Homeostasis - BEHS Science


Chapter 44:

Maintaining a Constant

Internal Environment


Body Temperature

Enzymes: Rxn rates inc. 2-3 times with each 10 0 C temp. inc.

(until denatured)

Each species has an optimal temp. range for metabolic rxns to be efficient

Thermoregulation  Organisms maintain their body temp within optimal range (various methods)

Heat Gain/Loss

Heat Gain/Loss (HIGH to LOW)

Conduction = molecule to molecule

(ie: hot concrete and feet in summer)

Convection = heat transfer b/t object and H


O or air moving across it

Radiation = electromagnetic waves transferred as heat (ie: suns rays)

Evaporative Cooling = lowers temp by releasing H sweating)


O as vapor (ie:

Ectotherms and Endotherms

Ectotherms (“cold-blooded”) maintain a temperature close to external temp.

•Low metabolic requirements (little heat generated)

•Most fish, reptiles, amphibians

Endotherms (“warm-blooded”) maintain a constant temp. that may vary significantly from external temp

(species dependent)

•High metabolic rate (lots of heat!)

•Humans, other mammals, and birds


Endothermic Advantages

Higher temp allows for inc. metabolic rate

More energy is generated

Can perform more vigorous activities for more sustained periods

Allows terrestrial life (more temp. fluctuations than H



Disadvantage : Require more frequent meals for higher aerobic resp. rate

Vasodilation and Vasoconstriction

Vasodilation  Blood vessels dilate (expand) in order to release more heat

Vasoconstriction  Blood vessels constrict in order to limit heat loss in the cold

Behavioral Controls

Basking in the sun to raise body temp


Migration to different climates


Inc or dec metabolic rate in hot/cold temps

Certain insects huddle to generate more heat




Blubber, fat

Reduces the loss of heat

Allows maintenance of higher body temps


Thermoregulation in Humans

Thermoregulation in Humans

Heat receptors in skin

Receptors send hot/cold signal to hypothalamus (brain)

Hypothalamus regulates vasodilation and vasoconstriction

Extreme Hot/Cold Environments

Cryoprotectants  Certain organisms (some frog eggs, arctic fish) have a biologically produced antifreeze

Heat shock proteins  Produced in response to heat. Bind to enzymes and other proteins to inhibit denaturization


Bears, squirrels go into a deep sleep during winter in order to avoid harsh conditions

Very low energy demands

Very low metabolic rate


Osmoregulation (Water Balance)

Organisms must balance their water and solute concentrations

Water uptake and loss must remain essentially equal

Cells could swell or shrivel

Water flows from high water potential (low [solute]) to low water potential (high [solute])


Osmoregulators maintain a constant solute concentration different from that of ext. environment

•Freshwater, terrestrial, some marine

•Costs energy (active transport)

Osmoconformers maintain solute concentration equal to that of surroundings

•Many marine invertebrates


Waste Disposal

Elimination of toxic materials is needed to maintain homeostasis

Nitrogenous wastes are very toxic to living cells

•Urea  Many terrestrial organisms, lowest toxicity, high energy requirement (humans)

•Uric Acid  Birds, insects reptiles, least water lost

•Ammonia  Fish, aquatic organisms, most toxic

Nitrogenous Waste

Selective Reabsorption and Secretion

Organisms will filter their wastes and reabsorb anything that may be of use

Accomplished in tubules (present in human kidneys)

May also secrete more waste materials into urine

Malpighian Tubules

Remove nitrogenous wastes from open circulatory system of insects

Vertebrate Urinary System


•Function in osmoregulation and reabsorption

•Contain a network of tubules

Renal Artery  Blood to kidney

Renal Vein  Blood from kidney

Ureter  Carries urine to bladder

Bladder  Stores urine

Urethra  Tube that exits the body

Vertebrate Kidneys

Two regions

•Renal cortex and renal medulla

Contains millions of nephrons

•Microscopic tubules

Glomerulus  Network of capillaries serving each nephron with a blood supply

Bowman’s Capsule  End of tubule that surrounds the glomerulus

Human Kidney


 nephron and collecting duct are lined by a transport epithelium process filtrate to form urine reabsorb solutes and water

•sugar, vitamins, and other organic nutrients from the initial filtrate and about 99% of the water reduce 180 L of initial filtrate to about 1.5 L of urine to be voided

Kidney Function

Proximal Tubule  NaCl and H


O reabsorption and pH regulation

Descending Loop of Henle  H


O reabsorption

Ascending Loop of Henle  NaCl reabsorption

Distal Tubule  K + and NaCl balance, pH regulation, some H


O reabsorbed

Collecting Duct  NaCl reabsorption,



O reabsorption

As it moves through the kidney, urine becomes more concentrated with unusable waste


Kidneys give terrestrial vertebrates the ability to regulate their osmotic balance

Without kidneys, life on land would not be possible