A2 Ultrafiltration and Selective Reabsorption

8/11/2019 A2 Ultrafiltration and Selective Reabsorption

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F214: Communication, Homeostasis and Energy

4.2.1 Ultrafiltration and Selective Reabsorption

describe and explain the production ofurine, with reference to the processes of

ultrafiltration and selective reabsorption;

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 theposterior pituitary gland;


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The Nephron

As fluid moves along thenephron, selective reabsorption

occurs.

Substances are reabsorbed back

into the tissue fluid and blood

capillaries surrounding thenephron tubule

The final product is urine

This passes into the pelvis and

down the ureter to the bladder


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Selective Reabsorption

All sugars, most

salts and some

water is

reabsorbed

water potential of

the fluid is

decreased by

addition of saltsand removal of

water

Water potential

increased as

salts are

removed by

active transport

Water potential

decreased again

by the removal of

water- ensuring

that urine has a

low waterpotential. Urine

has a higher

concentration of

solutes than blood

and tissue fluid


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Ultrafiltration


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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 Bowmans capsule, fluid from

the blood is pushed into the Bowmans capsule

The barrier between the blood in the capillaries, and the lumen of the Bowmans capsule

consists of:

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 gettingthrough (most proteins and all blood cells)

Podocytes- epithelial cells of the Bowmans 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 Bowmans capsule


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What is filtered out of the blood?

Blood plasma which includes

Water Amino acids

Glucose

Urea Inorganic ions (sodium, chloride, potassium)


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What is left in the capillary?

Blood cells Proteins

This makes the blood have a low (very negative) waterpotential 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


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Selective Reabsorption

Most reabsorption occurs

from the proximal

convoluted tubule where

85% of filtrate is

reabsorbed

All glucose and amino

acids, some salts andsome water are

reabsorbed


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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


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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


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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


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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 thedescending 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


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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 :

loss of water by osmosis

to the surrounding tissue

fluid

diffusion of the sodiumand chloride ions into the

tubule from surrounding

tissue fluid


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Water PotentialAs the fluid in the tubule ascends back

up towards the cortex, the waterpotential becomes higher (less

negative)

This is due to :

sodium and chloride ions

diffusing out of the tubule intothe 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


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Water PotentialThis 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 fromthe distal tubules and collecting ducts


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The Collecting Duct 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 collectingduct

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 permeabilityof 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


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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


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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 alower (more negative) water potential


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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


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Adjusting the Concentration of ADH

Osmoreceptors in the Hypothalalmus

monitor the bloodswater 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 isstored until needed


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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 isreleased

ADH is slowly broken down (it has a half life of 20 minutes)

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A2 Ultrafiltration and Selective Reabsorption