|
Kidney
structure and function (pt 2)
Monacia
Williams, Contributor
Well
hello again. How are you this week?
Good, I hope. Did you spend some time
studying the diagrams that were included
in last week's lesson? I hope you
did because you never know when they
might become useful. Remember that
most of the questions in the essay
section of Paper 02 usually begin
with "Using annotated diagrams".
This means that you will need to know
how to draw and label those diagrams.
This
week we will continue the lesson we
began last week. That lesson stopped
at the study of proximal convoluted
tubule so this week we move to the
next section of the nephron, the Loop
of Henle.
The
Loop of Henle
This
is the area of the tubule where water
is reabsorbed. The greater the length
of the Loop of Henle, the more water
that is reabsorbed from the filtrate.
This means that desert animals like
the kangaroo rat have very long Loops
of Henle causing the rat to drink
water only infrequently. Re-absorption
of water prevents dehydration of the
body.
Distal
convoluted tubule
This
area also facilitates the reabsorption
of water and mineral salts from the
filtrate. However, here, the reabsorption
is not only controlled by an osmotic
gradient, it is controlled by the
hormone ADH - the antidiuretic hormone.
The end product of this re-absorption
is urine.
In
man, the kidneys filter about 170
litres of water per day, yet only
1.5 litres of urine are excreted.
This means that most of the water
is reabsorbed. This happens because
the brain has the ability to monitor
the amount of water in the blood.
If the brain detects that the water
content of the blood has become low,
it will cause the pituitary gland
to secrete ADH. ADH will stimulate
the distal convoluted tubule to absorb
less water from the blood. This means
that the urine that leaves the tubule
and enters the collecting duct will
have less water and is, therefore,
more concentrated.
If
the brain detects a lot of water in
the blood, then the pituitary will
not be stimulated to produce ADH,
hence less water is absorbed by the
distal convoluted tubule. This means
that the urine that leaves the tubule
will have more water and will be dilute.
This control of the water content
of the body is known as osmoregulation.
Osmoregulation is the regulation of
the salt and water content of the
blood in order to ensure that the
concentration of the blood remains
constant. Daily activities such as
sweating and eating can change the
concentration of the blood. This control
of the water content is an example
of a negative feedback mechanism and
is a part of the homeostasis.
Collecting
duct
This
collects urine from several nephrons
at the same time. The collected urine
is sent to the bladder through the
ureter.
Bladder
The
bladder has muscular walls which can
stretch, enabling it to hold large
quantities of urine. The bladder has
a sphincter muscle which keeps it
tightly closed until it is full. Emptying
occurs when the muscle opens. Adults,
unlike young children, can consciously
control the operation of this muscle
and hence empty the bladder at will.
The
following table gives a comparison
of the composition of the blood in
the vessels going to and from the
kidney.
|
Composition
of the blood in the renal
artery and the renal vein
|
|
Renal
artery
|
Renal
vein
|
| Contains
more water |
Contains
less water-some is lost
in urine |
| Contains
a high concentration of
urea |
Contains
little or no urea-lost as
urine |
| Salt
concentration is high |
Salt
concentration is low |
| More
O2, less CO2 |
More
CO2 and less O2 |
|
As
seen in the first diagram, given two
weeks ago, all humans have a pair
of kidneys. However, it is quite possible
to survive on only one. However, if
both kidneys become damaged, it can
prove to be life threatening. A kidney
dialysis machine can be used to remove
the waste chemicals from the blood.
The process, which is known as dialysis,
lasts for about five hours and has
to be done two or three times per
week. The dialysis machine contains
a fluid, the dialysis fluid, which
runs counter current to the patient's
blood. Both the blood and the fluid
are separated by a partial membrane,
the dialysis membrane. Urea and other
wastes filter out of the blood by
diffusion and leave the machine into
the dialysis fluid. The patient's
blood is now cleaner and is allowed
to pass back into the vein.
Individuals
with diseased kidneys can also have
a kidney transplant. This involves
replacing one of the diseased kidneys
with a healthier one from a donor.
Even when the donor's genetic makeup
is close to the recipient, this carries
with it the fear of rejection by the
recipient. To prevent rejection, the
bone marrow of the patient is treated
with radiation to stop white blood
cell production. Radiation and drugs
help to suppress the patient's immune
system and prevent rejection of the
donated organ.
Well
there you have it, we have now completed
our study of excretion in plants and
animals. Have a wonderful week!
Monacia
Williams teaches at Glenmuir High
School. Send questions and comments
to kerry-ann.hepburn@gleanerjm.com
|
