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YEAR13 Wednesday 17th October 2012 Section 3
October 17, 2012, 11:54 am
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Osmoregulation

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(i) 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;
1. Dehydration
The osmoreceptors in the hypothalamus detects changes in the amount of water present in the blood. When the water potential of the blood is low the osmoreceptors lose water by osmosis. This causes them to stimulate neurosecretory cells (these release ADH.) If there is too little water (the blood is too concentrated) it tells the posterior pituitary gland to secrete ADH. This hormone has an effect on the kidney; ADH makes the kidney re-absorb water from the ultra-filtrate. The anti diuretic hormone binds to complementary receptors in the wall of the collecting duct which causes a series of enzyme controlled reactions. This results in vesicles causing aquaporins to embed themselves in the cell surface membrane. Higher levels of ADH make the kidney work harder to reabsorb more and more water. This results in the production of very small quantities of very concentrated urine. The result of reabsorbing water is to reduce the concentration of the blood. By negative feedback the pituitary makes less ADH.
.
2. Water logging
The hypothalamus detects that there is too much water in the blood. If the blood is too dilute, our cells will absorb water by osmosis and become “waterlogged”. Animal cells are in danger of swelling and bursting if they are placed in a solution which is too dilute. It is very important that the blood does not become so dilute that our cells are stressed by water logging.
When the blood becomes too dilute, our pituitary glands stop making ADH. If there is less ADH in the blood the cell surface membrane folds inwards to remove water permeable channels. The kidney stops reabsorbing water. Large volumes of very dilute urine are formed. This is just the opposite of what happens when your blood is too concentrated. When the concentration of the blood starts to rise, the pituitary gland starts to make ADH again. This is negative feedback again. Eventually the concentration of the blood will return to normal.

http://purchon.com/wordpress/biology/?page_id=170  SIMPLE VERSION

Information taken from A2 OCR Biology text book

Comment by laurabriggs1

Background: Osmoregulation is the control of water levels and salt levels in the body. Water and salt levels are needed to kept as close to a constant as possible in order to prevent any problems with osmosis occurring.

The wall lining of the collecting duct alters its permeability to suit the conditions of the need to use or conserve water.
ON A COLD DAY – LESS water is needed to be conserve, wall lining = LESS permeable. Less water reabsorbed = more urine produced.
ON A HOT DAY – MORE water is needed to be conserved, wall lining = MORE permeable. More water reabsorbed = less urine produced.

Permeability is altered by the walls of ducts responding to different concentrations of ADH (antidiuretic hormone) in the blood. The walls of the ducts have complementary membrane-bound receptors for ADH. ADH binds to the receptors and this initiates the chain of enzyme controlled reactions which results in the insertion of vesicles containing water-permeable channels into the membrane of the cell walls. This increases the permeability of the membrane to water.

If there is MORE ADH in the blood, there are MORE water-permeable channels. This allows more water to be reabsorbed, by osmosis, into the blood = less urine which has a lower water potential is excreted.
If there is LESS ADH in the blood then the cell surface membrane folds inwards to create new vesicles that remove water-permeable channels from the membrane. This REDUCES the permeability of the cell membrane = LESS water is reabsorbed into the blood. MORE urine of a higher water potential is excreted.

Water potential of the blood is monitored by osmoreceptors in the hypothalamus of the brain.
If the water potential of the blood is low, water leaves the receptors by osmosis causing them to shrink.
This stimulates neurosecretory cells in the hypothalamus, which send action potentials down their axons and cause the release of ADH.
These cells are specialised neurones that produce and release ADH. ADH flows down the axon to the terminal bulb in the posterior pituitary gland, where it is stored until needed.
ADH enters the capillaries that are in the PPG. It is then transported around the body and acts on the cells of the collecting ducts.
Once the water potential of the blood increases again, less ADH is released.
ADH is slowly broken down – it has a half life of about 20 mins. The ADH present in the blood is broken down and the collecting ducts receive less stimulation.

