Chapter 44: Osmoregulation and Excretion
by: Adrian Luna, Edgar Bolivar, and Carlos Dublado
Overview
• Systems of animals operate within a fluid environment
• Concentrations of water/solutes must be maintained so
systems could function properly, often against an animal’s
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
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
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
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 environment’s osmolarity
• Osmoregulators use energy to maintain osmotic
gradient; done so with active transport
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
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
44.2: An animal’s 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
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 (NH₄⁺) 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
• 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
The Influence of Evolution and
Environment on Nitrogenous
• The form of nitrogenous waste excreted depends on animal’s
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 animal’s habitat
• Amount of nitrogenous waste produced depends on the kind
and amount of food an animal eats
– Endotherms produce more nitrogenous wastes than ectotherms
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)
• 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
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
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 Bowman’s
capsule
• Blood comes into the glomerulus through the
afferent arteriole and the blood pressure
forces fluids into the Bowman’s 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)
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
Two Types of Nephrons
• 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
• 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
From Blood to Filtrate A Closer Look
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
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
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
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
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
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
44.5 The mammalian kidney’s 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
Solute Gradients and Water Conservation
• As filtrate flows through Bowman’s 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)
– Filtrate’s 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
• 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
medulla’s 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
• 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
Regulation of Kidney Function
• 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
• 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
• Summarizing the renin-angiotensinaldosterone 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
• 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
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