Structure and Function of a Nephron
Introduction
- Nephron: Basic functional unit of the kidneys
- Derived from Greek word Nephros (kidney)
- Each kidney: ~1 million nephrons
- Functions: Waste excretion, pH maintenance, blood pressure regulation
Basic Structure of a Nephron
- Location: Microscopic structures in the kidney
- Kidney regions: Outer cortex, inner medulla
- Nephrons span both cortex and medulla
- Length: 35-55 mm
- Two major portions: Bowman’s capsule and renal tubule
Bowman’s Capsule
- Closed, cup-like structure (discovered by Sir William Bowman)
- Encloses the glomerulus (cluster of blood vessels)
- Afferent vessel: Brings blood in
- Efferent vessel: Takes blood out
- Bowman’s capsule + glomerulus = Renal corpuscle
Renal Tubule
- Long, folded tube structure
- Emerges from Bowman’s capsule
- Ends into the duct system of the kidney
- Divided into three parts:
- Proximal convoluted tubule: Near Bowman’s capsule; highly folded
- Loop of Henle: Includes descending and ascending limbs; in renal medulla
- Distal convoluted tubule: Located far from Bowman’s capsule; also folded
Collecting Duct
- Distal convoluted tubules drain here
- Receives input from many nephrons
- Drains into larger collecting ducts, then renal pelvis, then ureter, bladder
Types of Nephrons
- Short cortical nephrons: Short loops of Henle; major part in cortex (80% of nephrons)
- Juxtamedullary nephrons: Long loops of Henle; in medulla (20% of nephrons)
Blood Supply to Nephron
- Capillaries: Network along nephron length
- Renal artery: Brings in impure blood
- Renal vein: Takes out filtered blood
Mechanisms of Urine Formation
- Filtration: At level of renal corpuscle via glomerular filtration barrier
- Blood enters through afferent vessel, filtered in glomerulus
- Filtration barrier: Blood vessel wall, basement membrane, podocytes with filtration slits
- Glomerular blood pressure drives filtration
- Glomerular filtrate moves into renal tubules
- Reabsorption: In renal tubules
- Most substances like glucose, sodium chloride reabsorbed back into blood
- Peritubular capillaries facilitate this
- Proximal convoluted tubule: 70% sodium chloride and water, 95% glucose and amino acids reabsorbed
- Loop of Henle: Concentration of urine, reabsorption of salts
- Secretion: Distal convoluted tubules
- Waste ions added to filtrate from blood (e.g., hydrogen, potassium, ammonium ions)
- Proximal convoluted tubule also involved in lesser secretion (e.g., urea, uric acid)
- Excretion: Final urine passed into collecting duct, then renal pelvis, then ureter/bladder
Concentration of Urine
- Outer cortex vs. inner medulla
- Loop of Henle: Key in creating high osmolarity in medulla
- Ascending limb: Pumps out ions, impermeable to water
- Descending limb: Permeable to water (aided by aquaporins)
- Overall: Extracts water, concentrates urine
Collecting Duct and Antidiuretic Hormone
- Collecting duct: Common drain for multiple nephrons
- Antidiuretic hormone (ADH): Regulates urine concentration
- Processed urine flows to renal pelvis, ureters, bladder, then urethra
Summary
- Nephron: Essential for multiple kidney functions
- Filtration, reabsorption, secretion, excretion: Key processes in urine formation
- ADH: Important hormone regulating urine concentration
Next Steps
- Upcoming video on the human excretory system incl. urethra and other organs
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