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CA Biology Standards #9a,f,g 1
Kidneys
A Case Study in Homeostasis
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Homeostasis All living things maintain stable internal conditions
External conditions are unstable, disorganized, changing Life is a fight against “entropy”
All organ systems of the human body contribute to homeostasis How? By keeping blood and tissue constituents and
values within a normal range. Why? Because abnormal ranges will lead to death.
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Urinary System Contributes to
homeostasis Keeps proper
levels of water, salts, etc.
Eliminates wastes
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Kidneys Key players in urinary system: the kidneys
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A Closer Peek Inside…
Unveiling the homeostatic magic:
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3 Steps to the Trick:
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1. Filtration: Separate Waste
Two main types of waste in the body: CO2 (Carbon dioxide)- produced by cells
Disposed of by lungs NH3 (Ammonia)- converted by liver into urea
NH2CONH2 Disposed of by kidneys
Filtration of urea from the bloodstream occurs in the glomerulus of the kidney
WARNING: Contents under pressure! Water, urea, glucose, vitamins, salts forced
out of glomerulus into Bowman’s capsule
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Filtration, cont’d: Glomerulus
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2. Reabsorption: Recycle
Filtration is too efficient: the body needs to reclaim some of the filtrate
Recovered filtrate leaves renal tubule, enters capillaries, and leaves through renal vein (and back to the body)
75% of water is returned by osmosis Glucose, sodium, potassium, calcium are
returned by active transport
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Reabsorption, cont’d
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3. Secretion
Other wastes, toxins from blood are secreted into the filtrate Fluid + wastes in tubule = urine Flows into collecting duct Urine is further concentrated in duct
because of water leaving by osmosis 99ml/100ml water retained this way
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Secretion, cont’d
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Secretion, cont’d Q: Who’s to blame
for this osmosis? A: The loop of Henle
Cells in the wall of the loop actively transport Cl- from the filtrate to the fluid between loops and collecting duct
High [Na+]/Low [H2O] outside duct . . .
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Discussion: ADH
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Reinforce
Movie: “kidney-G” Movie: “reabsorb_secrete-S” Movie: “hormones_kidney-S” Review: “neuron_NaKpump-G” Review: “neuron_transmit-G”