ANSWERS TO REVIEW QUESTIONS – CHAPTER 21
1.
Distinguish between the mechanisms and roles of passive homeostasis, equilibrium
homeostasis and negative feedback homeostasis. (pp. 479–481)
Homeostasis refers to the capacity of an animal to maintain a constant extracellular environment. Some
animals can achieve this passively simply by being in balance with their environment (passive
homeostasis). In the case in some marine animals whose extracellular fluids have a similar
concentration to sea water the process is called equilibrium homeostasis. In the slightly different case
of steady-state homeostasis, the animal produces a product (e.g. ammonia waste) that is lost into the
environment at a rate proportional to its concentration.
Passive homeostasis cannot cope with environmental change. Negative feedback homeostasis can. It
requires a sensor to detect change in an extracellular variable and compare this to a desired standard
and an effector system to bring about change if the detected value varies from the desired one. This
process is usually mediated in an integrating centre, which may be neuronal or hormonal in its
response.
2.
What are the principal differences in the solute and osmotic concentration of the
intracellular and extracellular fluids of animals? Why? (pp. 482–483 Table 21.1)
The osmotic concentration of extracellular fluid is the same as intracellular fluid. This must be so
because the flexible plasma membrane of a cell is permeable to water and so would not be able to
constrain any pressure difference (caused by differences in osmolarity) between the two compartments.
In contrast, the solute composition of intracellular fluid is always (and has to be) different to that of the
extracellular fluid. This ionic imbalance (intracellular fluid has higher K+ and lower Na+ and Cl–
concentrations) provides the basis for membrane voltage potentials and effectively allows the cell to
transport materials. Without this difference the cell could not function.
3.
Define osmoconform, osmoregulate, ionoconform and ionoregulate. Generally speaking,
which animals osmoconform and ionoconform, which osmoconform and ionoregulate, and
which osmoregulate and ionoregulate? (pp. 480–489)
Osmoconformers allow the osmotic concentration of their extracellular fluids to equal that of their
surroundings.
Osmoregulators are animals that regulate the osmotic concentration of their extracellular fluids to be
higher or lower than that of the surroundings.
Ionoconformers are marine animals that have ion concentration of their extracellular fluids similar to
that of seawater.
Ionoregulators are animals that use ion pumps to maintain their extracellular ion concentrations lower
than that of seawater.
Generally, most marine invertebrates osmoconform and ionoconform as this poses the least energy
expensive strategy in the constant environment that the sea presents. Cartilaginous fish osmoconform
(using urea) and ionoregulate. Marine lampreys and bony fish, and all freshwater animals, reptiles,
birds and mammals osmoregulate and ionoregulate.
4.
How do brine shrimp, which live in very hypersaline solutions, solve their ionic and osmotic
problems? (pp. 485, 491–492)
Brine shrimp which live in hypersaline water over five times more concentrated than sea water, both
ionoregulate and osmoregulate. Salt pumps located in the appendages in adults actively excrete salts
that are ingested or diffuse through their body wall. The body water lost by osmosis can be replenished
only by drinking the water in which they live, even though it increases their salt load and must
subsequently be excreted.
5.
Compare and contrast the advantages and disadvantages of ammonia, urea and uric acid as
the nitrogenous waste product for aquatic and terrestrial animals. (pp. 489–490 Figure
21.11)
Ammonia contains only one nitrogen atom per molecule so every nitrogen atom is equivalent to an
osmotic particle that requires water for excretion. Ammonia is also very toxic. However, it is extremely
soluble and virtually no energy is expended in its synthesis. This makes it ideal as an excretory product
for animals that live in large bodies of water and many aquatic animals excrete ammonia across their
body or gill surfaces. Urea is less toxic than ammonia and the excretion of urea requires less water than
ammonia because each molecule of urea contains two nitrogen atoms. However, the synthesis of urea
from ammonia requires the expenditure of energy, so there is a metabolic cost to urea excretion. Uric
acid contains four atoms of nitrogen per molecule so its excretion conserves even more water. It is also
highly insoluble and non-toxic but its synthesis requires even more energy expenditure than urea
synthesis. Terrestrial animals excrete either urea or uric acid with a very strong environmental and
phylogenetic influence to the pattern of waste excretion.
6.
How is urine formed by protonephridia, metanephridia, Malpighian tubules and nephrons?
How does the composition of the initial urine differ from that of the body fluids? (pp. 491–
492)
Protonephridia, metanephridia, Malpighian tubules and nephrons are all tubular excretory organs that
form urine by filtration and subsequently modify its composition by the active and passive reabsorption
of solutes and water, and secretion. Active reabsorption of useful solutes and water is very important
because the initial fluid is a simple filtrate and thus has essentially the same composition as blood
plasma and contains many ions, organic solutes and vitamins that are essential for survival. As such,
the initial urine does not contain blood cells and large molecules that do not pass through the filtration
apparatus.
