44

Question 1

osmoregulation

Answer 1

Homeostasis
thus requires osmoregulation, the general term for the
processes by which animals control solute concentrations and
balance water gain and loss.

Question 2

excretion

Answer 2

In safeguarding their internal fluid environment, animals
must also deal with a hazardous metabolite produced by the
dismantling of proteins and nucleic acids. Breakdown of
nitrogenous (nitrogen-containing) molecules releases ammonia, a very toxic compound. Several different mechanisms
have evolved for excretion, the process that rids the body of
nitrogenous metabolites and other metabolic waste products.

Question 3

osmolarity

Answer 3

osmosis occurs whenever two solutions separated by the membrane
differ in osmotic pressure, or osmolarity (total solute concentration expressed as molarity, that is, moles of solute per
liter of solution). The unit of measurement for osmolarity
used in this chapter is milliOsmoles per liter (mOsm/L). Seawater has an osmolarity of about 1,000 mOsm/L (equivalent
to a total solute concentration of 1 M), while the osmolarity
of human blood is about 300 mOsm/L.

Question 4

osmoconformer

Answer 4

• to be isoosmotic with its surroundings
• All osmoconformers are marine animals. Because an
  osmoconformer’s internal osmolarity is the same as that of its
  environment, there is no tendency to gain or lose water
• Many osmoconformers live in water that has a stable composition and hence have a constant internal osmolarity.

Question 5

osmoregulator

Answer 5

• to control internal osmolarity independent of that of its environment.
• Osmoregulation enables animals to live in environments
  that are uninhabitable for osmoconformers, such as freshwater and terrestrial habitats. To survive in a hypoosmotic
  environment, an osmoregulator must discharge excess water.
  In a hyperosmotic environment, an osmoregulator must instead take in water to offset osmotic loss. Osmoregulation
  also allows many marine animals to maintain an internal osmolarity different from that of seawater

Question 6

stenohaline

Answer 6

Most animals, whether osmoconformers or osmoregulators,
cannot tolerate substantial changes in external osmolarity and
are said to be stenohaline

Question 7

euryhaline

Answer 7

can survive large fluctuations in external osmolarity. Euryhaline osmoconformers include many barnacles and
mussels, which are continually covered and uncovered by
ocean tides

Question 8

Most marine invertebrates are osmoconformers. Their osmolarity is the same as that of seawater. They therefore face no substantial challenges in water balance. However,

Answer 8

because these
animals differ considerably from seawater in the concentrations of specific solutes, they must actively transport these
solutes to maintain homeostasis

Question 9

chlroide cells

Answer 9

Many marine vertebrates and some marine invertebrates
are osmoregulators. For most of these animals, the ocean is a
strongly dehydrating environment. For example, marine
fishes, such as the cod in Figure 44.3a, constantly lose water
by osmosis. Such fishes balance the water loss by drinking
large amounts of seawater. In ridding themselves of salts,
they make use of both their gills and kidneys. In the gills,
specialized chloride cells actively transport chloride ions (Cl)
out and allow sodium ions (Na) to follow passively. In the
kidneys, excess calcium, magnesium, and sulfate ions are excreted with the loss of only small amounts of water.

Question 10

The osmoregulatory problems of freshwater animals are the opposite of those of marine animals

Answer 10

The body fluids of freshwater
animals must be hyperosmotic because animal cells cannot tolerate salt concentrations as low as that of lake or river water.
Having internal fluids with an osmolarity higher than that of
their surroundings, freshwater animals face the problem of
gaining water by osmosis and losing salts by diffusion. Many
freshwater animals, including bony fishes, solve the problem of
water balance by drinking almost no water and excreting large
amounts of very dilute urine. At the same time, salts lost by diffusion and in the urine are replenished by eating and/or gill uptake

Question 11

anhydrobiosis

Answer 11

Extreme dehydration, or desiccation, is fatal for most animals.
However, a few aquatic invertebrates that live in temporary
ponds and in films of water around soil particles can lose almost all their body water and survive. These animals enter a
dormant state when their habitats dry up, an adaptation
called anhydrobiosis (“life without water”).

