1. Overview of excretion in mammals
Materials from the blood are transferred to the
nephrons where filtration, reabsorption and
secretion will occur. Excretion will occur at the
urethra. Remember: substances do not move
back to the lumen of the tubule from the
interstitial fluid because of small surface area in
the exterior side compared to interior (lumen
part)
1. Filtrate is produced when substances
from the blood is filtered in the
glomerulus and the Bowman’s capsule.
The concentration of this filtrate is the
same compared to the concentration of
the interstitial fluid in other parts of the
body.
2. The filtrate will move towards the
proximal tubule. Volume and
composition of the filtrate is changed
here. Production of H+
ions and NH3 to
balance the pH of the filtrate (produced
by the transport epithelium). Drugs and
poison are transferred from the
peritubular capillaries to the proximal
tubule.
Remember: the P. tubule reabsorbs
NaCl and H2O. The transport epithelium
in p tubule transport Na+
(active) and Cl-
(passive) into the interstitial fluid.
Water follows via osmosis.
Important: transferred back to the
capillaries: NaCl, Nutrients (active);
HCO3
-
, H2O, K+
(passively)
Secreted into the p. tubule: H+
(active);
NH3 (passive)
3. Water is reabsorbed greatly in the
descending part of the loop of Henle.
The transport epithelium that lines the
tubule is greatly permeable to water
but not to salt.
4. The thin ascending loop of Henle moves
salt from the filtrate passively. The
thick ascending loop of Henle moves
NaCl actively.
Important: animals with very long loop
of Henle or with juxtamedullary
nephrons conserve water efficiently
because of the mechanisms mentioned
in 3 and 4. The mechanism involve is
the countercurrent exchange of
substances. At upper part of the loop of
Henle concentration of solute is not as
high as you descend down the loop.
Water is reabsorb by the interstitial
fluid all the way down because of
varying change in osmolarity of the
interstitial fluid. The interstitial fluid
becomes more hypersomotic compared
to the filtrate as you descend because
the ascending loop of Henle transports
the NaCl in the filtrate.
5. The distal tubule acts on the secretion
and reabsorption of substances just like
the p tubule. It also controls the pH of
the filtrate by secretion of H+
and
reabsorption of HCO3
-
Important: reabsorbed: NaCL, HCO3
-
(active); H2O (passive)
Secreted: K+
and H+
(active)
6. The collecting duct determines how
much salt is excreted in the urine. It is
permeable to water but not to salts.

2. Important: reabsorbed: H2O, urea (due
to high concentration in the urine)
(passive) NaCl (active)
Conservation of water
Here filtrate concentration is always
compared to normal concentration of
interstitial fluid.
In the Bowman’s capsule: same
concentration because only filtration of
small substances occurred. (About 300
mosm/L)
In the descending loop of Henle:
increases from 300 to 1200 at the
bottom part of the loop (water is
greatly reabsorbed)
In the ascending limb: filtrate
concentration decreases
Importance: lose of water in the
ascending limb produces a
hyperosmotic filtrate. This
hyperosmotic filtrate will produce the
gradient that will move the salt from
the filtrate back to the interstitial fluid.
A gradient is produced between the
interstitial fluid and that of the filtrate.
Water will always move out from any
point in the descending limb because
the surrounding interstitial fluid will
always be hyperosmotic.
The surrounding capillaries do not
affect this gradient. It moves opposite
that of the limb of the loop of Henle.
In the Distal tubule: filtrate is
hypoosmotic.
In the collecting duct: because of
permeability to water the filtrate
becomes hyperosmotic along the way.
High concentration of urea in the
filtrate allows its diffusion to maintain
the gradient. Even though the filtrate
lost some solute along the way the
filtrate produced is still hyperosmotic
compared to interstitial fluid of the
body.
Nervous and hormonal control
The mammalian kidney has the ability
to adjust the volume and osmolarity of
urine through water and salt balance
and rate of urea production.

3. Water reabsorption through osmolarity
of blood.
Osmoreceptor in the hypothalamus
detects osmolarity of blood.
Hyperosmotic blood will trigger the
release of antidiuretic hormone (ADH).
ADH is produced by the hypothalamus
but is stored and released in the
pituitary gland.
ADH targets the transport epithelium of
the distal tubule and collecting duct.
(why not the descending loop of
Henle?)
The transport epithelium becomes
permeable to water. Water is
reabsorbed and decreases the
osmolarity of blood.
Negative feedback mechanism will
stimulate the osmoreceptors in the
hypothalamus to inhibit release of ADH.
Drinking water also decreases release of
ADH.
Alcohol disrupts the release of ADH.
Thereby, producing dilute urine.
Water reabsorption through blood
pressure or low blood volume.
Decrease blood pressure or blood
volume may be a result of dehydration
or low salt intake.
JGA or the juxtaglomerular apparatus
monitors the blood pressure in the
afferent arteriole. Low blood pressure
will stimulate the JGA to release renin
in the bloodstream.
Renin will convert angiotensinogen into
angiotensin II.
Angiotensin II can increase blood
pressure and volume in different ways.
It can increase blood pressure by
constriction of arterioles. It can also
increase raise blood pressure and
volume by stimulating the proximal
tubule to reabsorb more water and
NaCl. It can also stimulate the release of
aldosterone found in the adrenal
glands.
Aldosterone acts on the distal tubule
that stimulates reabsorption of Na+
and
water.

4. The RAAS also function in a negative
feedback mechanism. Decrease in
blood pressure an volume stimulate
production of rennin and aldosterone.
Increase in blood pressure and volume
inhibit the release of these hormones.
ADH- through blood osmolarity
RAAS- through blood volume and
pressure
This is important because an animal can
reabsorb water even without a change
in blood osmolarity.
ANF or atrial natriuretic factor opposes
action of rennin. It decreases blood
volume and pressure. It inhibits release
of renin and aldosterone.