The mammalian kidney: function 5.2.6

Question 1

What are the nephrons responsible for?

Answer 1

The formation of urine

Question 2

What are the two stages of urine formation?

Answer 2

• Ultrafiltration
• Selective reabsorption

Question 3

Where does ultrafiltration occur?

Answer 3

Bowmans capsule

Question 4

Where does selective reabsorption occur?

Answer 4

Proximal convoluted tubule

Question 5

Explain ultrafiltration briefly

Answer 5

Small molecules (such as amino acids, water, glucose, urea, and salt ions) are filters out of the blood capillaries of the glomerulus and into the Bowmans capsule to form filtrate (known as glomerular filtrate)

Question 6

How is the blood in the glomerular capillaries separated?

Answer 6

The capillaries are separated from the lumen of the Bowmans capsule by two cell layers with a basement membrane in between them

Question 7

What are the two cell layers that separate the glomerular capillaries and the lumen of the Bowmans capsule?

Answer 7

First = endothelium of the capillary - which are perforated by thousands of tiny membrane-lines holes
Next layer is the basement membrane which is made up of a network of collagen and glycoproteins
Second = epithelium of the Bowmans capsule, which have many tiny finger like projections with gaps in between (like claw clips) called podocytes

Question 8

What is the function of podocytes

Answer 8

The holes allow substances dissolved in blood plasma (glomerular filtrate) to pass into the Bowmans capsule, such as amino acids, salt, glucose, water.

However red and white blood cells, and platelets remain in the blood as they are too large to pass through the holes in the podocytes

Question 9

What is the function of the basement membrane

Answer 9

It is a layer between the endothelium and the podocytes and acts as a filter because it stops large protein molecules from getting through

Question 10

What happens after the necessary reabsorption of filtrate matter (amino acids, salts, glucose etc.)

Answer 10

The filtrate eventually leaves the nephron and is now referred to as urine

Question 11

Where does the urine flow after the kidneys?

Answer 11

It flows out of the kidneys, along the ureters, and into the bladder where it is temporarily stored

Question 12

How is the high blood pressure in the glomerulus formed?

Answer 12

The afferent arteriole (entering) is wider in diameter than the efferent arteriole (leaving), resulting in high blood pressure in the glomerulus

Question 13

What is the result of the high blood pressure in the glomerulus?

Answer 13

It causes the smaller molecules being carried in the blood to be forced out of the capillaries of the glomerulus and into the Bowmans capsule, where they form what is known as the filtrate

Question 14

Why does ultrafiltration occur (what causes it)?

Answer 14

Due to the differences in water potential between the plasma in the glomerular capillaries and the filtrate in the Bowmans capsule

Question 15

How does solute concentration affect water potential in the glomerulus and Bowmans capsule

Answer 15

The basement membrane allows most solutes within the blood plasma to filter into the Bowmans capsule however not large protein molecules. As a result solute concentration is higher (water potential lower) in the blood plasma of the glomerulus than in the filtrate of the Bowmans capsule.

Question 16

What is the result of the high solute concentration In the glomerulus?

Answer 16

Water moves down the water potential gradient from the Bowmans capsule into the blood plasma in the glomerular capillaries

Question 17

What is the effect of pressure gradient outweighing solute concentration?

Answer 17

The water potential of the blood plasma in the glomerulus is higher than the water potential of the filtrate in the Bowmans capsule therefore there is an overall movement of water down the water potential gradient from the blood into the Bowmans capsule

Question 18

Where does selective reabsorption occur?

Answer 18

In the proximal convoluted tubule

Question 18

Why does selective reabsorption occur?

Answer 18

As many of the substances that end up in the glomerular filtrate actually need to be kept by the body

Question 19

Why is it called selective reabsorption

Answer 19

As only certain substances are reabsorbed

Question 19

How do microvilli aid reabsorption

Answer 19

Increase the surface area for reabsorption

Question 19

What is the lining of the proximal convoluted tubule composed of?

Answer 19

A single layer of epithelial cells such as:
- Microvilli - increased surface area for reabsorption
- Co-transporter proteins - transports a specific solute such as glucose or amino acids across the plasma membrane
- High number of mitochondria - provides ATP for Na/K pumps in the basal membranes
- Tightly packed cells - means no fluid can pass between the cells and all substances must pass through the cells

Question 19

How do many co-transporter proteins in the luminal membrane aid reabsorption

Answer 19

As they co-transport specific solutes such as glucose and amino acids across the membrane

Question 19

How does many mitochondria aid reabsorption

Answer 19

Provides ATP for the Na/K pumps in the basal membranes of cells

Question 20

How does cells being tightly packed together aid reabsorption?

Answer 20

Means that no fluid can pass between the cells and all substances reabsorbed must pass through the cells

Question 20

State the process of selective reabsorption

Answer 20

• Blood capillaries are very close to PCT, with the basal membrane being closest to capillaries
• Na/K pumps move sodium ions out of epithelial cells
• This lowers the concentration of sodium ions in the epithelial cells
• Sodium ions move back into epithelial cells from PCT lumen
• These sodium ions do not diffuse freely yet they must pass through co transporter proteins in the membrane
• Each co-transporter protein co transports a sodium ion and another solute from the filtrate
• Once inside the epithelial cells these solutes diffuse down their concentration gradients, through the basal membranes, into the blood

Question 21

What are the molecules reabsorbed from the PCT during selective reabsorption?

Answer 21

• All glucose in the filtrate is reabsorbed into blood
• Amino acids, vitamins, and inorganic ions
• The movement of these solutes into the blood raises the water potential of the filtrate and so water moves into the blood by osmosis
• Significant amount of urea too as the concentration of urea in the filtrate is higher than in the capillaries

Question 22

Where are necessary salts reabsorbed back into the blood?

Answer 22

Loop of Henle

Question 23

What happens as salts are reabsorbed back into the blood?

Answer 23

Water follows by osmosis

Question 24

Where is water also reabsorbed into the blood?

Answer 24

From the collecting duct in different amounts depending on how much water the body needs at that time

Question 25

What occurs in the ascending limb of the loop of henle?

Answer 25

The ascending limb is impermeable to water
Na and Cl are pumped out by active transport into the medulla
Creates low water potential in the medulla

Question 26

What occurs in the descending limb of the loop of henle?

Answer 26

It is permeable to water yet impermeable to solutes - and so water moves down the water potential gradient (created by ascending limb) into the medulla by osmosis and is reabsorbed by the capillary network

Question 27

What occurs in the thin ascending limb of the loop of henle?

Answer 27

Na and Cl are able to diffuse out into the medulla, creating the water potential gradient

Question 28

Why is there a low water potential in the medulla?

Answer 28

Due to the high concentration of ions - such as sodium and chloride

Question 29

Describe the counter current mechanism in the loop of henle

Answer 29

Solute concentration at any part of the ascending tubule is lower than that in the descending limb which causes a build up of salt concentration in the surrounding tissues

Question 30

What occurs in the distal convoluted tubule?

Answer 30

Reabsorption

Question 31

What is the permeability of the walls of the nephron controlled by?

Answer 31

ADH (anti-diuretic hormone)

Question 32

What occurs in the DCT if the body lacks salt, and why does this occur?

Answer 32

Na and Cl are actively transported out if body lacks salt.
This causes more ions to follow and also water to move out by osmosis

Question 33

How does the glomerulus generate hydrostatic pressure?

Answer 33

As the lumen of afferent arteriole is wider than efferent arteriole