F. Kidney and the control of water balence Flashcards
What is the homeostatic control of water potential of the blood called
Osmoregulation
What is the structure that carries out osmoregulation
Nephron
Describe the structure of the mammalian kidney
- fibrous capsule
- cortex
- medulla
- renal pelvis
- ureter
- renal artery
- renal vein
What does renal mean
Relating to the kidney
What is the fibrous capsule
Outer membrane that protects the kidney
What is the cortex
Lighter coloured outer region made up if renal (Bowman’s capsule), convoluted tubles and blood vessels
What is the medulla
Darker coloured inner region made up of loops of Henle, collecting ducts and blood vessels
What is the renal pelvis
A funnel shaped cavity that collects urine into the ureter
What is the ureter
A tube that carries urine to the bladder
What is a renal artery
Supplies the kidney with blood from the heart via the aorta
What is the renal vein
Returns blood to the heart via the vena cava
Describe the structure of the nephron
- renal (Bowman’s) capsule
- proximal convoluted tubule
- loop of Henle
- distal convoluted tubule
- collecting duct
What is a renal (Bowman’s capsule)
- the closed end at the start of the nephron
- cup shaped and surrounded by a mass of blood capillaries (the glomerulus)
- inner renal capsule is made of specialised cells called podocytes
What is a proximal convoluted tubule
- series of loops surrounded by blood capillaries
- walls made of epithelial cells which have microvilli
What is the loop of Henle
- long hairpin loop that extends from the cortex into the medulla of the kidney and back again
- surrounded by blood capillaries
What is the distal convoluted tubules
- series of loops surrounded by blood capillaries
- walls made of epithelial cells but less than the proximal tubule
What is the collecting duct
- tube into which a number of distal convoluted tubules from a number of nephrons empty
- lined by epithelial cells and becomes increasingly wide as it empties into the pelvis of the kidney
Name all the blood vessels associated with a single nephron
- afferent arteriole
- glomerulus
- efferent arteriole
- blood capillaries
What is an afferent arteriole
- tiny vessel that ultimately arises from the renal artery
- supplies the nephron with blood
- the afferent arteriole enters the renal capsule of the nephron where it forms the glomerulus
What is the glomerulus
- a many branched knot of capillaries from which fluid is forced out of the blood
- the glomerular capillaries recombine to form the efferent arteriole
What is the efferent arteriole
- tiny vessel that levels the renal capsule
- smaller diameter than the afferent arteriole so causes an increase in blood pressure within the glomerulus
- efferent arteriole carries blood away from the renal capsule and later branches to form the blood capillaries
What is the blood capillaries (within the nephron)
- concentrated network of capillaries that surrounds the proximal convoluted tubule, the loop of Henle and the distal convoluted tubule
- from where they reabsorb mineral salts, glucose and water
- these capillaries merge together into venules (tiny veins) that in turn merge together to form the renal vein
Name an important function of the kidney
To maintain the water potential of plasma and hence tissue fluid (osmoregulation)
Name the series of stages of osmoregulation as carried out by the nephron
- the formation of glomerular filtrate by ultrafiltration
- reabsorption of glucose and water by the proximal convoluted tubule
- maintenance of a gradient of sodium ions in the medulla by the loop of Henle
- reabsorption of water by the distal convoluted tubule and collecting ducts
Describe the steps in the formation of glomerular filtrate by ultrafiltration
- blood enters kidney through renal artery which branches into many afferent arterioles
- each of which enters a renal (Bowman’s capsule) of a nephron
- walls of glomerular capillaries are made of endothelial cells with pores between them
- diameter of afferent arteriole is bigger than the efferent arteriole
- meaning there is a build up of hydrostatic pressure within the glomerulus
- water, glucose and mineral ions are squeezed out of the capillary to form the glomerular filtrate
What is squeeze out the glomerulus capillary
- water
- glucose
- mineral ion
- urea
What cannot leave the capillary
- red blood cells
- white blood cells
- platelets
Why can red blood cells/ large proteins not pass across into the renal capsule
They are too large
How is the movement of glomerular filtrate resisted
- hydrostatic pressure of the fluid in the renal capsule space
- low water potential of the blood in the glomerulus
What modifications reduce the resistance to allow movement of glomerular filtrate
