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

1. Glomerular filtration: H2O & solutes move from blood into nephron

2. Tubular reabsorption: Useful substances move from filtrate into blood

3. Tubular secretion: Wastes & excess substances move from blood into filtrate

4. 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

The filtration membrane is held in place by specialised podocytes

Auer Lab Life Sci Div Lawrence Berkley National Lab

Southern Illinois School of Medicine

© 2008 Paul Billiet ODWS

8

Step 1: Glomerular Filtration

Solute Blood Glomerular Filtrate

Water yes yesNaCl yes yesGlucose yes yesAmino Acids yes yesH+ ions yes yesPlasma Proteins yes noErythrocytes(blood cells)

yes no

Platelets yes no

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/ mg 100cm-3

Urea 0.03 0.03

Glucose 0.10 0.10

Amino acids 0.05 0.05

Salts 0.72 0.72

Proteins 8.00 0

© 2008 Paul Billiet ODWS

Filtrate cocktail anyone?

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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 ReabsorptionLoop 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/ mg 100cm-3

Urea 0.03 2.00

Glucose 0.10 0

Amino acids 0.05 0

Salts 0.72 1.50

Proteins 0 0

© 2008 Paul Billiet ODWS

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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

Variable permeability to water

Impermeable to water

Freely permeable to water

© 2008 Paul Billiet ODWS

The nephron osmoregulation

Variable permeability to water

Impermeable to water

Freely permeable to water

Na+ Na+ Na+

Active reabsorption

H2O H2O

Passive osmosis

80% of water reabsorbed

Ultrafiltration under pressure

© 2008 Paul Billiet ODWS

Variable permeability to water

Impermeable to water

Freely permeable to water

The nephron osmoregulation

Mo

re and

mo

re salty

H2O

H2O

H2O

H2O

H2O

H2O

Co

llecting

d

uct

Loop of

Henlé

H2O

H2O

Na+

Na+

Na+

© 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.

Homework