Download Chapter 44 Worksheet Osmoregulation and Excretion Water

Chapter 44 Worksheet
Osmoregulation and Excretion
Water Balance and Waste Disposal
1. Osmoregulation is the general process by which animals control solute concentrations and
balance water gain and loss. In the fluid environment of cells, tissues, and organs,
osmoregulation is essential. For physiological systems to function properly, the relative
concentrations of water and solutes must be kept within fairly narrow limits. In addition, ions
such as sodium and calcium must be maintained at concentrations that permit normal
activity of muscles, neurons, and other body cells. Osmoregulation is thus a process of
homeostasis. The ultimate function of Osmoregulation is to maintain the composition of the
cellular contents, but most animals do this indirectly by managing the composition of an
internal body fluid that bathes the cells. In insects and other animals with an open
circulatory system, this fluid is the hemolymph. In vertebrates and other animals with a
closed circulatory stem, the cells are bathed in an interstitial fluid that contains a mixture of
solutes controlled indirectly by the blood.
2. In most animals, osmotic regulation and metabolic waste disposal rely on one or more kinds
of transport epithelium- one or more layers of specialized epithelial cells that regulate solute
movements. Transport epithelia move specific solutes in controlled amounts in specific
directions. Transport epithelia are typically arranged into complex tubular networks with
extensive surface areas. Transport epithelia that function in maintaining water balance also
often function in disposal of metabolic wastes.
3. When proteins and nucleic acids are broken apart for energy or converted to carbohydrates
or fats, enzymes remove nitrogen in the form of ammonia. Ammonia is very toxic, in part
because its ion, ammonium, interferes with oxidative phosphorylation. Because ammonia
can be tolerated only at very low concentrations, animals that excrete nitrogenous wastes as
ammonia need access to lots of water. Therefore, ammonia excretion is most common in
aquatic species. Being highly soluble ammonia molecules easily pass through membranes are
readily lost by diffusion to the surrounding water. In many invertebrates, ammonia release
occurs across the whole body surface. In fishes, most of the ammonia is lost as NH4+ across
the epithelium f the gills; the kidneys excrete only minor amounts of nitrogenous waste.
4. Regardless of the type of nitrogenous waste, the amount produced y an animal is coupled to
the energy budget. Endotherms, which use energy at high rates, eat more food and produce
more nitrogenous waste than ectotherms. The amount of nitrogenous waste is also linked to
diet. Predators, which derive much of their energy from protein, excrete more nitrogen that
animals that rely mainly on lipids or carbohydrates as energy sources.
5. An animal’s nitrogenous wastes are correlated with its phylogeny and habitat. Because most
metabolic wastes must be dissolved in water when they are removed from the body, the
type and quantity of waste products may have a large impact on water balance. Nitrogenous
breakdown products of proteins and nucleic acids are among the most important wastes in
terms of their effect on osmoregulation. During their breakdown, enzymes remove nitrogen
in the form of ammonia, a small and very toxic molecule. In general, the kinds of nitrogenous
wastes excreted depend on an animal’s evolutionary history and habitat - especially water
availability. The amount of nitrogenous waste produced is coupled to the energy budget and
depends on how much and what kind of food an animal eats.
6. Animals that excrete nitrogenous wastes as ammonia need access to lots of water. This is
because ammonia is very soluble but can only be tolerated at very low concentrations.
Therefore, ammonia excretion is most common in aquatic species. Many invertebrates
release ammonia across the whole body surface. In fishes, most of the ammonia is lost as
ammonium ions (NH4+) at the gill epithelium. Freshwater fishes are able to exchange NH4+
for Na+ from the environment, which helps maintain Na+ concentrations in body fluids. The
main advantage of urea is its low toxicity, about 100,000 times less than that of ammonia.
