Biology 102 Chapter 51
Salt and Water Balance and Nitrogen Excretion
1. Distinguish between osmoregulators and osmoconformers.
---osmoregulation is how organisms regulate solute balance
and the gain or loss or water
---osmoconformers does NOT actively adjust its internal
osmolarity (isoosmotic with environment or able to
tolerate changes)
---osmoregulators are animals that must adjust internal
osmolarity since body fluids are NOT isoosmotic with
outside environment
--very costly in terms of energy
---adjust by discharging excess water/taking in water
2. Discuss the problems that marine organisms, freshwater
organisms, and terrestrial organisms face in maintaining
homeostasis, and explain what osmoregulatory adaptations serve as
solutions to these problems.
---most cartilaginous fishes are hyperosmotic to seawater
--maintain internal salt [] lower than sea water by
pumping salt out through rectal glands/kidneys
---hyperosmotic because of large [ ]s of urea
--water enters body by osmosis
--balance osmotic uptake of water by excreting urine
---marine bony fish are hypoosmotic to seawater
--lose water to environment
--drink large amounts of sea water/pump excess salt
out gill epithelium
--excrete only small amount of urine
---freshwater animals are hyperosmotic to their environment
--constantly take in water by osmosis
--contractile vacuoles in protests
--many compensate by excreting large amounts of very
dilute urine
--salts lost in process & replenished by eating
substances with high salt content or by active
uptake of sodium & chlorine ions from water
by gill epithelium
---terrestrial animals live in dehydrating environment
--cannot survive desiccation
---adaptations include protective outer layers (cuticles,
shells, multi-layered skin (dead, keratinized cells)
---drinking and eating moist foods
---some animals are nocturnal, active at night when cooler
---some can produce large amounts of metabolic water
---excretory system adapted to conserve water while
eliminating wastes
3. Explain the correlation between the type of nitrogenous waste
produced (ammonia, urea, or uric acid) by an organism and its
habitat.
---kind of nitrogenous wastes an animal excretes depends on
its evolutionary history and habitat
---most aquatic animals excrete wastes as ammonia
---very soluble in water, easily permeates membranes
---soft-bodied invertebrates: ammonia diffuses across body
surface into surrounding water
---in fishes, ammonia excreted as ammonium ions across
gill epithelium
---ammonia excretion unsuitable for animals in terrestrial
habitat
--so toxic can only be transported in animal & excreted
in very dilute solution
---urea is nitrogenous waste excreted by mammals & most
adult amphibians
--can be more []ed in body since it is 100,000x less
toxic than ammonia
--reduces water loss for terrestrial animals
--produced in liver by combining ammonia with CO2
--transported to kidneys via circulatory system
---sharks produce/retain urea as osmoregulatory agent
---uric acid is primary form of nitrogenous waste excreted
by land snails, insects, birds, and many reptiles
--much less soluble in water than ammonia/urea
--can be excreted as precipitate after reabsorption of
nearly all water from urine
--eliminated in paste-like form through cloaca in birds
and reptiles
---mode of reproduction important determining factor
--shelled egg embryo utilizes uric acid
--ammonia/uric acid would accumulate to toxic []s
---animal’s habitat & phylogenetic position influences
type of nitrogenous waste produced
4. Explain the role of transport epithelia in osmoregulation.
---blood or other body fluids exposed to a filtering device
---this filtering device composed of transport epithelia
--membranes of transport epithelia selectively permeable
---membranes retain proteins & other large molecules in
body fluid
---hydrostatic pressure (BP in animals) forces water and
small solutes (salts, sugars, amino acids, & nitrogenous
wastes) through device into excretory system
--aqueous solution in excretory system = filtrate
5. Describe how a flame-bulb (protonephridial excretory system)
functions.
---found in flatworms (no circulatory system/coelom)
---simple, tubular excretory system
---protonephridium: network of closed tubules lacking
internal openings that branch throughout body
--smallest branches capped by cellular flame bulb
---interstitial fluid passes through flame bulb & is
propelled by tuft of cilia along branched system of
tubules
---urine from system empties into external environment
through numerous openings called nephridiopores
---transport epithelia lining tubules absorb salts before
fluid exits body
---also found in rotifers, some annelids, larvae of
mollusks, and lancelets
6. Explain how the metanephridial excretory tubule of annelids
functions, and describe any structural advances over a
protonephridial system.
---each segment of most annelids contains pair of
metanephridia, excretory tubules that have internal
openings to collect body fluids
---coelomic fluid enters funnel-shaped nephrostome, which
is surrounded with cilia
---fluid passes through metanephridium and empties into
storage bladder that empties outside via nephridiopore
---nephrostome collects coelomic fluid from body segment
just anterior
---network of capillaries envelopes each metanephridium
---excretion of hypoosmotic, dilute urine offsets continual
osmosis of water from damp soil across skin
7. Explain how the Malpighian tubule excretory system contributed
to the success of insects in the terrestrial environment.
