What is the Structure and Function of a Nephron with step-by-step urine formation process?

Direct Answer

A Nephron is the microscopic structural and functional filtering unit of the human kidney. Each adult kidney contains approximately 1 to 1.2 million nephrons, responsible for removing metabolic nitrogenous wastes (urea, uric acid) from the blood, maintaining water-electrolyte balance (osmoregulation), and regulating blood pressure.

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Structural Components of a Nephron

Each nephron consists of two major structural divisions:

#### 1. Renal Corpuscle (Malpighian Body)

• Glomerulus: A high-pressure tuft of microscopic blood capillaries supplied by a wide *afferent arteriole* and drained by a narrower *efferent arteriole*.
• Bowman's Capsule: A double-walled cup-shaped epithelial sac that encloses the glomerulus to collect filtrate.

#### 2. Renal Tubule

• Proximal Convoluted Tubule (PCT): Highly coiled segment lined with microvilli brush border cells for maximum reabsorption.
• Loop of Henle: Hairpin-shaped loop with a thin descending limb (permeable to water) and a thick ascending limb (permeable to salts).
• Distal Convoluted Tubule (DCT): Coiled segment responding to hormones (Aldosterone and ADH/Vasopressin).
• Collecting Duct: Collects concentrated urine from multiple nephrons and drains into the renal pelvis.

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The Three Steps of Urine Formation

```mermaid

flowchart LR

A["1. Glomerular Ultrafiltration (Afferent High Pressure)"] --> B["2. Selective Tubular Reabsorption (PCT & Loop of Henle)"]

B --> C["3. Tubular Secretion (DCT: H+, K+, NH3)"]

C --> D["Final Urine Output (1-1.5 L/day)"]

```

#### Step 1: Glomerular Ultrafiltration

• Blood flows into the glomerulus under elevated hydrostatic pressure because the afferent arteriole is significantly wider than the efferent arteriole.
• Water, glucose, amino acids, urea, and electrolytes are forced across the filtration membrane (endothelial fenestrations, basement membrane, and podocyte slit pores) into Bowman's space.
• Blood cells and large plasma proteins are retained in the bloodstream.
• Glomerular Filtration Rate (GFR): $\approx 125\text{ mL/min}$ or 180 Litres per day.

#### Step 2: Selective Tubular Reabsorption

• Out of 180 L of filtrate formed daily, 99% is reabsorbed back into the peritubular capillaries, leaving only about 1.5 L of urine excreted.
• In the PCT: $100\%$ of glucose and amino acids, and $70-80\%$ of water and electrolytes ($\text{Na}^+, \text{Cl}^-$) are reabsorbed actively and passively.
• In the Loop of Henle: Concentrates the medulla through counter-current multiplication.

#### Step 3: Tubular Secretion

• Cells of the DCT and collecting duct actively secrete surplus hydrogen ions ($\text{H}^+$), potassium ions ($\text{K}^+$), and ammonia ($\text{NH}_3$) from the blood into the tubular filtrate.
• This step is crucial for maintaining proper blood pH ($7.35 - 7.45$) and electrolyte homeostasis.