3.6.4.3 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 carries out osmoregulation?

Answer 2

The kidney and in particular, the nephron

Question 3

What is the shape of the nephron?

Answer 3

A narrow tube closed at one end with two twisted regions separated by a long paperclip loop

Question 4

What is the structure of the nephron?

Answer 4

Renal capsule
Proximal convoluted tubule
Loop of Henle
Distal convoluted tubule
Collecting duct

Question 5

What is the renal capsule?

Answer 5

The closed end at the start of the nephron
It surrounds a mass of blood capillaries known as the glomerulus
The inner layer is made of specialised cells called podocytes

Question 6

What is the proximal convoluted tubule?

Answer 6

A series of loops surrounded by blood capillaries

It’s walls are made of epithelial cells and microvilli

Question 7

What is the loop of Henle?

Answer 7

A long hairpin loop which extends from the cortex into the medulla of the kidney (further towards the centre) and back again
It is surrounded by blood capillaries

Question 8

What is the distal convoluted tubule?

Answer 8

A series of loops surrounded by blood capillaries
Its walls are made of epithelial cells
Surrounded by fewer capillaries than the proximal tubule

Question 9

What is the collecting duct?

Answer 9

A tube into which a number of distal convoluted tubules from a number of nephrons empty
Lined with epithelial cells and becomes increasingly wide as it empties

Question 10

What are the four blood vessels in the nephron? (No explanation)

Answer 10

Afferent arteriole
Glomerulus
Efferent arteriole
Blood capillaries

Question 11

What is the afferent arteriole?

Answer 11

A tiny vessel which 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

Question 12

What is the glomerulus?

Answer 12

A many branched knot of capillaries from which the fluid is forced out of the blood
The glomerulus capillaries recombine to form the efferent arteriole

Question 13

What is the efferent arteriole?

Answer 13

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

Question 14

What are the blood capillaries in the nephron?

Answer 14

A concentrated network of capillaries that surrounds the proximal convoluted tubule and forms where they reabsorb mineral salts, glucose and water
The capillaries merge together into venules, tiny veins that in turn merge into the renal vein

Question 15

What are the stages of osmoregulation of the nephron?

Answer 15

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

Question 16

What is the process of the formation of glomerular filtrate by ultrafiltration?

Answer 16

The walls of the glomerulus capillaries are made up of epithelial cells with pores between them
As the diameter of the afferent arteriole is greater than that of the efferent arteriole, there is a build up of hydrostatic pressure
Therefore water, glucose and mineral ions are squeezed out of the capillary to form the glomerular filtrate
Blood cells and proteins do not pass through as they are too large

Question 17

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

Answer 17

Connective tissue and epithelial cells
Hydrostatic pressure of the fluid in the renal capsule space
The low water potential of the blood

Question 18

What modifications cause filtrate to move out of the glomerulus?

Answer 18

The inner layer called podocytes have spaces between them which allows filtrate to pass between these cells rather than through them
The endothelium has spaces between its cells to allow the movement of fluid

Question 19

During ultrafiltration of the glomerulus, where does filtrate move between?

Answer 19

Filtrate passes from the blood into the renal capsule

Useful substances are small so are reabsorbed

Question 20

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

Answer 20

They have microvili to provide a large surface area
They have infoldings at their bases to to give a larger surface area to transfer reabsorbed substances into blood capillaries
They have a high density of mitochondria to provide ATP for active transport

Question 21

What is the process for the reabsorption of glucose and water by the proximal convoluted tubule?

Answer 21

Sodium ions are actively transported out of the proximal convoluted tubule cells into blood capillaries which lowers the sodium ion concentration
Sodium ions diffuse down a concentration gradient from the lumen of the proximal convoluted tubule into the epithelial lining cells through carrier proteins via facilitated diffusion
These carrier proteins are of specific types, each of which carries another molecule along with the sodium ions
The molecules which have co-transported into the cells of the proximal convoluted tubule diffuse into the blood. This causes most valuable molecules to be absorbed

Question 22

What is the purpose of the loop of Henle?

Answer 22

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

Question 23

What are the two regions of the loop of Henle?

Answer 23

Descending limb: narrow, thin walls which are highly permeable to water
Ascending limb: wider, thick walls which are impermeable to water

Question 24

What is the process of the loops of Henle?

