The Kidneys and Osmoregulation (Chapter 14)

Question 1

Describe the structure of the 4 tubes in the kidney

Answer 1

1) each kidney receives blood from a renal artery
2) each kidney returns blood via a renal vein
3) the ureter (narrow tube) carries urine from kidney to the bladder
4) from the bladder, the ureter carries urine to the outside of the body

Question 2

Describe the structure of the kidney

Answer 2

• The whole kidney is covered by a fairly tough capsule, beneath which lies the cortex
• The central area is made up of the medulla
• Where the ureter joins, there is an area called the pelvis

Question 3

What are the 3 main areas of the kidney?

Answer 3

1) cortex
2) medulla
3) pelvis

Question 4

What is the kidney made up of?

Answer 4

1000s of tiny tubes (nephrons) and many blood vessels

Question 5

Describe the structure of the nephron

Answer 5

• One end of the tube forms a Bowman’s capsule (cup-shaped structure), which surrounds a glomerulus (tight network of capillaries) in the cortex
• From the capsule, tube then runs towards the centre of the kidney, first forming the PCT (a twisted region) and then the loop of Henle
• The tubule then runs back upwards the cortex, where it forms the DCT (another twisted region) before finally joining a collecting duct that leads down through the medulla and into the pelvis

Question 6

Describe the two blood vessels closely associated with the nephrons

Answer 6

• Each glomerulus is supplied with blood by an afferent arteriole (a branch of the renal artery)
• The capillaries of the glomerulus rejoin to form an efferent arteriole which leads off the form a network of capillaries running closely alongside the rest of the nephron
• Blood from these capillaries flows into a branch of the renal vein

Question 7

Describe the 2-stage process in which the kidney makes urine

Answer 7

1) ultrafiltration - involves filtering small molecules including urea, out of the blood and into the Bowman’s capsule
2) selective reabsorption - involves taking back any useful molecules from the fluid in the nephron as it flows along

Question 8

Where do molecules from the Bowman’s capsule flow?

Answer 8

Along the nephron towards the ureter

Question 9

How is the blood in the glomerular capillaries separated from the lumen of the Bowman’s capsule?

Answer 9

By 2 cell layers and a basement membrane

Question 10

Describe the structure of the Bowman’s capsule

Answer 10

1) first cell layer = the endothelium (lining) of the capillary, which has far more gaps than in any other capillaries
2) basement membrane - made up of a network of collagen and glycoproteins
3) second cell layer (inner lining the Bowman’s capsule) - formed from podocytes (epithelial cells) which have many tiny finer-like projections with gaps between them

Question 11

Why are the holes in the capillary endothelium and the gaps between the podocytes quite large?

Answer 11

To make it easy for substances dissolved in the blood plasma to get through from the blood into the Bowman’s capsule

Question 12

What is the function of the basement membrane?

Answer 12

• To stop large protein molecules and blood cells from getting through
• ∴ the basement membrane acts as a filter

Question 13

How are the concentrations of substances in the glomerular filtrate and blood plasma different?

Answer 13

They are almost identical, but almost no plasma proteins in glomerular filtrate

Question 14

What is the glomerular filtration rate?

Answer 14

The rate at which the fluid filters from the blood in the glomerular capillaries into the Bowman’s capsule

Question 15

What is the glomerular filtration rate determined by?

Answer 15

• The differences in water potential between the blood plasma in the glomerular capillaries and the filtrate in the Bowman’s capsule
• Water potential is lowered by the presence of solutes and raised by high pressures

Question 16

Why is the water potential in the blood plasma higher than the water potential of the contents of the Bowman’s capsule?

Answer 16

Inside the glomerular capillaries, the blood pressure is relatively high, bc the diameter of the afferent arteriole is wider than that of the efferent arteriole causing a head of pressure inside the glomerulus

Question 17

Why is the concentration of solutes in the blood plasma higher than the concentration of solutes in the filtrate in the Bowman’s capsule?

Answer 17

• While most of the contents of the blood plasma filter through the basement membrane and into the capsule, the plasma protein molecules are too big to get through and ∴ stay in the blood
• This difference in solute concentration tends to make the water potential in the blood capillaries lower than that of the filtrate in the Bowman’s capsule

Question 18

Why does water move down the water potential gradient from the blood into the Bowman’s capsule?

Answer 18

Overall, the effect of differences in pressure outweighs the effect of differences in solute concentration and ∴ the water potential of the glomerulus is higher than the water potential of filtrate in the capsule

Question 19

Why are substances reabsorbed into the blood as the fluid passes along the nephron?

Answer 19

Bc many of the substances in the glomerular filtrate need to be kept in the body

Question 20

Why is the process called selective reabsorption?

Answer 20

Bc only certain substances are reabsorbed

Question 21

Where does most of the reabsorption take place?

Answer 21

In the proximal convoluted tubule

Question 22

What is the lining of the PCT made of?

