Biology 2672a:
Comparative Animal
Physiology
Kidneys and tubules
Kidneys
 Regulation
of salts and water in
body
 Excretion of nitrogenous wastes
 Production of Urine
More concentrated: conserving
water/ excreting more salts
 More dilute: excreting more water

Glomerular filtration
Bowman’s capsule
Pressure
maintained by
vasoconstriction
of efferent
vessels
Primary Urine:
Dilute, no proteins etc.
Porous walls +
high pressure
Water and solutes
<10kDa out
Water, sugars, salts,
amino acids, Urea
(sometimes assisted by
active transport)
Large things (e.g. proteins) remain behind
Fig. 27.1b
An Amphibian nephron
Reabsorption of salts
Water reabsorption
modulated here
Concentrated
Urine
(permeable
distal tubule)
-antidiuresis
Dilute Urine
(impermeable
distal tubule)
-diuresis
Mammalian kidneys, the big
picture
Cortex
Medulla
Renal Pelvis
Ureter
Urine Flow
Blood  Tubules
Tubules  Collecting tubes Reabsorption,
Cunning osmotic
Filtration
trickery concentrates
waste products
Fig. 27.6a
The nephron – not quite a oneway journey…
Fig. 27.6
Thick ascending
loop of Henle
Salt Re-absorption
Collecting Duct
Urine out,
concentration
of definitive
Urine
Loop of Henle
Thin ascending loop
of Henle
Bowman’s capsule
Ultrafiltration,
Production of
primary urine
Thick
segment of
descending
loop of
Henle
Re-absorption of
sugars, amino
acids, water
Thin segment
of descending
loop of Henle
Fig. 27.6
Solute reabsorption
 In
thick segment of
descending limb of
loop of Henle
Glucose
 Amino Acids

 Water
 Also
some in the
thick ascending limb
Concentration gradient in kidney
Fig. 27.13
The concentration gradient
 Established
by active transport of
salts in loop of Henle
 Leads to a gradient of urea as
well
e.g. Fig. 27.12
Concentration of urine
Occurs in collecting
ducts
 Driven by osmotic
gradient across
kidney
 Both urea and salts
 Can be manipulated
by altering
permeability of
collecting duct to
water

Fig. 27.14a
Changing concentration of
definitive urine
Fig. 27.14
Concentrating Urine
 Essential
for water conservation
on land
 Allows the selective removal of
salts
 Expected to be particularly highly
developed in desert mammals…
Reducing excretory water loss
 Efficient
kidneys
Get rid of a lot of salt and wastes
per unit water
 Mammals, birds, insects

 Efficient
from gut

re-absorption of water
Dry Faeces
Predictions about desert
mammal kidneys
 Longer
loop of Henle = greater
concentration gradient

Expect desert mammals to have
longer loops of Henle and to
produce more concentrated urine
Cortex
Medulla
Medulla
Renal Pelvis
Medullary thickness is a
measure of the length
of the loops of Henle
 Medullar + Pelvis =
good measure of
concentrating power

Fig. 27.6a
Medullary thickness is positively correlated
to maximum urine concentration
Fig. 27.8
Medullary thickness is related to
body size and habitat
Fig. 27.9
Microvasculature of kidneys
Lab Rat
Sand Rat
Fig. 27.10a,c
Interspecific variation in urine concentration
correlates with habitat in large mammals, too
Mesic
Xeric
Table 28.2
Insects
 Highly
efficient (most successful
terrestrial animals)
 Open circulatory system

No high pressure filtering
 Malpighian
tubules
Marcello Malpighi (1628-1694)
Malpighian tubules
Foregut & Midgut
Hindgut
Malpighian tubules
 Anywhere
from 2 to 200,
depending on species
 A blind-ended tube with walls
exactly 1 cell thick
 Float in haemolymph
 Open into hindgut
Malpighian tubules
 No

high pressure filtration
Active transport-driven formation of
dilute urine
Cells
Haemolymph
Lumen
Fig 27.21
Haemolymph
Principal cell
Mitochondria packed into
evaginations
Stellate cell
Lumen
Haemolymph
K+ Channel

Proton pump
generates
electrochemical
gradient


V-ATPase (H+ pump)
Lumen
Requires ATP
K+ follows via
electrogenic
transporter
Haemolymph
Cl- Channel
Cl- follows K+
gradient
 Water follows
osmotic
gradient into
tubule lumen

Aquaporin
V-ATPase (H+ pump)
Lumen
Malpighian tubules summary

Active transport sets up ion gradients

Proton pump; K+, Cl-
Na+,K+-ATPase also involved
(breaking news!)
 Water follows
 Passive transport of nitrogenous
wastes, amino acids etc.
 Active transport of large molecules


Alkaloids, proteins etc.
Water and solute reabsorption
 Urine
from tubules is dilute and
contains lots of things the insect
doesn’t want to lose
 Reabsorption of water and
solutes in hindgut/rectum

Determines final concentration of
the urine
Reading for Thursday
 Thursday:
Guest lecture (Dr.
Scott MacDougall-Shackleton;
birdsong)

Reading on OWL
 Tuesday:

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