Blood Cleaning by the Kidneys - Part (2)

(2) Tubular Reabsorption

Only about 1% of the glomerular fitrate actually leaves the body because the rest (the other 99%) is reabsorbed into the blood while it passes through the renal tubules and ducts. This is called tubular reabsorption and occurs via three mechanisms. They are:

• Osmosis
• Diffusion, and
• Active Transport.

Reabsorption varies according to the body's needs, enabling the body to retain most of its nutrients.
The processes of tubular reabsorption occur in the following order (see also the diagram of a kidney nephron to trace the flow of fluid through the 3 stages outlined below):

In the PCT
Most of the volume of the fitrate solution is reabsobed in the proximal convoluted tubule (PCT). This includes some water and most/all of the glucose (except in the case of diabetics).

Most of the energy consumed by the kidneys is used in the reabsorption of sodium ions (Na⁺), which are solutes - that is, they are dissolved in the water component of the fitrate solution.
As the concentration of Na⁺ in the filtrate solution is high (about the same as the concentration of Na⁺ in blood plasma), Na⁺ moves from the tubular fluid into the cells of the PCT. In the cases of many Na⁺ ions this occurs with the help of symporters. Symporters simultaneously facilitate passage through the PCT membrane of both Na⁺ and another substances/solutes. Other such substances that are reabsorbed with Na⁺ in this way include glucose (an important type of sugar), amino acids, lactic acid, and bicarbonate ions (HCO₃^-). These then move on through cells via diffusion and/or other transport processes.
A short way to summarize the above is to say that solutes are selectively moved from the glomular filtrate to the plasma by active transport. (However, almost all glucose and amino acids, and high but variable amounts of ions, are reabsorbed again later - see the next section, below).

Following the movement of solutes (including Na⁺ ), water is then also reabsorbed by osmosis. About 80% of the filtrate volume is reabsorbed in this way. As this part of the reabsorption process is not controlled by the proximal tubule itself, it is sometimes called obligatory water reabsorption.

In the Loop of Henle

The remaining water (together with the dissolved salts and urea) passes from the PCT into the descending limb of Henle. It then passes along the Loop of Henle, and up the ascending limb of Henle.

The different permeability properties of the two limbs of the Loop of Henle, together with their counterflow arrangement, allows a countercurrent multiplication to generate a high solute concentration in the tissue fluid of the medulla (that is, outside of the tubules). The highest solute concentrations are generated deep in the medulla. This is explained as follows:

1. Descending Limb of Loop of Henle
   The epithelium lining of the descending limb of Henle is relatively permeable to water - but much much less permeable to the salts Na⁺ and Cl^-, and to urea. Therefore water gradually moves from the descending limb and into the interstitium (surrounding the tubules) as fluid flows through this part of the system of renal tubules.
2. Thin Ascending Limb of Loop of Henle
   The thin ascending limb of Henle differs from the descending limb in that it is impermeable to water (so the water that is inside the tubule at this stage generally remains inside it), but is highly permeable to Na⁺ and Cl^-, and somewhat permeable to urea. Therefore while the tubular fluid flows back towards the renal cortex, Na⁺ and Cl^- (which are more concentrated in the tubular fluid than in the interstital fluid) diffuse from the tubules into the interstitium. Some urea also enters the tubules at this stage - but the loss of NaCl from the tubular fluid greatly exceeds the gain in urea.
3. Thick Ascending Limb of Loop of Henle
   The thick ascending limb of Henle (and its continuation into the first part of the DCT), reabsorbs NaCl from the tubular fluid via a different transport process from that of the thin ascending limb of Henle.

The overall effect of the processes outlined above is that the concentation of the fluid inside the renal tubules that form the Loop of Henle is highest at the deepest part of the renal medulla, and is less concentrated in the renal cortex. This is what is meant by the "concentration gradient" of the Loop of Henle. The term "counter-current" is also used in descriptions of the Loop of Henle - and refers to the tubular fluid flowing in opposite directions along the descending and ascending limbs (as indicated by the thin red arrows in the diagram above.

In the DCT
The water, urea, and salts contained within the ascending limb of Henle eventually pass into the distal convoluted tubule (DCT).

The DCT reacts to the amount of anti-diuretic hormone (ADH) in the blood:

• The more ADH is present in the blood, the more water is re-absorbed into it. This happens because the presence of ADH in the blood causes the cells in the last section of the DCT (and associated tubules and collecting ducts) to become more permeable to water, therefore they allow more water to pass from the tubular fluid back into the blood. This results in more concentrated urine.
• The opposite is also true, i.e. if the level of ADH in the blood is reduced then the cells in the latter sections of the DCT (and associated tubules and collecting ducts) becomes less permeable to water therefore less water is able to pass from the tubular fluid back into the blood - which results in less concentrated urine.

The amount of ADH in the blood may be affected by conditions such as diabetes insipidus, or by consumption of diuretics* in the diet (*substances that occur in some foods and drinks).

Click here to view the next page: Blood Cleaning by the Kidneys - Part (3): Tubular Secretion.