Chapter 2: Renal Anatomy & Physiology

Gross Anatomy

The kidneys are paired retroperitoneal organs positioned between T12-L3 vertebral levels. Each kidney weighs approximately 120-170 grams and measures 10-12 cm in length, 5-6 cm in width, and 3-4 cm in thickness. The right kidney sits slightly lower than the left due to liver displacement. Blood supply comes from the renal arteries, which receive 20-25% of cardiac output despite the kidneys representing only 0.5% of body weight.

Internal Architecture

Cortex: The outer region, containing glomeruli, proximal and distal convoluted tubules. It has a granular appearance due to the high density of these structures.

Medulla: The inner region, composed of renal pyramids. It contains the loops of Henle and collecting ducts, responsible for the urine concentration gradient.

Pelvis: A funnel-shaped structure that collects urine from the major calyces and drains it into the ureter.

Microscopic Anatomy - The Nephron

Each kidney contains approximately one million nephrons, the functional units of the kidney. Understanding nephron anatomy is essential because different diseases affect specific segments, creating distinctive clinical patterns that guide diagnosis and treatment.

Components of the Nephron

1. The Glomerulus

A tuft of capillaries surrounded by Bowman's capsule, this is the site of ultrafiltration. The highly specialized glomerular filtration barrier consists of three layers:

Fenestrated Endothelium: Allows passage of solutes, plasma, and water but blocks blood cells.
Glomerular Basement Membrane (GBM): A negatively charged matrix that repels large proteins like albumin.
Podocyte Foot Processes: Interdigitating extensions of specialized epithelial cells that form filtration slits, providing the final barrier to protein leakage.

2. The Proximal Tubule

The workhorse of the nephron, the proximal convoluted tubule (PCT) reabsorbs approximately 65% of filtered sodium and water, 100% of filtered glucose and amino acids, and most filtered bicarbonate. It is also the primary site of organic acid and base secretion, making it a common target for drug-induced toxicity.

3. The Loop of Henle

This U-shaped segment creates the hypertonic medullary interstitium necessary for concentrating urine via the countercurrent multiplication mechanism.

Descending Limb: Highly permeable to water, impermeable to solutes.
Ascending Limb: Impermeable to water; the thick segment actively reabsorbs sodium via the Na-K-2Cl cotransporter (the target of loop diuretics).

4. The Distal Convoluted Tubule (DCT)

Responsible for the fine-tuning of sodium, potassium, and calcium balance. It's the site of action for thiazide diuretics and where parathyroid hormone regulates calcium reabsorption.

5. The Collecting Duct

The final site of urine processing, where multiple nephrons drain. It has two main cell types:

Principal Cells: Regulate sodium reabsorption and potassium secretion, primarily under the influence of aldosterone. Also the site of ADH-mediated water reabsorption.
Intercalated Cells: Fine-tune acid-base balance by secreting H+ or HCO3-.

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