Answer 22

A single layer of cuboidal epithelial cells

Question 23

What do the cuboidal epithelial cells of the PCT have to make them adapted for their function of reabsorption?

Answer 23

1) microvilli - to increase the surface area of the inner surface facing the lumen
2) tight junctions - to hold adjacent cells together so that the fluid cannot pass between the cells
3) many mitochondria - to provide energy for the Na-K pump proteins in the outer membranes of the cells
4) co-transporter proteins in the membrane facing the lumen
5) blood capillaries very close to the outer surface of the PCT

Question 24

Describe the blood in the capillaries close to the outer surface of the PCT

Answer 24

It has come directly from the glomerulus ∴ it has much less plasma in it than usual and has lost much of its water and many of the ions/other small solutes

Question 25

What are the basal membranes of the cells lining the PCT and what is a characteristic of them?

Answer 25

• Those nearest the blood capillaries

- They are folded to give a large surface area for carrier proteins

Question 26

What 2 kind of carrier proteins are in the basal membranes?

Answer 26

1) Na-K pump

2) co-transporter proteins

Question 27

How does the Na-K pump allow the movement of glucose and other solutes into the cells of the PCT from the lumen?

Answer 27

1) Na-K pumps in the basal membranes move Na+ ions out of the cells, into the blood and these are carried away in the blood
2) this lowers the [Na+] inside the cell, so that Na+ diffuse into the cell, down their concentration gradients from the fluid in the lumen of the PCT via co-transporter proteins
3) the passive movement of Na+ into the cell provides the energy to move glucose molecules against their concentration gradient into the cell

Question 28

What is the movement of glucose into the cells of PCT using Na+ an example of?

Answer 28

Indirect/secondary active transport, because the energy (as ATP) is used in the pumping of Na+, not in the moving of glucose

Question 29

How does glucose move from the cells of the PCT into the blood?

Answer 29

Once inside the cell, glucose diffuses downs its concentration gradient, through a co-transporter protein in the basal membrane, into the blood

Question 30

How much of the glucose is transported out of the PCT and into the blood?

Answer 30

All of it - normally no glucose is left in the filtrate ∴ there is no glucose in the urine

Question 31

What other solutes/substances are reabsorbed in the PCT?

Answer 31

Amino acids, vitamins, urea and many Na+ and Cl- ions

Question 32

How is water also reabsorbed into the blood in the PCT

Answer 32

1) the removal of the solutes from the filtrate in the PCT greatly increases its water potential and the movement of solutes into the blood decreases the water potential of the blood
2) ∴ a water potential gradient exists between the filtrate and blood
3) water moves down this gradient through the cells and into blood
4) the water and reabsorbed solutes are carried away, back into circulation
5) ∴ the reabsorption of solutes helps further uptake of water from the lumen of the PCT

Question 33

What is the order of movement of solutes/water in the PCT?

Answer 33

PCT lumen –> PCT cell –> basal membrane –> blood plasma

Question 34

Why is quite a lot of urea also reabsorbed in the PCT?

Answer 34

• Urea is a small molecule which passes easily through cell membranes ∴ the concentration of urea in the filtrate us considerably higher than that in the capillaries
• ∴ it diffuses passively through the cells of the PCT and into the blood

Question 35

What 2 substances are not reabsorbed in the PCT?

Answer 35

Uric acid and creatinine

Question 36

Why is creatinine not reabsorbed?

Answer 36

Creatinine is actively secreted by the cells of the PCT into its lumen

Question 37

What does the reabsorption of so much water from the filtrate in the PCT do to the volume of the liquid remaining?

Answer 37

It greatly reduces it - only 64% of fluid entering PCT passes onto the loop of Henle

Question 38

What is the function of the long loops of Henle that dip down into the medulla?

Answer 38

• To create a very high concentration of Na+ and Cl- in the tissue fluid of the medulla
• This enables a lot of water to be reabsorbed from the collecting duct as it flows through the medulla, allowing the production of a very concentrated urine
• ∴ water is conserved by the body, rather than lost in urine, helping to prevent dehydration

Question 39

What are the 2 parts of the loop of Henle and what is the difference between them?

Answer 39

1) descending limb - permeable to water and Na+/Cl-

2) ascending limb - impermeable to water

Question 40

Explain how water is reabsorbed in the loop of Henle

Answer 40

1) cells that lie the ascending limb actively transport Na+ and Cl- out of the fluid in the loop, into the tissue fluid, decreasing the water potential of the tissue fluid and increasing the water potential of fluid in the loop
2) as fluid flows down the descending limb, water from the filtrate moves down water potential gradient into the tissue fluid by osmosis and Na+ and Cl- diffuse into the loop, down their concentration gradient ∴ at the bottom of the loop, fluid contains much less water and many more Na+/Cl- than when it entered from the PCT
3) bc the fluid is so concentrated, it is relatively easy for Na+ and Cl- to leave it and pass to the tissue fluid, making the concentration in the tissue fluid even higher
4) the fluid in the loops continues to lose Na+/Cl- as it moves up the limb, making the fluid even less concentrated

Question 41

What does a longer loop mean?

