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SBI4U
Ms. Ho-Lau
10.2 Urine Formation in the Nephron
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4 Processes
glomerular filtration:
- move water and solutes (except proteins) from blood into nephron
- produces filtrate
tubular reabsorption:
- removes useful substances from filtrate and returns them to the blood
tubular secretion:
- moves additional wastes from blood into filtrate
water reabsorption:
- removes water from filtrate and returns it to the blood
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How Urine Forms
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1. Glomerular Filtration
moves water and dissolved substances from blood into
the Bowman’s capsule
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1. Glomerular Filtration
2 factors contribute to filtration:
1. the permeability of the capillaries of the glomerulus:
the pores in the walls of the capillaries allow water
and dissolved substances to pass through; but
impermeable to proteins and blood cells
2. blood pressure:
the high blood pressure within the glomerulus provides
the force for filtration
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1. Glomerular Filtration
1600L to 2000L of blood pass through kidneys each day
produces 180 L of filtrate
This filtrate is identical to blood plasma, minus proteins
and blood cells
Body has to reabsorb water, salts, and nutrients
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2. Tubular Reabsorption
proximal tubule and the loop of Henle
reabsorbs ~65% of the filtrate
involves active and passive transport
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2. Tubular Reabsorption2. Tubular Reabsorption: Proximal tubule
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2. Tubular Reabsorption: Proximal tubule
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2. Tubular Reabsorption:
The Loop of Henle
reabsorbs water and ions from filtrate
3 sections
Descending limb
Thin segment of ascending limb
Thick segment of ascending limb
Note: medulla is a very salty environment
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2. Tubular Reabsorption: The Loop of Henle
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2. Tubular Reabsorption: The Loop of Henle
• Descending limb
permeable to water
• Water diffuses from
filtrate to capillaries
(osmosis)
• [Na+] in filtrate ↑
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2. Tubular Reabsorption: The Loop of Henle
• Ascending limb
impermeable to water
but permeable to ions
• Water is not
absorbed
• Na+ diffuses from
filtrate into capillares
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2. Tubular Reabsorption: The Loop of Henle
• Na+ are actively transported out
of the filtrate into the capillaries
• replenish medulla’s salty
environment
• At this point, 2/3 of Na+ and
water from the filtrate has been
absorbed
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3. Tubular Reabsorption and Secretion:
Distal Tubule
Active reabsorption of Na+
Passive reabsorption of negative ions
Water reabsorbed by osmosis
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3. Tubular Reabsorption and Secretion:
Distal Tubule
Active secretion of K+ and H+ from blood into distal tubule to maintain blood pH
Antibiotics and other medications secreted from blood into distal tubule
Secretions are controlled by hormones
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4. Water Reabsorption: Collecting duct
Passive reabsorption of water
Hormones can alter the amount of
reabsorption in response to
hydration levels
Urine:
~1% of the volume of the original
filtrate