Question 12

Anhydrobiosis requires adaptations that keep cell membranes intact. Researchers are just beginning to learn how
tardigrades survive drying out, but studies show that

Answer 12

desiccated individuals contain large amounts of sugars. In
particular, a disaccharide called trehalose seems to protect the
cells by replacing the water that is normally associated with
proteins and membrane lipids. Many insects that survive
freezing in the winter also use trehalose as a membrane protectant, as do some plants resistant to desiccation

Question 13

The threat of dehydration is a major regulatory problem for terrestrial plants and animals:

Answer 13

Adaptations that reduce water loss are key to survival on land. Much
as a waxy cuticle contributes to the success of land plants, the
body coverings of most terrestrial animals help prevent dehydration. Examples are the waxy layers of insect exoskeletons,
the shells of land snails, and the layers of dead, keratinized skin
cells covering most terrestrial vertebrates, including humans.
Many terrestrial animals, especially desert-dwellers, are nocturnal, which reduces evaporative water loss because of the lower
temperature and higher humidity of night air.

Question 14

transport epithelia

Answer 14

In most animals, osmoregulation and metabolic waste disposal rely on transport epithelia—one or more layers of
epithelial cells specialized for moving particular solutes in
controlled amounts in specific directions. Transport epithelia
are typically arranged into complex tubular networks with
extensive surface areas. Some transport epithelia face the outside environment directly, while others line channels connected to the outside by an opening on the body surface
- also often function in disposal of metabolic wastes

Question 15

ammonia

Answer 15

some of the most significant waste
products are the nitrogenous breakdown products of proteins
and nucleic acids (Figure 44.8). When proteins and nucleic
acids are broken apart for energy or converted to carbohydrates or fats, enzymes remove nitrogen in the form of
ammonia (NH3). Ammonia is very toxic, in part because its
ion, ammonium (NH4
), interferes with oxidative phosphorylation. Although some animals excrete ammonia directly,
many species expend energy to convert it to less toxic compounds prior to excretion.

Question 16

excreting waste as ammonia

Answer 16

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 and are readily
lost by diffusion to the surrounding water.

Question 17

excreting waste as urea

Answer 17

Although ammonia excretion works well in many aquatic
species, it is much less suitable for land animals. Ammonia is
so toxic that it can be transported and excreted only in large
volumes of very dilute solutions.
- Instead, mammals, most adult amphibians, sharks, and some
marine bony fishes and turtles mainly excrete a different nitrogenous waste, urea. Produced in the vertebrate liver, urea
is the product of a metabolic cycle that combines ammonia
with carbon dioxide.

Question 18

The main advantage of urea is

Answer 18

its very low toxicity. Animals
can transport urea in the circulatory system and store it safely
at high concentrations. Furthermore, much less water is lost
when a given quantity of nitrogen is excreted in a concentrated
solution of urea rather than a dilute solution of ammonia.

Question 19

The main disadvantage of urea is

Answer 19

Animals
must expend energy to produce urea from ammonia. From a
bioenergetic standpoint, we would predict that animals that
spend part of their lives in water and part on land would
switch between excreting ammonia (thereby saving energy)
and excreting urea (reducing excretory water loss)

Question 20

excreting waste as uric acid

Answer 20

Insects, land snails, and many reptiles, including birds, excrete
uric acid as their primary nitrogenous waste. (Bird droppings,
or guano, are a mixture of white uric acid and brown feces.) Uric
acid is relatively nontoxic and does not readily dissolve in
water. It therefore can be excreted as a semisolid paste with very
little water loss. This is a great advantage for animals with little
access to water, but there is a cost: Uric acid is even more energetically expensive to produce than urea, requiring considerable ATP for synthesis from ammonia.

