control of blood water potential

proximal convoluted tubule - reabsorption of glucose and water

1. Na+ actively transported out of cells lining proximal convoluted tubule into blood capillaries
2. this decreases the Na+ concentration in these cells
3. Na+ from lumen of proximal convoluted tubule enter down their concentration gradient through different co-transporter proteins, each bringing other molecules through, e.g. glucose, amino acids, chloride ions
4. these molecules then diffuse into the blood via facilitated diffusion, microvilli increase SA

descending limb

• narrow, thin walls

• very permeable to water

ascending limb

- wide, thick walls
- not permeable to water
- more permeable to salts, e.g. Na+

cortex

bowmans capsule

medulla

• loops of Henle

• collecting ducts

renal pelvis

cavity which collects urine into ureter

stages of osmoregulation

• formation of glomerular filtrate at the Bowmans Capsule

• reabsorption of glucose and water by the PCT

• maintenance of gradient of Na+ by the loop of Henle (water reabsorption)

• reabsorption of remaining water by the DCT and collecting ducts (role of ADH)

describe and explain what happens in ultrafiltraton

- afferent arteriole narrows into efferent arteriole which increases hydrostatic pressure
- causing ultrafiltration of small molecules: e.g. H2O, glucose and mineral ions are forced out through basement membrane into filtrate
- filtrate passes out - through spaces between Podocytes and glomerular capillaries
- RBCs and larger proteins can’t - too large so stay in the blood

what substances will be present in the glomerular filtrate?

water, glucose, urea, amino acids, fatty acids, glycerol, ions, small proteins

the glomerular filtration rate is the total volume of filtrate formed per minute. explain the effect on the glomerular filtration rate of a large loss of blood from the body.

• blood pressure decreased

• less pressure forms less filtrate

what components of the blood are not normally present in the filtrate?

blood cells, platelets, proteins

where does ultrafiltration occur?

Renal capsule

describe how urea is removed from the blood

hydrostatic pressure causes ultrafiltration at renal capsule through basement membrane enabled by small size urea molecule

how much filtrate water is reabsorbed in the proximal convoluted tubule?

nearly 85%

how is the proximal convoluted tubule adapted?

by having cells that have:
- microvilli - large SA to reabsorb substances from filtrate
- carrier proteins in membrane for active transport
- mitochondria - high density to provide ATP for active transport

describe protein digestion

1. endopeptidases hydrolyse the peptide bonds between the amino acids in the centre of the polypeptide - this increases the SA by creating more ends for exopeptidases
2. exopeptidases hydrolyses the peptide bonds between the terminal amino acids of the peptides formed by endopeptidases, this creases dipeptides and single amino acids
3. membrane bound dipeptidases hydrolyse peptide bonds between the amino acids in these dipeptides

describe the processes involved in the absorption of the products of polypeptide digestion

- amino acids are absorbed down their conc gradient from the lumen into the blood by facilitated diffusion
- Na+ removed from epithelial cell by active transport using the sodium-potassium pump into blood
- maintains low conc of Na+ in epithelial cell (maintaining Na+ conc gradient between lumen and epithelial cell)
- amino acid moves into epithelial cell with Na+ via carrier protein in co-transport
- amino acid then moves into blood by facilitated diffusion

explain how a lack of insulin affects reabsorption of glucose in the kidneys of a person who does not secrete insulin

1. high concentration of glucose in blood
2. high concentration in filtrate
3. reabsorbed by facilitated diffusion
4. requires carriers
5. these are working at maximum rate
6. not all glucose is reabsorbed

explain how urea is concentrated in the filtrate

• reabsorption of water by osmosis

• at the proximal convoluted tubule

• at the distal convoluted tubule

• active transport of ions creates gradient

function of the loop of Henle

- creates a high concentration of Na+ and chloride ions in the tissue fluid of the medulla
- this allows water to be reabsorbed from the contents of the nephron as the pass through collecting duct

survival advantage of the loop of Henle

• very concentrated urine can be produced

• conserves water and prevents dehydration

explain the role of the loop of Henle in the absorption of water from the filtrate

1. Na+ and Cl- are actively transported out of ascending limb
2. this decreases water potential in tissue fluid (interstitial region)
3. water can’t leave ascending limb (it’s impermeable to water)
4. water can leave filtrate in the descending limb by osmosis into interstitial space
5. water absorbed into blood capillaries by osmosis
6. water leaves collecting duct
7. (add from notes)

interstitial space

highest water potential (at top) by cortex

counter-current system

• solute conc at any part of the ascending tubule is lower than that in the descending limb

