Kidney

Surface projections of the organs in the trunk, including kidneys

Detailed anatomical structures of the human kidney

Right kidney as seen on abdominal ultrasound

CT scan showing a human kidney

Histological section of kidney tissue under microscope

Light microscopy of kidney glomerulus

Kidney histology highlighting tubules

The kidneys are paired, bean-shaped organs in the vertebrate urinary system responsible for filtering blood and excreting waste products and excess fluids in the form of urine. They play a central role in homeostasis, including regulating electrolyte balance, blood pressure, and acid-base balance. The study of the kidneys is known as nephrology (from Greek nephros meaning "kidney"), while the term renal (from Latin renes) is the adjective often used in medical contexts.

Anatomy and Location

In humans, the kidneys are located in the posterior portion of the abdominal cavity, one on each side of the vertebral column. They lie between vertebral levels T12 and L3 and are considered retroperitoneal, meaning they are located behind the peritoneum. The right kidney typically lies slightly lower than the left to accommodate the overlying liver. Each kidney is topped by an adrenal gland.

The kidneys are partially protected by the 11th and 12th pairs of ribs and are cushioned by a double layer of fat: perirenal and pararenal fat. Rare anatomical variations such as a third kidney may occur.

Structure

Each adult kidney is approximately 11 cm long and 5 cm thick, weighing about 150 grams. The kidney has a concave medial border with an opening called the renal hilum, where the renal artery, renal vein, ureter, and nerves enter and exit.

Gross anatomy

• Renal capsule – A tough fibrous layer surrounding the kidney
• Renal cortex – The outer layer containing glomeruli and blood vessels
• Renal medulla – The inner region organized into cone-shaped renal pyramids
• Renal pelvis – Funnel-shaped structure collecting urine from calyces
• Ureter – Tube carrying urine to the bladder
• Calyces – Chambers that collect urine from the pyramids
• Papilla – Tip of each renal pyramid that empties into a calyx

Blood Supply

The kidneys receive about 20–25% of the cardiac output through the renal artery, a direct branch of the abdominal aorta. The renal artery branches into:

• Interlobar arteries → Arcuate arteries → Interlobular (cortical radial) arteries
• Afferent arterioles → Glomerulus → Efferent arterioles
• Peritubular capillaries and vasa recta

Filtered blood exits through the renal vein into the inferior vena cava.

Nephron

Main article: Nephron

Each kidney contains over a million nephrons – the microscopic functional units. Each nephron consists of:

• Glomerulus – site of filtration
• Bowman's capsule
• Proximal convoluted tubule
• Loop of Henle
• Distal convoluted tubule
• Collecting duct system

Processes:

• Filtration of blood plasma
• Reabsorption of vital substances (e.g., glucose, Na⁺)
• Secretion of waste (e.g., H⁺, drugs)
• Excretion as urine

Collecting Duct System

Main article: Collecting duct system

The collecting ducts transport urine through the renal medulla to the renal pelvis. Antidiuretic hormone (ADH) regulates water permeability:

• ↑ADH = concentrated urine
• ↓ADH = dilute urine

Deficiency or resistance to ADH can result in diabetes insipidus.

Physiological Functions

The kidneys filter roughly 180 liters of blood daily, forming 1.5–2 liters of urine. Functions include:

Waste Excretion

Removal of nitrogenous waste:

• Urea
• Uric acid
• Creatinine

Electrolyte and Fluid Balance

Regulation of:

• Sodium (Na⁺)
• Potassium (K⁺)
• Calcium (Ca²⁺)
• Phosphate
• Water balance

Acid-Base Balance

Maintenance of blood pH (~7.4) by:

• H⁺ excretion
• HCO₃⁻ reabsorption

Main transporters:

• NHE3, H⁺-ATPase, NBC1, AE1

Blood Pressure Regulation

Via the Renin–angiotensin–aldosterone system (RAAS):

• ↓Blood pressure → ↑Renin → ↑Angiotensin II → ↑Aldosterone → Na⁺ retention → ↑Blood volume → ↑Blood pressure

Hormone Secretion

• Erythropoietin – stimulates red blood cell production
• Renin – blood pressure regulation
• Calcitriol – active vitamin D for calcium homeostasis

Medical Terminology

• Nephro- (Greek) and Renal- (Latin) denote kidney-related terms
• Nephrectomy = surgical removal of a kidney
• Radical nephrectomy = removal of the kidney with surrounding structures

Diseases and Disorders

Congenital

• Polycystic kidney disease
• Horseshoe kidney
• Renal dysplasia
• Congenital hydronephrosis
• Duplicated ureter

Acquired

• Glomerulonephritis
• Nephrotic syndrome
• Pyelonephritis
• Diabetic nephropathy
• Kidney stones
• Renal cell carcinoma
• Lupus nephritis
• Minimal change disease
• Renal failure (acute or chronic)

Chronic Kidney Disease (CKD) Staging

CKD is staged by glomerular filtration rate (GFR):

• Stage 1: Normal GFR (>90), kidney damage
• Stage 2: GFR 60–89
• Stage 3: GFR 30–59, moderate dysfunction
• Stage 4: GFR 15–29, severe damage
• Stage 5: GFR <15, End-stage renal disease (ESRD), requires dialysis or transplant

Dialysis and Transplantation

• Dialysis removes waste via machine (hemodialysis) or abdominal lining (peritoneal dialysis)
• Kidney transplantation offers a curative option
  • First successful transplant: 1954, Boston
  • Types: Living donor and Deceased donor
• Immunosuppressive therapy is required post-transplant

Gallery

• 
  Posterior view of the left kidney
• 
  Schematic of a kidney nephron
• 
  Diagram showing nephron function
• 
  Mechanism of secretion and reabsorption in nephrons
• 
  Diagram of peritoneal dialysis setup

See Also

• Urology
• Nephrology
• Urinary system
• Kidney development
• Endocrine system
• Kidney transplantation
• Glomerular filtration rate