Urea can be transported and stored safely at high concentrations. This reduces the amount
of water needed for nitrogen excretion when releasing a concentrated solution of urea
rather than a dilute solution of ammonia. The main disadvantage of urea is that animals
must expend energy to produce it from ammonia. In weighing the relative advantages of
urea versus ammonia as the form of nitrogenous waste, it makes sense that many
amphibians excrete mainly ammonia when they are aquatic tadpoles. They switch largely to
urea when they are land-dwelling adults. Land snails, insects, birds, and many reptiles
excrete uric acid as the main nitrogenous waste. Like urea, uric acid is relatively nontoxic.
But unlike either ammonia or urea, uric acid is largely insoluble in water and can be excreted
as a semisolid paste with very small water loss. While saving even more water than urea, it is
even more energetically expensive to produce. Uric acid and urea represent different
adaptations for excreting nitrogenous wastes with minimal water loss. Mode of reproduction
appears to have been important in choosing between these alternatives. Soluble wastes can
diffuse out of a shell-less amphibian egg (ammonia) or be carried away by the mother’s
blood in a mammalian embryo (urea). However, the shelled eggs of birds and reptiles are not
permeable to liquids, which means that soluble nitrogenous wastes trapped within the egg
could accumulate to dangerous levels (even urea is toxic at very high concentrations). In
these animals, uric acid precipitates out of solution and can be stored within the egg as a
harmless solid left behind when the animal hatches. The type of nitrogenous waste also
depends on habitat. For example, terrestrial turtles (which often live in dry areas) excrete
mainly uric acid, while aquatic turtles excrete both urea and ammonia. In some species,
individuals can change their nitrogenous wastes when environmental conditions change. For
example, certain tortoises that usually produce urea shift to uric acid when temperature
increases and water become less available.
7. Water enters and leaves cells by osmosis, the movement of water across a selectively
permeable membrane. Osmosis occurs whenever two solutions separated by a membrane
differ in osmotic pressure, or osmolarity (moles of solute per liter of solution). The unit of
measurement of osmolarity is milliosmoles per liter (mosm/L). There is no net movement of
water by osmosis between isoosmotic (like isotonic) solutions, although water molecules do
cross at equal rates in both directions. When two solutions differ in osmolarity, the one with
the greater concentration of solutes is referred to as hyperosmotic (hypertonic) and the
more dilute solution is hypoosmotic (hypotonic). Water flows by osmosis from a
hypoosmotic solution to a hyperosmotic one.
8. One - available only to marine animals - is to be isoosmotic to the surroundings as an
osmoconformer. Although they do not compensate for changes in external osmolarity,
osmoconformers often live in water that has a very stable composition and hence have a
very constant internal osmolarity. In contrast, an osmoregulator is an animal that must
control its internal osmolarity, because its body fluids are not isoosmotic with the outside
environment. An osmoregulator must discharge excess water if it lives in a hypoosmotic
environment or take in water to offset osmotic loss if it inhabits a hyperosmotic
environment. Osmoregulation enables animals to live in environments that are
uninhabitable to osmoconformers, such as freshwater and terrestrial habitats. It also enables
many marine animals to maintain internal osmolarities different from that of seawater. Most
animals, whether osmoconformers or osmoregulators, cannot tolerate substantial changes
in external osmolarity and are said to be stenohaline. In contrast, euryhaline animals - which
include both some osmoregulators and osmoconformers - can survive large fluctuations in
external osmolarity. For example, various species of salmon migrate back and forth between
freshwater and marine organisms. The food fish, tilapia, is an extreme example, capable of
adjusting to any salt concentration between freshwater and 2,000 mosm/L, twice that of
seawater.
9. A distinct osmoregulatory strategy evolved in marine sharks and most other
chondrichthyans. Like bony fishes, sharks have an internal salt concentration much less than
that of seawater, so salt tends to diffuse into their bodies from the water, especially across
their gills. Unlike bony fishes, however, marine sharks are not hypoosmotic to seawater.
