F214: Communication, Homeostasis and Energy 4.2.1 Ultrafiltration and Selective Reabsorption describe and explain the production of urine, with reference.

F214: Communication, Homeostasis and Energy4.2.1 Ultrafiltration and Selective Reabsorption

• describe and explain the production of urine, 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 the posterior pituitary gland;

The NephronAs 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 increased as salts are removed by active transport

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

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:

• 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

• 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 :

• 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 PotentialAs the fluid in the tubule ascends back up towards the cortex, the water potential becomes higher (less negative)

This is due to :

• 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 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 from the distal tubules and collecting ducts

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 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)