Kidneys

Question 1

What is osmoregulation?

Answer 1

The homeostatic control of the water potential of the blood.
An optimum concentration of water and salts is maintained to ensure a constant water potential of blood plasma and tissue fluid.

Question 2

What is the structure of the kidney?

Answer 2

Fibrous capsule - outer membrane for protection.
Cortex - lighter coloured region made up of renal capsules, convoluted tubules and blood vessels.
Medulla - darker coloured inner region made up of loops of Henle, collecting ducts and blood vessels.
Renal pelvis - cavity that collects urine into the ureter.
Ureter - tube carries urine to bladder.
Renal artery - supplies blood to kidney via aorta.
Renal vein - returns blood via vena cava.

Question 3

What is the nephron?

Answer 3

A narrow tube up to 14mm long, closed at one end, with two twisted regions separated by a long hairpin loop.
Each nephron is made up of the renal (bowman’s) capsule, proximal convoluted tubule loop of Henle, Distal convoluted tubule, collecting duct.

Question 4

What is the renal capsule?

Answer 4

The closed end at the start of the nephron.
Cup-shaped and surrounds a mass of blood capillaries - the glomerulus.
The inner layer is made up of podocytes.

Question 5

What is the PCT?

Answer 5

A series of loops surrounded by blood capillaries.
Its walls are made of epithelial cells which have microvilli.
The DCT is the same, but has less capillaries.

Question 6

What is the loop of Henle?

Answer 6

A long hairpin loop that extends from the cortex into the medulla of the kidney and back again.
It is surrounded by blood capillaries.

Question 7

What is the collecting duct?

Answer 7

A tube into which a number of DCTs 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 8

What are the blood vessels of the nephron?

Answer 8

The afferent arteriole is a tiny vessel that arises from the renal arteriole and supplies the nephron with blood.
It enters the renal capsule of the nephron to form the glomerulus.
This is a branched knot of capillaries from which fluid is forced out of the blood.
They recombine to form the efferent arteriole - a tiny vessel that leaves the renal capsule.

Question 9

What are the efferent arterioles?

Answer 9

It has smaller diameter than the afferent so causes an increase in blood pressure in the glomerulus. They carry blood away from the renal capsule and later branches to form blood capillaries.
This is a concentrated network of capillaries that surrounds the PCT, the loop of Henle, and DCT, from where they reabsorb mineral salts, glucose and water.
The capillaries merge together into venules, that merge into the renal vein.

Question 10

How does the nephron cause osmoregulation?

Answer 10

The formation of glomerular filtrate by ultrafiltration.
Reabsorption of glucose by the PCT.
Maintenance of gradient of sodium ions in the medulla by the loop of henle.
Reabsorption of water by the DCT and collecting ducts.

Question 11

How does ultrafiltration form glomerular filtrate?

Answer 11

The diameter of the afferent arteriole is greater than the efferent arteriole,so there is a build up of hydrostatic pressure within the glomerulus.
So glucose, water and mineral ions are squeezed out of the capillary to form the glomerular filtrate.
Blood cells and proteins cannot pass as they are too large.

Question 12

What is the movement of filtrate out of the glomerulus resisted by?

Answer 12

By the: capillary epithelial cells.
Connective tissue and epithelial cells of the blood capillary.
Epithelial cells of the renal capsule.
The hydrostatic pressure of the fluid in the renal capsule space.
The low water potential of the blood in the glomerulus.

Question 13

What are the modifications to reduce the barrier to the flow of filtrate through the glomerular capillaries?

Answer 13

The inner layer of the renal capusle is made up of podocytes.
These cells have spaces between them which allows filtrate to pass beneath them and through gaps between their branches.
The endothelium of the glomerular capillaries has spaces 100nm wide between its cells. Fluid can again pass between, not through, the cells.

Question 14

What is the reabsorption of glucose by the PVT?

Answer 14

In the proximal convoluted tubule nearly 85% of filtrate is reabsorbed back into the blood.
Ultrafiltration removes small molecules.
Some, such as urea, are wastes, but most are useful so are reabsorbed.

Question 15

How are the epithelial cells the proximal convoluted tubules have adapted to reabsorb substances into the blood?

Answer 15

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.

Question 16

What is the process of reabsorption of glucose and water by the PVT?

