1. F214: Communication, Homeostasis and Energy 4. 2
F214: Communication, Homeostasis and Energy 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;

2. 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

3. Selective Reabsorption
All sugars, most salts and some water is reabsorbed
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 of the fluid is decreased by addition of salts and removal of water
Water potential increased as salts are removed by active transport

4. Ultrafiltration

5. 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 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

7. What is filtered out of the blood?
Blood plasma which includes
Water
Amino acids
Glucose
Urea
Inorganic ions (sodium, chloride, potassium)

8. 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

9. 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

10. 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

11. 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

12. 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

13. 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

14. 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

15. 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 :
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

16. 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

17. 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

18. 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

19. 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

20. 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

21. 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

22. 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)