Reabsorption and secretion

Question 1

In peritubular capillaries state if hydrostatic pressure and oncotic pressure are high or low. Why?

Answer 1

hydrostatic - low - overcoming resistance

oncotic - high - lost 20% fluid

Question 2

If oncotic pressure is more than hydrostatic pressure what process occurs?

Answer 2

reabsorption

Question 3

What is Tm?

Answer 3

maximum transport capacity of carrier due to saturation of carriers

Question 4

What substances have a carrier mediated transport system?

Answer 4

glucose, amino acids, sulphate and phosphate, organic acids

Question 5

If Tm is exceeded what happens?

Answer 5

excess enters the urine

Question 6

What is the renal threshold?

Answer 6

plasma threshold at which saturation occurs

Question 7

How much of plasma glucose is filtered and how much reabsorbed?

Answer 7

all of it is filtered

10mmoles/l

Question 8

What is the renal plasma threshold for glucose?

Answer 8

10mmoles/l

Question 9

Does the kidney regulate glucose?

Answer 9

no - insulin and the counter regulatory hormones regulate glucose

Question 10

What is the significance of the Tm of glucose in terms of the normal plasma volume

Answer 10

normal plasma volume is 5 and Tm is 10

set high above non-diabetic values to ensure the valuable nutrient is reabsorbed

Question 11

What is glycosuria a failure of?

Answer 11

insulin - not the kidney

Question 12

What other substance is Tm set high above the normal plasma concentration?

Answer 12

amino acids

Question 13

Give an example of substances the kidney does regulate by the tm mechanism. How?

Answer 13

phosphate and sulphate ions

Tm set close to plasma volume so any excess is excreted

Question 14

Apart from tm control by the kidneys what else can regulate phosphate?

Answer 14

PTH

Question 15

Where are sodium ions most abundant?

Answer 15

ECF

Question 16

What percentage of sodium is reabsorbed and what percent of this occurs in the proximal tubule?

Answer 16

99.5

65-75

Question 17

How is sodium reabsorbed?

Answer 17

active transport which establishes a gradient for sodium across the tubule wall

Question 18

Explain briefly the reabsorption of sodium

Answer 18

Na enters tubule cell from lumen by following its concentration gradient
Pumped out due to Na+K+ATPase on basolateral surface

Question 19

What drives the whole process of sodium reabsorption?Why?

Answer 19

sodium pumps

pumps sodium out of the cells to increase gradient for sodium to move passively into the cells

Question 20

What is permeability a property of? ion or membrane?

Answer 20

membrane

Question 21

How does sodium reabsorption occur due to permeability?

Answer 21

brush border of proximal tubule has higher permeability than most membranes in the body

Question 22

Why does the brush border of proximal tubule have high permeability for sodium? (2)

Answer 22

large surface area - microvilli

large number of sodium ion channels facilitate passive diffusion

Question 23

Why is the reabsorption of sodium key to the reabsorption of chlorine (among others)

Answer 23

establishes and maintains concentration gradient

Question 24

Why does water follow the sodium and chloride during reabsorption?

Answer 24

due to osmotic force

Question 25

What does water reabsorption do to the remaining substances in the tubule?

Answer 25

concentrates them due to loss of water

creates outgoing concentration gradient

Question 26

2 factors which reabsorption rate on non-actively reabsorbed (urea, calcium) depends on

Answer 26

amount of water removed from lumen - extent of concentration gradient
permeability of membrane to the solute

Question 27

Describe urea reabsorption

Answer 27

despite the concentration gradient being established the membrane is only moderately permeable so therefore only 50% is reabsorbed

Question 28

Despite the concentration gradient established for reabsorption what substances are not absorbed due to the impermeable membrane?

Answer 28

inulin and mannitol

Question 29

Does high or low sodium in the tubule facilitate glucose symport?

Answer 29

high

Question 30

How does glucose go against its concentration gradient to move from tubule lumen to proximal tubule cell?

Answer 30

sodium moving down its gradient uses SGLT protein to pull glucose into cell

Question 31

What protein does glucose use to exit the basolateral membrane of the proximal tubule cell?

Answer 31

GLUT

Question 32

What direction does secretion go?

Answer 32

peritubular capillaries –> tubule lumen

Question 33

Where is secretion NOT found?

Answer 33

Loop of henle

Question 34

Why is secretion important?

Answer 34

2nd route into tubule lumen

protein bound - not filtered and harmful substances - fast excretion

Question 35

Are Tm carrier mechanism specific?

Answer 35

no - drugs can use these which are originally made for endogenous substances

Question 36

Where are the drugs secreted?

Answer 36

early at proximal tubule

Question 37

Where is potassium found? in cells or in ECF?

Answer 37

in cells

Question 38

Normal potassium concentration

Answer 38

4 mmoles/l

Question 39

What happens in hyperkalaemia?

Answer 39

decrease resting membrane potential

VF and death

Question 40

What happens in hypokalaemia?

Answer 40

increase resting membrane potential

arrhythmias and death

Question 41

Where is potassium primarily reabsorbed?

Answer 41

proximal tubule

Question 42

If more potassium is ingested what happens?

Answer 42

more is excreted - more secretion

Question 43

What hormone also controls potassium?

Answer 43

aldosterone from adrenal cortex

Question 44

Does an increase or decrease in potassium increase aldosterone? How?

Answer 44

increase

ECF bathing aldosterone secreting cells stimulate its release and increase secretion

Question 45

What does the aldosterone do to the kidneys?

Answer 45

stimulate renal tubule potassium secretion

Question 46

What does aldosterone do to sodium?

Answer 46

stimulate reabsorption at distal tubule