Regulation of Tubular Reabsorption

Because it is essential to maintain a precise balance between tubular reabsorption and glomerular filtration, there are multiple nervous, hormonal, and local control mechanisms that regulate tubular reabsorption, just as there are for control of glomerular filtration. An important feature of tubular reabsorption is that reabsorption of some solutes can be regulated independently of others, especially through hormonal control mechanisms.

Glomerulotubular Balance—The Ability of the Tubules to Increase Reabsorption Rate in Response to Increased Tubular Load

One of the most basic mechanisms for controlling tubular reabsorption is the intrinsic ability of the tubules to increase their reabsorption rate in response to increased tubular load (increased tubular inflow). This phenomenon is referred to as glomerulotubular balance. For example, if GFR is increased from 125 ml/ min to 150 ml/min, the absolute rate of proximal tubular reabsorption also increases from about 81 ml/ min (65 per cent of GFR) to about 97.5 ml/min (65 per cent of GFR). Thus, glomerulotubular balance refers to the fact that the total rate of reabsorption increases as the filtered load increases, even though the percentage of GFR reabsorbed in the proximal tubule remains relatively constant at about 65 per cent.

Some degree of glomerulotubular balance also occurs in other tubular segments, especially the loop of Henle. The precise mechanisms responsible for this

Peritubular Capillary and Renal Interstitial Fluid Physical Forces

Hydrostatic and colloid osmotic forces govern the rate of reabsorption across the peritubular capillaries, just as these physical forces control filtration in the glomerular capillaries. Changes in peritubular capillary reabsorption can in turn influence the hydrostatic and colloid osmotic pressures of the renal interstitium and, ultimately, reabsorption of water and solutes from the renal tubules.

Normal Values for Physical Forces and Reabsorption Rate. As the glomerular filtrate passes through the renal tubules, more than 99 per cent of the water and most of the solutes are normally reabsorbed. Fluid and electrolytes are reabsorbed from the tubules into the renal interstitium and from there into the peritubular capillaries. The normal rate of peritubular capillary reabsorption is about 124 ml/min.

Reabsorption across the peritubular capillaries can be calculated as

Reabsorption = Kf x Net reabsorptive force

The net reabsorptive force represents the sum of the hydrostatic and colloid osmotic forces that either favor or oppose reabsorption across the peritubular capillaries. These forces include (1) hydrostatic pressure inside the peritubular capillaries (peritubular hydrostatic pressure [Pc]), which opposes reabsorption; (2) hydrostatic pressure in the renal interstitium (Pif) outside the capillaries, which favors reabsorption; (3) colloid osmotic pressure of the peritubular capillary plasma proteins (pc), which favors reabsorption; and (4) colloid osmotic pressure of the proteins in the renal interstitium (pif), which opposes reabsorption.

Figure 27-15 shows the approximate normal forces that favor and oppose peritubular reabsorption.

Peritubular capillary

32 mm Hg

Interstitial fluid

6 mm Hg

15 mm Hg

Tubular cells

Tubular lumen

Bulk flow

10 mm Hg Na+ (Atp) Net reabsorption V_V

pressure

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  • bertha
    What forces favor capillary reabsorption?
    8 months ago

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