The proximal tubule and the loop of Henle

Question 1

where does most reabsorption occur?

Answer 1

Proximal tubule

Question 2

how often is plasma filtered?

Answer 2

65 times per day

Question 3

what do the kidneys reabsorb?

Answer 3

99% of fluid
99% of salt
100% of glucose
100% of amino acids
50% of urea
0% creatinine

Question 4

Is reabsorption specific or non specific?

Answer 4

specific - unlike filtration which is non-specific

Question 5

what is glomerular filtrate?

Answer 5

modified filtrate of blood- contains ions and solutes at plasma concentration but lacks RBCs and large plasma proteins

Question 6

how does the osmolarity change between the fluid reabsorbed in the proximal tubule and the filtrate?

Answer 6

iso-osmotic - no change in osmolarity

Question 7

what is reabsorbed in the proximal tubule?

Answer 7

Sugars
Amino acids
Phosphate
Sulphate
Lactate

Question 8

what is secreted in the proximal tubule?

Answer 8

H+
Hippurates
Neurotransmitters
Bile pigments
Uric acid
Drugs
Toxins

Question 9

what are the two membranes of the wall of the nephron?

Answer 9

apical membrane - faces tubular fluid and lumen

Basolateral membrane - faces interstitial fluid

Question 10

what is paracellular reabsorption?

Answer 10

some substances are reabsorbed between adjacent epithelial cells through “leaky” tight junctions

Question 11

what is essential for Na+ reabsorption?

Answer 11

An energy-dependent Na+-K+ ATPase transport mechanism at the basolateral membrane

Question 12

what is iso osmotic fluid reabsorption across “leaky” proximal tubule epithelium due to?

Answer 12

(1) Standing Osmotic Gradient

(2) Oncotic Pressure Gradient

Question 13

what causes oncotic drag of peritubular plasma?

Answer 13

After losing 20% plasma , the plasma protein concentration is higher in peritubular capillaries and so helps drag fluid from lateral space into the blood

Question 14

what causes paracellular reabsorption of negative chloride ions?

Answer 14

the net movement of positive charge from the movement of Na+ from tubular fluid into blood through the transcellular route sets up an electrical gradient and so negative ions move

Question 15

why does the osmolarity not change?

Answer 15

water follows reabsorbed salt and so salt and water are absorbed in equal proportions

Question 16

how much glucose is reabsorbed in the proximal tubule?

Answer 16

100%

Question 17

what happens to rate of filtration if you increase plasma concentration?

Answer 17

rate of filtration increases

Question 18

why does reabsorption plateau?

Answer 18

Membrane transporter proteins are fully saturated with glucose

Question 19

what is the function of the loop of henle?

Answer 19

• Generates a cortico-medullary solute concentration gradient
• This enables the formation of hypertonic urine

Question 20

what is countercurrent flow?

Answer 20

opposing flow in two limbs

Question 21

what do the loop and vasa recta establish together?

Answer 21

hyper-osmotic medullary interstitial fluid

Question 22

what does the ascending limb reabsorb and what is its permeability to water?

Answer 22

• Along the entire length of the ascending limb Na+ & Cl- are being reabsorbed
• relatively impermeable to water

Question 23

what does the descending limb reabsorb and what is its permeability to water?

Answer 23

-does not reabsorb NaCl -highly permeable to water

Question 24

what does the selective permeabilities of the ascending and descending limbs enable?

Answer 24

an osmotic gradient to be established in the medulla

Question 25

what is the triple co-transporter?

Answer 25

transporter that allows the movement of Na+, K+ and Cl- from the filtrate to the interstitial fluid

Question 26

how is K+ recycled and what does this mean?

Answer 26

• Potassium is moved across both basolateral and luminal membranes so there is not net movement of potassium
• recycling means that NaCl is absorbed into the interstitial fluid

Question 27

what blocks the triple co-transporter?

Answer 27

loop diuretics

Question 28

why does water not cross the membrane?

Answer 28

the tight junctions are to small to fit water though and so water cannot follow the salt

Question 29

what happens when the triple co-transporter pumps solute from the ascending limb?

Answer 29

1. Solute removed from lumen of ascending limb (water cannot follow)
2. Tubular fluid is diluted and osmolality of interstitial fluid is raised
3. Interstitial solute cannot enter the descending limb
4. Water leaves the descending limb by osmosis
5. Fluid in the descending limb is concentrated

Question 30

what is the difference ins osmotic state from leaving proximal tube and entering distal tubule?

Answer 30

• its iso osmotic leaving the proximal tubule

- its hypo osmotic entering the distal tubule

Question 31

in a steady state, what is the osmolarities of the cortex and medulla?

Answer 31

• it remains at 300 in the cortex but progressively increases in osmolarity in the medulla

Question 32

what other contributes to half the medullary osmolarity?

Answer 32

urea cycle

Question 33

What is the purpose of countercurrent multiplication?

Answer 33

To concentrate the medullary interstitial fluid

Question 34

why is concentrating the medullary interstitial fluid essential?

Answer 34

To enable the kidney to produce urine of different volume and concentration according to the amounts of circulating antidiuretic hormone (ADH = vasopressin)

Question 35

what runs alongside the long loop of Henle of juxtamedullary nephrons?

Answer 35

vasa recta

Question 36

what does the loop of henle and vasa recta form?

Answer 36

countercurrent system

Question 37

how is the problem of essential blood flow through the medulla that washes away NaCl and urea minimised?

Answer 37

1) Vasa recta capillaries follow hairpin loops
2) Vasa recta capillaries freely permeable to NaCl and water
3) Blood flow to vasa recta is low (few juxtamedullary nephrons)

Question 38

what ensures that the solute is not washed away?

Answer 38

Passive exchange across the endothelium preserves medullary gradient - blood equilibrates at each layer.