Chapter 44: Osmoregulation and Excretion by: Adrian Luna, Edgar Bolivar, and Carlos Dublado
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- Chapter 44: Osmoregulation and Excretion by: Adrian Luna, Edgar
Bolivar, and Carlos Dublado
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- Overview Systems of animals operate within a fluid environment
Concentrations of water/solutes must be maintained so systems could
function properly, often against an animals external environment
Another problem = how to dispose of certain, sometimes toxic,
metabolic wastes Two homeostatic processes: Osmoregulation is how
animals regulate solute concentrations and balance gain and loss of
water Excretion is how animals get rid of nitrogen-containing waste
products of their metabolism
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- 44.1: Osmoregulation balances the uptake and loss of water and
solutes Like thermoregulation depends on balancing heat loss/gain,
osmoregulation depends on balancing uptake/loss of water/solutes
Osmoregulation based largely on movement of solutes between
internal fluids and external environment Must remove metabolic
waste products before accumulating to harmful levels
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- Osmosis All animals face same central problem of osmotic
balance Too much water = swell and burst Not enough water = shrivel
up and die Osmolarity (total solute concentration) Osmosis occurs
whenever two solutions of different osmotic pressure Isoosmotic two
solutions with same osmolarity separated by selectively permeable
membrane Hyperosmotic = solution with higher osmolarity Hypoosmotic
= solution with lesser (more dilute) osmolarity
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- Osmotic Challenges Two solutions to osmotic balance dilemma:
Osmoconformer = animal does not actively adjust its internal
osmolarity; usually isoosmotic to environment Osmoregulator =
animal must control its internal osmolarity because body fluids are
not isoosmotic to environment either discharge/take in water to
balance osmotic loss depending environments osmolarity
Osmoregulators use energy to maintain osmotic gradient; done so
with active transport
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- Land Animals Terrestrial animals have adaptated to reduce water
loss, due to threat of desiccation, allows survival on land Most
land animals still release considerable amounts of water through
gas exchange, urine, feces, and across their skin Balance kept by
drinking and eating moist foods and by using metabolic waters
derived from cellular respiration
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- Transport Epithelia Animals maintain composition of cellular
cytoplasm by managing an internal body fluid that bathes their
cells Maintenance of fluid composition depends on specialized
structures (transport epithelia or kidney) Transport epithelia = a
layer or layers of specialized epithelial cells, regulate solute
movements; essential to osmotic regulation and metabolic waste
disposal Move specific solutes in controlled amounts in specific
directions Joined by impermeable tight junctions and serve as a
barrier, ensuring solutes pass through their selectively permeable
membrane Transport epithelia often have dual functions of
maintaining water balance and disposing of metabolic wastes
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- 44.2: An animals nitrogenous wastes reflect its phylogeny and
habitat Most metabolic wastes have to be dissolved in water for
disposal, potentially have some kind of influence on water balance
depending on size/quantity Nitrogenous wastes are among the most
important wastes products in terms of effect on osmoregulation When
proteins and nucleic acids are broken down/converted into
fats/carbohydrates, enzymes remove nitrogen in the form of the
toxic molecule ammonia Ammonia could be converted into something
less toxic before secretion but only with the aid of ATP
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- Forms of Nitrogenous Waste Three different forms: ammonia,
urea, and uric acid differ in solubility, toxicity, and energy
costs Ammonia Is soluble but only tolerated at low concentrations
because of high toxicity animals that excrete it directly need
access to a lot of water common among aquatic species Molecules
easily pass through membranes and lost by diffusion to surrounding
water Many invertebrates release it across whole surface of body
Some fishes lose it as ammonium ions (NH) across the epithelium of
the gills Urea Since ammonia is highly toxic, can only be excreted
in large masses of dilute solutions containing water many
terrestrial and marine animals do not have access to sufficient
water to make up for loss
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- So mammals excrete urea, a substance produced in liver by a
metabolic cycle that combines ammonia with carbon dioxide has a
substantially lower toxicity than ammonia could be stored and
transported at much higher concentrations excretion requires less
water, is released in a concentrated solution rather than a dilute
one (like ammonia) a disadvantage is the expense of energy in the
synthesis of the substance Uric Acid Used by animals, like reptiles
and birds, and is relatively nontoxic like its counterpart urea It
is highly insoluble in water, usually excreted as a solid paste
with little water loss; it is more energetically expensive to make
than urea
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- The Influence of Evolution and Environment on Nitrogenous The
form of nitrogenous waste excreted depends on animals evolutionary
history and habit, especially with water Urea/uric acid are
adaptations for excretion with minimal water loss Mode of
reproduction has been a factor in determining form of nitrogenous
waste excreted Another factor is the animals habitat Amount of
nitrogenous waste produced depends on the kind and amount of food
