Control of blood water potential (A-level only)

Question 1

Osmoregulation

Answer 1

Osmoregulation is the control of the water potential in the blood.

Question 2

The kidneys

Answer 2

Osmoregulation takes place in the kidneys.

The kidneys absorb more or less water according to the water potential.

Question 3

High water potential

Answer 3

If blood water potential is too high, more water must be lost by excretion to return the water potential to normal.

The blood reabsorbs less water from the kidneys.

The urine is more dilute and water potential in the blood decreases.

Question 4

Low water potential

Answer 4

If blood water potential is too low, less water must be lost by excretion to return the water potential to normal.

The blood reabsorbs more water from the kidneys.

The urine is more concentrated and water potential in the blood increases.

Question 5

Structures of a nephron

Answer 5

Bowman’s Capsule
Afferent and efferent arterioles
Proximal convoluted tubule (PCT)
Loop of Henle
Collecting duct

Question 6

Bowman’s capsule

Answer 6

The Bowman’s capsule is the beginning of the tubules that make up the nephron.

The capsule surrounds a network of capillaries.

This network is called the glomerulus.

The first step of filtration of the blood to form urine takes place in the Bowman’s capsule.

This step produces the glomerular filtrate.

Question 7

Afferent and efferent arterioles

Answer 7

Blood flows into the glomerulus through the afferent arteriole and out of the glomerulus through the efferent arteriole.

The afferent arteriole is much wider than the efferent arteriole.

This means that the blood pressure in the capillaries is very high.

Question 8

Proximal convoluted tubule (PCT)

Answer 8

The PCT is the site of selective reabsorption.

After the glomerular filtrate has been produced in the Bowman’s capsule, glucose and water are reabsorbed into the bloodstream through the PCT.

Question 9

Loop of henle

Answer 9

The loop of Henle produces a low water potential in the medulla of the kidney.

The loop of Henle consists of an ascending limb and a descending limb.

The ascending limb is impermeable to water.

The descending limb is permeable to water.

Question 10

Collecting duct

Answer 10

Water is reabsorbed into the blood through the collecting duct.

The amount of water that is absorbed depends on the water potential of the blood.

If blood water potential is low, more water is reabsorbed.

If blood water potential is high, less water is reabsorbed.
This is osmoregulation.

Question 11

Steps in producing glomerular filtrate:

Answer 11

Pressure filtration
Capillary endothelium
Basement membrane
Podocytes
Glomerular filtrate

Question 12

Pressure filtration

Answer 12

The branch of capillary that enters the glomerulus is much wider than the branch that exits the glomerulus.

This creates a high blood pressure in the glomerulus.

The high blood pressure causes the fluid and its solutes (e.g. glucose, amino acids) in the blood to be forced out of the capillary.

This is called pressure filtration.

Question 13

Capillary endothelium

Answer 13

The fluid flows through the pores in the capillary endothelium.

Question 14

Basement membrane

Answer 14

Then the smaller molecules filter through slit pores in the basement membrane.

This is a mesh of collagen fibres and glycoprotein.

Most proteins and all blood cells are too big to pass through the slit pores.

Question 15

Podocytes

Answer 15

The substances finally pass between the epithelial cells of the Bowman’s capsule.

The epithelial cells, called podocytes, have finger-like projections that the substances can flow between.

Question 16

Glomerular filtrate

Answer 16

The fluid that has filtered from the capillaries to the Bowman’s capsule is called the glomerular filtrate.

The filtrate contains:
Water.
Amino acids.
Urea.
Glucose.
Inorganic ions.

Question 17

Stages of selective reabsorption into the bloodstream in the proximal convoluted tubule (PCT):

Answer 17

Sodium-potassium pumps
Co-transporter proteins
Reabsorption of glucose and amino acids
Reabsorption of water

Question 18

Sodium-potassium pumps

Answer 18

Na+ ions are actively transported out of the PCT epithelial cells and into the blood by sodium-potassium pumps.

K+ ions are also transported into the epithelium.

