Download The Kidney - csfcA2Biology

Homeostasis
The maintenance of a
constant environment within
a living organism
Homeostatic control systems
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Blood glucose levels
Thermoregulation
Osmoregulation by the kidney
What would happen if the body did
not regulate the above?
The homeostatic process
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A system needs to operate at a certain level called
the set point
Involves a detector that detects a stimulus (a
change in a level)
The detector informs a coordinator which triggers
an appropriate method of correcting the deviation
The coordinator communicates with an effector
which carries out the corrective procedure
Once the correction is made and the factor returns to
normal, information is feed back to the detector
which ‘switches off’
This is called negative feedback
Negative feedback response
The set point
Negative feedback loop restores the set point. Once
this has been adjusted then the corrective mechanism
is switched off.
The control of blood glucose
Osmoregulation by the kidney
The Kidney
The structure and function
of the kidney
The kidney is an organ of
excretion
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Excretion is the removal of the waste
products of metabolism (not digestion)
Carbon dioxide, urea and bile
The kidneys have two roles:
• They deal with the removal of urea
•
(nitrogenous waste)
Osmoregulation (maintaining a balance of
water and dissolved salts in the body)
The production of urea
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Urea is made by the liver by the breakdown of
unwanted and excess amino acids
The amino acids are deaminated (the NH2
group removed) and ammonia is made
Ammonia is very toxic therefore the body
converts the ammonia into urea which is
carried around the body in the blood and
removed by the kidneys
Deamination of amino acids
The remaining keto group can be used in respiration
either converted to acetyl CoA, pyruvic acid or a krebs
intermediate
Conversion of ammonia to urea
•The liver removes the amino group (deamination) and produces
ammonia which is toxic and needs to excreted as urea
• Carbon dioxide reacts with ammonia as part of the ornithine cycle to
produce urea
ammonia +carbon dioxide=urea and water
2NH3+CO2=CO(NH2)2+H20
Kidneys are approx. 11cm by 6cm by 2.5cm and surrounded by fat for
protection. They have filtered all the blood in the body every 22 minutes.
Fibrous capsule
Position of the nephron
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The nephron
is the
functional
unit of the
kidney
There are
about a
million in
each kidney.
The nephron undertakes several
processes
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The removal of waste includes:
• Ultra filtration in the Bowman’s capsule
• Selective re absorption in the proximal
convoluted tubule
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Osmoregulation includes:
• Concentration of salts by the loop of Henle in
•
the medulla of the kidney
Hormonal control (ADH) of the permeability of
the collecting duct cells
The process of ultra filtration
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Each nephron has a rich blood supply
Each Bowman’s capsule is supplied with blood by an afferent
arteriole
This branches inside the Bowman’s capsule to form the
glomerulus
Taking blood away from the capsule is the efferent arteriole.
The afferent arteriole is much wider than the efferent arteriole
So there is more blood carried to the glomerulus than is carried
away, and pressure in the glomerulus is high.
Small molecules such as water, glucose, urea and salts are
filtered out of the blood under high pressure and move into the
Bowman’s capsule
This is called glomerular filtrate
Ultra filtration in the Bowman’s
capsule
Filtration takes place in
the pores of the capillary
endothelium. Only
molecules of a certain size
can pass through.
The podocyte cells are
special cells lining the
Bowman’s capsule. They
leave a gap for the filtrate
to pass through easily.
Ultra filtration
Molecule or ion Approx. concentrations / g dm-3
water
protein
glucose
amino acids
urea
inorganic ions
Plasma
900.0
80.0
1.0
0.5
0.3
7.2
Filtrate
900.0
0.0
1.0
0.5
0.3
7.2
Selective re absorption in the
proximal convoluted tubule
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Water and solutes are removed from the plasma as it passes
through the glomerulus and forms glomerular filtrate
These solutes include some useful substances such as amino
acids, glucose and water, these must be reabsorbed back into
the body.
