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)
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.
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)
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 HCO3Important: reabsorbed: NaCL, HCO3- (active); H2O
Secreted: K+ and H+ (active)
The collecting duct determines how much salt is
excreted in the urine. It is permeable to water but
not to salts.
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)
Secreted into the p. tubule: H+ (active); NH3 (passive)
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.
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
reabsorbed 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.
In the descending loop of Henle: increases from 300
to 1200 at the bottom part of the loop (water is
In the ascending limb: filtrate concentration
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
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.
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
Nervous and hormonal control
Decrease blood pressure or blood volume may be a
result of dehydration or low salt intake.
The mammalian kidney has the ability to adjust the
volume and osmolarity of urine through water and
salt balance and rate of urea production.
Water reabsorption through osmolarity of blood.
JGA or the juxtaglomerular apparatus monitors the
blood pressure in the afferent arteriole. Low blood
pressure will stimulate the JGA to release renin in
Osmoreceptor in the hypothalamus detects
osmolarity of blood.
Renin will convert angiotensinogen into angiotensin
Hyperosmotic blood will trigger the release of
antidiuretic hormone (ADH).
Angiotensin II can increase blood pressure and
volume in different ways. It can increase blood
pressure by constriction of arterioles. It can also
increase raised 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.
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.
Aldosterone acts on the distal tubule that stimulates
reabsorption of Na+ and water.
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
renin. It decreases blood volume and pressure. It
inhibits release of renin and aldosterone.