3.6.4.3 - Control of blood water potential Flashcards
Nephron
- Basic structural and functional unit of the kidney (millions in the kidney)
> Associated with each nephron are a network of blood vessels
Different parts of the nephron
- Bowman’s / renal capsule
- Proximal convoluted
tubule - Loop of Henle
- Distal convoluted tubule
- Collecting duct
Summarise the role of the bowman’s capsule
Formation of glomerular filtrate (ultrafiltration)
Summarise the role of the Proximal convoluted tubule
Reabsorption of water and glucose (selective reabsorption)
Summarise the role of the loop of Henle
Maintenance of a gradient of sodium ions in the medulla
Summarise the role of the distal convoluted tubule
Reabsorption of water (permeability controlled by ADH)
Summarise the role of the collecting duct
Reabsorption of water (permeability controlled by ADH)
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, these are filtered by (and pass through) the capillary basement membrane as well as:
> fenestrations between capillary endothelial cells
> Podocytes - Large proteins / blood cells remain in blood
Describe the reabsorption of glucose
by the proximal convoluted tubule
- Na+ ions are actively transported out of the 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 the water potential of the capillaries
● So Water moves by osmosis down a water potential gradient into the capillary
Describe and explain how features of the cells in the Proximal Convoluted Tubule 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 Proximal Convoluted Tubule
● 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 that the water potential decreases down the medulla (compared to filtrate in collecting duct)
● Therefore a water potential gradient is maintained between the collecting duct and medulla
● Which maximises the reabsorption of water by osmosis from the filtrate
Describe the role of the loop of Henle in maintaining a gradient of sodium ions in the medulla
- In the ascending limb:
○ Na+ ions are 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 is reabsorbed by capillaries (so filtrate concentration increases)
○ Na+ ions
‘recycled’ → diffuses back in
Ascending limb (up)
- impermeable to water
- Na+ moves out (mostly by active transport)
Descending limb (down)
- water leaves by osmosis
- Na+ moves in (recycled)
Suggest why animals needing to conserve water have long loops of Henle (thick medulla)
● More Na+ ions move out (mostly by active transport)→ 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 does the loop of henle act as?
Counter current multiplier
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
When is more ADH produced/secreted?
When water potential in the blood is low
When is less ADH produced/secreted?
When water potential in the blood is high