This process is called selective re absorption.
All the glucose, amino acids, vitamins and many Na+ and Clions reabsorbed.
85% of the water is reabsorbed
Re absorption of glucose and salts is by active transport and
diffusion, water is reabsorbed by osmosis
Adaptations of the proximal tubule
cells
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The cells possess
microvilli to provide
a large surface area
for absorption
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Numerous
mitochondria are
present to provide
ATP for active
transport.
Research the mechanism of active transport in the PCT
Osmoregulation by the kidney
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The body needs to control the water potential of the blood
So far the kidney has filtered the blood and selectively re
absorbed 85% of the water back into the blood
There has been no mechanism so far to finely tune the
water content of the blood
The osmoregulatory function of the kidney undertakes
this role
We will consider:
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•
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The action of the loop of Henle
The homeostatic mechanism
The action of the hormone ADH (antidiuretic hormone) on
the distal convoluted tubule and the collecting duct
The Loop of Henle
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Its function is to create an area of high
solute concentration deep in the
medulla. (low water potential)
The collecting ducts of each nephron
pass through this area and so a lot of
water can potentially be reabsorbed from
the ducts by osmosis
The permeability of the collecting ducts
is controlled by a hormone ADH (anti
diuretic hormone)
Diagram showing the solute concentration in the medulla of the kidney
(milliosmoles per kilogram (mOsm/Kg)-thousandths of a mole of
molecules in 1kg of solution
The homeostatic mechanism
The set point
Negative feedback loop restores the normal water potential of the
blood. Once this has been adjusted then the corrective mechanism is
switched off.
Osmoregulation in the collecting
ducts of the kidney
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This acts as a process of negative feedback
The receptors for detecting changes in the water
potential of the blood are in the hypothalamus of the
brain (osmoreceptors)
The hypothalamus sends a nerve impulse to the
pituitary gland
The posterior lobe of the pituitary acts as the co
ordinator (releases hormone anti diuretic hormone
ADH)
This hormone travels in the blood to the cells lining
the collecting duct of the kidney
The cells of the collecting ducts of the kidney act as
the effector (may or may not become permeable)
Control of water re absorption by
ADH
With ADH
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The permeability of
the walls of the
distal convoluted
tubule and collecting
duct is controlled by
ADH
ADH makes the
walls of the these
more permeable so
that water moves
out and is
reabsorbed into the
blood
Without
ADH
Cells of the collecting duct
Lumen of the
collecting duct
Blood
Negative feedback mechanism in action
less water
second convoluted tubule
more water
reabsorbed into blood
and collecting ducts
reabsorbed into blood
less ADH
produced
blood
water
potential
becomes
more
negative
more ADH
released
receptors in
hypothalamus
becomes
less negative
becomes more
negative
normal water potential
of blood
blood
water
potential
becomes
less
negative
Describe how the body copes with a drop in
the water potential of the blood.
Describe how the
body copes with a
rise in the water
potential of the
blood
A fall in water potential of the
blood can be caused by:
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Reduced water intake
Sweating
Keeping exchange surfaces moist
Loss in faeces
Environmental adaptations in
animals
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Aquatic animals produce ammonia rather than
urea since it is highly soluble in water. The
ammonia diffuses out across the gills
Birds and insects secrete uric acid. This takes
lots of energy to produce but very little water is
needed for excretion. This conserves water
and allows them to live in dry environments.
Mammals excrete urea which is less toxic than
ammonia. Mammals have adapted the loop of
Henle to absorb more water in arid climates.
The kangaroo rat also reduce water loss by living in burrows during the day
which are cool and humid reducing water loss of evaporation.
Comparison of lengths of the Loop of Henle
Desert animals
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The kangaroo rate lives in the desert
where water in scarce
They live on water produced from
metabolic processes
They have an extra long loop of Henle to
increase the solute concentration in the
medulla and re absorb more water
producing more concentrated urine