Question 21

Using uric acid as a

waste product conveys a selective advantage because

Answer 21

(Although urea is much less harmful than ammonia,
it is toxic at very high concentrations.) Using uric acid as a
waste product conveys a selective advantage because it precipitates out of solution and can be stored within the egg as
a harmless solid left behind when the animal hatches

Question 22

Regardless of the type of nitrogenous waste, the amount

produced is coupled to the animal’s energy budget

Answer 22

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 than animals that rely mainly on lipids or carbohydrates as energy sources

Question 23

filtration

Answer 23

In the first step, body fluid (blood, coelomic fluid, or hemolymph) is brought in contact with the selectively permeable
membrane of a transport epithelium. In most cases, hydrostatic pressure (blood pressure in many animals) drives a
process of filtration. Cells, as well as proteins and other
large molecules, cannot cross the epithelial membrane and remain in the body fluid. In contrast, water and small solutes,
such as salts, sugars, amino acids, and nitrogenous wastes,
cross the membrane, forming a solution called the filtrate.

Question 24

reabsorption

Answer 24

The filtrate is converted to a waste fluid by the specific
transport of materials into or out of the filtrate. The process
of selective reabsorption recovers useful molecules and
water from the filtrate and returns them to the body fluids.
Valuable solutes—including glucose, certain salts, vitamins,
hormones, and amino acids—are reabsorbed by active transport.

Question 25

secretion

Answer 25

Nonessential solutes and wastes are left in the filtrate or
are added to it by selective secretion, which also occurs by
active transport. The pumping of various solutes adjusts the
osmotic movement of water into or out of the filtrate.

Question 26

excretion

Answer 26

In the

last step—excretion—the processed filtrate containing nitrogenous wastes is released from the body as urine.

Question 27

protonephridia

Answer 27

• flatworms have them; they form a network of dead-end tubules
• these tubules, which are connected to external openings, branch
  throughout the flatworm body, which lacks a coelom or body
  cavity. Cellular units called flame bulbs cap the branches of
  each protonephridium. Consisting of 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 moving cilia
  resemble a flickering flame, hence the name flame bulb.) The
  processed filtrate then moves outward through the tubules
  and empties as urine into the external environment. The
  urine excreted by freshwater flatworms has a low solute concentration, helping to balance the osmotic uptake of water
  from the environment

Question 28

metanephridia

Answer 28

Most annelids, such as earthworms, have metanephridia
(singular, metanephridium), excretory organs that collect fluid
directly from the coelom (Figure 44.12). Each segment of a
worm has a pair of metanephridia, which are immersed in coelomic fluid and enveloped by a capillary network. A ciliated funnel surrounds the internal opening. As the cilia beat,
fluid is drawn into a collecting tubule, which includes a storage bladder that opens to the outside

Question 29

Malpighian tubules

Answer 29

Insects and other terrestrial arthropods have organs called
Malpighian tubules that remove nitrogenous wastes and
that also function in osmoregulation (Figure 44.13). The
Malpighian tubules extend from dead-end tips immersed in
hemolymph (circulatory fluid) 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.
Water follows the solutes into the tubule by osmosis, and the
fluid then passes into the rectum. There, most solutes are
pumped back into the hemolymph, and water reabsorption by
osmosis follows. The nitrogenous wastes—mainly insoluble uric acid—are eliminated as nearly dry matter along with the
feces. Capable of conserving water very effectively, the insect
excretory system is a key adaptation contributing to these animals’ tremendous success on land.

Question 30

kidney

Answer 30

• In vertebrates and some other chordates, a specialized organ
  called the kidney functions in both osmoregulation and
  excretion. Like the excretory organs of most animal phyla,
  kidneys consist of tubules. The numerous tubules of these compact organs are arranged in a highly organized manner
  and are closely associated with a network of capillaries. The
  vertebrate excretory system also includes ducts and other
  structures that carry urine from the tubules out of the kidney
  and, eventually, the body.
• Vertebrate kidneys are typically nonsegmented. However,
  hagfishes, which are invertebrate chordates, have kidneys
  with segmentally arranged excretory tubules. This suggests
  that the excretory structures of vertebrate ancestors also may
  have been segmented.