Answer 16

Sodium ions are actively transported out of the cells lining the PVT into blood capillaries which carry them away. So concentration is lowered.
Sodium ions diffuse down a concentration gradient from the lumen of the PVT into the epithelial lining cells through special carrier proteins by facilitated diffusion.
The carrier proteins each carry another molecule - co-transport.
The molecules co-transported into the cells of the PVT then diffuse into the blood.

Question 17

What are the figures of reabsorption of water?

Answer 17

About 180dm^3 of water enters the nephrons each day.
Only 1dm^3 of this leaves the body as urine.
85% of reabsorption occurs in the PVT, the remainder is reabsorbed form the collecting duct as a result of the functioning of the loop of Henle.

Question 18

What is the function of the loop of henle?

Answer 18

It is responsible for water being reabsorbed form the collecting duct, thereby concentrating the urine so that is has a lower water potential thann the blood.
The concentration of the urine produced is directly related to the length of the loop of henle.

Question 19

What are the regions of the loop of henle?

Answer 19

The descending limb, which is narrow, with thin walls highly permeable to water.
The ascending limb, which is wider, with thick walls impermeable to water.

Question 20

How is the loop of henle a counter-current multiplier?

Answer 20

Sodium ions are actively transported out of the ascending limb.
This creates a low water potential in the region of the medulla between the limbs, water does not escape back due to impermeability of ascending walls.
Water instead passes out of filtrate through the permeable walls of the descending limb, by osmosis, into the space. Water is carried away by blood capillaries.
The filtrate moves down the descending limb as it lses water, lowering its water potential.

Question 21

How is the loop of henle a counter-current multiplier - collecting duct?

Answer 21

At the base of the ascending limb, sodium ions diffuse out of the filtrate, and actively pumped out as it moves up, increasing its water potential.
In the interstitial space between the ascending limb and collecting duct there is a high water potential in the cortex, and increasingly lower water potential the further into the medulla.
Water passes out of the collecting duct into the blood by osmosis as the filtrate moves down it.
The water potential is lowered in the filtrate, but also in the interstitial space, so water continues moving out along the whole length.

Question 22

What happens to the water out of the collecting duct?

Answer 22

It passes out through channel proteins specific to water (aquaporins).
ADH can alter the number of these channels and so control water loss.
By the time filtrate - urine, leaves the collecting duct, it has little water, so it has a lower water potential than the blood.

Question 23

What does the distal convoluted tubule do?

Answer 23

The cells of the walls have microvilli and mitochondria that allow them to reabsorb material rapidly from the filtrate by active transport.
Its role is to 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.
It does this by hormones altering its permeability.

Question 24

How does a rise in solute concentration occur?

Answer 24

Too little water being consumed.
Much sweating occurring.
Large amounts of ions being taken in.

Question 25

How does the body respond to the fall in water potential in cells?

Answer 25

Water loss from the osmoreceptor cells causes it to shrink, which causes the hypothalamus to produce antidiuretic hormone (ADH).
ADH passes to the pituitary gland, to the blood, to the kidney, where it increases the permeability to water of the walls of the DVT and collecting duct.
Specific protein receptors bind to ADH, activating the phosphorylase enzyme within the cell.

Question 26

How does the body respond to the fall in water potential in cells - phosphorylase?

Answer 26

Phosphorylase causes vesicles within the cell to move and fuse with its cell membrane, with pieces of plasma membrane with aquaporins.
Urea passes out of the collecting duct, further lowering water potential of the fluid around the duct.
So more water leaves the collecting duct and re-enters the blood.
This prevents the water potential from lowering, but doesn’t actually increases it.

Question 27

How else do osmoreceptors regulate water potential?

Answer 27

The osmoreceptors also send nerve impulses to the thirst centre of the brain, the encourage the individual to drink more water.
The osmoreceptors detect the rise in water potential and send fewer impulses to the pituitary gland.
This reduces the release of ADH, so the permeability of collecting ducts reverts to its former state.

Question 28

How does a fall in solute concentration occur?

Answer 28

Large volumes of water being consumed.
Salts used in metabolism or excreted not being replaced in the diet.

Question 29

How does the body respond to the rise in water potential?

Answer 29

Osmoreceptors detect the increase and increase the frequency of nerve impulses to the pituitary gland to reduce its release of ADH.
Less ADH, decreases the permeability of collecting ducts to water and urea.
Less water is reabsorbed into the blood.
More dilute urine is produced and the blood water potential falls.
Once returned to normal, the osmoreceptors causes tje pituitary gland to raises its ADH back to normal levels.