The control of blood water potential

Question 1

What is osmoregulation

Answer 1

The homeostatic control of the water potential of the blood.

Question 2

What are the basic structural and functional units of the kidneys called

Answer 2

nephrons

Question 3

What is the kidney made up of

Answer 3

• Fibrous capsule: an outer membrane that protects the kidney.
• Cortex: a lighter coloured outer region made up of renal (Bowman’s) capsules, convoluted tubules and blood vessels.
• Medulla: a darker coloured inner region made up of loops of Henle, collecting ducts and blood vessels.
• Renal pelvis: a funnel-shaped cavity that collects urine into the ureter.
• Ureter: A tube that carries urine to the bladder.
• Renal artery: supplies the kidney with blood from the heart via the aorta.
• Renal vein: returns blood to the heart via the vena cava.

Question 4

What is a Nephron

Answer 4

• The nephron is the functional unit of the kidney.
• It is a narrow tube up to 14mm long, closed at one end, with two twisted regions separated by a long hairpin loop.

Question 5

List and describe the things that each nephron is made up of

Answer 5

• renal (Bowman’s) capsule: the closed end at the start of the nephron. It is cup-shaped and surrounds a mass of blood capillaries known as glomerulus. The inner layer of the renal capsule is made up of specialised cells called podocytes.
• Proximal convoluted tubule: A series of loops surrounded by blood capillaries. Its walls are made of epithelial cells which have microvilli.
• Loop of Henle: a long, hairpin loop that extends from the cortex into the medulla of the kidney and back again. It is surrounded by blood capillaries.
• Distal convoluted tubule: a series of loops surrounded by blood capillaries. Its walls are made of epithelial cells, but it is surrounded by fewer capillaries than the proximal tubule.
• Collecting duct: a tube into which a number of distal convoluted tubules from a number of nephrons empty. It is lined by epithelial cells and becomes increasingly wide as it empties into the pelvis of the kidney.

Question 6

List and describe the blood vessels associated with each nephron

Answer 6

• Afferent arteriole: A tiny vessel that ultimately arises from the renal artery and supplies the nephron with blood. The afferent arteriole enters the renal capsule of the nephron where it forms the glomerulus.
• 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.
• Efferent arteriole: A tiny vessel that leaves the renal capsule. It has a smaller diameter than the afferent arteriole and so causes an increase in blood pressure within the glomerulus. The efferent arteriole carries blood away from the renal capsule and later branches to form the blood capillaries.
• Blood capillaries: A concentrated network of capillaries that surrounds the proximal convoluted tubule, the loop of Henle, and the distal convoluted tubule and from where they reabsorb mineral salts, glucose and water. These capillaries merge together into venules that in turn merge together to form the renal vein.

Question 7

List the series of stages by which the nephron carries out its role in osmoregulation

Answer 7

1)The formation of glomerular filtrate by ultrafiltration.
2) Reabsorption of glucose and water by the proximal convoluted tubule.
3) Maintenance of a gradient of sodium ions in the medulla by the loop of Henle.
4) Reabsorption of water by the distal convoluted tubule and collecting ducts.

Question 8

What are the walls of the glomerular capillaries made up of

Answer 8

Endothelial cells with pores between them

Question 9

Describe the formation of glomerular filtrate by ultrafiltration

Answer 9

• As the diameter of the afferent arteriole is greater than that of the efferent arteriole, there is a build up of pressure within the glomerulus.
• As a result, water, glucose and mineral ions are squeezed out of the capillary to form the glomerular filtrate.
• Blood cells and large proteins cannot pass into the renal capsule as they are too large.
• The movement of the filtrate out of the glomerulus is resisted by the:
  1) Capillary endothelial cells.
  2) Connective tissue and endothelial cells of the blood capillary.
  3) Epithelial cells of the renal capsule.
  4) The hydrostatic pressure of the fluid in the renal capsule space.
  5) The low water potential of the blood in the glomerulus.
• There are some modifications which resist this barrier to the flow of filtrate and thus said in its formation:
  1) The inner layer of the renal capsule is made up of highly specialised cells called podocytes. These cells have spaces between them which allows filtrate to pass beneath them and through gaps between their branches. Filtrate passes between these cells rather than through them.
  2) The endothelium of the glomerular capillaries has spaces up to 100 nm wide between its cells.Again, fluid can therefore pass between, rather than through, these cells.
• As a result, the hydrostatic pressure of the blood in the glomerulus is sufficient to overcome the resistance and so filtrate passes from the blood into the renal capsule.
• This filtrate, which contains urea, does not contain cells or plasma proteins which are too large to pass across the connective tissue.

