Transcript F214: Communication, Homeostasis and Energy 4.2.1 The Kidney

F214: Communication, Homeostasis and Energy
4.2.1 Ultrafiltration and Selective Reabsorption
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describe and explain the production of
urine, with reference to the processes of
ultrafiltration and selective reabsorption;
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explain, using water potential terminology,
the control of the water content of the blood,
with reference to the roles of the kidney,
osmoreceptors in the hypothalamus and the
posterior pituitary gland;
The Nephron
As fluid moves along the
nephron, selective reabsorption
occurs.
Substances are reabsorbed back
into the tissue fluid and blood
capillaries surrounding the
nephron tubule
The final product is urine
This passes into the pelvis and
down the ureter to the bladder
Selective Reabsorption
All sugars, most
salts and some
water is
reabsorbed
water potential of
the fluid is
decreased by
addition of salts
and removal of
water
Water potential
decreased again
by the removal of
water- ensuring
that urine has a
low water
potential. Urine
has a higher
concentration of
solutes than blood
and tissue fluid
Water potential
increased as
salts are
removed by
active transport
Ultrafiltration
Blood flows from the afferent arteriole, into the glomerulus, and leaves through the efferent
arteriole, which is narrower, meaning that blood in the glomerulus is at high pressure
As the blood in the glomerulus is at higher pressure than in the Bowman’s capsule, fluid from
the blood is pushed into the Bowman’s capsule
The barrier between the blood in the capillaries, and the lumen of the Bowman’s capsule
consists of:
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Endothelium- having narrow gaps between its cells that plasma can
pass through
Basement Membrane- made of a fine mesh of collagen fibres and glycoproteins
which act as a filter to stop molecules with a relative molecular mass of 69000 getting
through (most proteins and all blood cells)
Podocytes- epithelial cells of the Bowman’s capsule containing finger like projections
called major processes. These ensure that there are gaps between the cells allowing
fluid to pass into the lumen of the Bowman’s capsule
What is filtered out of the blood?
• Blood plasma which includes
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Water
Amino acids
Glucose
Urea
Inorganic ions (sodium, chloride, potassium)
What is left in the capillary?
• Blood cells
• Proteins
This makes the blood have a low (very negative) water
potential which ensures some fluid is retained in the
blood
The very low water potential of the blood in the
capillaries helps to reabsorb water at a later stage
Selective Reabsorption
• Most reabsorption occurs
from the proximal
convoluted tubule where
85% of filtrate is
reabsorbed
• All glucose and amino
acids, some salts and
some water are
reabsorbed
Specialised for Selective Reabsorption
• Microvilli on the cell surface membrane of
the tubule provides a large surface area
• Co-transporter proteins in the membrane
transport glucose and amino acids in
association with sodium ions by facilitated
diffusion
• The opposite membrane (close to blood
capillaries) is folded to increase surface
area and contains sodium-potassium
pumps that pump sodium out and
potassium in
• Cell cytoplasm has many mitochondria
indicating that energy is required as ATP
How does Selective Reabsorption Occur?
