Excretion and Kidney

Question 1

Describe the location of the kidneys in humans and draw and label a diagram to show the human urinary system.

Answer 1

Location:
- The kidneys are attached to the top of the abdominal cavity. They are surrounded by a thick, protective layer
of fat.

Urinary system:

• The kidneys are supplied with blood at arterial pressure by the renal arteries that branch off from the abdominal aorta.
• Blood is removed by the renal vein that drains into the inferior vena cava.
• From the kindey, the urine travels down the ureter into the bladder and then out via the urethra.

Question 2

Name the 3 main areas of the kidney and describe the role of each area.

Answer 2

• Cortex:
  Dark outerlayer where the blood is filtered. Has a very dense capillary network that carries the blood from the renal artery to the nephrons.
• Medulla:
  Lighter in colour,. Contains tubules of the nephrons that form the pyraminds of the kidney and the collecting ducts.
• Pelvis:
  Pelvis is latin for basin. The central chamber when urine collects before passing out down the ureter.

Question 3

Draw and label a diagram to show the internal structure of a kidney.

Answer 3

(pp. 421)
- Very outer layer is the capulse, then the cortex.
- The inner layer is the medulla. In the medulla are nephrons (which form kidney pyramids).
- The inner section of the medulla is the pelvis.
- The renal artery and vein run on top of the pelvis.

Question 4

Name the functional unit of the kidney.

Answer 4

Nephron.

Question 5

Draw and label a diagram of a nephron.

Answer 5

• Glomerulus is a bed of capillaries which is fed blood by the renal artery.
• The glomerulus is inside the Bowman’s capsule which leads to the proximal convoluted tubule.
• The descending limb of the loop of henle follows the PCT and then the ascending limb.
• Ascending limb of the loop of henle to the distal convoluted tubule, which then leads to the collection ducts.

The glomerulus, Bowman’s capsule, PCT and DCT are in the cortex. The loop of henle and the collecting duct are in the medulla.

Question 6

Define the term “Bowman’s capsule”.

Answer 6

A cup shaped structure that contains the golmerulus and is the site of ultra filtration in the kidney.

Question 7

Define the term “glomerulus”.

Answer 7

A bed of capillaries within the Bowman’s capusule. Is under very high pressure as this is where ultrafiltration happens.

Question 8

Define the term “proximal convoluted tubule”.

Answer 8

The first twisted section of the nephron after the Bowman’s capsule where many substances are reabsorbed into the blood.

Question 9

Define the term “loop of Henle”.

Answer 9

A long loop of tubule that creates a steep concentration gradient across the medulla.

Question 10

Define the term “distal convoluted tubule”.

Answer 10

The second twisted section of the nephron where the permeability of the walls varies in response to the ADH level of the blood.

Question 11

Define the term “collecting duct”.

Answer 11

The final part of the tubule that passes through the renal medulla and the place where hypertonic urine is produced if needed. The permeability of the wallls is affected by ADH levels and it is the main site of water balancing.

Question 12

Describe the functions of the kidney.

Answer 12

• Removing waste products from the blood.

- Osmoregulation

Question 13

Define the term “osmoregulation”.

Answer 13

The balancing and control of the water potential of blood.

Question 14

Label and annotate a photomicrograph of kidney tissue at low and high power to show key histological features.

Answer 14

x230
- The glomeruli are white blobs with lots of RBCs visible.
see pp.423

Question 15

Describe the role of the glomerulus.

Answer 15

• Ultrafilitration
• The glomerulus is supplied with blood by a wide afferent (incoming) arteriole from the renal artery. The blood leaves through a narrower efferent (outward) arteriole and as a result a high pressure is built up in the capillaries of the glomerulus.
• This pressure forces blood out through the capillary walls.
• The fluid then passes through the basement membrane (a network of collagen fibres and other proteins). This acts as a sieve which prevents anything greater than 69,000 relative molecular mass passing through.
• Most of the plasma contents pass through but the blood cells and many proteins are retained because of their large size.

Question 16

Describe the role of the Bowman’s capsule.

Answer 16

• Part of ultrafiltration.
• Bowman’s capsule has specialised cells called Podocytes that act as an additional filter. They have small extensions called pedicels which wrap around the capillaries forming slits that make sure any cells, platelets or plasma proteins that have got through the basement membrane do not pass through into the tubule.
• The filtrate which enters the capsule contains glucose, salt, urea and other substances at the same concentrations that they were in blood plasma.

