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
- Bowman’s / renal capsule - Formation of glomerular filtrate (ultrafiltration)
- Proximal convoluted tubule - Reabsorption of water and glucose (selective reabsorption)
- Loop of Henle - Maintenance of a gradient of sodium ions in the medulla
- Distal convoluted tubule and 5. 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, 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
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
Describe the role of antidiuretic
hormone (ADH) in osmoregulation
- Attaches to receptors on collecting duct (and
distal convoluted tubule) - Stimulating addition of channel proteins
(aquaporins) into cell-surface membranes - So increases permeability of cells of collecting
duct and DCT to water - So increases water reabsorption from collecting
duct / DCT (back into blood) by osmosis - So decreases volume and increases
concentration of urine produced
This applies to a decrease in water potential of the blood (eg. increased sweating, reduced water intake,
increased salt intake). To explain how the body responds to an increase in water potential, it is the opposite.