- inner layer of the renal capsule is made up of podocytes which have spaces between them allowing filtrate pass between these cells rather than through them
- the endothelium of glomerular capillaries has spaces between its cells
Describe the steps in the reabsorption of glucose and water by the proximal convoluted tubule
- Na + ions are actively transported out of the cells lining the proximal convoluted tubule into blood capillaries which carry them away
- Na+ ion concentration of these cells is therefore low
- Na + ions diffuse down a concentrated gradient from the lumen of the proximal convoluted tubule into the epithelial lining cells but only through special carrier proteins by facilitated diffusion
- these carrier proteins are of specific types each of which carries another molecule (e.g glucose or amino acids or chloride ions) along with the sodium ions (co-transport)
- the molecules which have been co-transported into the cells of the proximal tubule diffuse into the blood
- as a result all the glucose and most mother valuable molecules are reabsorbed as well as water
How are proximal convoluted tubules adapted to reabsorb substances
- microvilli to provide a large surface area to reabsorb substances from filtrate
- infoldings at their bases to give a large surface area to transfer reabsorbed substances into blood capillaries
- a high density of mitochondria to provide ATP for active transport
In what way are Na+ ions transported out the cells lining in the proximal convoluted tubule
Active transport
How do ions such as glucose amino acids and chloride ions move out of the lumen of the proximal convoluted tubule
Co-transport with Na+ ions
What is the loop on Henle responsible for
For water being reabsorbed from the collecting duct, thereby concentrating urine so that it has a lower water potential than the
Name the two regions of the loop of Henle
- descending limb
- ascending limb
What is the descending limb
-narrow with thin walls that are highly permeable to water
What is the ascending limb
-wider with thick walls that are impermeable to water
What is a counter current multiplier
A counter current mechanism system is a mechanism that expends energy to create a concentration gradient
How does the loop of Henle act as a counter-current system
- Na + ions are actively transported out of the ascending limb (using ATP from mitochondria)
- this creates low water potential in the medulla between the two limbs (interstitial region)
- water cannot pass out via osmosis of the ascending limb as walls are too thick
- walls of descending limb are very permeable to water and so it passes out the filtrate (by osmosis) into the interstitial space
- this way water enters the blood capillaries
- filtrate progressively loses water as it moves down the descending limb lowering its water potential (reaching lowest at hairpin)
- at base of ascending limb Na + ions are actively pumped out of the filtrate as they move up the ascending limb progressively increasing the water potential
- in the interstitial space between ascending limb and the collecting duct there is a gradient of water potential (highest water potential in the cortex lower further into medulla)
- collecting duct is permeable to water and so as the filtrate moves down it water passes out via osmosis (due to ions in the interstitial space)
- this water moves into blood vessels by osmosis
How is Na + ions moved out of the ascending limb of the loop of Henle
Via active transport using ATP provided by the many mitochondria in the cells of its walls
What is the water potential like in the interstitial between the two limbs
- low water potential
- as Na+ ions are being actively pumped out of the ascending limb
If the water potential in the interstitial between the two limbs is low, why does water not move out the ascending limb
-thick walls are almost impermeable to water
What happens in the descending limb
- water moves out via osmosis from a high water potential in the descending limb to the high water potential in the interstitial space
- as the walls of the descending limb is thin and permeable
When does water move into the capillaries
-when water moves via osmosis into the interstitial space from the descending limb/collecting duct
How does the counter-current multiplier ensure that there is always a water potential gradient drawing water out of the tubule
- as water passes out of the filtrate its water potential is lowered
- however the water potential is also lowered in the interstitial space
- meaning moving down the collecting duct/tubule there is always a concentration gradient
What do the cells in the walls of the distal convoluted tubule have?
