Resorption and Secretion

Question 1

What is the filtration fraction?

Answer 1

20% of plasma filtered into Bowman’s capsule in the glomerulus

Question 2

Why do the peritubular capillaries have a higher oncotic pressure?

Answer 2

20% plasma is filtered into Bowman’s capsules and so the remaining blood in the efferent arteriole has a higher concentration of plasma proteins

Question 3

What is oncotic pressure?

Answer 3

A form of osmotic pressure induced by proteins driving resorption of water from the tubule

Question 4

What are the starling forces of the peritubular capillaries?

Answer 4

Low P(PC) and high oncotic pressure - in favour of reabsorption

Question 5

What is the percentage of molecules are reabsorbed from the renal tubule?

Answer 5

99% H2O, 100% Glc, 99.5% Na and 50% urea (mainly at proximal convoluted tubule)

Question 6

How are molecules transported during reabsorption?

Answer 6

Via carrier mediated transport system as there are no channel transports

Question 7

What is permeability of a substance determined by?

Answer 7

Number and type of transporters

Question 8

What is transport maximum (Tm)?

Answer 8

Maximum transport capacity of carriers which is met by the saturation of the carriers

If Tm is excreted -> excess substrate enters the urine

Question 9

What is the renal threshold?

Answer 9

Renal threshold = plasma threshold

It is the plasma concentration at which saturation occurs

Question 10

Describe the filter and reabsorption of Glucose

Answer 10

• Glucose is freely filtered, so all plasma Glc is filtered, but in healthy people all is reabsorbed so no Glc will pass into the urine.
• In man for plasma glucose up to 10 mmoles/l, all will be reabsorbed.

Question 11

What happens to glucose reabsorption when [Glc] exceeds Tm?

Answer 11

Once it has met the renal plasma threshold for Glc (10mmoles/L), the extra is excreted in the urine (glycosuria)

In diabetes, [glucose] is way above the Tm, where is healthy levels, Tm is set way above normal [Glc]

Question 12

Do the kidneys regulate [glucose]?

Answer 12

No, insulin and counter-regulatory hormones are responsible for its regulation

Question 13

What ions are the kidneys responsible to regulate?

Answer 13

Sulphate and phosphate ions

These are reabsorbed is altered to meet the bodys needs; anything that exceeds Tm value would be too much for the body and so is excreted. Tm is set so that normal [plasma] causes saturation.

Question 14

Describe how the kidneys regulate phosphate ions

Answer 14

PTH regulated Ca levels, and when there is a decrease in plasma [Ca] PTH decreases phosphate reabsorption (by reducing the number of channels) so that it doesn’t bind to Ca, allowing more free Ca.

Question 15

How much Na is absorbed in the proximal tubule?

Answer 15

65-75%

Question 16

Describe how Na is reabsorbed across the tubule wall

Answer 16

Via active transport (not Tm) establishing a gradient for Na across the tubule wall.

Na+ moves into the tubule cell via passive diffusion (high conc. to low), where a channel protein actively pumps Na into the interstitial fluid. This decreases [Na+] in epithelial cells, increasing the gradient for Na+ to move into the cells.

Question 17

Why does the proximal tubule have a higher permeability to Na+ than the rest of the body?

Answer 17

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+.

Question 18

Describe the Cl and H2O reabsorption

Answer 18

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+.

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

Question 19

What does the rate of reabsorption of solutes depend on?

Answer 19

• Amount of H2O removed, which will determine the extent of the concentration gradient.
• The permeability of the membrane to any particular solute.

Question 20

Other than establishing concentration gradients, what is reabsorption of Na also important for?

Answer 20

Active transport of Na+ is also important for carrier mediated transport systems for other substances. Substances such as glucose, amino acids etc, share the same carrier molecule as Na+ (symport).

Question 21

What is the third renal process?

Answer 21

Tubular secretion:
Transports substances from the peritubular capillaries into the tubule lumen and therefore provide a second route into the tubule.

Question 22

What is secretion important for?

Answer 22

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

Question 23

Are carrier mechanisms specific to a substance?

Answer 23

No, 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).

Choline, creatinine etc, can be used for morphine and atropine.

Question 24

What is the normal ECF [K+]?

Answer 24

4mmoles/L

Question 25

What occurs in hyperkalaemia?

Answer 25

If 5.5mmoles/L = hyperkalaemia -> decreases resting membrane potential of excitable cells and eventually ventricular fibrillation and death.

Question 26

What occurs in hypokalamia?

Answer 26

< 3.5 mmoles/l = hypokalaemia -> increase resting membrane potential ie hyperpolarises muscle, cardiac cells -> cardiac arrhythmias and eventually death.

Question 27

Describe renal handling of K+

Answer 27

K+ filtered at the glomerulus is reabsorbed, primarily at the proximal tubule.

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

Question 28

What hormone controls K+?

Answer 28

Aldosterone

Question 29

How does aldosterone control K+?

Answer 29

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