Dehydration dooms most animals, but some aquatic invertebrates living in temporary ponds
and films of water around soil particles can lose almost all their body water and survive in a
dormant state, called anhydrobiosis, when their habitats dry up. For example, tardigrades, or
water bears, contain about 85% of their weight in water when hydrated but can dehydrate
to less than 2% water and survive in an inactive state for a decade until revived by water. The
threat of desiccation is perhaps the largest regulatory problem confronting terrestrial plants
and animals. Humans die if they lose about 12% of their body water. Adaptations that reduce
water loss are key to survival on land. Most terrestrial animals have body coverings that help
prevent dehydration. Including waxy layers in insect exoskeletons, the shells of land snails,
and the multiple layers of dead, keratinized skin cells. Being nocturnal also reduces
evaporative water loss. Despite these adaptations, most terrestrial animals lose considerable
water from moist surfaces in their gas exchange organs, in urine and feces, and across the
skin. Land animals balance their water budgets by drinking and eating moist foods and by
using metabolic water from aerobic respiration.
Excretory Systems
10. While excretory systems are diverse, nearly all produce urine by a two-step process. First,
body fluid (blood, coelomic fluid, or hemolymph) is brought in contact with the selectively
permeable membrane of a transport epithelium. Second, the composition of the collected
fluid is adjusted by selective reabsorption, recovers useful molecules and water from filtrate
and returns them to the body fluids, or secretion of solutes, which occurs by active
transport.
11. Flatworms, which lack a coelom or body cavity, have excretory systems called
protonephridia. The protonephridia form a network of dead-end tubules connected to
external openings; the tubules branch throughout the body. Cellular units called flame bulbs
cap the branches of each protonephridium. Formed from a tubule cell and a cap cell, each
flame bulb has a tuft of cilia projecting into the tubule. During filtration, the beating of the
cilia draws water and solutes from the interstitial fluid through the flame bulb, releasing
filtrate into the tubule network. The processed filtrate then moved outward through the
tubules and empties as urine into the external environment. The urine excreted by
freshwater flatworms has a how solute concentration, helping to balance the osmotic uptake
of water from the environment.
12. Most annelids, such as earthworms, have metanephridia, excretory organs that open
internally to the coelom. Each segment of a worm has a pair of metanephridia, which are
immersed in coelomic fluid and enveloped by a capillary network. The metanephridia of an
earthworm have both excretory and osmoregulatory functions. As urine moves along the
tubule, the transport epithelium bordering the lumen reabsorbs most solutes and returns
them to the blood in the capillaries. Nitrogenous wastes remain in the tubule and are
excreted to the outside. Earthworms inhabit damp soil and usually experience a newt uptake
of water by osmosis through their skin. Their metanephridia balance the water influx by
producing urine that is dilute (hypoosmotic to body fluids). Flatworms have an excretory
system called protonephridia, consisting of a branching network of dead-end tubules.
Metanephridia, another tubular excretory system, consist of internal openings that collect
body fluids from the body through a ciliated funnel, the nephrostome, and release the fluid
through the nephridiopore.
13. Insects and other terrestrial arthropods have organs called Malpighian tubules that remove
nitrogenous wastes and also function in Osmoregulation. The Malpighian tubules extend
from dead-end tips immersed in hemolymph to openings into the digestive tract. The
filtration step common to other excretory systems is absent. Instead, the transport
epithelium that lines the tubules secretes certain solutes, including nitrogenous wastes, from
the hemolymph into the lumen of the tubule.
14. Nephrons and associated blood vessels are the functional unit of the mammalian kidney. The
mammalian excretory system centers on paired kidneys which are also the principal site of
water balance and salt regulation. Each kidney is supplied with blood by a renal artery and
drained by a renal vein. Urine exits each kidney through a duct called the ureter. Both
ureters drain into a common urinary bladder. The mammalian kidney has two distinct
regions an outer renal cortex and an inner renal medulla. The nephron, the functional unit of
the vertebrate kidney consists of a single long tubule and a ball of capillaries called the
glomerulus. The blind end of the tubule forms a cup-shaped swelling, called Bowman’s
capsule, which surrounds the glomerulus.
