Transcript Excretion - CAPE Biology Unit 1 Haughton XLCR 2013

Excretion
Substitute teacher: Omar Murray
Teacher: Mrs.Haughton
Objectives

Outline the function of the nephron in
filtering blood. Mention:
–
Ultra filtration in the glomerulus
–
Factors affecting glomerulus filtrate
–
Selective reabsorption in the proximal
convoluted tubule
–
Description of the regions of the Loop of
Henle and functions
–
Selective reabsorption in the distal
convoluted tubule.
–
Urine collection in the collection duct.
The human kidney
The basic unit of structure and function
of the kidney is the nephron and its
associated blood supply. Each kidney, in a
human, contains an estimated 1 million
nephrons each having an
approximate
length of 3cm. The total length of tubules in
each kidney is about 120km. This offers an
enormous surface area for the exchange of
materials.
Each nephron is composed of six regions, each
having its own particular structure and function:
1.
Renal corpuscle, composed of renal capsule (
bowman’s capsule) and glomerulus;
2.
Proximal convoluted tubule;
3.
Descending limb of the loop of Henle;
4.
Ascending limb of the loop of Henle;
5.
Distal convoluted tubule;
6.
Collecting duct.
Ultrafiltration
The first step in the formation of urine is
ultrafiltration of the blood. This takes place in
the bowman’s capsule. Ultrafiltration is
filtration under pressure. The pressure comes
from the blood pressure and is know as
hydrostatic pressure, or pumping pressure.
Blood enters the glomerulus at high pressure
direct from the heart via the dorsal aorta, renal
artery and finally an arteriole.
Glomerulus
The glomerulus is a knot of capillaries in the
renal capsule. The diameter of the capillaries in the
glomerulus is less than that of the arterioles, so as
the blood enters the narrow capillaries pressure
rises. Water and small solute molecules are
squeezed out of the capillaries through the
epithelium of the renal capsule and into the
interior capsule. Large molecules like proteins, as
well as red blood cells and platelets , are left behind
in the blood.
Filtration
takes place through three layer:
Endothelium
of the blood capillary- this is very
thin and is perforated with thousands of pores of
about 10nm in diameter. They occupy up to 30%
of the area of the wall. The pores are not a
barrier to plasma proteins because they are too
large
Basement
membrane of the blood capillaries- all
epithelial cells rest on a basement membrane. It consist
of a meshwork of fibers, including collagen fibers.
Water and small solute molecules can pass through
spaces between the fibers. Red blood cells and platelets
are too large. Protein molecules are repelled by negative
charges on the fibers.

Epithelium of the renal capsule- this is made of
cells which are highly modified for
filtration,
called podocytes. Each cells has many foot-like
extensions
projecting from neighboring cells .
They fit together loosely, leaving slits called slit
pores or filtration slits about 25nm wide. The
filtered fluid can pass through these slits.
Glomerular filtrate
About 20% of the plasma is filtered into
the capsule. Of the 3 layers, the basement
membrane is the main filtration barrier. The
filtered fluid in the capsule is called
glomerular filtrate. It has a chemical
composition similar to that of blood plasma. It
also contains glucose, amino acids, vitamins,
ions, nitrogenous waste, some hormones and
water. Blood passing from the glomerulus has
a lower water potential due to the increase
concentration of proteins and a reduced
hydrostatic pressure.
Factors affecting
glomerular filtrate
(GF)

Hydrostatic pressure

Solute potentials

Blood pressure(higher the blood pressure
the higher the filtrate rate.)

Vasodilatation(decreasing resistance to
the flow of blood to the glomerulus
thereby increasing filtrate rate)

Vasoconstriction(increase resistance in
the efferent arteriole slows rate blood
exit glomerulus thereby more blood is
filtered)
Hydrostatic pressure
The filtration pressure forcing fluid
out of the glomerulus depends not only
on the hydrostatic pressure of the blood,
but also on the pressure of the
glomerular filtrate. If this equaled the
hydrostatic pressure of the blood they
would cancel each other out(plasma
would not enter the nephron). In fact,
the
hydrostatic
pressure
of
the
glomerular filtrate is much less than
that of the blood, although not zero.
Solute potential
Similar to the hydrostatic pressure, the
solute potential on either side of the filtration
barrier will affect the fluid flow. As you know
water tends to move from a less concentrated
solution to a more concentrated solution. As the
blood flows from the afferent arteriole to the
efferent arteriole through the glomerulus it loses
water and other substances, but proteins remain
in the blood increasing the concentration by 20%
as a result of the water lose. This makes the
solute potential, of the blood more negative and
tends to decrease (GFR) . The greater the water
potential of the blood compared with the GF, the
greater the filtration pressure and the GFR.
Selective reabsorption in the
proximal convoluted tubule
The proximal convoluted tubule cells are adapted
for reabsorption as followed:

Large surface area due to microvilli and basal
channels;

Numerous mitochondria;

Closeness of blood capillaries
Over 80% of the glomerular filtrate is
reabsorbed here, including all the glucose, amino
acids, vitamins, hormones and about 80% of the
sodium chloride and water. The mechanism of
reabsorption is as follows

Glucose, amino acids, and ions diffuse into the
cells of the proximal convoluted tubule from the
filtrate and are actively transported out of the
cells into the spaces between them and the basal
channels. This is done by carrier proteins in the
cells surface membranes.

