Download Excretion PPT

Excretory System
Excretion

rids the body of
metabolic
wastes
4 Excretory Organs:
1. Skin–excretes perspiration (water, salt & some urea)
 not to get rid of wastes as much as to cool us down
2. Liver

excretes bile
pigments,
cholesterol and
excess fatsoluble vitamins
3. Lungs

excrete CO2

produce urine
(water, nitrogenous
wastes, inorganic
salts)
 urea is the primary
nitrogenous waste
in humans
 (produced by liver
but excreted by
kidneys)
4. Kidneys
Urea Production (by liver)




Overall reaction of the urea cycle:
O
\\
2 NH3 + CO2 --- H2N-C-NH2 + H2O

ammonia

(ammonia comes from the breakdown of
amino acids)
urea
Anatomy
Urinary System – Gross
Anatomy
a. Kidneys
 size of a fist
 produce urine


Kidney transplant in
a cat
b. Ureters


- muscular tubes
- carry urine to bladder by peristalsis
c. Urinary Bladder
- can hold up to 600 mL
 hollow, muscular, expandable organ
 contracts when emptying
 contains stretch receptors, telling you it is full

Bladder Cancer

passes urine to
the outside
 short in females
(4 cm); longer
in males (20
cm)
 two sphincters
control the flow
of urine from
the bladder
d. Urethra
e. Renal
Cortex

outer layer
surrounding
renal medulla
 Site of
filtration,
tubular reabsorption
and secretion
f. Renal Medulla
-
cone-shaped tissue masses called renal pyramids
Site of water re-absorption
g. Renal Pelvis

central cavity that connects with the ureter
h. Nephrons (renal or kidney tubules)
- over 1,000,000
/ kidney
 several will be
connected to
the same
collecting duct
(enter the renal
pelvis)
Blood
Supply
to the
Nephron:
Blood Supply to the Nephron:
The renal artery supplies the kidney
 from the renal artery, an afferent arteriole
leads to the
glomerulus
 (a knot of
capillaries
inside the
glomerular
capsule)
(Bowman’s
Capsule)

blood leaves the
glomerulus via
the efferent
arteriole which
joins the
peritubular
capillaries
(surrounds the
rest of the
nephron)
a.
b.
c.
d.
e.
f.
g.
h.
i.
j.
k.
l.
Glomerular (Bowan’s)
capsule
Glomerlus
Afferent arteriole
Efferent arteriole
Proximal convoluted
tubule
Ascending limb
Decending limb
Loop of Henle
Peritubular capillary
Distal convoluted
tubule
Collecting duct
Renal medulla
Urine formation: Four steps:
1. Glomerular Filtration (in cortex)
 Blood enters the Glomerular capsule
(Bowman’s capsule)

“filterable blood components” (water, nitrogenous
wastes, nutrients, salts) leave the glomerulus and
enter the capsule. (= glomerular filtrate)


Water and dissolved nutrients, salts and urea enter the capsule
due to the glomerular blood pressure (about 60mmHg)
large molecules and formed elements cannot enter the
glomerulus (too big) and therefore, they leave the glomerulus
via the efferent arteriole
- approximately 180 litres of water are
filtered per day
 - only 1.8 liters of water are excreted

2. Tubular
Reabsorption
(in cortex)
- molecules and
ions are passively
and actively
reabsorbed from
the nephron to
the peritubular
capillary
- Na+ is actively
reabsorbed while Clfollows passively
 - water will follow
the NaCl (passively)
to balance the
osmolarity of the
blood
 (approximately 6070% of Na+ are
reabsorbed at the
proximal convoluted
tubule)


- specific carrier
molecules actively reabsorb
specific nutrients (glucose,
amino acids, vitamins)

ex. Glucose - there are MANY glucose
carrier molecules to ensure that glucose is not
lost

patients with uncontrolled diabetes
mellitus have too much glucose in the
blood, therefore, there will not be
enough carrier molecules to reabsorb it
all.

too much glucose left in
the filtrate results in
increased osmolarity in
the filtrate compared to
the blood, therefore, H2O
will stay in the filtrate to
“dilute” it.
 this accounts for the
frequent urination and
constant thirst of patients
that have not controlled
their diabetes

therefore, the proximal convoluted tubule
reabsorbs most water, nutrients and required salts
(ions)
 - things that are NOT reabsorbed are: some
water, much nitrogenous waste, and excess salts
(ions)
3. Tubular
Secretion (in
cortex)

a second
way that
substances
are removed
from the
blood and
added to the
tubular fluids

