15.5 Structure & function of kidney

Question 1

What are the two important homeostatic roles of the kidneys in the body?

Answer 1

Excretion (of urea), and osmoregulation (balancing and controlling water potential in the blood)

Question 2

What are the three main areas of the structure of the kidney?

Answer 2

Cortex, medulla and pelvis

Question 3

What is the cortex?

Answer 3

The cortex is the dark outer layer. This is where the filtering of the blood takes place and has a dense capillary network carrying blood from renal artery to nephrons.

• contains : glomerulus, the Bowman’s capsule, proximal convoluted tubule, and distal convoluted tubule of nephron

Question 4

What is the medulla?

Answer 4

Medulla is lighter in colour as it contains the tubules of nephrons that form the pyramids of the kidney and collecting ducts.

• contains the loop of Henle and collecting duct of nephron

Question 5

What is the pelvis?

Answer 5

The pelvis of the kidney is the central chamber where the urine collects before passing out down the ureter.

Question 6

What are nephrons?

Answer 6

Nephrons are tiny tubules that make up the main functional structures of the kidneys.
In the nephrons the blood is filtered and filtered material is returned to the blood. Responsible for urine formation

Question 7

What are the main structures of the nephron?

Answer 7

Bowman’s capsule, Proximal convoluted tubule, Loop of Henle, Distal convoluted tubule, collecting duct

Question 8

What is the Bowman’s capsule?

Answer 8

It is a cup-shaped structure that contains the glomerulus, a tangle of capillaries. More blood goes into the glomerulus than leaves it due to the ultra filtration processes that take place.

Question 9

What is the proximal convoluted tubule?

Answer 9

The first coiled region of the tubule after the Bowman’s capsule, found in the cortex of the kidney. This is where many of the substances needed by the body are reabsorbed into the blood.

Question 10

What is the loop of Henle?

Answer 10

• A long loop of tubule that creates a region with a very high solute concentration in the tissue fluid deep in the kidney medulla.
• The descending limb runs from the medulla to bottom of loop.
• The ascending limb runs from the bottom to the cortex.

Question 11

What is the distal convoluted tubule?

Answer 11

• A second twisted tubule where the fine-tuning of the water balance of the body takes place.
• The permeability of the walls to water varies in response to the levels of the antidiuretic hormone (ADH) in the blood.
• The distal convoluted tubule also regulates ion balance and pH of the blood.

Question 12

What is the collecting duct?

Answer 12

• The urine passes down the collecting duct through the medulla to the pelvis.
• The walls are also sensitive to ADH.

Question 13

What is ultrafiltration?

Answer 13

The removal of nitrogenous waste of the blood resulting in the formation of tissue fluid in the capillary beds of the body.

Question 14

What is the afferent arteriole?

Answer 14

The arteriole that takes blood into each glomerulus.

Question 15

What is the efferent arteriole?

Answer 15

The arteriole that takes the filtered blood away from the glomerulus.
- it has a smaller diameter

Question 16

What is the basement membrane made up of?

Answer 16

• It is made up of a network of collagen fibres and other proteins that make up a second “sieve”.

Question 17

Describe the process of ultrafiltration.

Answer 17

1) The glomerulus is supplied with blood by a relatively wide afferent (incoming) arteriole from the renal artery.
2) The blood leaves through a narrower efferent (outward) arteriole and as a result there is considerable pressure in the capillaries of the glomerulus.
3) This forces liquid and small molecules in the blood out the capillary wall and in the Bowman’s capsule.
4) The liquid and small molecules pass through three layers to get into the Bowman’s capsule and enter the nephron tubule- the capillary endothelium, the basement membrane and the epithelium of the Bowman’s capsule.
5) Most of the plasma contents can pass through the basement membrane but the blood cells and many proteins are retained in the capillary because of their size.

Question 18

Which three layers does the blood pass through to get into the Bowman’s capsule?