Answer 24

Sodium ions actively transported out of the ascending limb using ATP provided by mitochondria
This causes a low water potential in the medulla region between the two limbs. No water moves into this region as the walls are impermeable
In the descending limb where the walls are permeable, water enters the interstatial space which then enters blood capillaries by osmosis and is carried away
The filtrate progressively loses water as it moves down the descending limb, reaching its lowest water potential at the tip
At the base of the ascending limb, sodium ions diffuse out of the filtrate and as it moves up they are also actively pumped out which raises the water potential
In the interstatial space between the ascending limb and the collecting duct, there is a water potential gradient with the lowest potential reaching into the medulla
The collecting duct is permeable to water and so as the filtrate moves down it, water passes out. This water passes by osmosis into the blood vessels that occupy this space, and is carried away
As water passes out of the filtrate, its water potential is lowered. However water is lowered in the intestinal space so water continues to move out by osmosis down the whole length of the collecting duct. The counter current multiplier ensures there is always a water potential gradient drawing water out of the tubule

Question 25

What is the structure of the loop of Henle?

Answer 25

Filtrate from the proximal convoluted tubule goes into the start of the loop, moving from the cortex into the medulla
Descending limb with walls permeable to water
Filtrate then moves into the ascending limb which is impermeable to water
Then the filtrate moves into the distal convoluted tubule
Then filtrate moves into the collecting duct with walls that vary in their permeability to water

Question 26

What allows water to move in the collecting duct?

Answer 26

Water moves out of the collecting duct through channel proteins that are specific to water

Question 27

How does ADH affect osmosis in the collecting duct?

Answer 27

Antidiuretic hormone can alter the number of these channels and so control water loss
By the time the filtrate, now called urine, leaves the collecting duct on its way to the bladder, it has lost most of its water and so has a lower water potential than the blood

Question 28

What is the distal convoluted tubule? What is its role?

Answer 28

The cells making up the distal convoluted tubule have microvilli and many mitochnodria that allow them to reabsorb material rapidly from the filtrate by active transport
The main 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
The wall therefore changes in permeability under the influence of various hormones

Question 29

Why is the loop of Henle called a counter-current multiplier?

Answer 29

The filtrate in the collecting duct with a lower water potential meets interstatial fluid that has a lower water potential
Therefore there is a constant flow of water into the interstatial fluid and then into the blood

Question 30

What causes a low water potential in the blood?

Answer 30

Too little water being consumed
Large amounts of sweating
Large amounts of ions being taken in e.g. sodium chloride

Question 31

What is the process for the body’s response to a fall in water potential?

Answer 31

Osmoreceptors in the hypothalamus detect the fall in water potential
When water potential is low, water is lost from these receptors which causes them to shrink
This change causes the hypothalamus to produce ADH
ADH passes into the pituitary gland then into the capillaries
ADH passes into the kidney through the blood where it increases the permeability to water of the cell-surface membranes of the collecting duct and distal convoluted tubule
Protein receptors on the cell surface membrane of the distal convoluted tubule and collecting duct bind to ADH, leading to the activation of an enzyme phosphorylase
The activation of phosphorylase causes vesicles to fuse with the cell-surface membrane
These vesicles contain pieces of plasma membrane that have numerous water channel proteins so fusing increases the number of water channels and increases the permeability

Question 32

What is the cortex?

Answer 32

An outer region of the kidney make up of renal capsules, convoluted tubules and blood vessels

Question 33

What is the medulla?

Answer 33

The inner region of the kidney made of loops of Henle, collecting ducts and blood vessels

Question 34

What is the simplified process of the loop of Henle?

Answer 34

Sodium moves out of the ascending limb
Low water potential in the region between the limbs
The walls are impermeable
In the descending limb, the walls are permeable so water moves into the interstitial space
The filtrate loses water as it moves down the descending limb
At the start of the ascending limb, sodium diffuses and, further up, is actively transported out
In the interstitial space, the lowest water potential reaches into the medulla
Collecting duct = permeable to water so water passes out of the filtrate going down the collecting duct and into the blood where it is carried away
The water potential of the filtrate is lowered but water is lowered in the interstitial space so the water potential gradient still causes movement. This is a countercurrent multiplier

Question 35

What is another name for the renal capsule?

Answer 35

The Bowman’s capsule