Question 31

where is ADH released from

Answer 31

A combination of nervous and hormonal controls manages
the osmoregulatory function of the mammalian kidney. One
key hormone in this regulatory circuitry is antidiuretic
hormone (ADH), also called vasopressin. ADH is produced
in the hypothalamus of the brain and stored in the posterior
pituitary gland, located just below the hypothalamus. Osmoreceptor cells in the hypothalamus monitor the osmolarity
of blood and regulate release of ADH from the posterior
pituitary

Question 32

ADH when blood osmolarity rises

Answer 32

more ADH is released into the bloodstream. When ADH reaches
the kidney, its main targets are the collecting ducts. There, ADH
brings about changes that make the epithelium more permeable
to water. The resulting increase in water reabsorption concentrates urine, reduces urine volume, and lowers blood osmolarity
back toward the set point. (Only the gain of additional water in
food and drink can fully restore osmolarity to 300 mOsm/L.) As
the osmolarity of the blood subsides, a negative-feedback mechanism reduces the activity of osmoreceptor cells in the hypothalamus, and ADH secretion is reduced

Question 33

ADH when blood osmolarity is reduced

Answer 33

A reduction in blood osmolarity below the set point has
the opposite set of effects. For example, intake of a large volume of water leads to a decrease in ADH secretion to a very
low level. The resulting decrease in permeability of the collecting ducts reduces water reabsorption, resulting in discharge of large volumes of dilute urine. (Diuresis refers to
increased urination, and ADH is called antidiuretic hormone
because it opposes this state.)

Question 34

ADH influences water uptake in the kidney’s collecting

ducts by

Answer 34

regulating the water-selective channels formed by
aquaporins. Binding of ADH to receptor molecules leads to a
temporary increase in the number of aquaporin proteins in
the membranes of collecting
duct cells (Figure 44.20). Additional channels recapture more
water, reducing urine volume.

Question 35

ADH mutations

Answer 35

Mutations that prevent ADH production or that inactivate the ADH receptor gene block the increase in channel
number and thus the ADH response. The resulting disorder
can cause severe dehydration and solute imbalance due to
production of urine that is abnormally large in volume and
very dilute. These symptoms give the condition its name:
diabetes insipidus

Question 36

RAAS

Answer 36

A second regulatory mechanism that helps maintain homeostasis by acting on the kidney is the renin-angiotensinaldosterone system (RAAS). The RAAS involves the
juxtaglomerular apparatus (JGA), a specialized tissue
consisting of cells of and around the afferent arteriole that
supplies blood to the glomerulus

Question 37

JGA

Answer 37

a specialized tissue
consisting of cells of and around the afferent arteriole that
supplies blood to the glomerulus (Figure 44.22). When
blood pressure or blood volume in the afferent arteriole
drops (for instance, as a result of dehydration), the JGA
releases the enzyme renin. Renin initiates a sequence of
chemical reactions that cleave a plasma protein called
angiotensinogen, ultimately yielding a peptide called
angiotensin II

Question 38

angiotensin II effects

Answer 38

Functioning as a hormone, angiotensin II raises blood pressure by constricting arterioles, which decreases blood flow to
many capillaries, including those of the kidney. Angiotensin
II also stimulates the adrenal glands to release a hormone
called aldosterone. This hormone acts on the nephrons’ distal tubules and collecting duct, making them reabsorb more
Na+ and water, thus increasing blood volume and pressure.

Question 39

angiotensin II drugs

Answer 39

Because angiotensin II acts in several ways that increase
blood pressure, drugs that block angiotensin II production
are widely used to treat hypertension (chronic high blood
pressure). Many of these drugs are specific inhibitors of angiotensin converting enzyme (ACE), which catalyzes the
second step in the production of angiotensin II.
- renin released from the JGA acts on angiotensinogen (in the blood), forming angiotensin I. ACE
in vascular endothelium, particularly in the lungs, then splits
off two amino acids from angiotensin I, forming active angiotensin II. Blocking ACE activity with drugs prevents angiotensin II production and often lowers blood pressure into
the normal range.

