Reabsorption and Secretion

Question 1

How does reabsorption occur in the peritubular capillaries?

Answer 1

In peritubular capillaries:

PPC very low because hydrostatic P overcoming frictional resistance in efferent arteriols

osmotic pressure high compared to normal, loss of 20% plasma concentrates plasma protein

osmotic pressure >> PPC only reabsorption

Question 2

what is the only thing to happen in the glomerular capillaries?

Answer 2

PGC >> Pp

Only filtration occurs at glomerular capillaries

Question 3

Since aorund 20% of the plasma has filtered into Bowman’s capsule in the glomerulus, (filtration fraction), the blood remaining in the efferent arteriole and then the peritubular capillaries has what?

Answer 3

a higher concentration of plasma proteins and therefore increase osmotice pressure

Question 4

As a consequence, the net result of the low P_PC and the high oscmotic pressure causes what?

Answer 4

the net result of the low PPC and the high osmotic pressure is that the balance of Starling’s forces in the peritubular capillaries is entirely in favour of reabsorption

Question 5

what substances are reabsorbed and where about are they mainly reabsormbed?

Answer 5

99% H2O, 100% glucose, 99.5% Na+, 50% urea filtered at the glomerulus are reabsorbed within the tubule, mainly at the proximal convoluted tubule

Question 6

many substances are reabsorbed by what?

Answer 6

Many substances are reabsorbed by carrier mediated transport systems

eg glucose, amino acids, organic acids, sulphate and phosphate ions

Question 7

do carries have a maximum transport capacity?

Answer 7

Carriers have a maximum transport capacity T_m which is due to saturation of the carriers

Question 8

what happens if T_m is exceeded?

Answer 8

If T_m is exceeded, then the excess substrate enters the urine

Question 9

what do carrier proteins allow?

Answer 9

Carrier protein enables larger molecules such as glucose to cross the membrane

Question 10

Capacity is limited by number of _______

Answer 10

carriers

Question 11

what is the renal threshold?

Answer 11

Renal threshold = plasma threshold at which saturation occurs

Question 12

One of the most important substances to consider is glucose.

Describe the titration curve for glucose

Answer 12

1. Glucose is freely filtered, so whatever its [plasma] that will be filtered.
2. In man for plasma glucose up to 10 mmoles/l, all will be reabsorbed.

Beyond this level of plasma [glucose], it appears in the urine = Renal plasma threshold for glucose.

(If plasma [glucose] = 15 mmoles/l, 15 will be filtered, 10 reabsorbed and 5 excreted.)

Question 13

does kidney regulate glucose?

Answer 13

Kidney does NOT regulate [glucose], (insulin and the counter-regulatory hormones responsible for its regulation)

Question 14

What is the normal level of glucose ocncentration and why is it this?

Answer 14

Normal [glucose] of 5 mmoles/l, so Tm is set way above any possible level of (non-diabetic) [glucose]. Ensures that all this valuable nutrient is normally reabsorbed

Question 15

is the appearance of glucose in someones urine a sign of failing kidneys?

Answer 15

The appearance of glucose in the urine of diabetic patients = glycosuria, is due to failure of insulin, NOT, the kidney. N.B. Any patient with glucose in their urine should be followed up

Question 16

What is the Tm of amino acids and why is this again?

Answer 16

For amino acids, Tm is also set so high that urinary excretion does not occur, regulated by insulin and counter-regulatory hormones

Question 17

does the kidneys regulate any substances?

Answer 17

Kidney does regulate some substances by means of the Tm mechanism, eg sulphate and phosphate ions

This is because Tm is set at a level whereby the normal [plasma] causes saturation

Any ­increase above the normal level will be excreted, therefore achieving its plasma regulation

(Also subject to PTH regulation for phosphate, PTH decrease reabsorption)

Question 18

Na+ ions are the most abundant in the ECF, a very large amount is filtered every day

180 l/day x 142 mmoles/l = 25560 mmoles/day, 99.5% is reabsorbed

where is most of it rebabsorbed and how is it reabsorbed?

Answer 18

65-75% of Na+ion reabsorption occurs in the proximal tubule

Not reabsorbed by a Tm mechanism, but by active transport, which establishes a gradient for Na+ across the tubule wall

Question 19

what is the process of Na+ activley being pumped out across the tubule wall?

Answer 19

Active Na+pumps are located on the basolateral surfaces, where there is a high density of mitochondria

This decreases [Na+] in the epithelial cells, increasing the gradient for Na+ ions to move into the cells passively across the luminal membrane

It is the Na+ pumps which drive the whole process

Question 20

WAIT A MINUTE!!! Na+ is not permeable at cell membranes!

how does it cross the brush border of proximal tubule cells?

Answer 20

The brush border of the proximal tubule cells has a higher permeability to Na+ ions than most other membranes in the body, partly because of the enormous surface area offered by the microvilli and the large number of Na+ ion channels, which facilitate this passive diffusion of Na+

This reabsorption of Na+ ions is key to the reabsorption of the other components of the filtrate

Question 21

How does negative ions cross the proximal tubular membrane?

