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Homeostasis
• the steady-state physiological condition of
the body
• Ability to regulate the internal environment
• important for proper functioning of cells
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Homeostasis
• Thermoregulation
– how organisms regulate their body temperature
• Osmoregulation
– how organisms regulate solute balance and gain
or loss of water
• Excretion
– how organisms get rid of nitrogen-containing
waste products of metabolism, such as urea
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Homeostasis
• Maintenance usually involves negative
feedback loops
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Feedback mechanisms in human thermoregulation
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Figure 44.4 The relationship between body temperature and ambient
(environmental) temperature in an ectotherm and an endotherm
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Figure 44.5 Countercurrent heat exchangers
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Excretion
• Nitrogen-containing wastes from the
metabolism of proteins and nucleic acids
are particularly bad.
• the nitrogenous waste product is ammonia
(NH3)
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Ammonia
• most efficient to excrete directly
• Very toxic, soluble in water
– must be excreted in dilute solutions
• Excreted by most aquatic organisms
• diffuses across body surface or gills
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Terrestrial animals
• Can’t afford to lose a lot of water
• excrete substances that can be excreted in
more concentrated form
• use energy to convert ammonia to a less
toxic molecule
– urea or uric acid
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Urea
• Much less toxic
• excreted by many terrestrial animals
• produced in liver
– metabolic cycle combines ammonia & carbon
dioxide
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Figure 44.13 Nitrogenous wastes
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Uric Acid
• Excreted by some land snails, insects, birds
& reptiles
• Not soluble in water
• excreted as a precipitate after water has
been reabsorbed
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Urea vs Uric Acid
• Both adaptations to conserve water
• depends on mode of reproduction…
• animals with shelled eggs excrete uric acid
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Osmoregulation
• Cells cannot survive a net gain or loss of
water
– common problem to all animals
– solutions differ
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Osmoregulation
Two basic solutions:
• Be isotonic to the environment
– osmoconformers
• Actively discharge (in hypotonic
environments) or take in ( in hypertonic
environments) water
– osmoregulators
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Marine Environments
• Most marine invertebrates are
osmoconformers
– may still regulate specific ion concentrations
• Most marine vertebrates osmoregulate
– Chondrichthyes
• isotonic but lower salt conc. & high urea conc.
– Osteichthyes
• hypotonic to environment
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Osmoregulation in a saltwater fish
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Figure 44.12 Salt-excreting glands in birds
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Freshwater Environments
• Problem of water entering body via osmosis
• Protozoa (amoeba & paramecium)
– use contractile vacuoles
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Freshwater Environments
• Freshwater Bony Fish
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Osmoregulation in a freshwater fish
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Terrestrial Environments
• Many adaptations to prevent water loss
– shells, layers of dead skin, waxy cuticle,
exoskeletons, scales, etc.
• Drink water & eat moist foods
• Specialized organs to conserve water
– ex: kidneys
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The human excretory system
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Mammalian Excretory System
• renal artery and renal vein
• Urine exits the kidneys through the ureter
• The ureters of both kidneys enter the
urinary bladder
• Urine leaves the body via the urethra
– Sphincter muscles between the bladder and
urethra control urination
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The human kidney
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Kidney
• outer renal cortex & inner renal medulla
• within each region are microscopic
excretory tubules called nephrons,
collecting ducts and capillaries
• the nephron is the functional unit of the
kidney
• renal pelvis
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The nephron within the human kidney
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The nephron and collecting duct
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Function of Nephron
• 1. Filtration of blood
– blood pressure forces any small molecules from
the blood into the lumen in the bowman’s
capsule
– a nonselective process with regard to small
molecules
– filtrate initially consists of water, urea, salts,
glucose, vitamins, etc.
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Function of Nephron
• 2. Secretion
– substances are transported into the filtrate
– most commonly occurs in the proximal and
distal tubules
– a very selective process – involving both
passive and active transport
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Function of Nephron
• 3. Reabsorption
– the selective transport from the filtrate to the
interstitial fluid or blood plasma
– Sugars, vitamins, organic nutrients and water
are all reabsorbed
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Figure 44.22 The nephron and collecting duct: regional functions of the transport
epithelium
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Function of Nephron
• 4. Excretion
– Get rid of the wastes
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Key functions of the nephron
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Figure 44.23 How the human kidney concentrates urine: the two-solute model (Layer
1)
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Figure 44.23 How the human kidney concentrates urine: the two-solute model (Layer
2)
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Figure 44.23 How the human kidney concentrates urine: the two-solute model (Layer
3)
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Characteristics of Urine
• The kidneys can produce a hypertonic urine
when it is necessary
• can excrete a hypotonic urine
• Water and salt reabsorption are subject to
nervous and hormonal control
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ADH (Antidiuretic Hormone)
• released when the solute concentration of
the blood rises
• makes the transport epithelium of the distal
tubules and the collecting ducts more
permeable to water
• alcohol inhibits production of ADH
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Hormonal control of the kidney by negative feedback circuits
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Evolution of the Vertebrate
Kidney
• 1st – in freshwater fish
• Fish, amphibian & reptile kidneys can only
produce urine that is isotonic or hypotonic
to their body fluids
• Terrestrial reptiles – can reabsorb water in
cloaca
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Evolution of the Vertebrate
Kidney
• Only birds and mammals have loops of
Henle in their nephrons
– Hypertonic urine
– Mammals have more juxtamedullary nephrons
than birds
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