypogastric nerve, ilioinguinal nerve
  - Left kidney: 11^th and 12^thribs
  - Right kidney: 12^thrib

Abdominal organs Peritoneal relations in the abdomen

Internal macrostructure

Renal cortex
- 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
Renal medulla
- 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. ^[1]
  - 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)
Renal sinus
- 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

Internal macrostructure of the kidney Microstructure of the kidney

The kidneys receive 20–25% of the cardiac output. Despite this, the medulla is relatively hypoxic and vulnerable to ischemic damage.

Surrounding structures

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
Perirenal fascia
- Definition: a single multilaminated connective tissue structure that envelops each kidney and adrenal gland along with a layer of perirenal fat
- Divisions
  - Anterior perirenal fascia
  - Posterior perirenal fascia
- 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

	Description
Arteries	The right and left renal arteries branch laterally from the aorta at the level of L1–2, just inferior to the superior mesenteric artery, and enter the kidney at the hilum. Accessory renal arteries are present in 30% of individuals, they also branch laterally from the aorta and enter at the hilum. The renal arteries give off branches to the adrenal gland (inferior suprarenal artery) and ureter. Renal blood flow: renal artery → anterior and posterior branches → segmental arteries → lobar arteries → interlobar arteries → arcuate arteries → interlobular arteries → afferent arteriole → glomerulus → efferent arteriole → vasa recta (→ venous system)
Veins	A single vein exits each kidney at the hilum anterior to the renal artery and drains into the vena cava. The left renal vein is longer than the right renal vein because it runs a longer course to the inferior vena cava, which is situated on the right side of the body. The left renal vein crosses the midline between the aorta and the superior mesenteric artery (SMA). The left suprarenal and gonadal veins drain into the left renal vein, whereas on the right, they drain directly into the vena cava.
Lymphatics	Drain into the paraaortic lymph nodes
Sympathetic innervation	Sympathetic trunk → renal plexus surrounding each renal artery
Parasympathetic innervation	None ^[2]^[3]

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.

Abdominal aorta and inferior vena cava Vasculature of kidney and glomerular apparatus

Microscopic anatomy

Nephron

Description
- The functional unit of the kidney, composed of a renal corpuscle and a renal tubule
- There are approx. 1 million nephrons per kidney ^[4]
Function
- 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

Microstructure of the kidney

Renal corpuscle

Description
- 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
Components
- 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.
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
Layer	Description	Clinical relevance	Function
Layer	Description	Clinical relevance	Size barrier	Charge barrier
Fenestrated glomerular capillary endothelium	Porous capillary endothelium (fenestrated endothelium)	Preeclampsia	Molecules > 100–50 nm	Negatively charged glycoproteins (e.g., with heparan sulfate on the GBM) prevent the entry of negatively charged molecules (e.g., albumin).
Glomerular basement membrane (GBM)	Consists of 3 layers Lamina rara interna: covers the endothelium and contains heparan sulfate Lamina densa: consists of type IV collagen and laminin Lamina rara externa: covers podocytes and contains heparan sulfate	Goodpasture syndrome Alport syndrome Diabetic glomerulosclerosis	Molecules > 70 nm
Podocytes	Form the visceral epithelial layer of the Bowman capsule Foot processes surround the glomerular capillaries and leave filtration slits (slit diaphragms)	Minimal change disease	Molecules > 50–60 nm

The glomerular filtration barrier ensures that large and/or negatively charged molecules in the blood cannot pass into the Bowman capsule.

Cross section of renal corpuscle Structure of the renal corpuscle Glomerulus capillaries Glomerular filtration barrier

Renal tubules

Overview of the renal tubules
Segments		Location	Microscopy	Function
Proximal convoluted tubule (PCT)		Renal cortex	Tall, cuboidal cells with brush border (long apical microvilli) Basal invaginations with multiple mitochondria Contains lysosomes	Each segment has a distinct epithelial lining and function Receive the ultrafiltrate from the renal corpuscle Transport and concentrates the ultrafiltrate Form urine via reabsorption, secretion, and excretion of substances See ”Physiology of the kidney.”
Loop of Henle	Thin descending loop of Henle	Renal medulla	Smaller diameter than PCT Flat, squamous epithelium (resembles capillary endothelium)
Loop of Henle	Thick ascending loop of Henle	Renal medulla	Wider diameter than the descending segment Epithelium with variable short brush border and tight junctions (impermeable to water)
Distal convoluted tubule		Renal cortex	Directly adjacent to the nephron's glomerulus Squamous epithelium of variable thickness with short brush border and tight junctions (impermeable to water)
Connecting tubule and collecting duct		Renal cortex and renal medulla	Connecting tubule collects urine from several nephrons Cuboidal or columnar epithelium Principal cells with tight junctions (with aldosterone receptors) α-Intercalated cells and β-intercalated cells with tight junctions

Structure of renal parenchyma

Juxtaglomerular complex

Definition
- 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)
Components
- Juxtaglomerular cells
  - Modified smooth muscle cells located in the afferent arterioles
  - Function: renin synthesis
- Macula densa
  - 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 ^[3]
  - 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

Cross section of renal corpuscle Macula densa

Function

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.”

Embryology

General

Origin: intermediate mesoderm
Development begins at the 4^th week of embryonic development.
Precursor cells migrate cranial to caudal.
The urogenital ridge gives rise to the pronephros, mesonephros, and metanephros.

Stages ^[5]

Pronephros: the first embryonic excretory organ (rudimentary in human embryos)
- Arises in the cervical region of the embryo during the 3^rd–4^th 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 4^th week of embryonic development
- Cranial end: functions as interim kidney during the 1^st trimester
  - Forms Bowman capsule
  - Functions until the end of the 2^nd 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 5^th week of embryonic development
- Canalization is complete by the 10^th week of embryonic development.
- Maturing of the kidneys continues until week 35–36 of embryonic development.
- Components
  - 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
- 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.

Renal embryology (stages) Genital embryology (first stage) Renal embryology (derivatives)

Clinical significance

References

Layton AT. Recent advances in renal hypoxia: insights from bench experiments and computer simulations. American Journal of Physiology. 2016.
$Neural Control of Renal Function.
Standring S. Gray's Anatomy: The Anatomical Basis of Clinical Practice. Elsevier Health Sciences ; 2016
Denic A, Lieske JC, Chakkera HA, Poggio ED, Alexander MP, Singh P, Kremers WK, Lerman LO, Rule AD. The Substantial Loss of Nephrons in Healthy Human Kidneys with Aging. J Am Soc Nephrol. 2017; 28 (1): p.313-320.doi: 10.1681/ASN.2016020154 . | Open in Read by QxMD
Jain S. Kidney Development and Related Anomalies. Elsevier ; 2014: p. 2701-2715
Feher JJ. Quantitative Human Physiology. Academic Press ; 2017
Kurklinsky AK, Rooke TW. Nutcracker phenomenon and nutcracker syndrome.. Mayo Clinic proceedings. 2010; 85 (6): p.552-9.doi: 10.4065/mcp.2009.0586 . | Open in Read by QxMD
Vaziri ND. Mechanism of erythropoietin-induced hypertension.. Am J Kidney Dis. 1999; 33 (5): p.821-8.doi: 10.1016/s0272-6386(99)70413-0 . | Open in Read by QxMD

3 free articles remaining

You have 3 free member-only articles left this month. Sign up and get unlimited access.

Have an account? Log In Start free trial

Evidence-based content, created and peer-reviewed by physicians. Read the disclaimer