Question 17

Define the term “ultrafiltration”.

Answer 17

The process by which the blood plasma is filtered through the walls of the Bowman’s capsule under pressure.

Question 18

Define the term “ultrafiltrate”.

Answer 18

The fluid that passes into the tubules of the nephron after having been filtered through the Bowman’s capsule under high pressure. It contains nothing greater than 69,000 relative molecular mass.

Question 19

Define the term “podocyte”.

Answer 19

Podocytes are cells in the Bowman’s capsule wrap around capillaries of the glomerulus, preventing any molecules larger than 69,000 relative molecular mass reaching the tubules.

Question 20

Draw and label a diagram to show the microstructure of the interface between the glomerulus and the Bowman’s capsule.

Answer 20

• To the left is the capillary endothelial cells with gaps between them.
• Next layer is the basement membrane.
• Next layer is the Bowman’s capsule with podocyte cells.
• Podocytes attach pedicels to capillaries.
  (see pp.424)

Question 21

Compare the composition of the blood and the ultrafiltrate.

Answer 21

The filtrate which enters the Bowman’s capsule contains glucose, salt, urea and other substances at the same concentrations that they were in blood plasma.

Cells, platelets and plasma proteins greater than 69,000 RMM are not present in the ultrafiltrate.

Question 22

Describe the role of the proximal convoluted tubule.

Answer 22

• First stage of selective reabsorption.
• Once the substances have been removed from the nephron they diffuse into the capillary network which surround the tubules down steep concentration gradients.
• These conc grads are maintained by a constant flow of blood through the capillaries.
• The filtrate that reaches the loop of henle at the end of PCT is isotonic ( at same conc) with the tissue fluid surrounding the tubules. Also isotonic with blood in the surrounding capillaries.
• At this stage, over 80% of the glomerular filtrate has been reabsorbed back into the blood.

Question 23

What substances are reabsorbed in selective reabsorption?

Answer 23

• All glucose
• All amino acids
• All hormones
• All vitamins
• All mineral ions

Question 24

What substances are reabsorbed in the proximal convoluted tubule?

Answer 24

• All glucose
• All amino acids
• All hormones
• All vitamins
• All mineral ions
• – These are reabsorbed by active transport.
• After these have been reabsorbed some water is reabsorbed by osmosis down the water potential gradient (WPG).

Question 25

Explain how the cells of the proximal convoluted tubule are adapted for their function.

Answer 25

• Cells lining the proximal convoluted tubule are covered with microvilli (increase SA for reabsorption).
• Also have many mitochondria to provide ATP for active transport.

Question 26

Describe the role of the loop of Henle. (Detail of counter-current multiplier.)

Answer 26

• Enables the kidney to produce urine more concentrated than their own blood.
• Different areas of the loop have different permeability to water. Central to its role.
• It acts as a counter-current multiplier, using energy to produce concentration gradients that result in the movement of substances such as water from area to another.
• Cells use ATP to transport ions using active transport and this produces a diffusion gradient in the medulla.
• The changes that take place in the loop of henle are dependent on high concs of sodium and chloride ions in the tissue fluid in the medulla. These high concs are the result of events in the ascending limb of the loop of henle.

Question 27

Describe what happens in the descending limb of the loop of henle.

Answer 27

• Leads on from PCT.
• Region where water moves out the filtrate down a concentration gradient.
• The upper part is impermeable to water but the lower part (which runs down into the medulla) is permeable to water.
• The concentration of sodium and chloride ions in the tissue fluid of the medulla is high (because a lot is diffusing out/ being actively transported out of the ascending limb of the loop of henle).
• The filtrate entering the descending limb of LOH is isotonic (at some conc) with the blood.
• As it travels down the limb, water passes out of the loop into the tissue fluid by osmosis down a WPG. It then moves down a WPG into the blood of the surrounding capillaries.
• Descending limb is not permeable to sodium and chloride ions and no active transport takes place. The fluid that reaches the bottom of the LOH is very concentrated and hypertonic (higher conc) to the blood in capillaries.

Question 28

Describe what happens in the ascending limb of the loop of henle.

Answer 28

• First section of the ascending limb of the LOH is very permeable to sodium and chloride ions and they diffuse out down a conc grad.
• In the second section, sodium and chloride ions are actively pumped out into the medulla tissue fluid against the conc grad.
• Causes a very high conc of sodium and chloride ions in the medulla tissue fluid (provides WPG for descending limb).
• The ascending limb is not permeable to water so water cannot follow the sodium and chloride ions by osmosis.
• This means the filtrate in the ascending limb becomes increasingly dilute (as sodium and chloride ions are moving out and water is staying in). Filtrate at the top of the ascending limb is hypotonic (lower conc) to the blood.