Comment by erinmeredithh

Information taken from OCR A2 Biology (Heinemann) book

Comment by erinmeredithh

Osmoregulation – the regulation of water concentration in the blood, this controls how much water is available for cells to absorb

http://www.google.co.uk/imgres?um=1&hl=en&client=firefox-a&sa=N&rls=org.mozilla:en-US:official&biw=1024&bih=600&tbm=isch&tbnid=NDrjT5qLr7PjTM:&imgrefurl=http://www.biology-online.org/4/2_water_homeostasis.htm&docid=2Eb6Vjlk0QfLcM&imgurl=http://www.biology-online.org/images/water_regulation.gif&w=465&h=198&ei=8pB-UJf1LeSg0QWBhoCADg&zoom=1&iact=rc&dur=446&sig=116754602623128865144&page=2&tbnh=113&tbnw=266&start=12&ndsp=16&ved=1t:429,r:12,s:0,i:145&tx=116&ty=18

• Water levels in the blood decrease
• This is detected by osmoreceptors
o Cells sensitive to osmotic pressure
 Osmotic pressure is affected by concentration of solutes in the body
o Expand or contract as osmotic pressure changes
 low levels of solutes
 receptors swell
• Water moves away from the blood into the cells
o Many osmoreceptors are found in the:
 Hypothalamus
 Liver
 Kidneys
• The hypothalamus sends messages to the pituitary gland
• Pituitary gland secretes ADH
o Anti diuretic hormone
 Peptide made up of 9 amino acids
 Secreted from the posterior pituitary
o ADH binds to receptors of cells in collecting duct (medulla)
 Promotes reabsorption of water into the circulation
 Stimulates insertion of aquaporins (water channels) in the tubules
 Transport pure water through tubular cells into the blood
o Reduces water loss in urine
• Water levels return back to a normal level

• Water levels in the blood increase
• This is detected by osmoreceptors
o Cells sensitive to osmotic pressure
 Osmotic pressure is affected by concentration of solutes in the body
o Expand or contract as osmotic pressure changes
 High levels of solutes
 receptors contract
• Water moves out of osmoreceptors into the blood via osmosis
o Many osmoreceptors are found in the:
 Hypothalamus
 Liver
 Kidneys
• The hypothalamus sends messages to the pituitary gland
• Pituitary gland secretes less ADH
o Anti diuretic hormone
 Peptide made up of 9 amino acids
 Secreted from the posterior pituitary
o ADH does not bind to receptors of cells in collecting duct (medulla)
 Less water is reabsorbed into the bloodstream
 The tubules remain impermeable to water, less is lost into the interstitial fluid
 Water is retained in the PCT to travel into the collecting duct
o More water remains in urine
• Water levels return back to a normal level

Comment by rosiepatterson

Brief overview of osmoregulation

Osmoregulation can be defined as the process through which the body can control the level of water and ions in it. It is facilitated by the kidney and the pituitary gland (housed in the hypothalamus) affects the rate at which water absorption occurs through the release of Antidiuretic hormone (AHD) which increases the rate at which the kidneys extract water from the kidneys. This usually occurs when a human is dehydrated. It simply works by increasing the permeability of the distal convoluted tubule resulting in the desired effect of water leaving the tubule at a higher rate than would otherwise have been achieved. Similarly it increases the permeability of the collecting duct which further increases reabsorption of water.

Here is a diagram of the negative feedback system employed by the pituitary gland and its resulting effect on the amount of ADH produced.

Comment by hasssankhan

(i) 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;

What you may be asking is osmoregulation? Osmoregulation controls the amount of salt and water in your body.
Water can be gained from food, drink and respiration. Water is lost in urine, sweating, faeces and vapour vapour in the air you breathe out.

Altering the permeability of the collecting duct:
ADH (antidiuretic hormone) effects the permeability of the walls of the collecting duct. The wall contains receptors for ADH which it binds to and causes and chain of enzyme controlled reactions. Ultimately the final result inserts vesicles containing water permeable channels, known as aquaporins, into the cell surface membrane. This means that the walls are more water permeable, therefore more ADH more water permeable channels. Thus more water is reabsorbed via osmosis and less urine passes out the body. Therefore the urine is more concentrated.
If there is less ADH cell membrane folds inwards to create new vesicles that remove aquaporins from the water so water permeability of the cell walls decreases so more dilute urine passes.

Adjusting the concentration of ADH in the blood:
Water potential of blood is monitored by osmoreceptors in the hypothalamus of the brain.
When water potential of the blood is low the osmoreceptor cells lose water by osmosis which causes them to shrink and stimulate neurosecretory cells in the hypothalamus which secrete ADH. ADH flows down the axon to the terminal bulb in the posterior pituitary gland where it is stored.
They release ADH when stimulated and then enter the blood capillaries running through porterior pituitary gland and travels around the body.

