Definition for kidney
The kidneys are a pair of bean-shaped organs located in the retroperitoneal space on either side of the vertebral column. Each kidney filters blood, removes waste products and excess water as urine, regulates electrolytes and acid-base balance, and produces hormones like erythropoietin and renin.
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Here is a comprehensive definition and overview of the kidney, drawn from authoritative medical textbooks:
Kidney - Definition and Overview
What is the Kidney?
The kidneys are paired ovoid, reddish-brown retroperitoneal organs situated in the posterior abdomen on each side of the vertebral column. Each adult kidney measures 10-12 cm in length, 5.0-7.5 cm in width, and 2.5-3.0 cm in thickness, weighing approximately 125-170 g in males (10-15 g less in females). - Campbell Walsh Wein Urology
Location and Relationships
- The kidneys lie on the psoas muscles with their upper poles more medial and posterior than the lower poles.
- Due to downward displacement by the liver, the right kidney sits 1-2 cm lower than the left, occupying L1-L3; the left kidney spans T12-L3.
- Each kidney is enclosed in Gerota's fascia (a fibro-fatty layer), which provides protection against bleeding.
- The kidneys shift inferiorly by approximately 3 cm during inspiration and with change from supine to erect posture. - Campbell Walsh Wein Urology
Structure
The kidney has two main zones:
- Cortex - outer layer
- Medulla - inner layer, divided into outer and inner medulla; the medullary tip (papilla) protrudes into the renal pelvis.
The specific components of the kidney are:
- Nephrons - approximately 1 million per kidney (established during prenatal development; cannot be regenerated after birth)
- Collecting ducts - formed when several nephrons join; cortical collecting ducts descend into the medulla and open as papillary ducts into the renal pelvis
- Microvasculature - the renal artery branches into interlobar arteries → arcuate arteries → cortical radial arteries → afferent arterioles → glomerular tufts. No arteries directly penetrate the medulla. - Comprehensive Clinical Nephrology, 7th Edition
The Nephron - Basic Functional Unit
The nephron consists of:
- A renal (Malpighian) corpuscle - Bowman's capsule surrounding the glomerulus
- A glomerulus - a tuft of capillaries receiving blood from an afferent arteriole, exiting via an efferent arteriole
- A tubular system - proximal tubule → loop of Henle (descending thin limb, ascending thin limb, thick ascending limb) → distal tubule → collecting duct - Goodman & Gilman's Pharmacological Basis of Therapeutics
Functions
| Function | Detail |
|---|---|
| Filtration | Two kidneys produce ~120 mL ultrafiltrate/min; >99% is reabsorbed |
| Excretion | Removes metabolic waste (urea, creatinine, drugs) |
| Reabsorption | ~65% of filtered Na⁺ reabsorbed in proximal tubule; loop of Henle reclaims ~25% more |
| Urine concentration | Loop of Henle creates osmotic gradient; collecting duct concentrates final urine |
| Endocrine | Produces erythropoietin (red cell production), renin (blood pressure regulation), and activates vitamin D |
| Acid-base balance | Regulates bicarbonate and hydrogen ion excretion |
The kidneys consume 7% of total-body oxygen despite comprising only 0.5% of body weight - a reflection of their enormous metabolic activity. - Goodman & Gilman's Pharmacological Basis of Therapeutics
Key Facts Summary
- Number: 2 (paired)
- Shape: Ovoid / bean-shaped
- Color: Reddish-brown
- Position: Retroperitoneal
- Size: ~10-12 cm long; ~150 g each
- Functional unit: Nephron (~1 million per kidney)
- Urine output: ~1 mL/min (from 120 mL/min filtered)
- Enclosure: Gerota's fascia
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