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Active transport needed in absorption of nutrients?

Wondering if someone can explain this to me...

Good blood supply to villi means the nutrients that have diffused into the blood will be transported away and the concentration gradient will remain high, which should allow the same rate of diffusion.

So why am I being told active transport is needed?

If anyone can help thanks

Reply 1

barrinalo

Some substance can be absorbed through diffusion some cant. For example I think glucose must be pumped in through a glucose transporter. Some stuff like ions can diffuse in. Also if there are too much nutrients, then even with adequate blood supply sometimes it cant maintain the gradient. I think.

Reply 2

Tangmashi

Ok, so what you are saying is that although the flow of blood from hepatic portal is high, the diffused nutrients may not be carried away quick enough and the concentration can balance in the ileum and blood. Yes that makes sense thanks!

I don't think I need to know about particular substances and ions right now, though something to do with sodium (?) and active transport sounds familiar.

One more question you might know:

Glucose and amino acids enter liver and the liver distributes them to the organs. But the fats go through lymph b/c they are too big molecules (?), then they enter blood after liver. So how are they distributed around the body and what are they used for exactly?

Reply 3

barrinalo

Yes sodium ions are used along with secondary active transport for the glucose transporter. Actually the villi have a lacteal which is part of the lymph system. The lipids will enter the lymph system via the lacteal. They don't go into the lymph because they are too big, the body just processes the fat differently. At first I thought it is due to solubility, since lipids are not soluble in water, but then lipids in the lymph system must also be dissolved in water. They are bound to chylomicrons to help them become soluble. I am not aware that they enter the blood after the liver. I would assume they are distributed via the lymph system and the blood as well. Lipids are used for a variety of body functions, for example the production of phospholipids for use in cell membrane, adipose tissue for protection and energy storage, metabolised to release energy, production of lipid based hormones such as testosterone etc.