29. Renal Handling of Glucose, Sodium and Inulin Flashcards
Graph
Glucose
Fig. 29.1 Renal handling of glucose, inulin and sodium
Glucose
> In health, glucose is completely
reabsorbed so its concentration
falls to zero along the length of the
PCT
(sharp decline in the first part of the
PCT, with slower decline further down).
Fig. 29.1 Renal handling of glucose, inulin and sodium
Sodium
> Sodium is almost completely
reabsorbed and is followed by passive
diffusion of water,
so its concentration remains unchanged (straight line).
Fig. 29.1 Renal handling of glucose, inulin and sodium
Inulin
> I nulin is filtered but not reabsorbed,
so its concentration rises (sloped positive curve
Fig. 29.1 Renal handling of glucose, inulin and sodium
Inulin
> I nulin is filtered but not reabsorbed,
so its concentration rises (sloped positive curve
What are the transport mechanisms involved?
> Passive diffusion down
chemical or electrical gradients,
e.g. sodium ions from
tubular lumen into tubular cell.
> Facilitated diffusion down
chemical or electrical gradients:
• Co-transport (symport) of glucose, amino acids, bicarbonate and other electrolytes with sodium
• Antiport mechanism involving
Na+ and H+ across the tubule wall.
> Active transport against
chemical or electrical gradients,
e.g. movement of sodium from
tubular cell into the interstitium
via the Na+-K+-ATPase pump.
The driving force behind the reabsorption of sodium and other ions is the Na+-K+-ATP ase pump, which acts at the basilar wall of the tubular cell.
It extrudes 3 Na+ into the
interstitium in exchange
for 2 K+ that are
pumped into the cell.
This creates a Na+
concentration gradient between
the tubular lumen
and
cell such that sodium
can diffuse passively
into the cell from the lumen.
The movement of other molecules and
electrolytes is coupled to sodium
reabsorption by antiport and symport
mechanisms.
Water diffuses along its
concentration gradient.
The K+ that is pumped into the
cell passively diffuses
out into the interstitium along its
concentration gradient.
How much glucose is filtered by the kidney, and how much can be reabsorbed?
> 100% of plasma glucose is filtered.
> Active transport mechanisms become
saturated at higher concentrations
of solute and their maximum
rate of transport (reabsorption) is reached.
This is known as the transport maximum (Tm) for a given solute.
> Glucose reabsorption is proportional to the amount filtered, and hence to its plasma concentration multiplied by the GFR, up to the transport
maximum, which is about 180 g/dL
or 10 mmol/L of
glucose in venous plasma.
> Once the renal threshold for glucose is reached, not all the filtered glucose is reabsorbed and glucose starts to appear in the urine.
What physiological factors increase the GFR?
These are the same factors that
govern filtration across any capillary bed:
> Permeability of capillaries
> Size of capillary bed (surface area)
> Hydrostatic and osmotic pressure
gradients across the capillary wall
(Starling’s forces).
The GFR is proportional to glomerular capillary pressure, which in turn depends on:
> Local autoregulation,
mediated by renal sympathetic nerves
> Mean arterial pressure
