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RS11 Renal System - 3 General Considerations - Coggle Diagram
RS11
Renal System - 3
General Considerations
General Considerations
Reabsorption
denotes the active transport of solutes and the passive movement of water from the tubular lumen into the peritubular capillaries i.e. it is the removal of a substance from the filtrate (e.g.Na+reabsorption from PCT).
The Filtered Load
i.e. the amount of solute transported across the
glomerular membranes per unit time.
Mathematically, filtered load
= GFR x plasma concentration of the solute (Px)
= mL/min x mg/mL
= mg/min.
The Excretion Rate.
It is the amount of a substance that appears in the
urine per unit time.
Mathematically, excretion rate
= urine flow rate (V) x urine concentration of the substance (Ux)
= V x Ux
= mL/min x mg/mL
= mg/min.
If the filtered load exceeds the excretion rate, net reabsorption of that
substance has occurred.
Renal Tubular Transport Maximum (Tm).
It refers to the maximal
amount of a given solute that can be transported (reabsorbed or secreted) per minute by the renal tubules. The limit is due to saturation of the specific transport systems involved.
The highest attainable rate of reabsorption is called the maximum
tubular reabsorptive capacity and is designated 'Tr' (or Tm).
Substances that are reabsorbed by an active carrier mediated
process and that have a Tm include: phosphate ion (HPO42–),
Sulphate (SO42–), glucose, amino-acids, uric acid, albumin,
acetoacetate, β-hydroxybutyrate and α-ketoglutarate.
Na+ REABSORPTION
Filtered load of sodium is 575-580 gm/day;
approx. 98% of which is reabsorbed:
60% by PCT,
20% by thick ascending limb of loop of Henle,
15% gets reabsorbed by DCT,
2-3% by CT and
the remaining 2% is excreted in urine.
Reabsorption from PCT
• Na+ reabsorption across the PCT occurs in 2 steps:
Step 1: Through apical (luminal) membrane i.e. from the tubular lumen into the tubular epithelial cells. Na+ is transported through apical
membrane passively, down an electrical and chemical gradient by facilitated diffusion.
• This step involves two processes:
i. First, Na+ undergoes ‘passive transport’ with the secondary active transport of glucose or amino acid via symporters.
ii. Next, Na+
reabsorption occurs coupled with H+ secretion (by Na+ -H+ exchange). The transport of Na+
in the PCT is not associated with K+
secretion. To maintain electroneutrality the entry of Na+ into the tubular cell is balanced by 2 mechanisms:
a. (mainly) by passive inward diffusion of Cl- down its concentration gradient (Clis the only quantitatively reabsorbable anion in the filtrate), or
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