Tubular Processing of the Glomerular Filtrate

After the initial filtration performed on the barrier formed by capillary glomerular basement membrane and Bowman's capsule, begin important steps: reabsorption and tubular secretion. The reabsorption is particularly important since, as seen, the kidney filters approximately 180 liters of plasma a day. Imagine if it was necessary to replace all the liquid in the body throughout the day.

The processes of tubular reabsorption and secretion involving passage through two barriers: the tubular epithelium and endothelial cells that form the peritubular capillaries. The passage occurs by a transcellular route (across the apical membrane, facing the lumen of the tubule and the basolateral membrane, facing the interstitium) or paracellular route (through the matrix of tight junctional complexes, that bind to each epithelial cell its neighbor).

The principal transport mechanisms by which these processes occur is the movement by diffusion, by osmosis, moves through channels and transporters.

Let us examine the possible routes in passing water / solutes from blood to lumen and opposite.

The transcellular route

We will use as an example the reabsorption of sodium (Na). First, is the removal of active sodium via Na-K-ATPase via the basolateral membrane of the cell into the interstitium (this creates a low sodium concentration into the cell, allowing it to move from the lumen into the cell by a variety of simportes, antiportes and channels. excess sodium in the interstitial side causes there to be movement of anions to balance the positive charge. accumulation of sodium and anions in the interstitial space produces osmotic gradient between lumen and interstitium causing movement of water (which going into the interstitium). accumulation of salt and water in the interstitium promotes mass flow of water and solutes into the peritubular capillaries, driven by Starling forces.

The paracellular route

As the water flows sodium and anions, through the epithelium, the volume remaining in the lumen decreases. Therefore, any solution that has not specifically been transported by the transcellular route will be more concentrated. If the tight junctional complexes are permeable to the substance in question is "leakage" of the substance by diffusion.

This fact occurs with various solutes, such as urea, calcium and magnesium in the proximal tubule.

Limit the rate of active transport

All solute is transported by active has its maximum transfer lumen into the interstitial or vice versa. This limit is due to the saturation of specific transport systems involved, and occurs when the amount of solute which reaches the tubule exceed the capacity of carrier proteins and specific enzymes involved in the transport process.

An example that can be given is that of the glucose. Your maximum transport revolves around 320mg/min. If a larger amount that reach the tubules probably part will not be reabsorbed and therefore we have a condition called glucosuria (glucose is excreted in the urine).

The Passive Transport

The passive transport (one who does not expends energy) is also important in mechanisms of exchanges between the means already mentioned.

Water, for example has constantly osmosis through the less concentrated to the highest concentration. However, osmosis does not occur everywhereof renal tubules and since the thick ascending limb of Henle's loop, the water permeability does not exist. Moreover there is a variation in the rate of water permeability depending on the presence of ADH in the distal tubules and collecting tubules.

Much of the osmotic flow occurs through the paracellular route, but can also occur through the cells themselves through channels called aquaporins.

Water reabsorption is closely linked to reabsorption by active transport of sodium.

Another example of a passive reabsorption that can be given is the chloride ion. This often suffers diffusion when the sodium is reabsorbed via tubular epithelial cell.

Overview of events happening in the renal tubules

Region
Event
Bowman's capsule
Filtration
Proximally convoluted tubule
Reabsorption and secretion
Slender branch Henle's loop
Reabsortion
Thick branch Henle's loop
Reabsortion
Distal convoluted tubule
Reabsorption and secretion
The collecting duct
Reabsorption and secretion

Some questions about renal physiology

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