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6.4.3 - Control of Blood Water Potential, Normal water potential of the…
6.4.3 - Control of Blood Water Potential
Kidney structure
Renal artery
carries oxygenated blood (including salt and urea) to kidneys
Renal vein
Carries deoxygenated blood (with salt and urea removed) away from the kidneys
Kindey
regulates water content of blood and filters blood
Ureter
carries urine from kidneys to bladder
Bladder
stores urine (temporarily)
Urethra
releases urine otside of the body
Capsule
The kidney is surrounded by a fairly tough outer layer known as the fibrous capsule
Beneath the fibrous capsule, the kidney has three main areas:
cortex
contains the glomerulus, as well as the Bowman’s capsule, proximal convoluted tubule (PCT), and distal convoluted tubule (DCT) of the nephrons
medulla
contains the loop of Henle and collecting duct of the nephrons
renal pelvis
where the ureter joins the bladder
Nephrons
Each kidney contains thousands of microscopic tubes, known as nephrons
The nephron is the functional unit of the kidney
The nephrons are responsible for the formation of urine
Osmoregulation - process by which the water potential of the blood is maintained at an optimum level
The kidney carries out this process by controlling the amount of salt and water excreted in the urine
Ultrafiltration
The blood in the glomerular capillaries is separated from the lumen of the Bowman’s capsule by two cell layers with a basement membrane in between them
Endothelium of the capillary
this is made up of a network of collagen and glycoproteins
The basement membrane
hese epithelial cells have many tiny finger-like projections with gaps in between them and are known as podocytes
Epithelium of the Bowman’s capsule
each capillary endothelial cell is perforated by thousands of tiny membrane-lined circular holes
Creating hydrostatic pressure
Arterioles branch off the renal artery and lead to each nephron
The blood is at a high pressure because it is coming directly from the renal artery (connected to the aorta)
The afferent arteriole is wider than the efferent arteriole which causes a high hydrostatic pressure in the glomerulus
In each nephron the blood vessels form a knot of capillaries (the glomerulus) sitting inside the cup-shaped Bowman’s capsule
The capillaries get narrower as they get further into the glomerulus which further increases the pressure on the blood moving through them
Forming the filtrate
High hydrostatic pressure in the capillaries causes the smaller molecules (in the blood) to be forced out of the capillaries and into the Bowman’s capsule. Where they form what is known as the glomerular filtrate
What is in the filtrate?
The main substances that form the glomerular filtrate are: amino acids, water, glucose, urea and inorganic ions (mainly Na+, K+ and Cl-)
Red and white blood cells and platelets remain in the blood as they are too large to pass through the holes in the capillary endothelial cells.
The basement membrane acts as a filter as it stops large protein molecules from getting through
How does this occur?
Overall, the effect of the pressure gradient outweighs the effect of solute gradient
The water potential of the blood plasma in the glomerulus is higher than the water potential of the filtrate in the Bowman’s capsule
Blood flows through the glomerulus, there is an overall movement of water down the water potential gradient from the blood into the Bowman’s capsule
selective reabsorption
occurs as the glomerular filtrate travels through the proximal convoluted tubule (PCT), through the loop of Henle and along the distal convoluted tubules (DCT)
The useful substances (~85% of filtrate) leave the tubules (of the nephron) and enter the capillary network that surrounds them
Structure of PCT
Lots of mitochondria: provide ATP for the active transport
Microvilli: increases surface area and speed up the rate of diffusion of substances into the epithelial cell
Folded basal membrane: increases surface area and speed up the rate of diffusion of substances out of the epithelial cell.
Glucose
Sodium ions (Na+) are actively transported out of the cells lining the PCT lowering the Na+ concentration in these cells
Na+ now diffuse down the concentration gradient from the lumen of PCT into the epithelial lining cells but only through special carrier proteins by facilitated diffusion
Some of these carrier proteins are sodium-dependent glucose transport proteins which carries glucose into the cells with the Na+.
Once in these lining cells of the PCT glucose can now diffuse into the blood
Water
Glucose and sodium ions are transported back into the blood (via active transport and diffusion)
As a result: The water potential of blood decreases
Water moves into blood by osmosis (down water potential gradient).