- microvilli
- mitochondria
- allows them to reabsorb material rapidly from the filtrate (from capillaries)
What is the main role of the distal convoluted tubule
- make final adjustments to the water and salts that are reabsorbed
- to control pH of the blood by selecting which ions to reabsorb
How does the distal convoluted tubule control the pH/ water salt concentration
- altering permeability of its walls
- which is altered under the influence of different hormones
What does the counter current flow of the loop of Henle mean
- the filtrate in the collecting duct with a lower water potential meets interstitial fluid with an even lower water potential
- this means the concentration gradient exists for the whole collecting blood
- if the two flows were in the same direction less of the water would enter the blood
How is the homeostatic control of the osmoregulation in the blood achieved
-by a hormone that acts on the distal convoluted tubule and the collecting duct
What may cause a decrease in water potential
- too little water being consumed
- much sweating
- large of ions being taken in
How does the body respond to the fall in water potential of the blood
- when water potential of the blood is low water is lost from these osmoreceptor cells (in the hypothalamus) by osmosis
- due to the water loss the osmoreceptor cells shrink causing the hypothalamus to produce ADH
- ADH passes to the pituitary gland from where it is secreted into the capillaries
- ADH passes in the blood to the kidney where it increases the permeability to water of the cell surface membrane of the cells that make up the walls of the distal convoluted tube and collecting duct
- specific glycoprotein receptors on the cell-surface membrane of these cells bind to ADH molecules which causes the activation of the enzyme phosphorylase
- this causes the vesicles within the cell to move and fuse with the cell-surface membrane
- the vesicles contain pieces of plasma membrane that have numerous aquaporins which means when the vesicles fuse with the cell surface membrane the cell-surface membrane is more permeable to water
- ADH also increases the permeability of the collecting duct to urea which therefore passes out further lower the water potential of the fluid around the duct
- combined effect is more water leaves the collecting duct by osmosis down water potential gradient and re-enters blood, increasing blood water potential
- osmoreceptor in the hypothalamus detect rise in water potential and send fewer impulses to the pituitary gland
- pituitary gland reduces release of ADH and the permeability of the collecting ducts to water and urea (reverting to former state)
Where are osmoreceptors found
In the hypothalamus of the brain
How do osmoreceptors detect a decrease in water potential of the blood
- when water potential of the blood is low water is lost from these osmoreceptor cells by osmosis
- due to water loss osmoreceptor cells shrink causing the hypothalamus to produce ADH
What does ADH stand for
antidiuretic hormone
Where is ADH released
From the pituitary gland
What is the effect of ADH on the kidney
Increases the permeability to water of the cell surface membrane of the cells that make up the walls of the distal convoluted tubule and the collecting duct
How does ADH increase the permeability to water of the cell surface membrane of the cells that make up the walls of the distal convoluted tubule and the collecting duct
- ADH binds to specific protein receptors on the cell-surface membrane of the cells that make up the walls of the distal convoluted tubule and the collecting duct
- this leads to the activation of an enzyme called phosphorylase
- this causes vesicles within the cell to move and fuse with the cell surface membrane
- vesicles contain pieces of plasma membrane that have numerous water channel proteins (aquaporins)
- when vesicles fuse with the cell-surface membrane density of aquaporins increased so it is more permeable to water
ADH also increases the permeability of what ?
- the collecting ducts permeability to urea
- urea passes out the collecting duct
- lowering the water potential of the fluid around the duct
- more water leaves via osmosis down a water potential gradient and re-enters the blood
What do osmoreceptors also do to increase blood water potential
Send nerve impulses to the thirst centre of the brain to encourage the individual to seek out and drink more water
What may cause an increase in water potential
- large volumes of water being consumed
- salts used in metabolism or excreted not being replaced in diet
How does the body respond to the rise in water potential
- osmoreceptors in the hypothalamus detect the rise in water potential and increase the frequency of nerve impulses to the pituitary gland to reduce its release of ADH
- Less ADH in the blood leads to a decrease in the permeability of the collecting ducts to water and urea
- less water is reabsorbed into the blood from the collecting duct
- more dilute urine is produced and water potential of the blood falls
- when water potential of the blood has returned to normal the osmoreceptors in the hypothalamus cause the pituitary to raise its ADH release back to normal