an animal eats Endotherms produce more nitrogenous wastes than
ectotherms
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- 44.3: Diverse excretory systems are variations on a tubular
theme Excretory system largely in charge of managing osmotic
balance, depends on regulating solute movement between the internal
fluids and external environment Central to homeostasis because it
disposes of metabolic wastes and controls body fluid composition by
adjusting rates of solute loss Excretory Processes Urine (fluid
waste) produced by nearly all excretory systems involving several
steps Excretory tubules collect fluid (by filtration) called
filtrate through selectively permeable membranes of transport
epithelium from the body fluids (e.g. blood and hemolymph)
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- Large molecules unable to diffuse, hydrostatic pressure (blood
pressure) forces water and small solutes like salts, sugars, and
nitrogenous wastes into excretory tubules Filtration is
nonselective, essential small molecules need to be recovered and
returned to body fluids; done so with selective reabsorption, which
uses active transport to absorb valuable solutes Nonessential
solutes (excess salts and toxins) are left in the filtrate or added
by selective secretion (also uses active transport) The movement of
solutes helps adjust osmotic movement of water in/out of filtrate
Filtrate is then excreted out of the system and body as urine
Survey of Excretory Systems Excretory systems differentiate, but
all revolve around the structure of a complex network of tubules
that provide for exchange of water and solutes
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- Concept 44.4 Nephrons and associated blood vessels The
Excretory system is centered on the kidneys the site of water
balance and salt regulation Mammals have two bean-shaped kidneys
Blood supply comes from the renal artery The renal vein drains the
blood Urine exits the kidneys through the ureter which drains into
the urinary bladder and exits the body through the urethra
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- Structure and Function of the Nephron and Associated Structures
Kidneys have 2 regions: An outer renal cortex An inner renal
medulla The nephron is a tubule with a ball of capillaries at one
end called the glomerulus Each kidney has about 1 million nephrons
Surrounding the glomerulus is the Bowmans capsule
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- Blood comes into the glomerulus through the afferent arteriole
and the blood pressure forces fluids into the Bowmans capsule
Porous capillaries and special cells of the capsule (podocytes) are
permeable to water and small solutes but not blood cells or large
molecules like plasma proteins Filtration is non-selective, meaning
the Bowman's capsule is filled with both wastes and molecules
(salts, glucose, amino acids and vitamins)
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- Pathway of the Filtrate From Bowman's capsule the filtrate
passes through 3 regions: Proximal tubule The loop of Henle, a
hairpin turn with a descending and ascending limb And the distal
tube From each nephrons distal tubule, the filtrate empties into a
collecting duct From the collecting duct, the filtrate flows into
the renal pelvis which is drained by the ureter
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- Cortical nephrons have reduced loops of Henle and are confined
to the renal cortex, they make up 80% of a mammals nephrons
Juxtamedullary nephrons make up the other 20%, they have well
developed loops extending deeply into the renal medulla Only
mammals and birds have juxtamedullary nephrons, the nephrons of
other vertebrates lack a loop Henle altogether Juxtamedullary
nephrons allow mammals to make urine that is hyperosmotic to body
fluids an adaptation important to water conservation Two Types of
Nephrons
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- As capillaries converge and leave the glomerulus they form the
efferent arteriole which divides again forming peritubular
capillaries that surround the proximal and distal tubules The
capillaries that extend downwards form the vasa recta They also
form a loop with the ascending and descending vessels conveying
blood in opposite directions Although they associate, there is no
direct movement of materials They're immersed in interstitial fluid
through which substances can diffuse between capillaries and
filtrate Exchange is usually facilitated by blood and filtrate
flow
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- 1) Proximal tubule secretion and reabsorption alter volume and
composition of filtrate. Cells in the tubule can synthesize and
secrete ammonia to neutralize acid The proximal tubule also
reabsorbs about 90% of bicarbonate(HCO3-) an important buffer Drugs
and other poisons processed in the liver pass from the peritubular
capillaries into the interstitial fluid to be secreted across the
epithelium of the proximal tube into the nephrons lumen Conversely,
valuable nutrients like glucose, amino acids and potassium (K+) are
actively and passively transported from the filtrate to the
peritubular capillaries From Blood to Filtrate A Closer Look
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- One of the most important functions of the proximal tubule is
the reabsorption of most NaCl (salt) and water Salt diffuses into
the cells of the transport epithelium whose membranes actively
transport Na+ This positive transport is balanced by the passive
transport of Cl- out As salt passes through, water follows by
osmosis To stop water and salt from coming back into the tubule,
the outside of the epithelium has a smaller surface area than the
side facing the lumen, instead, they diffuse into the
capillaries
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- 2)In the descending limb of the loop of Henle reabsorption of
water continues as the filtrate passes Here the transport