Question 10

Describe the reabsorption of glucose and water by the promixal convoluted tubule

Answer 10

• In the proximal convoluted tubule nearly 85% of the filtrate is reabsorbed back into the blood as there are useful molecules in the filtrate.
• How this happens:
• sodium ions are actively transported out of the cells lining in the proximal convoluted tubule into blood capillaries which carry them away.
• The sodium ion concentration in these cells is therefore lowered.
• Sodium ions now diffuse down a concentration 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 (glucose, amino acids, chloride ions etc) along with the sodium ions by co-transport.
• The molecules which have been co-transported into the cells of the proximal convoluted tubule then diffuse into the blood.
• As a result, all the glucose and most other valuable molecules are reabsorbed as well as water.

Question 11

How much water enters the nephrons and is reabsorbed per day on average

Answer 11

• About 180 dm^3 of water enters the nephrons each day.
• Of this, only about 1 dm^3 leaves the body as urine.
• 85% of the reabsorption of water occurs in the proximal convoluted tubule.
• The remainder is reabsorbed from the collecting duct as a result of the functioning of the loop of Henle.

Question 12

Describe what the loop of Henle is

Answer 12

• The loop of Henle is a hairpin-shaped tubule that extends into the medulla of the kidney.
• It is responsible for water being reabsorbed from the collecting duct, thereby concentrating the urine so that it has a lower water potential than the blood.
• Th concentration of the urine produced is directly related to the length of the loop of Henle.

Question 13

Describe the two regions that the loop of henle is made up of

Answer 13

1) The descending limb which is narrow, with thin walls that are highly permeable to water.
2) The ascending limb which is wider, with thick walls that are impermeable to water.

Question 14

Describe how the loop of Henle acts as a counter-current multiplier

Answer 14

1) Sodium ions are actively transported out of the ascending limb of the loop of Henle using ATP provided by the many mitochondria in the cells of its wall.
2) This creates a low water potential (high ion concentration)in the region of the medulla between the two limbs (called the interstitial region).
3) In normal circumstances water would pass out of the ascending limb by osmosis. However, the thick walls are almost impermeable to water and so very little escapes.
4) The walls of the descending limb, however, are very permeable to water and so it passes out of the filtrate, by osmosis, into the interstitial space. This water enters the blood capillaries in this region by osmosis and is carried away.
5) The filtrate progressively loses water in this way as it moves down the descending limb, lowering its water potential. It reaches its lowest water potential in the tip of the hairpin.
6) At the base of the ascending limb, sodium ions diffuse out of the filtrate and as it moves up the ascending limb these ions are also actively pumped out and therefore the filtrate develops a progressively higher water potential.
7) In the interstitial space between the ascending limb and the collecting duct there is a gradient of water potential with the highest water potential (lowest ion conc) in the cortex and an increasingly lower water potential (higher ion conc) the further into the medulla one goes.
8) The collecting duct is permeable to water and so, as the filtrate moves down it, water passes out of it by osmosis. The water passes by osmosis into the blood vessels that occupy this space, and is carried away.
9) As water passes out of the filtrate its water potential is lowered. However, the water potential is also lowered in the interstitial space and so water continues to move out by osmosis down the whole length of the collecting duct. The counter-current multiplier ensures that there is always a water potential gradient drawing water out of the tubule.