Answer 21

Negative ions such as Cl- diffuse passively across the proximal tubular membrane down the electrical gradient established and maintained by the active transport of Na+

Question 22

The active transport of Na+ out of the tubule followed by Cl- creates what?

Answer 22

The active transport of Na+ out of the tubule followed by Cl- creates an osmotic force, drawing H2O out of the tubules

Question 23

By H2O being removed by osmosis, what happens to the substances left in the tubule?

Answer 23

H2O removed by osmosis from the tubule fluid concentrates all the substances left in the tubule creating outgoing concentration gradients

Question 24

the rate of re-absorption of these non-actively reabsorbed solutes depends on what things?

Answer 24

a) amount of H2O removed, which will determine the extent of the concentration gradient
b) the permeability of the membrane to any particular solute

For some substances eg inulin and mannitol, the tubular membrane is impermeable

Question 25

How permeable is the tubular membrane in response to urea?

Answer 25

Tubule membrane is only moderately permeable to urea, so that only about 50% is reabsorbed, the remainder stays in the tubule

Question 26

can all substances be reabsorbed if a concentration gradient is established?

Answer 26

So despite a concentration gradient being established favouring their reabsorption, they cannot gain access through the tubule membrane so that all that is filtered stays in the tubule and passes out in the urine

Question 27

what is it that establishes the gradient in which other things pass down?

Answer 27

It is the active transport of Na+ that establishes the gradients down which other ions, H2O and solutes pass passively

Question 28

what happens if active transport is decrease of sodium?

Answer 28

Anything which decreases active transport eg decreased BF = disruption of renal function

Question 29

why is sodium important for carrier mediated transport systems?

Answer 29

Importance of active transport of Na+ also for carrier mediated transport systems

Substances such as glucose, amino acids etc, share the same carrier molecule as Na+ (symport)

Question 30

High [Na+] in the tubule ________ and low [Na+] ______ glucose transport

Na+ reabsorption also linked to______ ion reabsorption (A/B)

Answer 30

High [Na+] in the tubule facilitates and low [Na+] inhibits glucose transport

Na+ reabsorption also linked to HCO3- ion reabsorption (A/B)

Question 31

the third renal process is tubular secretion, what is it?

Answer 31

Secretory mechanisms transport substances from the peritubular capillaries into the tubule lumen and thereofre provide a second route into the tubule

Question 32

what is tubular secretion important for?

Answer 32

Important for substances that are protein-bound, since filtration at glomerulus is very restricted. Also for potentially harmful substances, means can be eliminated more rapidly

Question 33

are carrier mechanisms specific?

Answer 33

Tm-limited carrier-mediated secretory mechanisms known for a large number of endogenous as well as exogenous substances such as drugs??

Carrier mechanisms are not very specific so that eg organic acid mechanism, which secretes lactic and uric acid can also be used for substances such as penicillin, aspirin and PAH (para-amino-hippuric acid)

Similarly, organic base mechanism for choline, creatinine etc, can be used for morphine and atropine

All of these substances are secreted at the proximal tubule

Question 34

sudy this diagram - shows how secretion fits into everything

Answer 34

Question 35

is K+ important?

Answer 35

K+ is the major cation in the cells of the body and the maintenance of K+ balance is essential for life

Normal ECF[K+] around 4mmoles/l

Question 36

What happens if K+ is too high or too low?

Answer 36

If it ­increased to 5.5mmoles/l = hyperkalaemia = decreased resting membrane potential of excitable cells and eventually ventricular fibrillation and death. Remember the Nernst equation!

If [K+] < 3.5 mmoles/l = hypokalaemia = increased resting membrane potential ie hyperpolarizes muscle, cardiac cells = cardiac arrhythmias and eventually death

Question 37

We are normally in K+ balance, even though a normal diet contains more than enough K+ for the body’s needs. Beware the killer banana!

How is K+ managed?

Answer 37

Renal handling of K+ is complex. K+ filtered at the glomerulus is reabsorbed, 1°ily at the proximal tubule

Question 38

What are changes in K+ excreiton due to?

Answer 38

due to changes in its secretion in the distal parts of the tubule. Any increase ­in renal tubule cell [K+] due to increased ingestion will = K+ secretion, while any decrease in intracellular [K+] = reduced secretion

Question 39

K+ secretion is also regulated by what?

Answer 39

K+ secretion is regulated by the adrenal cortical hormone aldosterone

Question 40

how does aldosterone affect K+ secretion?

Answer 40

An increase ­in [K+] in ECF bathing the aldosterone secreting cells stimulates aldosterone release which circulates to the kidneys to stimulate increase ­in renal tubule cell K+ secretion

Question 41

Aldosterone also stimulates ___ reabsorption at the distal tubule but by a different reflex pathway

Answer 41

Na+

Question 42

H+secretion: H+ions are actively secreted from the tubule cells (not the peritubular capillaries) into the lumen A/B Balance