Answer 41

That the fluid at the bottom of the loop can become more concentrated

Question 42

Although the fluid near the top of the ascending limb is not very concentrated, why is it still relatively easy to actively remove Na+/Cl-?

Answer 42

Because the tissue fluid becomes less concentrated higher up and the concentrations are never very different

Question 43

What is the counter-current multiplier and what does it enable?

Answer 43

• The mechanism with 2 limbs of the loop running side by side, with the fluid flowing down in 1 and up in the other
• It enables the maximum concentration of solutes to be built up both inside and outside the tube at the bottom of the loop

Question 44

Why is urea also concentrated in the tissue fluid of the medulla?

Answer 44

Because the cells of the ascending limb and cels lining the collecting duct are permeable to urea, which diffuses into the tissue fluid

Question 45

What happens in the collecting duct?

Answer 45

1) the fluid in the nephron passes once again through regions of the tissue fluid with a high solute concentration and low water potential (but can be higher than water potential of blood)
2) ∴ water moves out of the collecting duct, by osmosis, until the water potential of the urine is the same as the water potential of the tissue fluid in the medulla
3) the degree to which this happens is controlled by ADH

Question 46

What are the functions of the 2 parts of the distal convoluted tubule?

Answer 46

1) the first part functions the same as the ascending limb of the loop of Henle
2) the second part functions the same as the collecting duct

Question 47

What happens in the second part of the DCT and in the collecting duct?

Answer 47

1) Na+ are actively transported from the fluid int the nephron into the tissue fluid and then passed into the blood
2) K+ are actively transported into the tubule
3) the rate at which Na+ and K+ are moved into and out of the fluid in the nephron can be varied, and helps to regulate the concentration of Na+ and K+ in the blood

Question 48

What is osmoregulation?

Answer 48

The control of the water potential of body fluids (important part of homeostasis)

Question 49

What parts of the body does osmoregulation involve?

Answer 49

Hypothalamus, posterior pituitary gland and the kidneys

Question 50

What are osmoreceptors?

Answer 50

Specialised sensory neurones in the hypothalamus which constantly monitor the water potential of the blood

Question 51

What happens when osmoreceptors detect a decrease un the water potential of the blood below a set point?

Answer 51

1) nerve impulses are sent along the neurones to where they terminate in the posterior pituitary gland
2) these impulses stimulate the release of molecules of antidiuretic hormone (ADH), a peptide hormone, which enter the blood in capillaries and are carried all over the body

Question 52

What is the effect of ADH?

Answer 52

To reduce the loss of water in the urine by making the kidneys reabsorb as much water as possible

Question 53

What are the target cells for ADH?

Answer 53

The cells of the collecting duct

Question 54

Where on the cells does ADH act?

Answer 54

ADH acts on the CSMs of the cells, making them more permeable to water than usual by increasing the number of aquaporins in the CSMs

Question 55

What are aquaporins?

Answer 55

Water-permeable channels

Question 56

How does ADH increase the permeability of the CSMs if the cells of the collecting duct to water?

Answer 56

1) ADH molecules bind to receptor proteins on the CSMs, which in turn activate enzymes (phosphorylase) inside the cells
2) the cells contain ready-made vesicles that have many aquaporins in their membranes
3) the activated enzymes cause the vesicles to move towards the CSM and fuse with it ∴ increasing the permeability of the membrane to water

Question 57

How do aquaporins increase the reabsorption of water into the blood?

Answer 57

• As the fluid flows down the collecting duct, molecules of water move through the aquaporins, out of the tubule and into the tissue fluid, and then into the blood
• The tissue fluid in the medulla has a very low water potential and the fluid in the collecting ducts has a very high water potential ∴ the fluid in the collecting duct loses water and becomes more concentrated with a low volume

Question 58

What happen when there is an increase in the water potential of the blood?

Answer 58

1) the osmoreceptors are no longer stimulated and the neurones in the posterior pituitary gland stop secreting ADH
2) ∴ the aquaporins are moved out of the CSM of the collecting duct cells, back into the cytoplasm as part of the vesicles, making the collecting duct cells impermeable to water
3) ∴ the fluid flows down the collecting duct without losing any water, so a dilute urine collected in the pelvis
5) large volumes of dilute urine are produced, in order to keep the water potential of the blood constant

Question 59

Why do the collecting duct cells not respond immediately to the reduction in ADH secretion by the posterior pituitary gland?

Answer 59

Bc it takes some time for the ADH already in the blood to be broken down and for aquaporins to be removed from the CSM - 20 mins