control of blood water potential

describe how urea is removed from the blood

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

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

glomerular filtrate

water, glucose, urea, fatty acids, etc. will be present

blood cells, platelets and proteins are not normally present in the filtrate

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)

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

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 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

the loop of Henle

descending limb: narrow, thin walls and very permeable to water

ascending limb: wide, thick walls, not permeable to water and more permeable to salts, e.g. Na+

function:
- 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:
- 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

counter current multiplier
- active transport of Na+ and Cl- 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

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

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

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 (water channel proteins) 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

blood water potential too high

how:
- drink lots of water
- insufficient ions

what happens:
- 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

blood water potential too low

how:
- don’t drink enough water
- sweat too much
- take in lots of ions, e.g. NaCl

what happens:
- 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

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

permeability of membrane to water is increased

more water absorbed from collecting duct permeability to

smaller volume of urine

urine becomes more concentrated

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

ADH causes vesicles containing aquaporins to be inserted into membrane

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

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

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