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Homeostasis
• the steady-state physiological condition of the body
• Dynamic constancy of 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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Positive Feedback Example: Childbirth
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Negative 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
Endotherms use metabolism to generate body heat and maintain a body temperature around a set point
Ectotherms must use behavior to alter body temperature
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Figure 44.5 Countercurrent heat exchangers
How do Endotherms conserve heat?•Feathers provide insulation in birds•Fur and/or fat provide insulation in mammals•Water’s high heat capacity makes it harder to stay warm in aquatic habitats
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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
• Freshwater Bony 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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Mammalian Excretory System • renal artery and renal vein• Urine exits the kidneys
through the ureters• 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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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
• Mammalian 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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