Loop of Henle
main function is to reabsorb sodium ions and water and create a concentration gradient in the medulla of the kidney
located in the medulla (inner layer) of the kidney
Made up of two limbs
Descending limb - has thin cells making up its wall so is permeable to water
Ascending limb - has thick cells making the wall impermeable to water
movement of substances within the loop of henle
Sodium ions are actively transported out of ascending limb into the interstitial region.
Theses sodium ions then diffuse into the descending limb
he ascending limb is impermeable to water so water stays inside ascending limb.
This creates a low water potential (high ion concentration) in the interstitial fluid in the medulla
The walls of the descending limb are permeable to water so water leaves the descending limb and enters the medulla.
This water is reabsorbed into the blood through the capillary network
Water is lost from the filtrate as it moves down the descending limb.
At the bottom of the loop (in the hairpin) sodium ions diffuse out into the medulla further lowering the water potential
As the filtrate moves up the ascending limb sodium ions are actively pumped out of the filtrate.
The water potential of the filtrate progressively increases
In the interstitial space between the ascending limb and collecting duct a gradient of water potential has been established.
Highest water potential in the cortex lowering as it enters the medulla
The collecting duct is permeable water so as the filtrate moves down it water passes out of it via osmosis and into the blood vessels that occupy this space (so are carried away)
As water passes out of the filtrate its water potential is lowered.
However, the water potential of the interstitial space is also lowered (as water enters the blood) so water continues to move out via osmosis
Counter-current multiplier
ensures that there is always a water potential gradient drawing water out of the tubule
Osmoregulation
monitoring water potential
Specialised sensory receptors, known as osmoreceptors, monitor the water potential of the blood
These osmoreceptors are found in an area of the brain known as the hypothalamus
The osmoreceptors send impulses to the pituitary gland when they detect a decrease in the water potential of the blood
The pituitary gland secretes a hormone (ADH) which increases the reabsorption of water
A low water potential of the blood also stimulates the thirst centre of the hypothalamus
ADH and altering water potential
ADH (antidiuretic hormone) alters the permeability of the distal convoluted tubule and the collecting duct
ADH molecules enter the blood and travel throughout the body
ADH causes the kidneys to reabsorb more water
This reduces the loss of water in the urine
How does ADH increase water reabsorption?
ADH causes the luminal membranes (those facing the lumen of the nephron) of the collecting duct cells to become more permeable to water
ADH does this by causing an increase in the number of aquaporins (water-permeable channels) in the luminal membranes of the collecting duct cells
Aquaporins are the water transporters
The diffusion of water across biological membranes is facilitated by special channel proteins called aquaporins
Aquaporins are designed to have hydrophilic pores which allow water molecules to move through them
The effect of ADH on the kidneys
ADH released by pituitary gland and travels in blood to the kidney.
In the kidney; ADH binds to specific receptors in cell surface membrane of epithelial cells of collecting duct
The binding of ADH activates a signalling cascade. Phosphorylase is activated
that leads to the phosphorylation of the aquaporin molecules which activates the aquaporins
Vesicles containing aquaporins move towards the cell surface membrane
Vesicles containing aquaporins fuse with the cell surface membrane.
The cell surface membrane contains more aquaporins and so is more permeable to water
Water moves through the aquaporins down the water potential gradient, into the concentrated tissue fluid and blood plasma, by osmosis
The collecting duct
Collecting duct cells contain vesicles, the membranes of which contain many aquaporins
ADH molecules bind to receptor proteins
This results in the phosphorylation of the aquaporin molecules
Vesicles (with aquaporin-containing membranes) move towards the luminal membrane of the collecting duct cells
Vesicles fuse with the luminal membranes
Water moves through the aquaporins; out of filtrate into the tissue fluid and blood plasma
Normal water potential of the blood
Increased water intake, decreased salt/ion intake
Increase in water potential detected by osmoreceptors in the hypothalamus
pituitary gland releases less ADH
walls of DCT and collecting duct become less permeable to water
More water leaves the body in dilute urine
Decreased water intake, increased salt/ion intake
decrease in water detected by hypothalamus
Pituitary gland releases more ADH
Walls of DCT and collecting duct become more permeable to water
less water leaves the body in more concentrated urine