Once in these spaces and channels they enter
the extremely permeable blood capillaries by
diffusion and are carried away from the
nephron.

The constant removal of these substances from
the proximal convoluted tubule cells creates a
diffusion gradient between the filtrate in the
proximal tubule and the cells, down which
further substances pass. Once inside the cells
they are actively transported into the spaces
and channels and the cycle continues.
The loop of Henle
The function of the loop of Henle is to
conserve water. The longer the loop of Henle,
the more concentrated the urine that can be
produced. The urine of a human can be 4 to 5
times as concentrated as the blood. The loop of
Henle , together with the capillaries of the
vasa recta and collecting duct, creates and
maintains an osmotic gradient in the medulla
which extends from cortex to the tips of the
pyramids.
`
The loop of Henle has three distinct
regions, each with its own function.

The descending limb which has thin walls

The thin ascending limb this is the lower
half of the ascending limb and has thin
walls like the descending limb;

The thick ascending limb this is the upper
half of the ascending limb and has thick
walls.
The
descending
limb
is
highly
permeable to water and permeable to most
solutes. Its function is to allow substances to
diffuse easily through its walls.
Both parts of the ascending limb are
almost totally impermeable to water. The cells
in the thick part can actively
reabsorb
sodium, chloride, potassium and other ions
from the tubule. Normally water would follow
by osmosis the movement of these ions into
the cells, but this cannot occur because the
cells are permeable to water as stated. The
fluid in the ascending limb therefor becomes
very dilute by the time it reaches the distal
convoluted tubule.
The distal convoluted
tubule and collecting
duct
In the last two regions of the nephron,
the distal convoluted tubule and the collecting
duct, fine tuning of the body fluid composition
is achieved. Fine control of the precise
amounts of water and salts reabsorbed is
important in osmoregulation. This is one role
of the distal convoluted tubule and collecting
duct.
The cells of the distal
convoluted tubule have a similar
structure to those of the proximal
tubule, with microvilli lining the
inner surface to increase the
surface area for reabsorption , and
numerous mitochondria to supply
energy for active transport.
The collecting duct carries fluid from
the outer region of the medulla, next to the
cortex, to the pyramids. As the fluid moves
down the collecting duct, the tissue fluid in the
medulla surrounding the duct gets more and
more concentrated. Water therefore leaves the
collecting duct by osmosis. The final
concentration of the urine can be as high as
the medulla, about 1200 units, although the
actual amount of water lost is controlled by
ADH.
ADH and the formation of
urine
The body maintains the solute potential of
the blood at an approximately steady state by
balancing water uptake from the diet with water
lost in evaporation, sweating, egestion and
urine. The precise control of solute potential,
however, is achieved primarily by the effect of a
hormone called antidiuretic hormone (ADH).
Diuresis is the production of large amounts of
dilute urine. Antidiuresis is therefor the
opposite. ADH is antidiuretic in its effects, so
has
the
effect
of
making
urine
more
concentrated. It is also know as vasopressin.
ADH is made in the hypothalamus and
passes the short distance to the posterior
pituitary
gland
by
a
process
called
neurosecretion. When the blood become more
concentrated (solute potential more negative),
as in a situation where too little water has been
drunk, excessive sweating has occurred or large
amounts of salt have been eaten, osmoreceptors
in the hypothalamus detect a fall in blood
solute potential. Osmoreceptors are special
receptors which are extremely sensitive to
changes in blood concentration. They set up
nerve impulses which pass to the posterior
pituitary gland where ADH is released.
In the presence of ADH, the increased number
of water channels allows water to from the
glomerular filtrate into the cortex and the medulla
by osmosis, reducing the volume of the urine and
making it more concentrated.
ADH also increases the permeability of the
collecting duct to urea, which diffuses out of the
urine to the tissue fluid of the medulla. Here it
increases the osmotic concentration, resulting in the
removal of an increased volume of water from the
thin descending limb.
The opposite occurs when there is a high
intake of water. The solute potential of the
blood begins to get less negative. ADH is
inhibited, the walls of the distal convoluted
tubule and collecting duct becomes impermeable
to water, less water is reabsorbed as the filtrate
passes through the medulla and a large volume
of water of dilute urine is excreted. Failure to
release sufficient ADH leads to a condition
known as diabetes insipidus in which large
quantities of dilute urine are produced
(diuresis). The fluid lost in the urine has to be
replaced by excessive drinking.