• causes a build up of salt conc in the surrounding tissues

• water moves out by osmosis

collecting duct

• the last chance to reabsorb the water

• here the amount of water reabsorbed in to the blood can be controlled

• the duct then flows into the renal pelvis, down the ureter and into the bladder

counter current multiplier

- active transport of Na+ and Cl- ions in the epithelial cells of ascending loop of Henle (out of the filtrate and into the surrounding tissue)
- Na+ and Cl- transported against conc gradient
- this creates and maintains a wp gradient, so water moves out of the collecting duct by osmosis
- the water can then be reabsorbed back into the blood

explain how the loop of Henle maintains the gradient of ions which allows water to be absorbed from filtrate in the collecting duct

• epithelial cell of tubule cells carry out active transport

• transport Na+ out of filtrate against conc gradient into tissue fluid

• maintains wp gradient for water reabsorption

• countercurrent multiplier

anti-diuretic hormone

- if dehydrated the wp of blood decreases and gets too low
- detected by osmoreceptors in the hypothalamus
- water moves from osmoreceptor into blood by osmosis (osmoreceptor decrease in volume)
- causes posterior pituitary gland to secrete more ADH

ADH and the collecting duct

- ADH travels in the blood to the kidneys, where it binds to a cell surface receptor on DCT or collecting duct (CD)
- ADH causes vesicles containing aquaporins to fuse with membrane of collecting duct cells so aquaporins are inserted into membrane
- increase in permeability of the membrane to water
- water enters CD cell through aquaporins by osmosis down a wp gradient from cell to capillary via interstitial fluid
- so ADH increases water reabsorption
- smaller volume of urine
- urine becomes more concentrated

how does blood water potential get too high?

• drink lots of water

• insufficient ions

what happens when blood water potential gets too high?

- detected by osmoreceptors in the hypothalamus
- less ADH secreted from posterior pituitary gland
- decreases permeability of collecting duct and distal convoluted tubule to water and urea
- more water leaves the body (less is reabsorbed back into the blood - dilute urine)
- osmoregulation → normal blood wp

how does blood water potential get too low?

• don’t drink enough water

• sweat too much

• take in lots of ions, e.g. NaCl

what happens when blood water potential gets too low?

- detected by osmoreceptors in the hypothalamus
- more ADH secreted from posterior pituitary gland
- increases water permeability of collecting duct and distal convoluted tubule (inserts aquaporins)
- less water leaves the body, more is reabsorbed back into the blood
- increase collecting duct permeability to urea, urea leaves, decreases wp around duct, water leaves duct
- osmoregulation → normal blood wp

the role of ADH

- ADH is passed in the blood to the kidneys, where it binds to a cell surface receptor
- this activates phosphorylase
- phosphorylase causes vesicles containing aquaporins to bind with cell surface membrane of collecting duct
- increases the permeability of membrane to water and therefore allows more water to be reabsorbed
- when wp too high, less ADH released meaning fewer aquaporins are fused with the cell membrane

aquaporins

water channel proteins

why does the cell volume of an osmoreceptor decrease when a person is dehydrated?

1. wp of blood will decrease

2. water moves from osmoreceptor into blood by osmosis

how does the secretion of ADH affect urine produced by the kidneys

1. permeability of membrane to water is increased

2. more water absorbed from collecting duct permeability to

3. smaller volume of urine

4. urine becomes more concentrated

how does ADH increase the movement of water from the lumen of the collecting duct into the blood?

1. ADH causes vesicles containing aquaporins to be inserted into membrane
2. water enters cell through aquaporins by osmosis down a wp gradient from cell to capillary via interstitial fluid

which hormone causes the decrease in the water content in the distal convoluted tubule?

ADH

how does having a long loop of Henle and secreting large amounts of ADH adapt desert mammals to live in desert conditions?

explain how the structure of protein molecules allows them to form channels through which only water molecules can pass

each protein has a specific tertiary structure which gives a specific shape to inside of channel

explain how the cells of the collecting duct are able to absorb water from the filtrate through the protein channels in their plasma membrane

• more negative wp inside tubule cells

• water enters by osmosis

ADH release

- when the wp is too high, less ADH is released meaning fewer aquaporins are fused with the cell membrane
- alcohol suppresses ADH production, this causes the kidneys to produce more dilute urine, it can lead to dehydration
- ecstasy increases ADH production, this causes the kidneys to reabsorb water, it can result in the body having too much water

distal convoluted tubule

in the distal convoluted tubule Na+ are actively pumped out of nephron into blood

reabsorption of water from the collecting duct

• tissue fluid deep in the medulla as a very low wp

• water moves down the collecting duct and the wp gradient decreases further into the medulla

• water enters the blood by osmosis and is conserved