- H+, ammonium
ions, creatinine,
and drugs (ex.
Penicillin) move
(actively) from the
blood in the
peritubular
capillary to the
distal convoluted
tubule
4. Re-absorption of Water:

- dependent mostly on the loop of Henle and
the collecting duct (in the MEDULLA of the
kidney)

- The loop of Henle
sets up an osmotic
gradient by actively
pumping NaCl into
the extracellular fluid
surrounding the loop
in the MEDULLA
- the concentration gradient gets greater
as the loop travels deeper into the
medulla
 - as the filtrate travels down the
descending loop, water diffuses out
because of the artificial gradient in the
medulla


- the descending loop is impermeable to
salts, therefore the concentration of
salts begins to build up inside the tubule
as you travel down the descending loop

- the thin portion of
the ascending loop
is permeable to
salts, but
impermeable to
water and urea.
Therefore, salt will
diffuse out of the
tubule into the
extracellular
space.

- this contributes to the artificial concentration gradient
of salts in the medulla
 - urea also diffuses into the extracellular space near
the end of the loop, but it comes from the collecting
duct that runs parallel to the loop

(The extra salt and urea at this end of the
tubule are what caused the water to diffuse out
from the descending loop)

- as the filtrate reaches the large portion of the
ascending loop, salt must be actively pumped out.

salt is also actively pumped out as the
ascending loop goes through the cortex,
and becomes the distal tubule. Water
will follow the salt out of the distal tubule
because now, the distal tubule is
permeable to water.

[ at the distal tubule, ammonia, H+, K+,
and drugs are actively pumped INTO
the tubule from the efferent arteriole (=
tubular secretion)]
Tubular secretion of H+ is
important in maintaining control
of the pH of the blood
* When the pH of the blood starts to
drop, more hydrogen ions are secreted.
* If the blood should become too
alkaline, secretion of H+ is reduced.

Finally, the last bit of water can be reabsorbed if
necessary as the collecting duct passes through the
region of the medulla that is very concentrated in salts.
Regulatory Functions of the Kidney:
1. Antidiuretic hormone (ADH) (secreted
from the posterior pituitary) controls how
much (or how little) water is reabsorbed
at the distal convoluted tubule and the
collecting duct.

If the blood solute concentration needs
to be decreased, ADH is produced; this
makes the collecting duct and the distal
tubule VERY permeable to water.
(increases blood volume)

If the blood solute concentration needs
to be increased, ADH is not produced;
the absence of ADH makes the
collecting duct and the distal tubule less
permeable to water. (decreases blood
volume)
2. Renin-AngiotensinAldosterone Homeostatic System
When blood volume (and therefore blood
pressure) is too low to promote
glomerular filtration, the
“juxtaglomerular apparatus” (in the
kidneys) secretes renin.
renin (an enzyme) changes
angiotensinogen (produced by the liver)
into angiotensin I.
 angiotensin I gets converted into
angiotensin II
 angiotensin II is a strong vasoconstrictor
and also stimulates the adrenal cortex
to release aldosterone.

Aldosterone (a hormone) promotes the
reabsorption of sodium ions, which
causes more water to reabsorbed.
 Therefore, aldosterone (from the
adrenal cortex) acts to increase blood
volume and blood pressure.

- a hormone
secreted by the
heart when
stretch
receptors in the
atria indicate
that the blood
volume is too
large.
 - ANH inhibits
the secretion of
renin and
aldosterone
3. Atrial
natriuretic
hormone (ANH)

Therefore, ANH promotes the excretion
of sodium ions, (which means that water
will follow). (= decreasing blood volume
and blood pressure.)
Diuretics - agents that increase the flow
of urine
 Examples:
 Alcohol - inhibits the secretion of ADH
 Caffeine - increases glomerular filtration
rate and decreases tubular reabsorption of sodium ions.


Some diuretic drugs prescribed to fight
high blood pressure, inhibit the reabsorption of sodium ions, which
decreases the blood pressure.