1. SBI4U Urine Formation in the Nephron

2. 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?

4. 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

5. Remember… filtrate?

6. 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

7. 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

8. 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?

9. 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

10. 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

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

12. 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

13. 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

14. 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

15. 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)

16. 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

17. Reabsorption of Water

18. 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

19. Pathway of Filtrate through the Nephron
Bowman’s Capsule  proximal tubule 
loop of Henle 
distal tubule  collecting duct

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

21. 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

22. 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

23. 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.

24. Homework