6. Obligatory reabsorption and secretion in the proximal convoluted tubule.pptx

Question 1

Functions of the proximal tubule

Answer 1

Recovery of ions, sugars, amino acids, peptides and a considerable amount of total water
Secretes components for urinary excretion
Metabolises amino acids

Question 2

What are the two pathways molecules and ions might take across the proximal tubule epithelium?

Answer 2

1. Transcellular route (through the cell body)

2. Paracellular route ( through the leaky ‘tight’ junctions between cell bodies

Question 3

What are the forces involved in obligatory reabsorption from the proximal tubule?

Answer 3

1. ION GRADIENTS across the basolateral membrane – active transport 3Na out 2K in
2. This sets up an ELECTROCHEMICAL GRADIENT of about -3mV (tubule lumen negative; causes paracellular efflux of cations)
3. OSMOTIC GRADIENT set up by pumping Na out of the cell into the interstitial space
4. Water moving along the paracellular path due to osmotic pressure drags solutes along with it. Known as SOLVENT DRAG
5. Chemical concentration of solutes left behind when water leaves the tubule facilitates a CHEMICAL GRADIENT

Question 4

Different transport mechanisms at the proximal convoluted tubule cell?

Answer 4

Sodium pump: Sets up Na+ concentration gradient. Gradient used by cell to transport other substances
Co-transport: Moves substances with Na into the cell
Counter transport (exchange): Moves substances with sodium out of tubule

Question 5

Movements of components occurring at the proximal convoluted tubule?

Answer 5

1. The sodium pump decreases [Na+]i
2. Sodium conc. gradient is used by the Na+/H+ exchanger to transport H+ OUT of the cell (against its conc. gradient)
3. H+ combines with filtered bicarbonate to produce carbonic acid which breaks down to H2O and CO2
4. H2O and CO2 diffuse into the cell
5. H2O and CO2 produce H+ and bicarbonate
6. H+ leaves cell (see note 2) into tubule lumen
7. On basolateral membrane chloride, bicarbonate and potassium leave down their concentration gradients
8. On basolateral membrane Ca2+ is exchanged for Na+. Ca2+ leaves the cell against its conc. Gradient
9. On apical membrane, Ca2+ enters through a Ca2+ channel. Also via paracellular route
10. Entry of other solutes: coupled to Na+ entry on apical membrane; facilitated diffusion on the basolateral membrane.
11. Solvent drag: Due to osmotic gradient from lumen to ISF. Movement of water (solvent) drags other ions through the paracellular route

Question 6

What is “transport maximum”?

Answer 6

The limit of how much solute can be moved across the proximal convoluted tubule.
A.k.a. Tm or Tmax
Measured in mg/min or mmol/min (rate)

Question 7

How does exceeding Tmax affect reabsorption and excretion?

Answer 7

Ion/component remains in fluid in tubule and is excreted in urine

Question 8

Amount filtered (mg/min) =

Answer 8

plasma conc (mg/ml) x GFR (ml/min)

Question 9

Threshold is the point at which the amount filtered is equivalent to ____

Answer 9

Threshold is the point at which the amount filtered is equivalent to Tmax.

Above threshold, substance appears in the urine. E.g. in diabetes the conc of glucose in urine is proportional to the plasma concentration

Question 10

Amount of solute filtered is proportional to the amount present in the ______

Answer 10

Amount of solute filtered is proportional to the amount present in the plasma

Question 11

The amount of solute appearing in the urine is the amount filtered from glomerulus plus ???

Answer 11

The amount secreted

Question 12

What happens to the following solutes in the proximal convoluted tubule after they have been filtered in the glomerulus:
Urea?
Lipid soluble substances?
Phosphate?
Protein?

Answer 12

Urea: simple diffusion reabsorbs 50-60% (rest lost)

Lipid-soluble substances: simple diffusion

Phosphate: sodium-linked transport. Activity of carriers changed by parathyroid hormone

Protein: small amount digested to amino acids within the tubule cells

Question 13

What substances are obligatorily reabsorped in the proximal convoluted tubule?

Answer 13

100%: glucose, amino acids, lactate
90%: bicarbonate
65%: water and sodium
55%: potassium
50%: chloride

Question 14

What is clearance?

Range?

Answer 14

The volume of plasma which is cleared of substance per unit time (ml/min)
Theoretical value

Range:
Zero (i.e. fully reabsorbed e.g. glucose, or never filtered e.g. protein)
—->
Equivalent of RPF (all substances filtered ends in urine)

Question 15

What 3 renal processes determine and modify composition of urine?

How are this values used to calculate the amount secreted?

Answer 15

Glomerular filtration

Tubular reabsorption

Tubular secretion

Amount secreted= Amounted filtered - amount reabsorbed + amounts secreted

Question 16

Filtration fraction =

Answer 16

F.F. = GFR/RPF

Question 17

What qualities must a substance have to have it’s GFR measured?

Ideal substance?

Answer 17

To measure GFR the substance must be:

• Freely filtered at the glomerulus
• Neither secreted or reabsorbed
• Not metabolised
• Not toxic

Inulin – ideal substance but is a plant sugar
and needs to be infused to establish constant
plasma concentrations

Question 18

What is the of the clearance ratios of {substance] and inulin?

Answer 18

Clearance [substance]/ clearance ilulin

If ratio = 1. Neither secreted or reabsorbed
If ratio> 1. Substance secreted
If ratio < 1. Substance reabsorbed

Question 19

Use of clearance ratios?

Answer 19

Determine renal transport mechanism (ie. net absorption and net secretion)

Question 20

Clearance =

Answer 20

Clearance = (urine concentration of X x urine flow rate)/ plasma concentration of X