NEPHRON

The nephron is the basic structural and functional unit of the kidney. It is responsible for filtering blood, removing waste, and maintaining fluid and electrolyte balance in the body. Here’s everything you need to know about the nephron:

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1. Structure of the Nephron

A nephron is composed of two main parts: the renal corpuscle and the renal tubule.

a) Renal Corpuscle

• The renal corpuscle is where blood filtration begins.
• Components:
  1. Glomerulus: A network of capillaries that filters blood. It allows water, ions, and small molecules to pass through while retaining blood cells and large proteins.
  2. Bowman’s Capsule: A cup-shaped structure that surrounds the glomerulus and collects the filtered fluid (filtrate).

b) Renal Tubule

• The renal tubule processes the filtrate to form urine.
• Sections:
  1. Proximal Convoluted Tubule (PCT):
     • Reabsorbs nutrients like glucose, amino acids, and water.
     • Most of the water and sodium ions are absorbed here.
  2. Loop of Henle:
     • Descending Limb: Permeable to water, allowing water to be reabsorbed.
     • Ascending Limb: Impermeable to water but actively reabsorbs sodium and chloride ions.
  3. Distal Convoluted Tubule (DCT):
     • Regulates ion balance, pH, and water content of the filtrate under hormonal control (e.g., aldosterone and parathyroid hormone).
  4. Collecting Duct:
     • Collects urine from multiple nephrons.
     • Adjusts water reabsorption based on the body’s hydration levels (regulated by antidiuretic hormone (ADH)).

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2. Functions of the Nephron

• Filtration:
  • Blood enters the glomerulus, and plasma is filtered into the Bowman’s capsule, forming the filtrate.
• Reabsorption:
  • Useful substances (e.g., glucose, amino acids, water, and ions) are reabsorbed back into the blood from the tubule.
• Secretion:
  • Waste products (e.g., urea, creatinine, and hydrogen ions) are actively secreted into the tubule.
• Excretion:
  • The final product, urine, is excreted from the body via the collecting ducts, ureters, bladder, and urethra.

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3. Types of Nephrons

• Cortical Nephrons:
  • Located mainly in the renal cortex.
  • Have shorter loops of Henle.
  • Account for about 85% of all nephrons.
• Juxtamedullary Nephrons:
  • Located near the medulla.
  • Have longer loops of Henle, extending deep into the medulla.
  • Essential for concentrating urine and conserving water.

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4. Blood Supply to the Nephron

• Afferent Arteriole: Brings blood to the glomerulus.
• Efferent Arteriole: Carries blood away from the glomerulus.
• Peritubular Capillaries:
  • Surround the proximal and distal tubules for nutrient and ion exchange.
• Vasa Recta:
  • Specialized capillaries around the loop of Henle in juxtamedullary nephrons.

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5. Hormonal Regulation in the Nephron

• Antidiuretic Hormone (ADH):
  • Increases water reabsorption in the collecting duct.
• Aldosterone:
  • Promotes sodium reabsorption and potassium secretion in the distal tubule.
• Parathyroid Hormone (PTH):
  • Increases calcium reabsorption in the distal tubule.
• Renin-Angiotensin System (RAS):
  • Regulates blood pressure and fluid balance.

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6. Key Processes in the Nephron

a) Glomerular Filtration:

• Occurs in the glomerulus.
• Filters approximately 180 liters of fluid daily, but most of it is reabsorbed.

b) Tubular Reabsorption:

• 99% of filtrate is reabsorbed.
• Reabsorption occurs primarily in the PCT and loop of Henle.

c) Tubular Secretion:

• Eliminates waste products and maintains acid-base balance.

d) Urine Concentration:

• Controlled by the loop of Henle and the collecting duct.
• Long loops of Henle create a concentration gradient in the medulla, which allows for water conservation.

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7. Clinical Relevance

• Kidney Diseases:
  • Glomerulonephritis: Inflammation of the glomerulus.
  • Chronic Kidney Disease (CKD): Progressive loss of kidney function.
• Diabetes and Nephrons:
  • High blood sugar damages nephrons, leading to conditions like diabetic nephropathy.
• Dialysis:
  • A treatment for kidney failure where a machine performs the filtering function of nephrons.

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8. Summary of Nephron Functions

Part of Nephron	Function
Glomerulus	Filters blood plasma.
Bowman’s Capsule	Collects filtrate from the glomerulus.
Proximal Convoluted Tubule	Reabsorbs nutrients, ions, and water.
Loop of Henle	Creates a concentration gradient for urine concentration.
Distal Convoluted Tubule	Regulates ion and pH balance.
Collecting Duct	Concentrates urine and reabsorbs water (under ADH).