The blood carries the nutrients from the gastrointestinal tract, where they are absorbed toward the body tissues, where they are used, changed into other substances or stored. The blood also carries the breakdown products of the nutrients’ metabolism toward the liver and kidneys from where they are excreted via the stool and urine.
Some nutrients, for example glucose, amino acids, some minerals (sodium, potassium) and alcohol, can travel via the blood independently–without the assistance of other substances.
Other nutrients can travel via the blood only with the help of carriers, which are mostly proteins. For example, the protein transferrin carries iron and transcobalamin carries vitamin B12.
Lipids (fatty acids, triglycerides and cholesterol) are carried in the blood as part of complex molecules called lipoproteins, for example low-density lipoproteins (LDL) and high density lipoproteins (HDL).