15. Weaving back and forth across the cortex and medulla is the nephron, the function unit of a
vertebrate kidney. Each nephron is supplied with blood by an afferent arteriole, a branch of
the renal artery that divides into the capillaries of the glomerulus. The capillaries come
together as they leave the glomerulus forming an efferent arteriole. This vessel subdivides
again into the peritubular capillaries, which surround the proximal and distal tubules.
Additional capillaries extend downward to form the vasa recta, a loop of capillaries that
serves the loop on Henle. The tubules and capillaries are immersed in interstitial fluid, where
substances diffuse.
16. Filtrate becomes urine as it flows through the mammalian nephron and collecting duct.
Secretion and reabsorption in the proximal tubule substantially alter the volume and
composition of filtrate. Reabsorption of water continues as the filtrate moves into the
descending limb of the loop of Henle.
17. The mammalian kidney’s ability to conserve water is a key terrestrial adaptation. The
mammalian kidney can produce urine much more concentrated than body fluids, thus
conserving water. In a mammalian kidney, the cooperative action and precise arrangement
of the loops of Henle and the collecting ducts are largely responsible for the osmotic gradient
that concentrates the urine. Two solutes, NaCl and urea, contribute to the osmolarity of the
interstitial fluid which causes the reabsorption of water in the kidney and concentrates the
urine.
18. The loop of Henle has several qualities of a countercurrent system, such as those
mechanisms that maximize oxygen absorption by fish gills or reduce heat loss in
endotherms. The countercurrent system involving the loop of Henle expends energy to
actively transport NaCl from the filtrate in the upper part of the ascending limb of the loop.
Such countercurrent systems, which expend energy to create concentration gradients, are
called countercurrent multiplier systems. The countercurrent multiplier system involving the
loop of Henle maintains a high salt concentration in the interior of the kidney, enabling the
kidney to form concentrated urine.
19. The juxtamedullary nephrons, have well-developed loops that extend deeply into the renal
medulla. Only mammals and birds have juxtamedullary nephrons; the nephrons of other
vertebrates lack loops of Henle. It is the juxtamedullary nephrons that enable mammals to
produce urine that is hyperosmotic to body fluids, conserving water. The juxtamedullary
nephrons, which maintain an osmotic gradient in the kidney and use that gradient to excrete
hyperosmotic urine, are the key to understanding the physiology of the mammalian kidney
as a water-conserving organ. The juxtamedullary nephron is a key adaptation to terrestrial
life, enabling mammals to get rid of salts and nitrogenous wastes without squandering
water. The remarkable ability of the mammalian kidney to produce hyperosmotic urine is
completely dependent on the precise arrangement of the tubules and collecting ducts in the
renal cortex and medulla. The kidney is one of the clearest examples of how the function of
an organ is inseparably linked to its structure.
20. One important aspect of the mammalian kidney is its ability to adjust both the volume and
osmolarity of urine, depending on the animal’s water and salt balance and the rate of urea
production. With high salt intake and low water availability, a mammal can excrete urea and
salt with minimal water loss in small volumes of hyperosmotic urine. If salt is scarce and fluid
intake is high, the kidney can get rid of excess water with little salt loss by producing large
volumes of hypoosmotic urine (as dilute as 70 mosm/L). This versatility in osmoregulatory
function is managed with a combination of nervous and hormonal controls. Regulation of
blood osmolarity is maintained by hormonal control of the kidney by negative feedback
circuits.
21. The South American vampire bat, Desmodus rotundas, illustrates the flexibility of the
mammalian kidney to adjust rapidly to contrasting osmoregulatory and excretory problems.