Question 40

renin-angiotensin-aldosterone system

Answer 40

The renin-angiotensin-aldosterone system operates as part of
a complex feedback circuit that results in homeostasis. A
drop in blood pressure and blood volume triggers renin release from the JGA. In turn, the rise in blood pressure and
volume resulting from the various actions of angiotensin II
and aldosterone reduces the release of renin

Question 41

ADH vs RAAS

Answer 41

• Both increase water reabsorption in the kidney, but they counter different osmoregulatory problems.
• The release of ADH is a response to an increase in blood
  osmolarity, as when the body is dehydrated from excessive
  water loss or inadequate water intake. However, a situation
  that causes an excessive loss of both salt and body fluids—a
  major wound, for example, or severe diarrhea—will reduce
  blood volume without increasing osmolarity.
• This will not affect ADH release, but the RAAS will respond to the drop in
  blood volume and pressure by increasing water and Na reabsorption. Thus, ADH and the RAAS are partners in homeostasis. ADH alone would lower blood Na concentration by
  stimulating water reabsorption in the kidney, but the RAAS
  helps maintain the osmolarity of body fluids at the set point
  by stimulating Na reabsorption.

Question 42

ANP

Answer 42

Another hormone, atrial natriuretic peptide (ANP),
opposes the RAAS. The walls of the atria of the heart release
ANP in response to an increase in blood volume and pressure. ANP inhibits the release of renin from the JGA, inhibits NaCl reabsorption by the collecting ducts, and
reduces aldosterone release from the adrenal glands. These
actions lower blood volume and pressure. Thus, ADH, the
RAAS, and ANP provide an elaborate system of checks and balances that regulate the kidney’s ability to control the osmolarity, salt concentration, volume, and pressure of blood.
The precise regulatory role of ANP is an area of active
research.

Question 43

proximal tubule

Answer 43

. Reabsorption in the proximal tubule
is critical for the recapture of ions, water, and valuable nutrients
from the huge volume of initial filtrate. NaCl (salt) in the filtrate
diffuses into the cells of the transport epithelium, where Na is
actively transported into the interstitial fluid. This transfer of
positive charge out of the tubule drives the passive transport of
Cl, as well as the movement of more Na from the lumen into
the cells of the tubule wall by facilitated diffusion and cotransport mechanisms

Question 44

proximal tubule: As salt moves from the filtrate to the interstitial fluid,

Answer 44

water follows by osmosis. The salt and water then diffuse
from the interstitial fluid into the peritubular capillaries. Glucose, amino acids, potassium ions (K), and other essential
substances are also actively or passively transported from the
filtrate to the interstitial fluid and then into the peritubular
capillaries.

Question 45

proximal tubule: pH

Answer 45

Processing of filtrate in the proximal tubule helps maintain a relatively constant pH in body fluids. Cells of the transport epithelium secrete H into the lumen of the tubule but
also synthesize and secrete ammonia, which acts as a buffer
to trap H in the form of ammonium ions (NH4
). The more
acidic the filtrate, the more ammonia the cells produce and
secrete, and a mammal’s urine usually contains some ammonia from this source (even though most nitrogenous waste is
excreted as urea). The proximal tubules also reabsorb about
90% of the buffer bicarbonate (HCO3
) from the filtrate, contributing further to pH balance in body fluids.

