chapter 15 p5

Question 1

Distal convoluted tubule: p1

Answer 1

Balancing the water needs of the body takes place in the distal convoluted tubule and the collecting duct.
These are the areas where the permeability of the walls of the tubules varies with the levels of ADH.
The cells lining the distal convoluted tubule also have many mitochondria so they are adapted to carry out active transport.

Question 2

Distal convoluted tubule: p2

Answer 2

If the body lacks salt, sodium ions will be actively pumped out of the distal convoluted tubule with chloride ions following down an electrochemical gradient.
Water can also leave the distal tubule, concentrating the urine, if the walls of the tubule are permeable in response to ADH.
The distal convoluted tubule also plays a role in balancing the pH of the blood.

Question 3

Study tip
Remember that the amount of reabsorption that occurs in the

Answer 3

proximal tubule is always the same - the fine-tuning of the water balance takes place further along the nephron.

Question 4

The collecting duct:
p1

Answer 4

The collecting duct passes down through the concentrated tissue fluid of the renal medulla (Figure 3 and 6).
This is the main site where the concentration and volume of the urine produced is determined.
Water moves out of the collecting duct by diffusion down a concentration gradient as it passes through the renal medulla.
As a result the urine becomes more concentrated.

Question 5

The collecting duct:
p2

Answer 5

The level of sodium ions in the surrounding fluid increases through the medulla from the cortex to the pelvis.
This means water can be removed from the collecting duct all the way along its length, producing very hypertonic urine when the body needs to conserve water.
The permeability of the collecting duct to water is controlled by the level of ADH, which determines how much or little water is reabsorbed.

Question 6

How long is your loop of Henle?

Answer 6

The ability of animals to produce very concentrated urine depends on several factors, one is the length of the loop of Henle.
As you have seen the loop of Henle develops the concentration gradient across the kidney medulla, meaning water can leave the collecting duct all the way through, concentrating the urine as it goes.
Fish have no loop of Henle and cannot produce urine that is more concentrated than their blood.
Desert animals tend to have lots of nephrons that have very long loops of Henle that travel deep into the medulla.

Question 7

The mammalian kidney has a vital excretory function -

Answer 7

it removes urea, the nitrogenous waste product of metabolism from the body.

Question 8

he kidney, however, also plays another important homeostatic role in the body - it is the main organ of

Answer 8

osmoregulation.

This involves controlling the water potential of the blood within very narrow boundaries, regardless of the activities of the body.
Eating a salty meal, drinking large volumes of liquid, exercising hard, running a fever, or visiting a very hot climate can all put osmotic stresses on the body.
It is very important to keep the water potential of the tissue fluid as stable as possible, because if water moves into or out of the cells by osmosis it can cause damage and even death.

Question 9

Osmoregulation:
p1

Answer 9

• Every day the body has to deal with many unpredictable events.
• The water potential of the blood has to be maintained regardless of the water and solutes taken in as you eat and drink, and the water and mineral salts lost by sweating, in defecation, and in the urine.
• Changing the concentration of the urine is crucial in this dynamic equilibrium.
• The amount of water lost in the urine is controlled by ADH in a negative feedback system.
• ADH is produced by the hypothalamus and secreted into the posterior pituitary gland, where it is stored.
• ADH increases the permeability of the distal convoluted tubule and, most importantly, the collecting duct to water.
• You will be concentrating on the effect of ADH on the collecting duct walls.

Question 10

typical water gains and losses in a human

Answer 10

Question 11

The mechanism of ADH action:

Answer 11

ADH is released from the pituitary gland and carried in the blood to the cells of the collecting duct where it has its effect.
The hormone does not cross the membrane of the tubule cells - it binds to receptors on the cell membrane and triggers the formation of cyclic AMP (CAMP) as a second messenger inside the cell.
A second messenger is a molecule which relays signals received at cell surface receptors to molecules inside the cell. The CAMP causes a cascade of events:

Question 12

The CAMP causes a cascade of events:

Answer 12

Vesicles in the cells lining the collecting duct fuse with the cell surface membranes on the side of the cell in contact with the tissue fluid of the medulla.

The membranes of these vesicles contain protein-based water channels (aquaporins) and when they are inserted into the cell surface membrane, they make it permeable to water.

This provides a route for water to move out of the tubule cells into the tissue fluid of the medulla and the blood capillaries by osmosis.
The more ADH that is released, the more water channels are inserted into the membranes of the tubule cells.
This makes it easy for more water to leave the tubules by diffusion, resulting in the formation of a small amount of very concentrated urine.
Water is returned to the capillaries, maintaining the water potential of the blood and therefore the tissue fluid of the body.

Question 13

When ADH levels fall…

Answer 13

the reverse happens:

Levels of cAMP fall, then the water channels are removed from the tubule cell membranes and enclosed in vesicles again.
The collecting duct becomes impermeable to water once more, so no water can leave.
This results in the production of large amounts of very dilute urine, and maintains the water potential of the blood and the tissue fluid.

Question 14

diagram of ADH in the hypothalamus

Answer 14

Question 15

Negative feedback control and ADH:

Answer 15

The permeability of the collecting ducts is controlled to match the water requirements of the body very closely.
This is brought about by a complex negative feedback system that involves osmoreceptors in the hypothalamus of the brain.
These osmoreceptors are sensitive to the concentration of inorganic ions in the blood and are linked to the release of ADH.

