6.4.3 Control of blood water potential Flashcards
Describe the structure of a nephron
● Nephron = basic structural and functional unit of the kidney (millions in the kidney)
● Associated with each nephron are a network of blood vessels
Summarise the role of different parts of the nephron
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Describe the formation of glomerular filtrate
- High hydrostatic pressure in glomerulus
○ As diameter of afferent arteriole (in) is wider than efferent arteriole (out) - Small substances eg. water, glucose, ions, urea forced into glomerular filtrate, filtered by:
a. Pores / fenestrations between capillary endothelial cells
b. Capillary basement membrane
c. Podocytes - Large proteins / blood cells remain in blood
Describe the reabsorption of glucose
by the proximal convoluted tubule
- Na
+ actively transported out of epithelial cells to
capillary - Na
+ moves by facilitated diffusion into epithelial
cells down a concentration gradient, bringing
glucose against its concentration gradient - Glucose moves into capillary by facilitated
diffusion down its concentration gradient
Describe the reabsorption of water
by the proximal convoluted tubule
● Glucose etc. in capillaries lower water potential
● Water moves by osmosis down a water potential
gradient
Describe and explain how features of the cells in the PCT allow the rapid
reabsorption of glucose into the blood
● Microvilli / folded cell-surface membrane → provides a large surface area
● Many channel / carrier proteins → for facilitated diffusion / co-transport
● Many carrier proteins → for active transport
● Many mitochondria → produce ATP for active transport
● Many ribosomes → produce carrier / channel proteins
Suggest why glucose is found in the urine of an untreated diabetic person
● Blood glucose concentration is too high so not all glucose is reabsorbed at the PCT
● As glucose carrier / cotransporter proteins are saturated / working at maximum rate
Explain the importance of maintaining a gradient of sodium ions in the
medulla (concentration increases further down)
● So water potential decreases down the medulla (compared to filtrate in collecting duct)
● So a water potential gradient is maintained between the collecting duct and medulla
● To maximise reabsorption of water by osmosis from filtrate
Describe the role of the loop of Henle in maintaining a gradient of sodium
ions in the medulla
- In the ascending limb:
○ Na
+ actively transported out (so filtrate concentration decreases)
○ Water remains as ascending limb is impermeable to water
○ This increases concentration of Na
+
in the medulla, lowering water potential - In the descending limb:
○ Water moves out by osmosis then reabsorbed by capillaries (so filtrate concentration increases)
○ Na
+
‘recycled’ → diffuses back in
The loop of Henle acts as ?
as a countercurrent multiplier (you
Ascending limb :
- impermeable to water
- na+ moves out ( by AT)
Depending limb :
- water leaves by osmosis
- Na+ moves in (recycled)
Suggest why animals needing to conserve water have long loops of Henle
(thick medulla)
● More Na
+ moved out → Na
+ gradient is maintained for longer in medulla / higher Na
+ concentration
● So water potential gradient is maintained for longer
● So more water can be reabsorbed from collecting duct by osmosis
Describe the reabsorption of water by the distal convoluted tubule and
collecting ducts
● Water moves out of distal convoluted tubule & collecting
duct by osmosis down a water potential gradient
● Controlled by ADH which increases their permeability
What is osmoregulation?
Control of water potential of the blood (by negative feedback)
Describe the role of the hypothalamus in osmoregulation
- Contains osmoreceptors which detect increase OR decrease in blood water potential
- Produces more ADH when water potential is low OR less ADH when water potential is high
Describe the role of the posterior pituitary gland in osmoregulation
Secretes (more / less) ADH into blood due to signals from the hypothalamus
