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Summary Chapter 28; Renal Tubular Reabsorption and Secretion
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chapter 28 of guyton and hall physiology
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Summary of reabsorption in renal tubules
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Chapter 28; Renal Tubular Reabsorption andSecretion
Excretion = Filtration – Reabsorption + Secretion.
Filtration = GFR x Plasma concentration. This equation assumes that all the solute is
filtered into the nephron.
For example, if glucose has a GFR of 180 L/day, and a plasma conc. of 1 g/L, then 180
x 1 = 180 g/day of glucose is filtered.
Excretion is determined by the balance between filtration and reabsorption, and that
balance is maintained by the body to prevent unnecessarily large changes to
excretion.
Glucose and amino acids are completely reabsorbed and not excreted.
Reabsorption and excretion of ions like Na+, Cl- and HCO3- varies depending on
bodily needs.
Urea and creatinine are excreted more than they are reabsorbed, so there is net
excretion.
Solutes get reabsorbed through the following pathway:
Lumen to tubular epithelial cells (transcellular route) via the luminal/brush
border/apical membrane, then across the cell, then out into the interstitium via the
basolateral membrane, then into the peritubular capillaries.
Some solutes pass in-between the cells instead of through them (paracellular route).
Na+ Reabsorption:
1. Sodium-potassium ATPase pump actively transports Na+ ions out of the
epithelial cells and into the interstitium via the basolateral membrane, while
moving K+ ions into the cells.
2. This makes the tubular cells negative (-70 mV), and low in Na+. Therefore, Na+
ions in the lumen move into the tubular cells by diffusion down a concentration
and electrical gradient.
3. The capillary hydrostatic and colloid osmotic pressures favor the passive
movement of Na+ ions into the peritubular capillaries from the interstitium. This
passive movement is known as ultrafiltration/bulk flow.
, Glucose Reabsorption:
1. Glucose is actively transported into the tubular cells via the luminal membrane
by SGLT (sodium glucose co-transporter). These are SGLT2 (in the early part of
the proximal tubule, and responsible for most of the glucose uptake) and SGLT1
(latter part of the proximal tubule).
2. The glucose diffuses out of the cells and into the interstitium via the basolateral
membrane with the help of glucose transporters GLUT2 (early part of proximal
tubule) and GLUT1 (distal part of proximal tubule).
3. Uptake of glucose by peritubular capillaries occurs through ultrafiltration.
Na+/K+ ATPase pump, provides the energy that causes Na+ diffusion across the
luminal membrane. The Na+ ions diffuse into the tubular cells via the SGLT, thus
driving them, and causing glucose uptake along with its own.
As the SGLT depends on the Na+/K+ ATPase pump, it is a SECONDARY active
transport system, and the Na+/K+ ATPase pump is a PRIMARY one.
Certain solutes, such as glucose, can only be reabsorbed up to a certain limit (due to
the limited number of transporters for that solute). This limit is known as the
‘transport maximum’.
Glucose has a roughly normal conc. of 100 mg/ml. However, as it increases in plasma
and reaches 250 mg/ml, also known as its ‘threshold’, it starts appearing in the urine.
But, there are still some transporters present that are free to reabsorb glucose.
However, when ALL the transporters in all the tubules are unavailable, which for
glucose occurs at 375 mg/ml, the transport maximum is reached, and glucose
reabsorption is paused.
This occurs in patients with uncontrolled diabetes mellitus.
Substances that exhibit passive diffusion don’t have a transport maximum, because
their reabsorption depends on the electrochemical gradient, permeability of the
membrane, and the time they’re in contact with the luminal membrane. This is why
they undergo ‘gradient-time transport’ instead.
The reason this occurs, with Na+ for example, is because Na+ ions that are
transported end up back leaking into the tubular lumen. This means that the transport
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