Explain how changes in the

concentrations of substances in the blood

are brought about by the proximal

convoluted tubule.

Selective reabsorption occurs from the proximal

convoluted tubule (PCT), where 85% of the filtrate is

reabsorbed. All of the glucose, amino acids and

some salts are reabsorbed along with some of the

waste. The membrane contains co-transporter

proteins which transport glucose and amino acids by

facilitated diffusion. Microvilli on the cell surface

membrane increase the surface area for

reabsorption.

Fill in the spaces…

ADH is a hormone that is produced by specialised nerve cells known as osmoreceptor hormones. These cells detect changes in the water potential of the blood flowing through the hypothalamus. If the water potential of the blood is too low, then ADH is released. ADH is not secreted immediately into the blood but passes along the axon of specialised nerve cells to the posterior pituitary gland, where it is released into the blood. ADH acts on the cells of the collecting duct. The ADH molecule attaches to receptors on the membrane of these cells and causes protein channels known as aquaporins to insert themselves into the membrane. Water passes through these channels by osmosis and a smaller concentration of more concentrated urine is produced.

Suggest where ADH is removed from

the blood and what happens to it.

As a protein: ADH undergoes deamination

(removing an amine group from amino acids to form

ammonia), and the ornithine (ammonia is converted

to urea) in the hepatocytes.

As a small molecule, ADH is dealt with in the kidney.

It is ultrafiltrated from the blood because it is small. It

is not reabsorbed but excreted.

Why will a diet high in protein result in a

high concentration of urea in urine?

A high intake of protein will result in a high level of

amino acids. These amino acids can’t be stored so

they are deaminated, converting them to ammonia.

The ammonia enters the ornithine cycle to be

converted to urea. The increased blood plasma

concentration of urea leads to more urea in urine.

What is the effect on the composition

of the blood if the kidneys fail?

If the kidneys can’t filter, substances remain in the

blood. So, there will be high levels of urea and

water.

Explain the need for close matching

of the donated kidney to the

recipient.

If the kidney isn’t closely matched, it will be

recognised as foreign which causes rejection by the

immune system. The use of immuno-suppressant

drugs will help to reduce the risk of rejection.

Also, a specific size could be needed (for example, if

the recipient is a small child)

Explain how a pregnancy

test indicates pregnancy.

The stick (which you urinate on), tests for hCG. hCG is

a small hormone so it can pass from the blood into the

filtrate (Bowmans Capsule). There are already

monoclonal antibodies on the stick, which are marked

with blue dye. The hormone binds to the

complementary antibody, and a line becomes visible.

The first line is for ‘control’/comparison, and the second

line indicates pregnancy.

Describe the features of the glomerulus and the

Bowmans Capsule which allow them to perform

their function effectively.

Their function is ultrafiltration.

The afferent arteriole is wider in diameter than the efferent arteriole.

There is a higher hydrostatic pressure in the glomerulus.

The endothelium has gaps to allow/prevent the passage of substances (e.g. fenestrations prevent red blood cells exiting).

The basement membrane only allows small molecules to pass through.

Podocytes of the Bowmans Capsule have finger like projections which ensure gaps to allow the passage of substances.

How does the structure of an aquaporin

prevent the passage of ions?

The ions are too long to pass through the channel.

The shape isn’t compatible.

The positive charge in the channel repel the

positively charged ions.

Explain the role of the loop of Henle in the

production of urine.

The loop of Henle causes a decrease in the water

potential in the medulla. In the ascending limb,

sodium and chlorine ions are actively transported

outwards, but the descending limb is permeable to

water so water is removed from here. As a result,

the water potential of the tissues surrounding the

collecting duct is lower than fluid inside it, so water is

removed from urine.

Describe the sequence of events that

results in the water potential of the blood

plasma returning to normal.

The osmoreceptors in the hypothalamus detect the low water

potential in the blood. So, ADH is produced by the

hypothalamus. The ADH passes to and from the pituitary

posterior gland before being released into the blood. ADH then

acts on the collecting duct and binds to receptors in the

plasma membrane of collecting duct cells. This activates the

phosphorylase enzyme and causes vesicles with aquaporins

which bind with the plasma membrane. This increases the

permeability to water, so water is reabsorbed by osmosis. We

then begin to feel thirsty, so the water potential of the blood

rises, switching off the release of ADH.

Explain how ultrafiltration

takes place in the kidney.

Blood flows into the glomerulus from the afferent arteriole,

which is wider in diameter than the efferent arteriole. This

difference in diameter ensures the blood in the capillaries is

under increased pressure. The high pressure forces liquid and

small molecules out into the Bowmans capsule. The

endothelium of the capillaries has small gaps to ensure

substances can pass through. The basement membrane is

made of glycoproteins which ensures large proteins cannot

pass through – it is a selective barrier. Epithelial cells of the

Bowmans capsule called podocytes have finger like

projections which ensure fluid can pass into the lumen of the

Bowmans capsule.

Describe the homeostatic mechanisms tht

would normally prevent glucose

appearing in the urine.

It The rise in plasma glucose concentration is

detected by cells in the pancreas. The beta cells in

the islets of Langerhans consequently produce more

insulin. The insulin is secreted into the blood, so the

cells take up more glucose. The glucose is

converted to glycogen. This increases the rate of the

use of glucose in respiration, but the glucose

concentration remains below threshold value in the

glomerular filtrate. It is reabsorbed at the PCT.