6.4.3 Kidneys Flashcards
(18 cards)
Describe how ultrafiltration produces glomerular filtrate
- there is a higher hydrostatic pressure in the glomerular capillaries than in the Bowman’s capusle
- therefore small molecules are forced out of the capillaries
- they pass through the fenestrations between the capillary endothelial cells
- then they pass through the basement membrane
- then they pass through the gaps between the podocytes in the epithelium of the Bowman’s capsule
- The large molecules are too large to get through so they stay in the glomerular capillaries
What causes the high hydrostatic pressure in the glomerular capillaries?
The diameter of the afferent arteriole is wider than the efferent arteriole
Complete this table to show which substances are found in each place
0
Glucose, water and amino acids are ‘reabsorbed’ at the PCT. What does this mean? Where do they move from and where do they go to?
• Go from the filtrate back into the blood
Use a flow chart to describe how is glucose is reabsorbed:
• Glucose is reabsorbed by ACTIVE TRANSPORT
o The Na+/K+ pump actively pumps Na+ out of the epithelial cells of the PCT into the blood
o This reduces the concentration of Na+ in the epithelial cells
o Therefore Na+ and glucose are co-transported through a symport protein from the filtrate into the epithelial cell
o So the concentration of glucose in the epithelial cell increases
o Glucose moves by facilitated diffusion from the epithelial cell into the blood
How much of the glucose from the filtrate is moved back into the blood?
• All of it! (unless someone is diabetic)
Which other molecules are moved by the same mechanism?
• Amino
How is water transported out of the filtrate?
• It moves by facilitated diffusion through aquaporins, by osmosis
What is an aquaporin?
• A channel protein which allows water to pass through
State and explain 3 ways that the cells of the PCT are adapted for selective reabsorption to take place
- Microvilli – increase the surface area in contact with the filtrate
- Lots of mitochondria – to produce ATP for active transport
- Lots of co-transport proteins which can transport specific molecules from the filtrate into the epithelial cells
Explain how a gradient of sodium ions is maintained in the medulla by the loop of Henle
- The epithelial cells of the ascending limb actively transport Na+ ions out of the filtrate and into the interstitial fluid (against their concentration gradient)
- This reduces the water potential in the tissue fluid
- Water cannot move out of the ascending limb because it is impermeable to water
- Some of the Na+ ions diffuse into the descending limb
- Water moves out of the descending limb by osmosis
- So the concentration of Na+ ions increases lower down the descending limb
- The Na+ diffuse out of the bottom of the ascending limb into the tissue fluid, causing the water potential to be lowest deeper in the medulla
Explain how the Loop of Henle allows the reabsorption of water from the distal convoluted tubule and collecting ducts
- There is active transport of sodium ions out of the ascending limb of the loop of Henle (against their concentration gradient)
- Therefore, the water potential of the filtrate, in DCT and the collecting duct, is less negative than the tissue fluid next to it
- Therefore, water will move out of the filtrate from the DCT and CD into the interstitial fluid by osmosis
- Since the water potential decreases deeper in the medulla this means that water continue to be reabsorbed by osmosis along the whole length of the collecting duct
What is the impact of having a longer Loop of Henle?
• The longer the loop of Henle means
o an increase in sodium ion concentration in the medulla AND
o the sodium ion gradient is maintained for longer in the medulla
• Therefore, the water potential gradient is maintained for longer
• So more water reabsorbed from loop of Henle and collecting duct by osmosis
What is ADH? What sort of molecule is it?
- Antidiuretic hormone
* It is a protein
What is an osmoreceptor and where are they found?
- An osmoreceptor is a cell which is sensitive to changes in water potential
- They are found in the hypothalamus
What happens to osmoreceptors when the water potential of the blood is too low?
- The water potential in the blood is lower than the cells
- So water moves from the osmoreceptor to the blood via osmosis
- This causes the osmoreceptor to shrivel/shrink
Write a flow chart to describe how the body responds to a low blood water potential?
- the low water potential is detected by the osmoreceptors in the hypothalamus
- Increased frequency of impulses to the posterior pituitary gland
- So the posterior pituitary gland releases more ADH
- ADH travels in the blood
- ADH binds to COMPLEMENTARY receptors on target cells in the Collecting Duct and the Distal Convoluted Tubule
- This causes vesicles containing aquaporins to fuse with the cell surface membranes of the target cells
- This increases the permeability of the cells to water
- So more water moves out from the collecting duct and the DCT to the tissue fluid by osmosis (down a water potential gradient)
- The water is then reabsorbed into the capillaries
- Therefore the urine volume is reduced and it becomes more concentration
What does the presence of ADH do to the water potential of the blood?
- ADH prevents the water potential of the blood from falling any further
- NOTE: ADH cannot cause the water potential of the blood to become less negative than before. This is because ADH causes water that was lost from the capillaries to be reabsorbed into the capillaries.
