Unit
- Gross anatomy: The kidneys are paired retroperitoneal organs located on either side of the vertebral column at the T12-L3 level. Each kidney measures approximately 11-14 cm in length and weighs 120-170 grams. The right kidney sits slightly lower due to the liver's position.
- Cortical and medullary organization: The outer renal cortex contains glomeruli and convoluted tubules, while the inner medulla consists of renal pyramids and collecting ducts. This arrangement optimizes urine concentration and waste removal.
- Blood supply: Renal arteries branch from the abdominal aorta, delivering 20-25% of cardiac output. The high perfusion supports filtration and metabolic exchange. Peritubular capillaries and vasa recta surround nephrons for reabsorption and secretion.
- Nephron as functional unit: Each kidney contains about 1 million nephrons, each composed of a glomerulus and a tubule. The glomerulus filters blood, and the tubule modifies the filtrate through reabsorption and secretion.
- Collecting system: Urine drains from collecting ducts into minor calyces, then major calyces, and finally the renal pelvis. The ureter transports urine to the bladder via peristaltic contractions.
- Innervation: Sympathetic nerves from the renal plexus regulate blood flow and renin release. Parasympathetic input is minimal; the kidneys operate largely autonomously.
- Lymphatic drainage: Renal lymphatics follow blood vessels and drain to para-aortic nodes. They remove excess interstitial fluid and immune cells.
- Clinical relevance: Knowledge of kidney size and location aids in imaging interpretation. A palpable kidney may indicate hydronephrosis or polycystic disease.
- Filtration barrier: The glomerular filtration barrier comprises fenestrated endothelium, the basement membrane, and podocyte slit diaphragms. It restricts passage of cells and large proteins based on size and negative charge. Damage to this barrier, as in nephrotic syndrome, leads to proteinuria.
- Proximal tubule function: This segment reabsorbs 60-70 percent of filtered sodium and water, plus nearly all glucose and amino acids via active transport. It is also a major site for secretion of organic acids and bases, toxins, and drugs, making it crucial for pharmacokinetics.
- Loop of Henle and concentration: The descending limb is permeable to water but not solutes, while the ascending limb actively extrudes sodium, chloride, and potassium. This countercurrent multiplication creates a hyperosmotic medullary interstitium, enabling the collecting duct to concentrate urine under ADH influence.
- Endocrine functions: The kidneys act as endocrine organs, releasing erythropoietin to regulate red cell mass, activating vitamin D to control calcium and phosphate metabolism, and secreting renin to initiate the RAAS cascade, thereby influencing systemic blood pressure and fluid balance.
- Acid-base regulation: The kidneys maintain acid-base homeostasis by reclaiming filtered bicarbonate in the proximal tubule and excreting titratable acids and ammonium in the distal nephron. This process adjusts slowly but provides complete compensation for metabolic acid-base disturbances.
Terminology
- Glomerulus
- A tuft of capillaries where blood filtration begins.
- Nephron
- The microscopic structural and functional unit of the kidney.
- Renal cortex
- The outer portion of the kidney containing glomeruli and proximal tubules.
- Renal medulla
- The inner region containing loops of Henle, collecting ducts, and pyramids.
Quick Revision
- Kidneys are retroperitoneal, each with cortex and medulla.
- Blood supply is high (20-25% cardiac output) via renal arteries.
- Nephrons are the functional units; each kidney has ~1 million.
- Urine drains from collecting ducts to renal pelvis and ureter.