Question 15

Through what does the water that passes out of the collecting duct by osmosis pass through and what can alter these

Answer 15

• the water that passes out of the collecting duct by osmosis does so through channel proteins that are specific to water (aquaporins).
• Antidiuretic hormone (ADH) can alter the number of these channels and so control water loss.

Question 16

Describe the adaptations of the distal convoluted tubule and it’s roles

Answer 16

• The cells that make up the walls of the distal convoluted tubule have microvilli and many mitochondria that allow them to reabsorb material rapidly from the filtrate, by active transport.
• The main role of the distal tubule is the make final adjustments to the water and salts that are reabsorbed and to control the pH of the blood by selecting which ions to reabsorb.
• To achieve this the permeability of its walls becomes altered under the influence of various hormones.

Question 17

Why is the counter-current principle important for the kidney and osmoregulation

Answer 17

• it means that, although the water potential gradient between the collecting duct and interstitial fluid is small, it exists for the whole length of the collecting duct.
• There is therefore a steady flow of water into the interstitial fluid and hence the blood.

Question 18

What does the hormone involved in the homeostatic control of osmoregulation act on

Answer 18

The distal convoluted tubule and the collecting duct.

Question 19

What lowers the blood water potential and what may cause it

Answer 19

• A rise in solute (glucose, proteins, sodium chloride, mineral ions) concentration lowers blood water potential.
• This may be cause but too little water being consumed.
• It may be caused by lots of sweating
• It may be caused by large amount of ions such as sodium chloride being taken in.

Question 20

Describe how the body responds to a fall in water potential

Answer 20

• Cells called osmoreceptors in the hypothalamus of the brain detect the fall in water potential.
• It is thought that, when the water potential of the blood is low, water is lost from these osmoreceptor cells by osmosis.
• Due to this water loss the osmoreceptor cells shrink, a change that causes the hypothalamus to produce antidiuretic hormone (ADH).
• ADH passes the posterior pituitary gland, from where it is secreted into the capillaries.
• ADH passes in the blood to the kidney, where it increases the permeability to alter of the cell-surface membrane of the cells that make up the walls of the distal convoluted tubule and the collecting duct.
• Specific protein receptors on the cell-surface membrane of these cells bind to ADH molecules, leading to activation of the enzyme phosphorylase within the cell.
• The activation of phosphorylase causes vesicles within the cell to move to, and fuse with, its cell-surface membrane.
• These vesicles contain pieces of plasma membrane that have numerous water channel proteins (aquaporins) and so when they fuse with the membrane the number of water channels is considerably increased, making the cell-surface membrane much more permeable to water.
• ADH increases the permeability of the collecting duct to urea, which therefore passes out, further lowering the water potential of the fluid around the duct.
• The combined effect is that more water leaves the collecting duct by osmosis, down a water potential gradient, and re-enters the blood.
• As the reabsorbed water came from the blood in the first place, this will not, in itself, increase the water potential of the blood, but merely prevent it from getting lower.
• The osmoreceptors also send nerve impulses to the thirst centre of the brain, to encourage you to seek out and drink more water.
• The osmoreceptors in the hypothalamus detect the rise in water potential and send fewer impulses to the pituitary gland.
• The pituitary gland reduces the release of ADH and the permeability of the collecting ducts to water and urea and reverts to its former state.

Question 21

What raises blood water potential and what may cause this

Answer 21

• A fall in the solute concentration of the blood.
• This could be caused by large volumes of water being consumed.
• This could also be caused by salts used in metabolism or excreted not being replaced in the diet.

Question 22

Describe how the body responds to a rise in water potential

Answer 22

• The osmoreceptors in the hypothalamus detect the rise in water potential and increase the frequency of nerve impulses to the pituitary gland and reduce its release of ADH.
• Less ADH, via 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 the water potential of the blood falls.
• when the 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 levels.

Question 23

How is the proximal convoluted tubule adapted to reabsorb substances into the blood

Answer 23

By having epithelial cells that have:
- microvilli to provide a large surface area to reabsorb substances from the 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