Renal Physiology: Reabsorption part 1

As I mentioned in my last post, if filtration was all there was to it, we'd pee every four minutes. Thankfully reabsorption is a thing! Roughly all sugars, 99.5% of salts and 99% of water is reabsorbed so that we don't pee ourselves to death.

General Stuff to do with Reabsorption

When substances are reabsorbed, they need to pass either through or between the cells of the tubules, pass through the interstitial space and go into the blood. Substances that pass through the cells obviously need to pass through two sides of the cell: the luminal membrane (that faces the inside of the tubule) and the basolateral membrane (which faces the outside of the tubule). Substances can move across or between cells down concentration gradients if there are pores or channels permeable to that particular substance. There are also pumps that can move substances up concentration gradients if need be.

Proximal Tubule Reabsorption

I'm going to talk about sodium reabsorption first, as it actually drives the reabsorption of a lot of other stuff.

Around 67% of sodium is reabsorbed in the proximal tubule. A Na⁺/K⁺ pump on the basolateral membrane pumps sodium out of the cell and potassium into the cell, so that the Na⁺ concentration inside the cell is low. This allows Na⁺ to passively diffuse into the cell via Na⁺ channels located on the luminal membrane. (As for the potassium- that goes back out via K⁺ channels in the basolateral membrane.)

Aside from passive Na⁺ channels, there are also cotransporters that transport Na⁺ along with other stuff. Glucose and amino acids are reabsorbed via Na⁺/glucose cotransporters and Na⁺/amino acid transporters, respectively. There are also transporters that transport Na⁺ and H⁺, but in opposite directions, allowing H⁺ to be secreted while Na⁺ is reabsorbed. As there are only a finite amount of these transporters, however, only a finite amount of glucose and amino acids can be reabsorbed. The highest amount that can be reabsorbed is called the Tm. For example, the Tm of glucose is 375mg/min- any excess that comes through will be excreted in the urine.

Cl^- is reabsorbed as a consequence of Na⁺ reabsorption. (See, I told you that sodium reabsorption drives the reabsorption of a lot of other stuff!) You see, reabsorption of Na⁺ creates an electrochemical gradient, with more positive stuff in the blood. This causes Cl^- to diffuse down the electrochemical gradient. Cl^- actually passes through the gap junctions between cells lining the tubule.

H₂O is yet another substance reabsorbed as a consequence of Na⁺ reabsorption. The reabsorption of Na⁺ creates an osmotic gradient for water to become reabsorbed.

In my next post, I will talk about reabsorption in the Loop of Henle and how it allows us to produce urine of varying concentrations.