6.4.3 control of blood water potential Flashcards
what is the function of the nephron?
filters the blood to remove waste + selectively reabsorbs useful substances back into the blood
what does urine contain?
water, dissolved salts, urea, other dissolved substances
what is the first stage of filtering & reabsorption?
- ultrafiltration occurs due to high hydrostatic pressure.
- water & small molecules are forced out of the glomerulus capillaries into the renal capsule
what is the second stage of filtering & reabsorption?
selective reabsorption occurs in the PCT
what is the third and fourth stage of filtering & reabsorption?
Loop of Henle maintains a sodium ion gradient so water can be reabsorbed by the blood
what is the fifth and six stage of filtering & reabsorption?
- water moves out of the DCT + collecting duct to return black to the blood.
- collecting duct carries remaining liquid to the ureter
in ultrafiltration, how does blood enter the nephron?
through the afferent arteriole
what does the afferent arteriole split int, and what does this cause?
smaller capillaries - causes a high hydrostatic pressure of the blood
what is forced into the glomerulus during ultrafiltration?
water, glucose, amino acids, minerals and other small molecules
what remains in the blood during ultrafiltration, and how does this leave the nephron?
- large proteins and blood cells are too big to pass through gaps in the capillary endothelium, so remain in blood
- leaves via the efferent arteriole
what is selective reabsorption and where does it occur?
when most of the glomerular filtrate is reabsorbed back into the blood - occurs in the PCT
Adaptations of the PCT for selective reabsorption
- microvilli - large surface area for reabsorption
- lots of mitochondria - provide energy for active transport
first stage of selective reabsorption:
concentration of sodium ions in PCT cell decreases, as Na+ ions are actively transported.
second stage of selective reabsorption:
- due to the conc gradient, Na+ ions diffuse down the gradient from the lumen of the PCT into the cells lining the PCT (cotransport)
third stage of selective reabsorption:
- glucose can then diffuse from the PCT epithelial cell into the bloodstream.
is all the glucose reabsorbed in selective reabsorption?
YES
why is a Na+ gradient maintained in the Loop of Henle?
to enable the reabsorption of water
ascending limb of the loop of henle:
thicker walls
- walls are impermeable to water
descending limb of the loop of henle:
- thinner walls
- walls are permeable to water
what is the first stage of maintaining a Na+ gradient in the Loop of Henle?
- mitochondria in the walls of the cell provides energy to actively transport Na+ ions out of the ascending limb
what is the second stage of maintaining a Na+ gradient in the Loop of Henle?
- the accumulation of Na+ ions outside the nephron in the medulla lowers the water potential
what is the third stage of maintaining a Na+ gradient in the Loop of Henle?
- water diffuses out by osmosis into the interstitial space
- and then into the blood capillaries.
what is the fourth stage of maintaining a Na+ gradient in the Loop of Henle?
- at the base of the ascending limb, some Na+ ions are transported about by diffusion
- creating a dilute solution
why can water be reabsorbed at the DCT and the collecting duct?
- all Na+ ions have been actively transported out of the PCT
- when the filtrate reaches the PCT it is very dilute
- filtrate moves into the DCT and collecting duct
- therefore more water diffuses out of the DCT and collecting duct
Describe how ultrafiltration occurs in a glomerulus. (3)
- high blood/hydrostatic pressure
- small substances pass through - water and glucose
- through small gaps in the capillary endothelium
Describe how urea is removed from the blood. (2)
- hydrostatic pressure
- causes ultrafiltration
- through basement membrane
Explain how urea is concentrated in the filtrate. (3)
- water reabsorbed by osmosis
- at the DCT/CD
- active transport of ions creates a gradient
The urine of a non-diabetic person does not contain glucose. Explain why. (2)
- Leaves the blood at kidney
- reabsorbed into blood from kidney tubule
If the glomerular filtrate of a diabetic person contains a high concentration of glucose, he produces a larger volume of urine. Explain why. (3)
Glucose in filtrate lowers water potential
- lower wp gradient
- less water reabsorbed by osmosis
what is negative feedback?
mechanisms to restore any deviations from normal in a system back to its original state
what can cause blood to have too LOW water potential?
- too much sweating
- not drinking enough water
- too salty food
what is the corrective mechanism for blood with a LOW water potential?
- more water is reabsorbed by osmosis
- urine is more concentrated
what can cause blood to have too HIGH water potential?
- drinking too much water
- not eating enough salt
what is the corrective mechanism for blood with a HIGH water potential?
- less water is reabsorbed by osmosis
- urine is more dilute
what is the role of the hypothalamus + pituitary gland in maintaining blood water potential?
- changes in wp in the blood are detected by osmoreceptors found in the hypothalamus
what happens to osmoreceptors if the wp of blood is too LOW?
- water leaves the osmoreceptors by osmosis and they shrivel
- stimulates the hypothalamus to produce more ADH
what happens to osmoreceptors if the wp of blood is too HIGH?
- water enters the osmoreceptors by osmosis
- stimulates hypothalamus to produce less ADH
what does ADH do?
- causes an increase in the permeability of the walls of the collecting duct and the DCT
- more water leaves the nephron and is reabsorbed into the blood
what are aquaporins?
channel proteins for water to pass through
what does ADH do to aquaporins?
- binds to them
- activating a phosphorylase enzyme in cells
what does phosphorylase do?
- causes the vesicles containing the aquaporins to fuse with the cell membrane
- meaning the aquaporins embed
with more aquaporins in the cell membrane…
more water leaves the DCT and collecting duct, and is reabsorbed into the blood.
in osmoregulation, if the wp is too high, what happens first?
detected by osmoreceptors in hypothalamus
n osmoregulation, if the wp is too high, what happens once osmoreceptors detect increase?
hypothalamus releases less ADH
in osmoregulation, if the wp is too high, what happens once the hypothalamus releases less ADH?
DCT and collecting duct walls become less permeable to water
in osmoregulation, if the wp is too high, what is the urine like?
higher volume and more dilute
in osmoregulation, if the wp is too low, what happens first?
osmoreceptors in hypothalamus detect decrease
in osmoregulation, if the wp is too low, what happens once the osmoreceptors have detected the decrease?
- hypothalamus releases more ADH
- which is released into the blood by the pituitary gland
in osmoregulation, if the wp is too low, what happens when more ADH has been released?
DCT and collecting duct walls become less permeable to water
in osmoregulation, if the wp is too low, what happens when wall become less permeable?
- more H20 is reabsorbed
- urine is more concentrated and is a lower volume
Give the location of osmoreceptors in the body of a mammal. (1)
hypothalamus
When a person is dehydrated, the cell volume of an osmoreceptor decreases. Explain why. (2)
water potential of blood will decrease
- water moves from osmoreceptor into blood by osmosis
Stimulation of osmoreceptors can lead to secretion of the hormone ADH. Describe and explain how the secretion of ADH affects urine produced by the kidneys. (4)
- Permeability of membrane to water is increased
- more water is absorbed from the collecting duct
- smaller volume of urine
- urine becomes more concentrated
Explain how urea is concentrated in the filtrate. (3)
- water reabsorbed by osmosis
- at the DCT/CD
- active transport of ions creates a gradient
what is osmoregulation
control of the water potential of the blood