Comment by alexandracoupe

Osmoregulation can be defined by the physiological processes than an organism uses to maintain water balance. This could be to compensate for water loss, excess water gain, and maintaining the osmolarity of body fluids.

Excess of water:
Urine is more dilute. This is because the excess water causes an increase in blood volume and lowers osmotic pressure of the blood. This change disturbs the exchange of materials.
The two processes involved are:
a) ultra-filtration
• This process is increased due to the increased hydrostatic pressure of blood caused by the excess of water.
• Hence, more nephric filtrate is filtered from the glomerular capillaries into the Bowman’s capsule.

b) Decreased reabsorption
• Water is mainly reabsorbed through the collecting tubules. The permeability of the wall of the DCT and collecting tubules is controlled by ADH, which is released from the pituitary gland.
• A lack of this hormone results in the lowering of the permeability of the DCT and CT.
• Therefore reabsorption of Na+ is decreased, so subsequently is less water is reabsorped.

Shortage of water:
• The rate of ultra-filtration is decreased due to decreased blood volume and lower hydrostatic pressure.
• An increased amount of ADH is released which increases the permeability of the walls of the DCT and CT
• So there is less ultra-filtration and more reabsorbtion

http://www.tutorvista.com/content/biology/biology-iv/excretion/role-kidney-osmoregulation.php#

Comment by hamzamahmud

Osmoregulation
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.

Osmoregulation is the control of the levels of water and mineral salts in the blood.

Water enters the body in three ways:
– Food
– Drink
– Metabolism

Water is lost from the body in three ways:
– Urine
– Swear
– Water vapour in exhaled air
– Faeces

If your body is dehydrated you will feel thirsty because your hypothalamus has detected that your blood is too concentrated: as well as stimulating the pituitary gland to make ADH, it will stimulate you to drink. However, you will not be able to drink exactly the right amount of water to get your blood back to the exact concentration.

How much water you get out of your food depends upon what it is that you eat and how much you eat. You might end up very fat if you ate something every time that you felt thirsty. So the water content of your food does not help you to control your water content.

Changes in the amount of water present in the blood are detected by the hypothalamus. If there is too little water, it tells the pituitary gland to secrete ADH. This hormone has an effect on the kidney; ADH makes the kidney re-absorb water from the ultra-filtrate. Higher levels of ADH make the kidney work harder to reabsorb more and more water. This results in the production of very small quantities of very concentrated urine. The result of reabsorbing water is to reduce the concentration of the blood. By negative feedback the pituitary gland secretes less ADH.

The walls of the collecting duct respond do the level of 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 within the cell. Aquaporins are then inserted into the cell surface membrane, making the walls more permeable to water.

Comment by ebelen

Osmoregulation
Control of ADH
The concentration of the blood (water potential) is monitored by osmoreceptors in the hypothalamus. An osmoreceptor is a sensory end organ that responds to changes in osmotic pressure. (Definition from http://wordnetweb.princeton.edu/perl/webwn?s=osmoreceptor) The higher the concentration of the blood the less water there is in the blood.If the concentration is too high impulses are sent to the pituitary gland which then releases more ADH. The water levels will be brought back to normal and the impulses stop.

Adjusting the concentration of ADH in the blood
Osmoreceptors in the hypothalamus of the brain monitor water potential in the blood.
When the water potential is low or very negative, osmoreceptors cells lose water by osmosis and they shrink, stimulating neurosecretory cells in the hypothalamus.
Neurosecretory cells= specialised neurones that produce and release ADH. When they are stimulated they send action potentials down their axons causing the release of ADH.
ADH enters blood capillaries in the posterior gland which is transported around the body acting on cells of the collecting ducts.
If the water potential of the blood rises then less ADH will be released.
ADH will be slowly broken down.

Keywords:
Osmoregulation is the control of water levels and salt levels in the blood. It is a homeostatic mechanism.

ADH (Antidiuretic hormone) is released from the pituitary gland and acts on the collecting ducts in the kidneys to increase their reabsorption of water. The amount of water must be kept more or less the same to avoid cell damage as a result of osmosis. There needs to be a balance between the amount of water gained through your diet and cellular respiration and the amount of water lost by the body in sweating, evaporation, faeces and urine.

