Question

where in our body are proteins absorbed into the bloodstream after being broken down into amino acids.

Answer 1

Most amino acids are absorbed across the epithelial cells (enterocytes) of the small intestine — especially the duodenum and jejunum — after proteins are broken down by stomach and pancreatic proteases and brush-border peptidases.

Key points:
- Protein digestion: pepsin in the stomach, then pancreatic proteases (trypsin, chymotrypsin, carboxypeptidases) and brush‑border peptidases break proteins into amino acids, dipeptides and tripeptides.
- Uptake by enterocytes: amino acids are taken up at the apical membrane by Na+-dependent amino acid transporters; di‑ and tripeptides are taken up by the H+-coupled PEPT1 transporter and then hydrolyzed to free amino acids inside the cell.
- Entry to the bloodstream: amino acids exit the basolateral membrane into the hepatic portal vein via specific transporters and are carried to the liver.
- Note: intact proteins are generally not absorbed (except in newborns for some immunoglobulins); unlike fats, proteins do not enter the lymphatic system.