SBI4U Urine Formation in the Nephron

Reminders  the Nephron What part of the kidney is the top half of the nephron in? What part of the kidney is the lower half of the nephron in? What is the nephron surrounded by?

Four Processes in Urine Formation Glomerular filtration: H2O & solutes move from blood into nephron Tubular reabsorption: Useful substances move from filtrate into blood Tubular secretion: Wastes & excess substances move from blood into filtrate Water reabsorption: H2O moves from filtrate into blood

Remember… filtrate?

Step 1: Glomerular Filtration Afferent arteriole branches from renal artery to supply blood to glomerulus Small pores in capillaries only allow water, salts, nutrients, and wastes to pass through into the nephron = filtrate (pre-urine) Blood pressure is 4x higher to facilitate filtration

Filtration in the glomerulus Southern Illinois School of Medicine Auer Lab Life Sci Div Lawrence Berkley National Lab The filtration membrane is held in place by specialised podocytes © 2008 Paul Billiet ODWS

Step 1: Glomerular Filtration Solute Blood Glomerular Filtrate Water yes NaCl Glucose Amino Acids H+ ions Plasma Proteins no Erythrocytes (blood cells) Platelets Why can’t plasma proteins, blood cells, and platelets pass through the glomerulus and enter the nephron?

Blood plasma v Filtrate Component Plasma / mg 100cm-3 Filtrate Urea 0.03 Glucose 0.10 Amino acids 0.05 Salts 0.72 Proteins 8.00 Filtrate cocktail anyone? © 2008 Paul Billiet ODWS

Did you know? Each nephron has an independent blood supply (and there are millions of them!) The glomerulus is a high-pressure filter Each day, 1600 L to 2000 L of blood pass through your kidneys, producing 180 L of filtrate. This filtrate is identical to blood plasma, minus proteins and blood cells. The greater the blood pressure, the more nephric filtrate forms

Step 2: Tubular Reabsorption Proximal tubule 65% of filtrate is reabsorbed into body! Active transport: glucose, amino acids, Na+, K+ Passive transport: Cl-, osmosis

Step 2: Tubular Reabsorption Loop of Henle (reabsorption of H2O & ions) a) Descending limb of loop of Henle surrounded by salty medulla Cell membranes are permeable to H2O  H2O leaves filtrate to surrounding capillaries by osmosis Concentration of Na+ in filtrate  toward bottom of loop

Step 2: Tubular Reabsorption b) Ascending limb of Loop of Henle Thin segment  Cell membranes are impermeable to H2O but permeable to solutes Na+ diffuses out of filtrate into surrounding capillaries

Step 2: Tubular Reabsorption b) Ascending limb of Loop of Henle Thick segment  sodium ions are actively transported out of the filtrate into capillaries Replenishes medulla’s salty environment and makes filtrate less concentrated

Step 3: Tubular Secretion Distal Tubule Secretion of K+, H+ (for pH balance), and medications from blood into DT actively Some reabsorption continues Passive: Cl- by diffusion H2O by osmosis Active: HCO3- (for pH balance)

Step 4: Water Reabsorption Collecting Duct Located in medulla (salty) H2O reabsorbed into blood stream by osmosis Concentrates urine according to body needs Hormones (ADH) control permeability of membranes to H2O Filtrate is now called urine

Reabsorption of Water

Kidney reabsorption Component Filtrate / mg 100cm-3 Urine Urea 0.03 2.00 Glucose 0.10 Amino acids 0.05 Salts 0.72 1.50 Proteins © 2008 Paul Billiet ODWS

Pathway of Filtrate through the Nephron Bowman’s Capsule  proximal tubule  loop of Henle  distal tubule  collecting duct http://www.sumanasinc.com/webcontent/animations/content/kidney.html

The nephron functions Freely permeable to water Impermeable to water Variable permeability to water © 2008 Paul Billiet ODWS

The nephron osmoregulation Active reabsorption Variable permeability to water Impermeable to water Freely permeable to water Na+ Na+ Na+ H2O H2O Passive osmosis Ultrafiltration under pressure 80% of water reabsorbed © 2008 Paul Billiet ODWS

The nephron osmoregulation Variable permeability to water Impermeable to water Freely permeable to water Collecting duct More and more salty Loop of Henlé Na+ H2O H2O © 2008 Paul Billiet ODWS

Interesting Body Quirks Why is pee yellow? The yellow color in urine is due to chemicals called urobilins. These are the breakdown products of the bile pigment bilirubin. Bilirubin is itself a breakdown product of the heme part of hemoglobin from worn-out red blood cells. Most bilirubin is partly broken down in the liver, stored in the gall bladder, broken down some more in the intestines, and excreted in the feces (its metabolites are what make feces brown), but some remains in the bloodstream to be extracted by the kidneys where, converted to urobilins, it gives urine that familiar yellow tint.

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