Once inside the lumen of the nephron, small molecules, such as ions, glucose and amino acids, get reabsorbed from the filtrate:
- Specialized proteins called transporters are located on the membranes of the various cells of the nephron.
- These transporters grab the small molecules from the filtrate as it flows by them.
- Each transporter grabs only one or two types of molecules. For example, glucose is reabsorbed by a transporter that also grabs sodium.
- Transporters are concentrated in different parts of the nephron. For example, most of the Na transporters are located in the proximal tubule, while fewer ones are spread out through other segments.
- Some transporters require energy, usually in the form of adenosine triphosphate (active transport), while others don't (passive transport).
- Water gets reabsorbed passively by osmosis in response to the buildup of reabsorbed Na in spaces between the cells that form the walls of the nephron.
- Other molecules get reabsorbed passively when they are caught up in the flow of water (solvent drag).
- Reabsorption of most substances is related to the reabsorption of Na, either directly, via sharing a transporter, or indirectly via solvent drag, which is set up by the reabsorption of Na.
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The reabsorption process is similar to the "fish pond" game that you see in some amusement parks or state fairs. In these games, there is a stream that contains different colored plastic fish with magnets. The children playing the game each have a fishing pole with an attached magnet to catch the fish as they move by. Different colored fish have different prize values associated with them, so some children will be selective and try to grab the colored fish with the highest prize value. Now suppose our nephron is the stream, the filtered molecules are the various colored fish, and our children are the transporters. Furthermore, each child is fishing for a specific colored fish. Most children start at the beginning of the stream and some spread out further downstream. By the end of the stream, most of the fish have been caught. This is what happens as the filtrate travels through the nephron.
Two major factors affect the reabsorption process:
- Concentration of small molecules in the filtrate - the higher the concentration, the more molecules can be reabsorbed. Like our children in the fish pond game, if you increase the number of fish in the stream, the children will have an easier time catching them. In the kidney, this is true only to a certain extent because: There is only a fixed number of transporters for a given molecule present in the nephron. There is a limit to how many molecules the transporters can grab in a given period of time.
- Rate of flow of the filtrate - flow rate affects the time available for the transporters to reabsorb molecules. As with our fish pond, if the stream moves by slowly, the children will have more time to catch fish than if the stream were moving faster.
To give you an idea of the quantity of reabsorption across the nephron, let's look at the sodium ion (Na) as an example:
- Proximal tubule - reabsorbs 65 percent of filtered Na. In addition, the proximal tubule passively reabsorbs about 2/3 of water and most other substances.
- Loop of Henle - reabsorbs 25 percent of filtered Na.
- Distal tubule - reabsorbs 8 percent of filtered Na.
- Collecting duct - reabsorbs the remaining 2 percent only if the hormone aldosterone is present.