How is glucose absorbed from the gastrointestinal tract? How are blood glucose levels maintained?
Glucose is absorbed by sodium glucose co-transport mechanism.
Blood glucose levels are maintained by liver, insulin, glucagon and some other hormones.
Absorption of glucose The transport of nutrients from intestinal lumen into blood stream is called absorption.
The carbohydrate foods we eat, are digested into monosaccharides (glucose, fructose, galactose). Almost 80 percent of these monosaccharides are glucose. Glucose is absorbed in small intestine by absorptive cells.
The process of transport of glucose from intestinal lumen into the absorptive cell has two stages. In the first stage sodium ion from inside the cells are transported to interstitial fluid. This leads to low sodium concentration inside the cell.
Then starts the second stage. As a result of low sodium inside the cells, sodium ions are transported from intestinal lumen by facilitated diffusion (diffusion with the help of transport protein). The transport protein that helps in this case, has a peculiarity. It transports sodium ion with glucose. Actually this protein drags glucose along with sodium ion from the lumen into the cell.
Once into the cell, other transport proteins and enzymes cause facilitated diffusion of glucose through basal and lateral membranes of the cell into interstitial fluid and from there into the blood.
Glucose absorption :
Regulation of blood glucose level In a normal person the blood glucose level is narrowly controlled by following mechanisms:
- Liver acts as a blood glucose buffer system. When after a meal blood glucose rises and insulin is secreted, two thirds of the glucose absorbed from gut is stored in the liver in the form of glycogen. Then during succeeding hours, liver releases glucose back into blood to maintain a narrow range of blood glucose level.
- When blood glucose concentration rises too high, insulin is secreted; which decreases the level. Conversely when glucose level drops too low glucagon is secreted and restores it. These two are pancreatic hormones.
- In case of drop of glucose level for a long period, epinephrine (adrenaline), growth hormone and cortisol are secreted from adrenal gland, which restores the level.