please explain the procees of urine formation and working of nephron in detail
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, HCO3−
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