• Sodium ion concentration is reduced as Sodium-potassium pumps remove
sodium ions from the cells lining the proximal convoluted tubule
• Sodium ions transported into the cell with glucose or amino acids by
facilitated diffusion
• As concentration rises, they are able to diffuse out of the opposite side of
the cell into the tissue fluid- active transport may also support this process
• from the tissue fluid, they diffuse into the blood and are carried away
• Reabsorption of salts, glucose and amino acids reduces the water
potential in the cells (makes it more negative) and increases the water
potential in the tubule fluid (towards zero)- this means water will enter
the cells from the tubule fluid and then be reabsorbed into the blood by
osmosis
Water Reabsorption
• After selective reabsorption in the proximal
convoluted tubule, the loop of Henle creates a
low (very negative) water potential in the
medulla to ensure more water is reabsorbed
from the collecting duct
Loop of Henle
• Consists of a descending limb into the medulla and
an ascending limb back out to the cortex
• Allows salts (sodium and chloride ions) to be
transferred from the ascending limb to the
descending limb
• The overall effect is to increase the concentration of
salts in the tubule fluid so they diffuse out from the
ascending limb into the surrounding medulla tissue
giving a low (very negative) water potential
Water Potential
As the fluid in the tubule descends
into the medulla down the
descending loop, the water
potential becomes lower (more
negative)
This is due to :
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loss of water by osmosis
to the surrounding tissue
fluid
diffusion of the sodium
and chloride ions into the
tubule from surrounding
tissue fluid
Water Potential
As the fluid in the tubule ascends back
up towards the cortex, the water
potential becomes higher (less
negative)
This is due to :
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sodium and chloride ions
diffusing out of the tubule into
the tissue fluid at the base
higher up the tubule, sodium
and chloride ions are actively
transported out into the tissue
fluid
wall of the ascending limb is
impermeable to water so it
cannot leave the tubule
the fluid loses salts but not
water as it moves up the
ascending limb
Water Potential
This arrangement is known as the hairpin
countercurrent multiplier system.
It increases the efficiency of salt transfer
from the ascending limb to the descending
limb
This causes a build up of salt in the
surrounding tissue fluid
‘Student Speak’ Water moves out of the
descending limb, making the fluid in the
tubule very salty. Salt then diffuses out of
the base of the ascending limb as it is at
high concentrations (very negative water
potential), and then is transported out
using active transport at the top of the
ascending limb
The removal of ions from the ascending
limb makes the urine very dilute and water
can then be reabsorbed by the body from
the distal tubules and collecting ducts
The Collecting Duct
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From the top of the ascending limb, the tubule fluid passes through the distal
convoluted tubule where active transport adjusts the concentration of various salts
The fluid has a high water potential (contains a lot of water) and flows into the collecting
duct
The collecting duct carries fluid into the medulla which contains a lot of salts (low/very
negative water potential)
As the fluid passes through, water moves by osmosis, from the tubule fluid into the
surrounding tissue
It then enters the blood capillaries by osmosis and is carried away
The amount of water absorbed depends
on the permeability of the walls of the
collecting duct
By the time the urine reaches the pelvis it
has a low (very negative) water potential
and the concentration of urea and salts is
high
Osmoregulation
• Osmoregulation is the control of water and salt
levels in the body
• Water is gained from food, drink and
metabolism
• Water is lost in urine, sweat, water vapour in
exhaled air and faeces
The Collecting Duct and ADH
• The walls of the collecting duct can be made more or less permeable
according to the needs of the body
• The walls of the collecting duct respond to levels of antidiuretic hormone
(ADH) in the blood
• Cells in the wall have membrane bound receptors for ADH
• The ADH binds to these receptors and causes a chain of enzyme controlled
reactions inside the cell
• The end result is to insert vesicles containing water permeable channels
(aquaporins) into the cell surface membrane
• This makes the walls more permeable to water
• More ADH in the blood means more aquaporins are inserted allowing
more water to be reabsorbed, and less, more concentrated urine with a
lower (more negative) water potential
Less ADH
• Less ADH in the blood means that less water is
reabsorbed
• The cell surface membrane folds inwards to
create new vesicles that remove the
aquaporins from the membrane
• The wall is less permeable and more water
passes out in urine with a higher (less
negative) water potential
Adjusting the Concentration of ADH
• Osmoreceptors in the Hypothalalmus
monitor the blood’s water potential
• When the water potential is low,
these cells lose water by osmosis and
shrink, stimulating Neurosecretory
cells (specialised nerve cells)
• Neurosecretory cells produce ADH in
their cell body, which flows down the
axon to the terminal bulb in the
posterior pituitary gland where it is
stored until needed
Adjusting the Concentration of ADH
• When the Neurosecretory cells are stimulated, action
potentials are sent down the axons, causing a release of ADH
• ADH enters blood capillaries running through the posterior
pituitary gland and is transported round the body and acts on
the wall of the collecting duct
• When water potential rises (less negative) less ADH is
released
• ADH is slowly broken down (it has a half life of 20 minutes)