This species feeds on the blood of large birds and mammals by making an incision in the
victim’s skin and then lapping up blood from the wound. Because they fly long distances to
locate a suitable victim, they benefit from consuming as much blood as possible when they
do find prey—so much so that a bat would be too heavy to fly after feeding. The bat uses its
kidneys to offload much of the water absorbed from a blood meal by excreting large volumes
of dilute urine as it feeds. Having lost enough water to fly, the bat returns to its roost in a
cave or hollow tree, where it spends the day. In the roost, the bat faces a very different
regulatory problem. Its food is mostly protein, which generates large quantities of urea, but
roosting bats don’t have access to drinking water. Their kidneys shift to producing small
quantities of highly concentrated urine, disposing of the urea load while conserving as much
water as possible. The vampire bat’s ability to alternate rapidly between producing large
amounts of dilute urine and small amounts of very hyperosmotic urine is an essential part of
its adaptation to an unusual food source.
22. Birds, like mammals, have kidneys with juxtamedullary nephrons that specialize in
conserving water. However, the nephrons of birds have much shorter loops of Henle than do
mammalian nephrons. Bird kidneys cannot concentrate urine to the osmolarities achieved by
mammalian kidneys. The main water conservation adaptation of birds is the use of uric acid
as the nitrogen excretion molecule. The kidneys of other reptiles, having only cortical
nephrons, produce urine that is, at most, isoosmotic to body fluids. However, the epithelium
of the cloaca helps conserve fluid by reabsorbing some of the water present in urine and
feces. Also, like birds, most other terrestrial reptiles excrete nitrogenous wastes as uric acid.
In contrast to mammals and birds, a freshwater fish must excrete excess water because the
animal is hyperosmotic to its surroundings. Instead of conserving water, the nephrons
produce a large volume of very dilute urine. Freshwater fishes conserve salts by reabsorption
of ions from the filtrate in the nephrons. Amphibian kidneys function much like those of
freshwater fishes. When in fresh water, the skin of the frog accumulates certain salts from
the water by active transport, and the kidneys excrete dilute urine. On land, where
dehydration is the most pressing problem, frogs conserve body fluid by reabsorbing water
across the epithelium of the urinary bladder. Marine bony fishes, being hypoosmotic to their
surroundings, have the opposite problem of their freshwater relatives. In many species,
nephrons have small glomeruli or lack glomeruli altogether. Concentrated urine is produced
by secreting ions into excretory tubules. The kidneys of marine fishes excrete very little urine
and function mainly to get rid of divalent ions such as Ca2+, Mg2+, and SO42-, which the fish
takes in by its incessant drinking of seawater.
23. The liver produces hormones like insulin and glugagon to regulate blood glucose. When the
brain detects blood glucose level that is too high, it sends a signal to the pancreas to secrete
insulin. Insulin decreases the blood glucose by: increasing permeability of plasma membrane
to glucose, converting more glucose to glycogen in the liver, and halting the conversion of
other substances like fats and amino acids to glucose in the liver. When the brain detects
that blood glucose is too low, it sends a signal to the pancreas to secrete glucagon. This
converts glycogen (stored glucose) into glucose in the LIVER. Thus increasing blood glucose
level. Therefore liver is an organ important in maintaining blood glucose level -->
homeostasis.
Key terms
Homeostasis- They steady-state physiological condition of the body.
Osmoregulation- Regulation of solute concentrations and water balance by a cell or organism.
Excretion- The disposal of nitrogen-containing metabolites and other waste products.
Transport epithelium- Layer(s) of epithelial cells that regulate solute movements and are
essential components of osmotic regulation and metabolic waste disposal.
Ammonia- A small, very toxic molecule (NH3) produced by nitrogen fixation or as a metabolic
waste product of protein and nucleic acid metabolism.
Urea- A soluble nitrogenous waste produced in the liver by a metabolic cycle that combines
ammonia with carbon dioxide.
Uric acid- A product of protein and purine metabolism and the major nitrogenous waste of
product of insects, land snails, and many reptiles. Uric acid is relatively nontoxic and largely
insoluble.
Osmolarity- Solute concentration expressed as molarity.
Osmoconformer- An animal that is isoomotic with its environment.
Osmoregulator- An animal that controls its internal osmolarity independent of the external
environment.
Stenohaline- Referring to organisms that cannot tolerate substantial changes in external
osmolarity.
