It is in the proximal convoluted tubule that we reabsorb most of the materials that we need to reabsorb from the filtrate. The reabsorption of this material is called tubular reabsorption.
When we talk about reabsorption, it is clear that the materials we are taking back up must eventually get back into the blood. But it doesn't go directly into the blood from the tubules. Instead, it is taken out of the filtrate by the cells that make up the tubules and sent into the interstitial fluid of the cortex or medulla. From there, these molecules will diffuse into the blood through the peritubular capillaries. The peritubular capillaries are more porous than most capillaries and also have very low blood pressure (because they are the second set of capillaries); therefore, they are quite good at allowing material to diffuse back into them.
Let's now talk about what, specifically, the proximal convoluted tubule (PCT) does and how it does it.
The cells that line the PCT have to be able to pick up dissolved solutes (like electrolytes and glucose). In order to do that, they need to have protein channels in their membranes. Remember, for anything hydrophilic (polar) and small to be transported across a membrane you need a protein channel. Some of the materials will be moving along their concentration gradients, while some will be moving against their concentration gradients. If they are moving along their gradient, they will move through a passive transport channel (facilitated diffusion). If they are moving against their gradient, they will be moving through an active transport channel (also called a pump). Since most of what the PCT needs to pick back up requires protein channels in the membrane, the PCT cells are packed full of these channels.
I have tried to illustrate this for you in this picture. Here you can see a simple cuboidal epithelium, like we would find in the PCT. The apical surface of the PCT cells is folded into microvilli, to increase surface area. A lot of surface area is necessary because the protein channels have to lie in the membrane and must be exposed to the filtrate in order to do their job.
The epithelial cells are connected to each other by tight junctions. These tight junctions block filtrate from sneaking between the PCT epithelial cells. That is important if we are going to have selective re-uptake of materials from the filtrate. Without the tight junctions, the nitrogenous wastes could get back into the blood through these areas!
The basal side of the membrane also has some folds (but I didn't draw them). Note that both the apical and basal membranes are loaded with channels.
You should be able to imagine that an item within the filtrate, Na+, for example, could be transported across the apical membrane into the cell, and then across the basal membrane, out of the cell, through protein channels. In this manner, Na+ can be reabsorbed from the filtrate into the interstitial fluid. Eventually it can diffuse into the blood through the peritubular capillaries.
Every solute item that needs to be reabsorbed has protein channels just for it. There are glucose channels and Na+ channels and K+ channels and H+ channels. And more. Therefore, as fluid passes through the PCT, its solute molecules are being picked out of it.
This explains how reabsorption of water occurs. You see, as solute is picked out of the filtrate, the filtrate gets more dilute... or hypotonic! Remember, water always flows passively through osmosis, and the rule for osmosis is that water flows down its osmotic gradient. The osmotic gradient is from hypotonic to hypertonic. Let's follow some filtrate down the PCT...
Actually, the solute is picked out of the filtration continuously the entire way through the PCT. But any difference at all in solute concentration between the lumen of the tubule and the interstitial fluid is enough to draw the water out of the PCT. Since water and solute leaves the PCT, less filtrate enters the loop of Henle than entered the PCT; but the filtrate that does enter the loop of Henle is isotonic to the cortical interstitial fluid.
Your book uses this diagram to describe what I just wrote about.
Basically it shows a number of different solutes in the filtrate that are selectively taken up by the PCT cells through protein channels. The reabsorption of these solutes (many are shown) leads to osmosis.
The figure has more information in it, too... like it explains which ions move by which type of transport. And, actually, in doing so it gets so vague that it isn't totally correct. You will see when you do the Marieb CDs that the PCT cells are loaded with sodium-potassium pumps on their basal membranes. These pumps ensure that the PCT cells themselves maintain very low sodium ion concentrations within them. Therefore, sodium ions move passively across the apical membrane (because there's a higher concentration of sodium ions in the filtrate than in the cells). Of course, then they are pumped actively across the basal membrane-- indicating a higher concentration of sodium ions in the interstitial fluid than in the cell.
By the time the filtrate has gotten through the PCT, 70% of all the materials that need to be reabsorbed have been reabsorbed! (Marieb says 65%). Not too shabby!
By the way... there is a little bit of tubular secretion that also occurs here... but we won't go into it now.
© 2011 STCC Foundation Press