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Excretion Wikispaces by MikeJenny


 Topic 12
All living organisms need to be able to excrete their waste
products, because they would otherwise build up and
become toxic.

Plants excrete oxygen, animals excrete carbon dioxide
and nitrogenous waste that come from proteins.

The way an animal disposes of its nitrogenous waste
depends on its evolution and habitat.
Types of nitrogenous waste:

Ammonia: (NH3) very toxic, must be disposed of quickly. Very
soluble in water and diffuses across the cell membrane of animals
that live in the water. Some can excrete through their bodies,
others, like fish, excrete across their gills.

Uric Acid: non-toxic, relatively insoluble, can be precipitated out
into a solid, and stored for later excretion. It takes more energy to
produce this molecule, but the animal does not loose water in
disposing of it. Birds, reptiles, insects and some amphibians that
live in the desert excrete uric acid

Urea: non-toxic, soluble. Produced in the liver and
transported via the blood to the kidneys for excretion.
Mammals, sharks, bony fish
                  THE NEPHRON
                                Efferent arteriole-
Afferent                        going out
arteriole going
into glomerulus

 vasa recta
 There are four main processes in excretion:





 Takes place between the glomerlus and the Bowman’s

1. Blood enters the glomerular capillaries under high pressure.
This pressure results from the fact that the efferent vessel
(exiting the renal artery) is narrower than the afferent vessel
(entering the renal artery).
2. The high hydrostatic pressure causes fluid and small
molecules to be pushed out of the capillaries into the capsule.

3. Specialized structures: fenestrated capillaries (tiny holes)
and podocytes in the basement membrane of the capsule
facilitate this movement.
4. Large molecules like proteins and cells can’t move out.

5. The concentration of salts, amino
acids, glucose, vitamins and urea in
the filtrate are equal to those in the

 6. Filtration is passive and

 Specialized cells of the
 capsule called podocytes
 wrap around the capillaries
 like fingers so that the entire
 surface is covered
 The filtrate moves from the capsule to the Proximal Tubule


1. Water and solutes from the filtrate need to get reabsorbed.
The volume and concentration of the filtrate will be altered in
the proximal tubule

2. Some substances move back into the capillary by diffusion:
HCO3, water, K+, Cl-

3. Some are actively transported: Na ( this sets up concentration
gradient so water can move), glucose, amino acids

1. Some substances are secreted into the filtrate from the

2. Ammonia, toxins, H+ ions and drugs are actively transported.

3. Urea moves by diffusion
The filtrate now moves down the descending limb of the Loop
of Henle

1. The epithelium here is permeable to water but not to solutes.

2. The concentration of water in the interstitial fluid is lower than
the filtrate (hyperosmotic) so water continues to move out of the
descending loop, making the filtrate more concentrated

3. The filtrate is at its greatest concentration (1200mosm/L) at the
bottom of the Loop.
 The filtrate now moves up into the ascending limb.

1. The epithelium in the ascending limb is permeable to
solutes but not to water.

2. The filtrate is very concentrated as it enters the ascending
limb and so Na+ and CL- move out down their concentration
gradients but water can’t move in, due to the selective
permeability of the cells.
3. About 1/2 way up the ascending limb salts are actively
transported out

4. The filtrate becomes less concentrated as it moves up, until
it is even less concentrated than it was when it entered the
descending limb
 The passive and active movement of solutes ensures that the
 interstitial fluid will have a high osmolarity.

The filtrate now moves into the distal tubule

The distal tubule’s function is to regulate the concentration
of K+, Na+ and Cl-. It also regulates pH by controlling
secretion and reabsorption of H+ and HCO3 - ions
 The filtrate now enters the collecting duct

 1. Filtrate goes from the medulla to the renal pelvis then
 to the ureter.

2. The cells here are permeable to water but not salts, so
water moves out due to high osmolarity in the interstitial

3. Na+ and Cl- can be actively reabsorbed if needed. This is
dependent on hormonal control.

4. Some urea diffuses out as the filtrate becomes more
concentrated. This helps to keep the interstitial fluid very
The steep concentration gradient that is maintained in the
interstitial fluid around the nephron means that mammals
can excrete concentrated urine and therefore conserve water.
This is achieved by the :
                 Countercurrent multiplier system
 A countercurrent exchange system is one in which the
 opposite flow of adjacent fluids maximizes transfer rates.

The system is also a multiplier system because the nephron
achieves a steeper concentration gradient of solute
concentration in the the medulla than would be possible by
just the counter current mechanism.

There is also a countercurrent system in the vasa recta, the
vessels that parallel the structures in the nephron.
    To review:
•As urine passes up the ascending loop solutes are actively
transported out- this increases the concentration in the interstitial
fluid up to 1200mosm) and makes the urine dilute (100 mosm.)

•This makes a steep gradient across the kidney: the cortex is
very isotonic (300mosm) and the medulla is very hypertonic

•The reason such steep differences can be achieved is due to
the countercurrent multiplier system. The active transport of
Na can only achieve a gradient of 200mosm, if you start with
300mosm, then it is only possible to get 500mosm in the
interstitial fluid. How does the concentration get to 1200??
 A. Loop is initially filled with
 filtrate that is isotonic to the
 interstitial fluid

B. Na is pumped out of the
ascending loop- this raises the
concentration outside and
lowers it inside to 200 mosms
C.Water moves out of the
descending limb by osmosis.
This raises the concentration
inside the tubule to 400mosm

D. New filtrate enters from the
glomerlulus- this pushes the
concentrated fluid (400) into the
ascending limb
E. Now there is a second round of
pumping. The Na pump produces
another 200mosm gradient across the
membrane. But it is starting from a
more concentrated solution, so the
concentration rises to 500mosm now.

F. In the third round of Na pumping,
the interstitial concentration rises to 700
mosm. This process continues. Notice
that the concentration is higher in the
medulla compared to the cortex.
So, the high osmotic gradient is achieved because:
1. Na is pumped out of the ascending loop by active
2. Water leaves the descending loop by osmosis
3. Glomerular filtration, driven by high pressure is
   constantly pushing new fluid into the loop
4. The vasa recta has its own countercurrent system which
   prevents it from dissipating the gradient in the interstitial
   fluid. So the concentration in the blood is high in the
   medulla and low in the cortex
The concentration of urine is regulated by hormones

ADH: antidiuretic hormone

 Other hormones can influence water and salt concentrations as well.

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