sbi4u urine formation in the nephron. reminders the nephron what part of the kidney is the top half...
TRANSCRIPT
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?
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
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
19
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