D4.2 - Formation of Urine PDF

D4.2 - Formation of Urine Formation of Urine Involves three processes: Glomerular formation: Moves water and solutes from blood plasma into nephron Tubular reabsorption: Removes useful substances from filtrate and returns them to blood for reuse Tubular secretion: Moves additio...

D4.2 - Formation of Urine Formation of Urine Involves three processes: Glomerular formation: Moves water and solutes from blood plasma into nephron Tubular reabsorption: Removes useful substances from filtrate and returns them to blood for reuse Tubular secretion: Moves additional wastes and excess substances from blood to filtrate Glomerular Filtration Blood moves into glomerulus - acts as a high pressure filter Dissolved solutes pass through walls of glomerulus into Bowman’s capsule, creating filtrate (liquid part that has passed through filter) ○ Filtrate is similar to blood plasma (has similar substances dissolved in it) Plasma proteins, platelets, and erythrocytes don’t pass into Bowman’s capsule because they are too large to pass through the membrane Tubular Reabsorption 65% of filtrate passes through proximal tubule (including Loop of Henle) is reabsorbed and returned to body Involves both active and passive transport mechanisms Tubular Reabsorption Loop of Henle’s function is to reabsorb water and ions from glomerular filtrate ○ Cells of descending limb are permeable to water and slightly permeable to ions ○ Water diffuses from filtrate into capillaries by osmosis ○ Na+ ion concentration inside tubule increases ○ Near bottom of loop and ascending tubule, the permeability changes so that it’s now impermeable to water and slightly permeable to ions ○ Na+ ions diffuse from filtrate into surrounding blood vessels At thick-walled portion of ascending limb, carrier molecules carry Na+ ions out of nephron into blood - via active transport, which are followed by passive diffusion of Cl-- and HCO3- ions due to electrical attraction This helps replenish salty environment of medulla which aids in the absorption of water from filtrate in descending limb Tubular Secretion Potassium ions, K+, actively secreted from capillaries into distal tubule Hydrogen ions, H+, actively secreted into distal tubule in order to maintain pH of blood Drugs and other substances not normally a part of body are secreted into distal tubule Filtrate is collected in collecting duct ○ Still contains large amount of water ○ Passive reabsorption of water from filtrate through osmosis ○ If dehydrated, the permeability to water in distal tubule and collecting duct is increased Causes more water to be reabsorbed - conserves water loss ○ Reabsorption of water causes filtrate to become 4x more concentrated Filtrate which is 1% of original filtrate volume is now called urine