Overview of excretion in mammals The transport epithelium that lines the
tubule is greatly permeable to water
Materials from the blood are transferred to the but not to salt.
nephrons where filtration, reabsorption and
secretion will occur. Excretion will occur at the 4. The thin ascending loop of Henle moves
urethra. Remember: substances do not move salt from the filtrate passively. The
back to the lumen of the tubule from the thick ascending loop of Henle moves
interstitial fluid because of small surface area in NaCl actively.
the exterior side compared to interior (lumen
part) Important: animals with very long loop
of Henle or with juxtamedullary
1. Filtrate is produced when substances nephrons conserve water efficiently
from the blood is filtered in the because of the mechanisms mentioned
glomerulus and the Bowman’s capsule. in 3 and 4. The mechanism involve is
The concentration of this filtrate is the the countercurrent exchange of
same compared to the concentration of substances. At upper part of the loop of
the interstitial fluid in other parts of the Henle concentration of solute is not as
body. high as you descend down the loop.
Water is reabsorb by the interstitial
2. The filtrate will move towards the fluid all the way down because of
proximal tubule. Volume and varying change in osmolarity of the
composition of the filtrate is changed interstitial fluid. The interstitial fluid
here. Production of H+ ions and NH3 to becomes more hypersomotic compared
balance the pH of the filtrate (produced to the filtrate as you descend because
by the transport epithelium). Drugs and the ascending loop of Henle transports
poison are transferred from the the NaCl in the filtrate.
peritubular capillaries to the proximal
tubule. 5. The distal tubule acts on the secretion
and reabsorption of substances just like
Remember: the P. tubule reabsorbs the p tubule. It also controls the pH of
NaCl and H2O. The transport epithelium the filtrate by secretion of H+ and
in p tubule transport Na+ (active) and Cl- reabsorption of HCO3-
(passive) into the interstitial fluid.
Water follows via osmosis. Important: reabsorbed: NaCL, HCO3-
(active); H2O (passive)
Important: transferred back to the
capillaries: NaCl, Nutrients (active); Secreted: K+ and H+ (active)
HCO3-, H2O, K+ (passively)
6. The collecting duct determines how
Secreted into the p. tubule: H (active); much salt is excreted in the urine. It is
NH3 (passive) permeable to water but not to salts.
3. Water is reabsorbed greatly in the
descending part of the loop of Henle.
Important: reabsorbed: H2O, urea (due gradient that will move the salt from
to high concentration in the urine) the filtrate back to the interstitial fluid.
(passive) NaCl (active) 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.
Conservation of water
In the Distal tubule: filtrate is
Here filtrate concentration is always
compared to normal concentration of
In the collecting duct: because of
permeability to water the filtrate
becomes hyperosmotic along the way.
High concentration of urea in the
In the Bowman’s capsule: same filtrate allows its diffusion to maintain
concentration because only filtration of the gradient. Even though the filtrate
small substances occurred. (About 300 lost some solute along the way the
mosm/L) filtrate produced is still hyperosmotic
compared to interstitial fluid of the
In the descending loop of Henle:
increases from 300 to 1200 at the
bottom part of the loop (water is Nervous and hormonal control
In the ascending limb: filtrate
concentration decreases The mammalian kidney has the ability
to adjust the volume and osmolarity of
Importance: lose of water in the urine through water and salt balance
ascending limb produces a and rate of urea production.
hyperosmotic filtrate. This
hyperosmotic filtrate will produce the
Water reabsorption through osmolarity
Water reabsorption through blood
pressure or low blood volume.
Osmoreceptor in the hypothalamus
detects osmolarity of blood.
Decrease blood pressure or blood
volume may be a result of dehydration
or low salt intake.
Hyperosmotic blood will trigger the
release of antidiuretic hormone (ADH).
JGA or the juxtaglomerular apparatus
monitors the blood pressure in the
ADH is produced by the hypothalamus afferent arteriole. Low blood pressure
but is stored and released in the will stimulate the JGA to release renin
pituitary gland. in the bloodstream.
ADH targets the transport epithelium of Renin will convert angiotensinogen into
the distal tubule and collecting duct. angiotensin II.
(why not the descending loop of
Angiotensin II can increase blood
pressure and volume in different ways.
The transport epithelium becomes It can increase blood pressure by
permeable to water. Water is constriction of arterioles. It can also
reabsorbed and decreases the increase raise blood pressure and
osmolarity of blood. volume by stimulating the proximal
tubule to reabsorb more water and
NaCl. It can also stimulate the release of
Negative feedback mechanism will aldosterone found in the adrenal
stimulate the osmoreceptors in the glands.
hypothalamus to inhibit release of ADH.
Aldosterone acts on the distal tubule
Drinking water also decreases release of that stimulates reabsorption of Na+ and
Alcohol disrupts the release of ADH.
Thereby, producing dilute urine.
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
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.