epithelium is freely permeable to water but not to salt or other
solutes Osmosis only occurs if the interstitial fluids hyperosmotic
to filtrate Osmolarity of the interstitial fluid Beacons greater
from the outer cortex to the inner medulla of the kidney ->
filtrate moving down the cortex to the medulla in the descending
limb continually loses water increasing solute concentration
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- 3)In the ascending limb of the loop of Henle the filtrate
reaches the tip of the loop and travels up the limb In contrast to
the descending limb, the ascending limb is permeable to salt but
not to water 2 specialized regions: Thin segment near the loop tip
where NaCl diffuse out Thick segment which actively transports
NaCl
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- 4)The distal tubule regulates K+ and NaCl concentrations by
varying the amount of K+ secreted into the filtrate and the NaCl
absorbed out Like the primal tubule, the distal tubule also
regulates pH by secreting H+ and absorbing bicarbonate
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- 5)The collecting duct carries filtrate through the medulla to
the renal pelvis By collecting NaCl the duct can control how much
salt is excreted through the urine Its degree of permeability is
usually under hormonal control, however it is permeable to water
but not to salt or urea in renal cortex -> because water is
taken out the filtrate becomes more concentrated/urea The inner
medulla becomes permeable to urea and because of its high
concentration some diffuses out of the duct and into the fluid
Along with NaCl, urea contributes to high osmolarity of
interstitial fluid in the medulla enabling kidneys to conserve
water by excreting urine that is hyperosmotic to general body
fluids
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- 44.5 The mammalian kidneys ability to conserve water loop of
Henle and collecting ducts largely responsible for osmotic gradient
that concentrates urine Two primary solutes: NaCl, deposited in
renal medulla by loops of Henle Urea, leaks across epithelium of
collecting duct in inner medulla
- Slide 37
- As filtrate flows through Bowmans capsule to proximal tubule,
osmolarity of 300 mosm/L Reabsorbs water and salt in the renal
cortex, volume decreases but osmolarity remains the same From
cortex to medulla in descending limb, water leaves tubule (osmosis)
Filtrates osmolarity increases as solutes become concentrated
Ascending limb is permeable to NaCl but not water NaCl diffuses to
maintain high osmolarity in interstitial fluid of renal medulla
Solute Gradients and Water Conservation
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- Countercurrent multiplier systems expend energy to create
concentration gradients loop of Henle expends energy to actively
transport NaCl from filtrate in upper part of ascending limb Vasa
recta is a countercurrent system Prevents capillaries from
dissipating gradient by carrying away high concentration of NaCl in
medullas interstitial fluid As descending vessel conveys blood
toward inner medulla, water is lost and NaCl diffuses into it In
ascending vessel, water reenters and salt diffuses out
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- When filtrate reaches distal tubule, it is hypoosmotic to body
fluids Descends toward the medulla, via collecting duct (permeable
to water, not salt) Concentrates salt, urea, and other solutes in
filtrate Before leaving kidney, urine may attain osmolarity of
interstitial fluid in inner medulla (as high as 1200 mosm/L) High
osmolarity allows solutes to be excreted with minimal water
loss
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- Osmoregulatory function managed with combination of nervous and
hormonal controls Hyperosmotic = low water, high salt Hypoosmotic =
high water, low salt Antidiuretic hormone (ADH) regulates water
balance Produced in hypothalamus of the brain stored in and
released from posterior pituitary gland Main targets of ADH are
distal tubules and collecting ducts, increases permeability of
epithelium to water When osmolarity of blood reaches a set point,
more/less ADH is released Regulation of Kidney Function
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- Juxtaglomerular apparatus (JGA) specialized tissue located near
afferent arteriole Supplies blood to glomerulus When blood
pressure/volume drops in afferent arteriole, enzyme renin initiates
chemical reactions, converts angiotensinogen to angiotensin II
raises blood flow to capillaries Stimulates proximal tubules to
reabsorb more salt and water, reducing amount of salt and water
excreted and raises blood volume/pressure Also stimulates adrenal
glands, releases aldosterone, allows distal tubules to reabsorb
more sodium (Na+) and water, increasing blood volume/pressure
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- Summarizing the renin-angiotensin- aldosterone system (RAAS)
Drop in blood pressure/volume triggers renin release from JGA Rise
in blood pressure/volume resulting from actions of angiotensin II
and aldosterone reduce renin ADH alone lowers Na+ concentration by
stimulating water reabsorption in the kidney RAAS maintains balance
by stimulating Na+ reabsorption
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- Atrial natriuretic factor (ANF) another hormone, opposes the
RAAS walls of atria of the heart release ANF in response to
increase in blood volume/pressure ANF lowers blood volume/pressure
Inhibits release of renin from JGA Inhibits NaCl reabsorption by
collecting ducts And reduces aldosterone release from adrenal
glands
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- 44.6 Adaptations of the vertebrate kidney Evolved in different
environments Variations in nephron structure/function allows for
different osmoregulation in various habitats All organs work
continuously, maintaining solute/water balance and excreting
nitrogenous wastes