Overview
This lecture covers the mechanisms of urine formation: filtration, reabsorption, and secretion, focusing on nephron structure, function, and regulation.
Filtration Process
- Filtration occurs in the renal corpuscle, made up of the glomerulus (capillary bed) and the glomerular (Bowman's) capsule.
- Blood enters the glomerulus via the afferent arteriole and leaves via the efferent arteriole.
- Filtration relies on high pressure due to a wider afferent and narrower efferent arteriole.
- Small molecules (water, salts, sugars, urea) exit the blood into Bowman's capsule as filtrate; large molecules (blood cells, proteins) remain in blood.
- Filtrate is not urine until it has been modified by later processes.
Structure and Function of Renal Tubules
- Renal tubules include the proximal convoluted tubule (PCT), loop of Henle, and distal convoluted tubule (DCT); the collecting duct is related but separate.
- The PCT has simple cuboidal cells with microvilli for primary reabsorption of water, salts, and nutrients.
- The loop of Henle (descending and ascending limbs) assists in reabsorption, particularly of water and salts.
- The DCT mainly handles secretion, moving unwanted substances from blood into filtrate.
- The collecting duct responds to antidiuretic hormone (ADH) to regulate water conservation via aquaporins.
Reabsorption and Secretion
- Reabsorbed substances return to blood via peritubular capillaries and vasa recta.
- Reabsorption ensures most water, nutrients, and useful ions return to blood.
- Secretion in the DCT adds wastes (urea, drugs, hydrogen ions) from blood into filtrate for removal.
- Once filtrate reaches the minor calyx, it becomes urine.
Juxtaglomerular Apparatus (JGA)
- The JGA helps regulate blood pressure, filtration rate, and kidney function.
- The JGA is located where the DCT meets the afferent and efferent arterioles.
- Juxtaglomerular cells (in afferent arteriole) monitor and adjust blood pressure.
- Macula densa cells (in DCT) monitor filtrate concentration.
- Mesangial cells help regulate filtration (function not fully understood).
Key Terms & Definitions
- Filtration — Movement of water and small solutes from blood into Bowman's capsule.
- Reabsorption — Process of moving useful substances from filtrate back into blood.
- Secretion — Transfer of additional wastes from blood into the renal tubules.
- Afferent arteriole — Blood vessel bringing blood into the glomerulus.
- Efferent arteriole — Blood vessel carrying blood away from the glomerulus.
- Podocytes — Foot-like cells covering glomerular capillaries, forming filtration slits.
- Peritubular capillaries/Vasa recta — Blood vessels involved in reabsorption around renal tubules.
- Aquaporins — Water channels in the collecting duct regulated by ADH.
- Juxtaglomerular apparatus (JGA) — Area where DCT contacts afferent/efferent arterioles, regulating kidney function.
Action Items / Next Steps
- Review images/diagrams of nephron structure and JGA.
- Prepare for physiology topics: osmotic pressure, permeability, and hormonal regulation of kidney function.