Question 19

Co-transporter proteins

Answer 19

Active transport of Na+ ions causes the concentration of Na+ ions inside the epithelial cells to decrease.

Na+ ions in the filtrate diffuse into the epithelial cells (down their concentration gradient) through co-transporter proteins.

Co-transporter proteins allow glucose and amino acids to be transported into the epithelial cells along with the Na+ ions.

Question 20

Reabsorption of glucose and amino acids

Answer 20

As glucose and the amino acids are co-transported into the PCT epithelial cells, their concentration increases inside the cells.

Glucose and the amino acids diffuse down the concentration gradient into the blood.

Blood pressure is relatively high so that the substances in the blood are carried away quickly.

This maintains a steep concentration gradient.

Question 21

Reabsorption of water

Answer 21

The movement of Na+ ions, glucose and amino acids into the bloodstream causes the water potential to decrease in the blood and increase in the PCT.

Water in the PCT diffuses into the blood through osmosis.

Any substances that are not reabsorbed are excreted as waste.

Question 22

The loop of henle allows water to be reabsorbed in the collecting duct. The steps involved are:

Answer 22

Top of the ascending limb
Bottom of the ascending limb
The descending limb
Reabsorption of water
Osmoregulation

Question 23

Top of the ascending limb

Answer 23

Na+ ions are actively transported out of the top of the ascending limb into the surrounding tissue fluid in the medulla.

This causes the solute concentration of the medulla to increase and the water potential to decrease.

The ascending limb is impermeable to water.

This means water inside the tubule cannot diffuse out.

Question 24

Bottom of the ascending limb

Answer 24

Na+ ions diffuse out of the bottom of the ascending limb into the medulla.

This further increases the solute concentration of medulla.

Question 25

Descending limb

Answer 25

The descending limb is permeable to water.

This means that water inside the tubule can diffuse out because there is a lower water potential in the medulla.

The water is reabsorbed by the bloodstream.

Question 26

Reabsorption of water

Answer 26

The overall effect of the descending and ascending limb is to create a high solute concentration and low water potential in the tissue fluid surrounding the collecting duct.

This causes the water inside the collecting duct to diffuse into the surrounding tissue fluid by osmosis.

The water is then reabsorbed into the bloodstream.

Question 27

Osmoregulation

Answer 27

The volume of water that is reabsorbed into the bloodstream depends on the permeability of the collecting duct.

The permeability of the collecting duct varies according to the water potential of the blood.

If the water potential is high, the collecting duct is less permeable and less water is absorbed in the blood.

If the water potential is low, the collecting duct is more permeable and more water is absorbed in the blood.

Question 28

ADH

Answer 28

Antidiuretic hormone (ADH) controls osmoregulation.

ADH influences the permeability of the distal convoluted tubule and collecting duct.

This controls how much water is reabsorbed from the kidney into the blood.

Question 29

Hypothalamus

Answer 29

Osmoreceptors in the hypothalamus (a section of the brain) monitor blood water potential.

If the water potential increases, water diffuses into the osmoreceptor cells and the cells swell.

If the water potential decreases, water diffuses out of the osmoreceptor cells and the cells shrink.

Question 30

Posterior pituitary gland

Answer 30

When osmoreceptors shrink, this is detected by the posterior pituitary gland.

The posterior pituitary gland then releases ADH into the blood.

Question 31

ADH role in the reabsorption of water

Answer 31

ADH is a hormone that binds to receptors on the cell membrane of epithelial cells of the distal convoluted tubule (DCT) and the collecting duct.

When ADH binds, vesicles containing aquaporins fuse with the cell membrane.

Aquaporins are protein channels for water.

Aquaporins increase the permeability of the DCT and collecting duct.

This means that more water is reabsorbed into the blood by osmosis.

Question 32

Urine

Answer 32

If more ADH is in the bloodstream, more water is reabsorbed from the nephron into the blood.

So the urine is more concentrated.

If less ADH is in the bloodstream, less water is reabsorbed from the nephron into the blood.

So the urine is more diluted.