Reabsorption & secretion Flashcards
Renal threshold is defined as
plasma threshold at which saturation occurs
Mechanisms that substances are reabsorbed from the nephron back into the blood
Carrier mediated transport proteins
What is Tm
maximum transport capacity
-transport rate at saturation
If maximum transport capacity (Tm) is exceeded, what happens to the excess substrate
excreted in urine
Renal plasma threshold for glucose is
> 10mmol/l
-beyond this, glucose will appear in the urine
Plasma glucose is reabsorbed up to what conc. (i.e. beyond, this glucose conc., it cannot be reabsorbed as renal plasma threshold for glucose is reached)
10mmol/l
Appearance of glucose in the urine of diabetic patients is due to the failure of what; NOT THE KIDNEY
insulin
What mechanism by the kidney regulates the levels of ions like sulphate and phosphate
Tm (maximun transport capacity) mechanism
-Tm is at a conc. where normal plasma conc. causes saturation of the carrier proteins so any increase above the normal level is excreted
What % of sodium filtered is reabsorbed + what percentage of this is reabsorbed mostly in the PCT
99.5%
75%
Unlike sulphate and phosphate ions which are reabsorbed by the kidney by a maximum transport capacity (Tm) mechanism, how is Na+ reabsorbed by the kidney
By Na/K pump expressed in the tubule membrane which actively transports Na+ back into the plasma
High Na+ conc. in tubule lumen and low Na+ conc. in the proximal tubule cells means sodium moves across the luminal membrane into the proximal tubule cells by…
then Na+ in the proximal tubule cells are reabsorbed into the ECF by
passive transport
active transport - Na/K pump
Na+ is not usually freely permeable at cell membranes (needs carrier proteins), so how can Na+ passively move from tubule lumen into proximal tubule cells
proximal tubule cells have a higher permeability to Na+ ions than most other membranes in the body because of the enormous surface area offered by the microvilli and the large number of Na+ ion channels, which facilitate the passive diffusion
Active transport of Na+ out of the tubule followed by Cl- creates an … … drawing … out of the tubules as well
osmotic force
water
H2O removed by osmosis from the tubule filtrate concentrates all the substances left in the tubule as fluid volume decreases which creates
outgoing conc. gradients of these remaining substances
Reabsorption of Na+ by active transport into the ECF creates a … gradient which drives … reabsorption as well, e.g. …
electrochemical
anion
Cl-
What are anions
negatively charged ions
Does PTH increase or decrease reabsorption of phosphate
decrease
increases it excretion in urine
Explain the significance of the active reabsorption of Na+ ions at the proximal tubule.
key to the reabsorption of the other components of the filtrate in the tubule as it establishes gradients for which other ions, water and solutes move down passively
Name some substances that are coupled to Na+ transport, i.e. share the same carrier protein (symporter)
Glucose
Amino acids
Apart from filtration and reabsorption, the final renal process is secretion - what exactly does this entail
Secretory mechanisms transport substances from the peritubular capillaries into the tubule lumen so providing a second route into the tubule, if didn’t get in through the glomerulus initially
Secretion of what substances from the peritubular capillaries into the tubule lumen is significant
Proteins/protein bound substances because glomerulus restricts the filtration of proteins into the tubule
K+ ions are primarily reabsorbed where in the nephron
proximal tubule
What are cations
positive ions
Major cation in the body
K+
Normal plasma K+
4mmol/l
Hyperkalaemia occurs when plasma K+ rises to
+
what effect does this have on resting membrane potential of excitable cells + consequence on these cells
5.5mmol/l
Decreases it so excessively depolarises cells which can cause ventricular fibrillation
Hypokalaemia occurs when plasma K+ lowers to
+
what effect does this have on resting membrane potential of excitable cells + consequence on these cells
<3.5mmol/l
Increases it so hyperpolarises cells like cardiac muscle –> arrhythmia
Increase dietary K+ has what effect on urine
Increased urinary loss to excrete K+
Although most K+ filtered through the kidneys is reabsorbed in the proximal tubule, its regulation occurs mostly where in the renal system
Collecting duct
What hormones increase K+ loss in urine
Aldosterone
ADH
Increase in ECF K+ conc. stimulates what hormone to be released and circulated to the kidneys to stimulate K+ secretion in urine
Aldosterone
Aldosterone promotes Na+ reabsorption where
Distal tubule
