Overview
This lecture explains glomerular filtration as a key kidney process, detailing nephron structure, main renal functions, and the filtration membrane's three selective layers.
Nephron Structure & Function
- The nephron is the kidney's functional unit, with about a million nephrons per kidney.
- The renal corpuscle consists of the glomerulus (a capillary network) and Bowman's capsule, initiating filtration.
- Blood enters via the afferent arteriole (wider) and exits via the efferent arteriole (narrower).
- Filtered fluid, now called filtrate, moves from the capsule to the proximal convoluted tubule (PCT), loop of Henle, distal convoluted tubule (DCT), and collecting duct.
Major Kidney Processes
- The kidney performs filtration (at the glomerulus), reabsorption (returns needed substances to blood), and secretion (final waste removal).
- Most filtered substances are reabsorbed; only select waste and excess ions are excreted.
- Peritubular capillaries surround the nephron for reabsorption and secretion.
Pressures in Filtration
- Hydrostatic pressure drives fluid through the glomerulus, similar to water pressure in a hose.
- Osmotic pressure involves water movement from areas of high to low concentration (osmosis).
- The size difference between afferent and efferent arterioles increases glomerular hydrostatic pressure.
Filtration Membrane Structure
- Filtrate crosses three layers:
- Fenestrated glomerular endothelium (small holes block large proteins/cells).
- Basement membrane (porous, further selective barrier).
- Filtration slits (gaps between podocyte extensions allow passage of small molecules).
- Only small molecules (water, ions, nitrogenous wastes) pass; large proteins and blood cells remain.
Substances in Filtrate
- Water and small ions (Na+, K+, Cl–) pass into the filtrate.
- Nitrogenous wastes (byproducts from cell metabolism) are the main substances eliminated.
- Most useful filtered substances are reabsorbed; only wastes are excreted.
Key Terms & Definitions
- Nephron — functional unit of the kidney that filters blood and forms urine.
- Renal corpuscle — structure composed of the glomerulus and Bowman's capsule.
- Filtration — process of moving fluid from blood into Bowman's capsule.
- Reabsorption — movement of filtered substances back into blood.
- Secretion — transfer of additional waste from blood to filtrate.
- Hydrostatic pressure — force exerted by fluid pushing out of the capillaries.
- Osmotic pressure — pressure caused by water moving across membranes due to solute differences.
- Filtration membrane — barrier with three layers (endothelium, basement membrane, filtration slits) that filters blood in the glomerulus.
- Podocytes — specialized cells with foot-like extensions forming filtration slits.
Action Items / Next Steps
- Review lab material on nephron structure and blood flow through the kidney.
- Study the three layers of the filtration membrane and what substances each allows to pass.