Question 46

As the filtrate passes through the proximal tubule,

Answer 46

materials to be excreted become concentrated. Many wastes leave
the body fluids during the nonselective filtration process and
remain in the filtrate while water and salts are reabsorbed.
Urea, for example, is reabsorbed at a much lower rate than are
salt and water. Some other toxic materials are actively secreted
into filtrate from surrounding tissues. (toxic molecules in proximal tubule are actively secreted from the transport epithelium into the lumen)

Question 47

aquaporin

Answer 47

• in descending limb of the loop of Henle
• Here numerous water channels
  formed by aquaporin proteins make the transport epithelium
  freely permeable to water. In contrast, there are almost no channels for salt and other small solutes, resulting in very low permeability for these substances

Question 48

Descending limb of the loop of Henle

Answer 48

• Reabsorption
  of water continues as the filtrate moves into the descending
  limb of the loop of Henle
• aquaporins
• For water to move out of the tubule by osmosis, the interstitial fluid bathing the tubule must be hyperosmotic to the
  filtrate. This condition is met along the entire length of the
  descending limb, because the osmolarity of the interstitial
  fluid increases progressively from the outer cortex to the
  inner medulla of the kidney. As a result, the filtrate loses
  water—and therefore its solute concentration increases—all
  along its journey down the descending limb.

Question 49

Ascending limb of the loop of Henle

Answer 49

The filtrate
reaches the tip of the loop and then travels within the ascending limb as it returns to the cortex. Unlike the descending
limb, the ascending limb has a transport epithelium studded
with ion channels, but not water channels. Indeed, this membrane is impermeable to water. Impermeability to water is very
rare among biological membranes and is critical to the function of the ascending limb.

Question 50

The ascending limb has two specialized regions:

Answer 50

a thin
segment near the loop tip and a thick segment adjacent to
the distal tubule. As filtrate ascends in the thin segment,
NaCl, which became concentrated in the descending limb,
diffuses out of the permeable tubule into the interstitial fluid.
This movement of NaCl out of the tubule helps maintain the
osmolarity of the interstitial fluid in the medulla. In the thick
segment of the ascending limb, the movement of NaCl out of
the filtrate continues. Here, however, the epithelium actively
transports NaCl into the interstitial fluid. As a result of losing
salt but not water, the filtrate becomes progressively more
dilute as it moves up to the cortex in the ascending limb of
the loop.

Question 51

distal tubule

Answer 51

The distal tubule plays a key role in regulating the K+ and NaCl concentration of body fluids. This
regulation involves variation in the amount of K+ secreted
into the filtrate as well as the amount of NaCl reabsorbed from
the filtrate. Like the proximal tubule, the distal tubule contributes to pH regulation by the controlled secretion of H+
and reabsorption of HCO3-.

Question 52

collecting duct

Answer 52

The collecting duct carries the filtrate through the medulla to the renal pelvis. The transport
epithelium of the nephron and collecting duct processes
the filtrate, forming the urine. One of this epithelium’s
most important tasks is reabsorption of solutes and water.
- the nephrons and collecting ducts have about
99% of the water and nearly all of the sugars, amino acids,
vitamins, and other organic nutrients are reabsorbed into
the blood

Question 53

producing concentrated urine

Answer 53

When the kidneys are conserving water, aquaporin channels in the collecting duct allow water molecules to cross the
epithelium. At the same time, the epithelium remains impermeable to salt and, in the renal cortex, to urea. As the collecting duct traverses the gradient of osmolarity in the
kidney, the filtrate becomes increasingly concentrated, losing more and more water by osmosis to the hyperosmotic
interstitial fluid. In the inner medulla, the duct becomes permeable to urea. Because of the high urea concentration in
the filtrate at this point, some urea diffuses out of the duct
and into the interstitial fluid. Along with NaCl, this urea
contributes to the high osmolarity of the interstitial fluid in
the medulla. The net result is urine that is hyperosmotic to
the general body fluids.

Question 54

producing dilute urine

Answer 54

In producing dilute rather than concentrated urine, the
kidney actively reabsorbs salts without allowing water to follow by osmosis. At these times, the epithelium lacks water
channels, and NaCl is actively transported out of filtrate. As
we will see shortly, the state of the collecting duct epithelium
is controlled by hormones that together maintain homeostasis for osmolarity, blood pressure, and blood volume.