7.
What processes are used to modify the composition of urine after it is formed by the
excretory tubules? (pp. 492–493 Figure 21.14)
The composition of the initial urine is extensively modified by the reabsorption of solutes and water
and by active secretion of specific waste products.
8.
Compare and contrast the structure and function of the excretory organs of earthworms,
crustaceans and insects. (pp. 494–495 Figures 21.15–21.17)
Annelid worms (earthworms) have metanephridia (rather than prototnephridia) that filter coelomic
fluid from the more anterior body segment. The antennal glands (or green glands) of crustaceans are
paired metanephridial excretory organs. A coelomic end sac forms urine by filtration of fluid from
blood vessels and solutes are reabsorbed during passage along a nephridial canal to form a dilute urine.
A bladder stores this urine and further reabsorbs solutes to further dilute the urine before it is voided
through an excretory pore. Marine crustaceans form urine that is iso-osmotic with blood and similar in
ion concentration; the nephridial canal may be short or absent. In contrast, insects have a variable
number of Malpighian tubules. Urine is formed in the blind ends of the tubules and is modified as it
passes along the tubules by the reabsorption of ions and the precipitation of uric acid. It is emptied into
the hindgut for further modification by the reabsorption of solutes and water to form a very dry urate
paste.
9.
Describe the structure of a mammalian nephron. What is the primary physiological function
of each part of the nephron? (pp. 496–499 Figures 21.19–21.22)
The first part of the nephron is composed of a dilated and invaginated double-walled cup. The outer
wall forms the renal, or Bowman’s capsule, while the inner wall is highly modified as podocytes,
which cover the glomerular capillaries but leave many slit-like spaces between their extensions. This
section of the nephron functions as a filter for the blood that passes through the capillaries. The filter
works because the diameter of the efferent arteriole is slightly smaller than that of the afferent arteriole,
causing a high pressure to be formed in the capillaries and effectively forcing the plasma through the
slits in the podocytes to form the glomerular filtrate. This filtrate next passes into the nephron tubule,
which extends from the renal capsule, and the first part of which is called the proximal convoluted
tubule. The proximal convoluted tubule is the site of much of the reabsorption of water and solutes that
must take place due to the very high rates of glomerular filtration. The rate of glomerular filtration is
about 125 mL min–1 in humans (or about 180 L per day) and this must be reabsorbed if we are not to
dehydrate very rapidly! The reabsorption is so efficient that over 99% of the glomerular filtrate,
including most of its solutes, is reabsorbed.
In mammals the next part of the nephron tubule forms a hairpin loop, the loop of Henle, which has
ascending and descending segments lying parallel to each other and in close proximity. The loop of
Henle is used to establish an osmotic concentration gradient in the renal medulla by the countercurrent
exchange of solutes and water between the thin descending and the thick ascending limbs. The length
of the loop is closely allied to the ability to produce concentrated urine, with desert animals having the
longest loops. The loop of Henle then runs into the distal convoluted tubule, which is mainly concerned
with the secretion of substances from the peritubular capillary blood. Finally the distal convoluted
tubule runs into the collecting duct which connects to the ureter that drains urine from the kidney. The
selective permeability of the collecting duct, under the control of antidiuretic hormone, allows the
osmoconcentration of the dilute urine that passes through it.
10. Explain how the loop of Henle establishes an osmotic concentration gradient in the renal
medulla of the mammalian kidney. How is the osmotic gradient used to osmotically
concentrate the urine? (pp. 498–499 Figure 21.22)
There are marked structural and functional differences between the descending and ascending limbs of
the loop of Henle. The thick section of the ascending limb actively transports Cl– out of the nephron
with Na+ passively following to maintain the electronic neutrality. This has the effect of reducing the
concentration of the fluid in this part of the nephron and the thick ascending limb’s walls are
impermeable to water. However, it does increase the solute concentration outside of the ascending
limb. In contrast, the descending limb lacks Cl– pumps and is permeable to water. Due to the increased
solute concentration outside its walls (the descending limb runs in close proximity but in the opposite
direction to the ascending limb) water passes from the descending limb out into the interstitium and
some solutes enter, thus increasing the concentration in the descending limb. The net effect of Cl –
pumping and countercurrent exchange in the loop of Henle is to establish a concentration gradient in
the renal medulla. Although the loops of Henle do not in themselves osmoconcentrate the urine (and in
fact the tubular fluid leaves the loop more dilute than normal body fluid) the osmotic gradient they
establish can be used, in conjunction with the collecting duct, to bring the concentration of the final
product to a very high level.