Explain the role of different segments of nephron in urine formation and osmoregulation with the help of a neat labelled diagram!

A Nephron has two parts:

• Glomerulus
• Renal tubule

Glomerulus

Tuft of capillaries formed by the afferent arteriole

Renal Tubule : Has many parts
(Bowman’s capsule      Proximal Convoluted Tubule (PCT)   Hairpin-shaped Loop of Henle      Distal Convoluted Tubule (DCT)   Collecting duct   Medullary pyramids → Renal pelvis.)

Glomerulus + Bowman’s Capsule = Malpighian body (Renal Corpuscle)

Urine Formation

Involves 3 basic steps:

• Glomerular filtration

• Re-absorption

• Secretion

Glomerular filtration

• Kidney filters 1100−1200 mL blood/min

• Filtration of blood occurs as it passes through three layers.

  • Endothelium of glomerular blood vessels

  • Epithelium of Bowman’s capsule

  • Basement membrane between these two layers

• Epithelial cells of Bowman’s capsule are called podocytes. Arrangement of these podocytes leaves extremely minute spaces called filtration slits or slit pores.

• Ultrafiltration occurs through these slits pores, i.e., all plasma components (except proteins) get filtered into the lumen of the Bowman’s capsule.

• Glomerular Filtration Rate (GFR): 125 mL/min or 180 L/day

• The glomerular filtration rate is regulated by the juxta glomerular apparatus which releases the hormone renin.

Re-absorption

• GFR = 180 L/day; Urine released = 1.5 L/day. Thus, 99% of filtrate is re-absorbed by the renal tubules.

• Re-absorption occurs through the epithelial cells of the various segments of the nephrons.

• Active Re-absorption: Glucose, Na ⁺, amino acids

• Passive Re-absorption: Nitrogenous waste, water

Secretion

• Tubular cells secrete substances such as H ⁺, K ⁺   and ammonia into the filtrate.

• Importance: Maintenance of ionic and acid-base balance of body fluids

Function of the Tubules

• Proximal Convoluted Tubule

  • Specialised for re-absorption as it is lined by a simple cuboidal brush border epithelium which increases the surface area for absorption

  • Re-absorbs all essential nutrients, electrolytes and water

  • Secretes H⁺, NH₄⁺, K⁺ ions and    to maintain pH

• Henle’s Loop

  • Minimum re-absorption occurs here

  • Helps in maintaining high osmolarity of the medullary fluid

  • Descending loop of Henle: Permeable to water and impermeable to electrolytes; Concentrates the filtrate

  • Ascending loop of Henle: Impermeable to water and permeable to electrolytes; Dilutes the filtrate

• DCT

  • Conditional re-absorption of Na⁺, water, HCO ₃⁻

  • Selectively secretes H⁺, K⁺, NH₃

  • Maintains pH and Na−K balance in blood

• Collecting Duct

  • Concentrates the urine by absorbing large amounts of water

  • Allows passage of urea into the medullary interstitial fluid to maintain osmolarity

  • Secretes H⁺ and K⁺   ions; hence, maintains pH and ionic balance