Question 55

In a mammalian kidney, the production of hyperosmotic

urine is possible only because

Answer 55

considerable energy is expended for the active transport of solutes against concentration gradients. The nephrons—particularly the loops of
Henle—can be thought of as energy-consuming machines
that produce an osmolarity gradient suitable for extracting
water from the filtrate in the collecting duct

Question 56

The two primary solutes affecting osmolarity are

Answer 56

NaCl, which is deposited in the renal medulla by the loop of Henle, and urea,
which passes across the epithelium of the collecting duct in
the inner medulla.

Question 57

filtrate’s volume and osmolarity

Answer 57

Filtrate passing from Bowman’s capsule to the
proximal tubule has an osmolarity of about 300 mOsm/L, the
same as blood. A large amount of water and salt is reabsorbed
from the filtrate as it flows through the proximal tubule in the
renal cortex. As a result, the filtrate’s volume decreases substantially, but its osmolarity remains about the same

Question 58

elbow of loop of Henle

Answer 58

As the filtrate flows from cortex to medulla in the descending limb of the loop of Henle, water leaves the tubule by osmosis. Solutes, including NaCl, become more concentrated,
increasing the osmolarity of the filtrate. The highest osmolarity (about 1,200 mOsm/L) occurs at the elbow of the loop of
Henle. This maximizes the diffusion of salt out of the tubule
as the filtrate rounds the curve and enters the ascending
limb, which is permeable to salt but not to water. NaCl diffusing from the ascending limb helps maintain a high osmolarity in the interstitial fluid of the renal medulla.

Question 59

countercurrent multiplier systems.

Answer 59

Notice that the loop of Henle has several qualities of a countercurrent system, such as the mechanisms that reduce heat
loss in endotherms. In those cases, the countercurrent mechanisms involve passive movement along either
an oxygen concentration gradient or a heat gradient. In contrast, 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

Question 60

What prevents the capillaries of the vasa recta from dissipating the gradient by carrying away the high concentration of NaCl in the medulla’s interstitial fluid?

Answer 60

As shown in
Figure 44.14, the descending and ascending vessels of the
vasa recta carry blood in opposite directions through the kidney’s osmolarity gradient. As the descending vessel conveys
blood toward the inner medulla, water is lost from the blood
and NaCl is gained by diffusion. These fluxes are reversed as
blood flows back toward the cortex in the ascending vessel,
with water reentering the blood and salt diffusing out. Thus, the vasa recta can supply the kidney with nutrients and other
important substances carried by the blood without interfering with the osmolarity gradient in the inner and outer
medulla.

Question 61

why does the kidney have such a high metabolic rate?

Answer 61

The countercurrent-like characteristics of the loop of
Henle and the vasa recta help to generate the steep osmotic
gradient between the medulla and cortex. However, diffusion
will eventually eliminate any osmotic gradient within animal
tissue unless gradient formation is supported by an expenditure of energy. In the kidney, this expenditure largely occurs
in the thick segment of the ascending limb of the loop of
Henle, where NaCl is actively transported out of the tubule.
Even with the benefits of countercurrent exchange, this
process—along with other renal active transport systems—
consumes considerable ATP.

Question 62

compare osmolarity of urine to that of interstitial fluid

Answer 62

Although isoosmotic to the inner
medulla’s interstitial fluid, the urine is hyperosmotic to blood
and interstitial fluid elsewhere in the body.

Question 63

As a result of active transport of NaCl out of the thick segment of the ascending limb,

Answer 63

As a result of active transport of NaCl out of the thick segment of the ascending limb, the filtrate is actually hypoosmotic
to body fluids by the time it reaches the distal tubule. Next the
filtrate descends again toward the medulla, this time in the collecting duct, which is permeable to water but not to salt. Therefore, osmosis extracts water from the filtrate as it passes from
cortex to medulla and encounters interstitial fluid of increasing
osmolarity. This process concentrates salt, urea, and other
solutes in the filtrate. Some urea passes out of the lower portion
of the collecting duct and contributes to the high interstitial osmolarity of the inner medulla.