Question 16

diagram of negative feedback

Answer 16

Question 17

When water is in short supply:

Answer 17

When water is in short supply in the body, the concentration of inorganic ions in the blood rises and the water potential of the blood and tissue fluid becomes more negative.
This is detected by the osmoreceptors in the hypothalamus.
They send nerve impulses to the posterior pituitary which in turn releases stored ADH into the blood.
The ADH is picked up by receptors in the cells of the collecting duct and increases the permeability of the tubules to water.
Water leaves the filtrate in the tubules and passes into the blood in the surrounding capillary network.
A small volume of concentrated urine is produced

Question 18

An excess of water:

Answer 18

When large amounts of liquid are taken in, the blood becomes more dilute and its water potential becomes less negative.
Again, the change is detected by the osmoreceptors of the hypothalamus.
Nerve impulses to the posterior pituitary are reduced or stopped and so the release of ADH by the pituitary is inhibited.
Very little reabsorption of water can take place because the walls of the collecting duct remain impermeable to water.
In this way the concentration of the blood is maintained - and large amounts of dilute urine are produced

Question 19

ADH, water balance, and blood pressure: p1

Answer 19

The osmoreceptors in the hypothalamus are not the only sensory receptors that exert control over the release of ADH.
It is also stimulated or inhibited by changes in the blood pressure, detected by baroreceptors in the aortic and carotid arteries.
These baroreceptors are also involved in the control of the heart rate.
A rise in blood pressure can often be caused by a rise in blood volume.
The increase in pressure is detected by the baroreceptors and in turn they prevent the release of ADH.

Question 20

ADH, water balance, and blood pressure: p2

Answer 20

This increases the volume of water lost in the urine, reducing the blood volume and so the blood pressure falls.
If the blood pressure falls it can be a signal that the blood volume has fallen.
If the baroreceptors detect a fall in blood pressure there is an increase in the release of ADH from the pituitary, so the kidneys respond to reduce water loss from the body.
Water is returned to the blood and a small amount of concentrated urine is produced.
People with severe diarrhoea or more than 20% blood loss often produce little or no urine. In each case explain why.

Question 21

Urine samples and diagnostic tests:

Answer 21

Urine contains water, urea, mineral salts - and much more. It contains the breakdown products of a whole range of chemicals, including hormones and any toxins taken into the body.
If you are affected by one of a number of different diseases, new substances will show up in your urine.
The presence of glucose in the urine is a well-known symptom of type 1 and type 2 diabetes.
If you have muscle damage, large amounts of creatinine will show up in your urine.

Question 22

Urine and pregnancy testing: p1

Answer 22

The human embryo implants in the uterus, around six days after conception.
The site of the developing placenta then begins to produce a chemical called human chorionic gonadotrophin (hCG).
Some of this hormone is found in the blood and the urine of the mother.
Until the 1960s, the most reliable available pregnancy test was to inject the urine from a pregnant woman into an African clawed toad (Xenopus laevis).

Question 23

Urine and pregnancy testing: p2

Answer 23

If she was pregnant, the hCG triggered egg production in the toad within 8-12 hours of the injection.
It could not be used until the woman was several weeks pregnant.
Modern pregnancy tests still test for hCG in the urine, but they rely on monoclonal antibodies.
Some are so sensitive that pregnancy can be detected within hours of implantation.

Question 24

Making monoclonal antibodies:

Answer 24

Monoclonal antibodies are antibodies from a single clone of cells that are produced to target particular cells or chemicals in the body.
A mouse is injected with hCG so it makes the appropriate antibody.
The B-cells that make the required antibody are then removed from the spleen of the mouse and fused with a myeloma, a type of cancer cell which divides very rapidly.
This new fused cell is known as a hybridoma.
Each hybridoma reproduces rapidly, resulting in a clone of millions of living factories’ making the desired antibody.
These monoclonal antibodies are collected, purified and used in a variety of ways.

Question 25

The main stages in a pregnancy test are as follows:
p1

Answer 25

the wick is soaked in the first urine passed in the morning - this will have the highest levels of hCG.
The test contains mobile monoclonal antibodies that have very small coloured beads attached to them.
They will only bind to hCG. If the woman is pregnant the hCG in her urine binds to the mobile monoclonal antibodies and forms a hCG/antibody complex (complete with coloured bead).
The urine carries on along the test structure until it reaches a window.

Question 26

The main stages in a pregnancy test are as follows:
p2

Answer 26

Here there are immobilised monoclonal antibodies arranged in a line or a pattern such as a positive (+) sign that only bind to the hCG/antibody complex. If the woman is pregnant, a coloured line or pattern appears in the first window.
The urine continues up through the test to a second window.
Here there is usually a line of immobilised monoclonal antibodies that bind only to the mobile antibodies, regardless of whether they are bound to hCG or not. This coloured line forms regardless of whether the woman is pregnant - it simply indicates that the test is working.

If the woman is pregnant, two coloured patterns appear. If she is not pregnant, only one appears.

Question 27

diagram of The main stages in a pregnancy test are as follows:

Answer 27