---Malpighian tubules are excretory organs of insects &
other terrestrial arthropods that remove nitrogenous
wastes from hemolymph and function in osmoregulation
---are outpocketings of gut that open into digestive tract
at midgut-hindgut juncture
---tubules dead-end at tips away from gut & are bathed in
hemolymph
---transport epithelium lining each tubule move solutes
(salts/nitrogenous wastes) from hemolymph into
tubule’s lumen
---accumulates nitrogenous wastes away from hemolymph
--water follows by osmosis
---fluid in tubule passes through hindgut to rectum
---salts/water reabsorbed across epithelium of rectum
--dry nitrogenous wastes are excreted with feces
---contributed to insect success because system conserves
water
8. Using a diagram, identify and give the function of each
structure in the mammalian excretory system.
---kidneys: pair of bean-shaped filtering organs
---blood enters each kidney via renal artery and exits via
renal vein
---kidneys receive 20% of all blood pumped with each
heartbeat
---urine exits kidney through duct called the ureter
---ureters of both kidneys drain into common urinary
bladder
---during urination, urine leaves body from urinary
bladder through tube called urethra
---sphincter muscles near junction of urethra and bladder
control urination
9. Using a diagram, identify and give the function of each part
of the nephron.
---each nephron consists of a glomerulus (capillary
cluster), surrounded by Bowman’s capsule, a proximal
tubule, a loop of Henle, a distal tubule, and a
collecting duct (receives filtrate from many nephrons)
---blood enters glomerulus via afferent arteriole and
leaves by efferent arteriole, which conveys it to
peritubular capillaries surrounding proximal and
distal tubules, and to the vasa recta, capillaries
surrounding the loop of Henle
---nephrons, collecting duct, and associated blood
vessels produce urine from filtrate (water and small
solutes) forced by blood pressure into Bowman’s
capsule from the glomerulus
---as filtrate travels from Bowman’s capsule to
collecting duct, its chemical makeup is changed
as substances pass via interstitial fluid between
nephron and surrounding capillaries
---filtrate processing continues in collecting duct
10. Describe and show the relationship among the processes of
filtration, secretion, and reabsorption.
FILTRATION
---blood pressure forces fluid (water, salts, urea, & other
small molecules) from glomerulus into lumen of Bowman’s
capsule
---porous capillaries and podocytes (specialized cells of
the capsule) nonselectively filter out blood cells and
large molecules
---any molecules small enough to be forced through
capillary wall enters nephron tubule
---filtrate at this point contains mixture of glucose,
salts, vitamins, nitrogenous wastes, & small molecules
in []s similar to blood plasma
---filtrate passes through proximal tubule, loop of Henle,
distal tubule, and empties into collecting duct
SECRETION
---filtrate joined by some substances transported across
the tubule epithelium from surrounding interstitial
fluid as it moves through nephron tubule
--adds plasma solutes to filtrate
---proximal & distal tubules most common sites of secretion
---secretion very selective process involving both passive
and active transport
--example: controlled secretion of H+ helps maintain
constant body fluid pH
REABSORPTION
---is selective transport of filtrate substances across
excretory tubule epithelium from filtrate back to
interstitial fluid
---reclaims small molecules essential to body
---occurs in proximal tubule, distal tubule, loop of Henle,
and collecting duct
---nearly all sugar, vitamins, organic nutrients are
reabsorbed
---in mammals and birds, water is also reabsorbed
11. Explain how the loop of Henle enhances water conservation by
the kidney.
---loop of Henle maintains the interstitial gradient of
NaCl
---filtrate [] of this salt increases by the loss of water
from the descending limb
---ascending limb then leaks the salt into interstitial
fluid
---additional salt is actively transported out of thick
segment of ascending limb
---filtrate is hypoosmotic
---as filtrate descends collecting tube, the increasing
osmolarity of interstitial fluid results in water loss
from filtrate
12. Describe the mechanisms involved in the hormonal regulation of
the kidney.
---antidiuretic hormone (ADH) enhances fluid retention by
increasing water permeability of epithelium of distal
tubules and collecting duct
--produced by hypothalamus/stored and released by
anterior pituitary gland
---ADH release triggered when osmoreceptor cells in
hypothalamus detect increased blood osmolarity due
to excessive loss of water from body
---juxtaglomerular apparatus (JGA) is specialized tissue
near afferent arterioles which carries blood to
glomeruli
---responds to decrease in blood pressure or volume, as
well as decrease in sodium ion [] by releasing enzyme
rennin into blood
---renin leads to conversion of inactive angiotensinogen to
active angiotensin II which functions as hormone
---angiotensin II directly increases blood pressure by
causing arteriole constriction
---also stimulates adrenal glands to release aldosterone,
which stimulates Na+ reabsorption by distal tubules
(water follows by osmosis)
--stimulates thirst centers in brain to induce drinking
--increases blood pressure and volume
---renin-angiotensin-aldosterone system (RAAS) part
of complex feedback circuit that functions in
homeostasis.
---hormone atrial natriuretic factor (ANF) opposes RAAS
--released by heart’s atrial walls in response to
increased blood volume and pressure
--inhibits release of renin from JGA, inhibits NaCl
absorption by collecting ducts, and reduces
aldosterone release from adrenal glands, and
decreases blood volume and lowers blood pressure