Half-life of a substance is the time taken for its concentration to drop to half its original value.

Source= OCR A2 Biology text book

Comment by frankiebarrick

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 SELECTIVE REABSORPTION

Osmoregulation
• Control of water levels and salt levels in the body
• These levels must be controlled to prevent problems with osmosis
• The amount of water that is reabsorbed depends on the permeability of the walls of the collecting duct
• For example, on a hot day, you need to conserve more water, so the collecting duct is more permeable
• This means that more water can be reabsorbed into the blood
• A smaller volume of urine will be produced

Altering permeability of collecting duct
• Walls of collecting duct respond to the level of ADH, antidiuretic hormone, in the blood
• Cells in the wall have membrane-bound receptors which bind to ADH, causing a chain of enzyme controlled reactions (synoptic link: the endocrine system, hormone, target tissue) that results in inserting vesicles containing water-permeable channels (aquaporins) into the cell surface membrane (synoptic link: exocytosis)
• Less ADH in the blood ⇒cell surface membrane folds inwards to create new vesicles that remove water permeable channels ⇒walls less permeable ⇒less water reabsorbed ⇒more water passes out in the urine

Adjusting the concentration of ADH in the blood (synoptic link: this is a negative feedback mechanism)
• Water potential of the blood monitored by osmoreceptors in the hypothalamus of the brain
• When the water potential is very negative, the osmoreceptor cells lose water by osmosis
• This causes them to shrink and stimulate neurosecretory cells (specialised neurones, produce and release ADH (n.b- ADH stored in the posterior pituitary gland)) in the hypothalamus, making these cells release ADH by sending action potentials down their axon

Summary- linking all these ideas together
➢ Increase in water potential of blood ⇒detected by osmoreceptors in hypothalamus ⇒less ADH released from posterior pituitary ⇒collecting duct walls less permeable ⇒less water reabsorbed into blood (and more urine produced) ⇒decrease in water potential of blood
➢ Decrease in water potential of blood ⇒detected by osmoreceptors in hypothalamus ⇒more ADH released from posterior pituitary ⇒collecting duct walls more permeable ⇒more water reabsorbed into blood (and less urine produced

Comment by benjamincharlespatterson

The movement of water in and out of tissues and cells is primarily controlled by water potential differences, as water molecules move down a water potential gradient across partially permeable membranes in osmosis. We may use this principle as human beings to regulate the water potential of our blood in order to maintain a constant, optimum potential. We may control the rate at which osmosis occurs by either altering the permeability of membranes to water, or controlling the potential gradient.
Osmoregulation is performed by negative feedback, and the most central component of this regulatory system are the osmoreceptor cells that can be found in the hypothalamus; the brain centre responsible for many of the functions that maintain homeostasis. The osmoreceptor cells detect the water potential of the blood that flows into the brain, and if the water potential is too low (i.e. the blood solutes are too concentrated), then in an attempt to retain water then the hypothalamus will stimulate the posterior pituitary gland to release anti-diuretic hormone (ADH) into the blood. This is important as a low water potential in the blood may cause cells, e.g. lymphocytes, to lose water and become crenated due to hypertonic conditions.
The anti-diuretic hormone works by altering the permeability of the collecting ducts through which urine will flow. Collecting duct cells in the kidney have complementary receptors to this ADH, and when stimulated will move aquaporin protein channels to the cell surface membrane and hence increase the partial permeability of the cells to water. In addition, the hypothalamus will signal to other cells in the brain that the body may be experiencing dehydration, and this triggers our instinct of thirst.
As the urine will have a comparatively high water potential compared to the blood in the vessels around the collecting duct, water molecules will return to the blood by osmosis down the water potential gradient. The collecting duct cells act as the partially permeable membrane.
Where the water potential is too high, a change will be detected by the hypothalamus and will not simulate the release of ADH. In an attempt to lose excess water from the body, without ADH then the cells of the collecting duct will become impermeable and hence much less water will be reabsorbed. This is important as hypotonic conditions in the blood may result in increased osmosis into blood cells and the consequent haemolysis of cells.

Source used: Biology 2 for OCR – Mary Jones, CUP

Comment by jonathanfunnell




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