Euyhaline-Referring to organisms that tolerate substantial changes in external osmolarity.
Anhydrobiosis- A dormant state involving loss of almost all body water.
Selective reabsorption- The cleansing of filtrate by re-adding some solutes to the body fluids
through active transport.
Secretion-The discharge of wastes from the body fluid into the filtrate.
Filtration- In excretory systems, the extraction of water and small solutes, including metabolic
wastes, from the body fluid.
Filtrate- Cell-free fluid extracted from the body fluid by the excretory system.
Protonephridium- An excretory system, such as the flame bulb system of flatworms, consisting
of flatworms, consisting of a network of tubules lacking internal openings.
Metanephridium- An excretory organ found in many invertebrates that typically consists of
tubules connecting ciliated internal openings to external openings.
Malphighian tubules- A unique excretory organ of insects that empties into the digestive tract,
removes nitrogenous wastes from the hemolymph, and functions in osmoregulation.
Renal artery- The blood vessel bringing blood to the kidney.
Renal vein-The blood vessel that carries blood away from the kidney.
Ureter- A duct leading from the kidney to the urinary bladder.
Urinary bladder-The pouch where urine is stored prior to elimination.
Urethra- A tube that releases urine from the mammalian body near the vagina in females and
through the penis in males; also serves in males as the exit tube for the reproductive system.
Renal cortex- The outer portion of the vertebrate kidney.
Renal medulla- The inner portion of the vertebrate kidney, beneath the renal cortex.
Nephron- Functional unit of vertebrate kidney - consists of a long tubule and ball of capillaries
called the glomerulus.
Glomerulus- A ball of capillaries surrounded by Bowman’s capsule in the nephron and serving
as the site of filtration in the vertebrate kidney.
Bowman’s capsule- A cup-shaped receptacle in the vertebrate kidney that is the initial,
expanded segment of the nephron where filtrate enters from the blood.
Proximal tubule- In the vertebrate kidney, the portion of a nephron immediately downstream
from Bowman’s capsule that conveys and helps refine filtrate.
Loop of Henle- The hairpin turn, with a descending and ascending limb, between the proximal
and distal tubules of the vertebrate kidney; functions in water and alt reabsorption.
Distal tubule- In the vertebrate kidney, the portion of a nephron that helps refine filtrate and
empties it into a collecting duct.
Collecting duct- The location in the kidney where processed filtrate, called urine, is collected
from the renal tubules.
Cortial nephrons- 80% of kidney’s nephrons. In mammals and birds, a nephron with a loop of
Henle located almost entirely in the renal cortex.
Juxtamedullary nephrons- In mammals and birds, a nephron with a loop of Henle that extends
far into the renal medulla.
Afferent arteriole- Branch of renal artery that supplies an individual nephron with blood.
Efferent arteriole- In the kidney, the blood vessel draining a nephron.
Peritubular capillaries- One of the tiny blood vessels that form a network surrounding the
proximal and distal tubules in the kidney.
Vasa recta- The capillary system in the kidney that serves the loop of Henle.
Antidiurectic hormone (ADH)- A peptide hormone, also known as vasopressin, that promotes
water retention by the kidneys. Produced in the hypothalamus and released from the posterior
pituitary, ADH also has activities in the brain.
Juxtaglomerular apparatus (JGA)- A specialized tissue in nephrons that releases the enzyme
rennin in response to a drop in blood pressure or volume.
Angiotensisn II- A peptide hormone that stimulates constriction of precapillary arterioles and
increases reabsorption of NaCl and water by the proximal tubules of the kidney, increasing
blood pressure and volume.
Aldeosterone- A steroid hormone that acts on tubules of the kidney to regulate the transport of
sodium ions (Ns+) and potassium ions (K+).
Renin-angiotensin-aldosterone system (RAAS) - Complex feedback circuit that functions in
homeostasis to conserve salt and water by regulating blood pressure.
Atrial natriuretic factor (ANF) - Hormone that opposes RAAS by reducing blood pressure by
inhibiting release of renin.