Question 29

Describe the roles of the distal convoluted tubule and explain how it can perform these roles.

Answer 29

• Osmoregulation happens here.
• The permeability to water of the DCT walls vary with the levels of ADH.
• Cells lining DCT also have many mitochondria so they can carry out active transport.
• 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.
• DCT also plays a role in balancing blood PH.

Question 30

Describe the role of the collecting duct and explain how it can perform this role.

Answer 30

• Collecting duct passes down through concentrated tissue fluid of the renal medulla.
• Main site where the concentration of the urine is determined.
• water moves out the collecting duct by osmosis down a WPG. Urine becomes more concentrated.
• 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.
• ADH will increase the amount of water reabsorbed (i.e. making more concentrated urine.) Less ADH will be released when the body is over-hydrated.

Question 31

Define the term “osmotic stress” and describe 5 factors that could put the body under osmotic stress.

Answer 31

Osmotic stress: a sudden change in the solute concentration around a cell, causing a rapid change in the movement of water across its cell membrane.

• Eating salty foods.
• Drinking large volumes of liquid.
• Exercising hard.
• Running a fever.
• Visiting a very hot climate.

Question 32

State 3 ways in which the body gains water and 4 ways in which water is lost from the body.

Answer 32

water gains:

• respiration in cells
• drinks
• food

water loss:

• exhaled
• faeces
• sweat
• urine

Question 33

State the main way (from the ways water can be gained by, or lost from, the body) the body can adjust its water balance.

Answer 33

• 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 the collecting duct.

Question 34

Describe what causes ADH to be released from the posterior pituitary gland.

Answer 34

• When osmoreceptors detect changes in the water potential of the blood.

Question 35

How does ADH start the process of osmoregulation?

Answer 35

• ADH is released from the pituitary gland into the blood and carried to the cells of the collecting duct.
• ADH binds to the receptors on the cell membranes of the tubule cells and triggers the formation of cyclic AMP (cAMP).

Question 36

Following the release of ADH, what is the role of cAMP in osmoregulation?

Answer 36

• cAMP is a second messenger molecule and it causes a cascade of events.
• 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-base 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 then the blood capillaries by osmosis.

Question 37

What is a second messenger molecule?

Answer 37

A molecule which relays signals received at cell surface receptors to molecules inside the cell. cAMP is a second messenger molecule.

Question 38

How does ADH effect the water retained or lost by the body?

Answer 38

The more ADH that is release, the more water channels are inserted into the membranes of the tubule cells. This makes the tubule cells more permeable to water so more diffuses out into the surrounding medulla tissue (it is retained in the body). This makes a small amount of very concentrated urine.

When ADH levels fall, the collecting duct and DCT are less permeable to water because there are less aquaporin channels. Therefore, large amounts of very dilute urine are produced.

Question 39

Draw a diagram to show the negative feedback loop that controls the water potential of the blood.

Answer 39

see pp.429

• The permeability of the collecting ducts is controlled to closely match the water requirements of the body.
• The negative feedback loop involves osmoreceptors in the hypothalamus that are sensitive to the concentration of inorganic ions in the blood and are linked to the release of ADH.

Question 40

How does the negative feedback loop respond when water is in short supply?

Answer 40

• When water is short supply, the concentration of inorganic ions in the blood is higher.
• This means the WP of blood and tissue fluid is more negative.
• The more negative WP is detected by osmoreceptors in the hypothalamus which send nerve impulses to the posterior pituitary gland.
• Posterior pituitary gland releases ADH into the blood.
• ADH binds the receptors on cell surface membranes of the tubule cells and increases their permeability to water.
• Water diffuses out of the filtrate in the tubules and passes into the medulla tissue fluid and then the capillary.
• A small volume of concentrated urine is produced.

Question 41

How does the negative feedback loop respond when water is in short supply?

Answer 41

• Blood becomes more dilute and its WP becomes less negative.
• Change detected by osmoreceptors.
• Nerve impluses to the posterior pituitary gland are reduced or stopped and so released of ADH is limited.
• Collecting ducts are less permeable to water because fewer aquaporins are inserted.
• Very little water is retained/ reabsorbed.
• Large amounts of dilute urine are produced.