Answer 18

1) The capillary endothelium
2) The BASEMENT MEMBRANE
3) Epithelium of the Bowman’s capsule

Question 19

What are podocytes?

Answer 19

Special cells on the wall of the Bowman’s capsule that act as an additional filter

Question 20

What are the extensions on podocytes called? And what do they do?

Answer 20

Pedicels.
- These extensions wrap around capillaries, forming slits that make sure any cells, platelets, or large plasma proteins that have managed to get through the epithelial cells and basement membrane do not get into the tubule itself.

Question 21

What does the filtrate usually consist of?

Answer 21

• Urea
• Water
• Glucose
• Salt

Question 22

Why is selective reabsorption needed?

Answer 22

• Ultrafiltration removes urea from the blood but it also removes a lot of water along with glucose, salt and other substances.
• Many of these substances are needed by the body e.g glucose is used for cellular respiration

Question 23

What substances are moved from the filtrate back into the blood in the PROXIMAL CONVOLUTED TUBULE?

Answer 23

• Glucose
• Amino acids
• Vitamins
• Hormones

Question 24

How are the substances transported from the filtrate back into the blood by the PROXIMAL CONVOLUTED TUBULE?

Answer 24

By active transport.

Question 25

How are sodium ions moved back by?

Answer 25

Active transport

Question 26

How are chloride ions and water transported back?

Answer 26

They follow passively down concentration gradients

Question 27

What are the adaptations of the proximal convoluted tubule?

Answer 27

• They are covered with microvilli, greatly increasing the surface area over which substances can be reabsorbed.
• They have many mitochondria to provide the ATP needed in active transport systems.

Question 28

What is the concentration of the filtrate when reaching the loop of Henle at the end of the proximal convoluted tubule COMPARED to the blood & tissue fluid?

Answer 28

Concentration of filtrate is ISOTONIC (same) compared to concentration in tissue fluid and blood

Question 29

How does the loop of Henle produce concentration gradients to move substances such as water from one area to another?

Answer 29

• The loop of Henle uses energy to produce concentration gradients that result in the movement of substances.
• It acts as a COUNTERCURRENT MULTIPLIER

Question 30

Describe the events that occur in the loop of Henle.

Answer 30

1) Near the top of the ascending limb, Na+ and Cl- ions are actively pumped out into the medulla. The ascending limb is impermeable to water, so the water stays inside the tubule. This creates a low water potential in the medulla, because theres a high concentration of urine.
2) Because there’s a lower water potential in the medulla than in the descending limb, water moves out of the descending limb into the medulla by osmosis. This makes the filtrate more concentrated (the ions can’t diffuse out- the descending limb isn’t permeable to them). The water in the medulla is reabsorbed into the blood through the capillary network.
3) Near the bottom of the ascending limb Na+ and Cl- ions diffuse out into the medulla, further lowering the water potential in the medulla. (The ascending limb is impermeable to water, so it stays in the tubule)
4) The first three stages massively increase the ion concentration in the medulla, which lowers the water potential. This is essential for the kidney to produce urine that is more concentrated than the blood.
5) The fluid left in the ascending limb becomes increasingly dilute, and by the time it reaches the top of the ascending limb it is hypotonic to the blood again.

Question 31

What is the role of the distal convoluted tubule?

Answer 31

It balances the water needs of the body

Question 32

What does the permeability of the walls of the distal convoluted tubule vary upon?

Answer 32

Levels of ADH

Question 33

How is the distal convoluted tubule adapted to its role?

Answer 33

The cells lining the distal convoluted tubule have many mitochondria so they are adapted to carry out active transport.

Question 34

What occurs in the DISTAL CONVOLUTED TUBULE?

Answer 34

• 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, when the walls of the tubule are permeable as a response to ADH.

Question 35

What occurs in the collecting duct?

Answer 35

• The collecting duct passes down through concentrated tissue fluid of the renal medulla.
• Water moves out of the collecting duct by diffusion down a concentration gradient as it passes through renal medulla, and so urine becomes more concentrated.