Many circulating plasma proteins are produced and secreted by the liver. The liver plays an important role in the uptake, storage, and distribution of both nutrients and vitamins from the bloodstream. It also maintains the blood glucose level and regulates circulating levels of very low-density lipoproteins (VLDLs). In addition, the liver degrades or conjugates numerous toxic substances and drugs, but it can be overwhelmed by such substances and damaged. The liver is also an exocrine organ; it produces a bile secretion that contains bile salts, phospholipids, and cholesterol. Finally, the liver performs important endocrine-like functions.
The liver produces most of the body’s circulating plasma proteins.
The circulating plasma proteins produced by the liver  include  albumins, which are involved in regulating plasma volume and tissue fluid balance by maintaining the plasma colloid osmotic pressure.  lipoproteins, in particular, VLDLs. The liver synthesizes most VLDLs, which participate in the transport of triglycerides from the liver to other organs. The liver also produces small amounts of other plasma lipoproteins, such as low-density lipoproteins (LDLs) and high-density lipoproteins (HDLs). LDLs transport cholesterol esters from the liver to other tissues. HDLs remove cholesterol from the peripheral tissues and transport it to the liver (see Folder 18.1).  glycoproteins, which include proteins involved in iron transport such as haptoglobin, transferrin, and hemopexin.  prothrombin and fbrinogen, important components of the blood-clotting cascade. nonimmune -globulins and -globulins, which also help maintain plasma colloid osmotic pressure and serve as carrier proteins for various substances (see Chapter 10, page 270).
The liver stores and converts several vitamins and iron.
Several vitamins are taken up from the bloodstream and are then stored or biochemically modified by the liver. They include  vitamin A (retinol), an important vitamin in vision. Vitamin A is the precursor of retinal, which is required for the synthesis of rhodopsin in the eye. The liver plays a major role in the uptake, storage, and maintenance of circulating levels of vitamin A. When the vitamin A levels in the blood decrease, the liver mobilizes its storage sites in the hepatic stellate cells (see page 637). Vitamin A is then released into the circulation in the form of retinol bound to retinolbinding protein (RBP). The liver also synthesizes RBP; RBP synthesis is regulated by plasma levels of vitamin A. Night blindness and multiple skin disorders are related to vitamin A deficiency.
 vitamin D (cholecalciferol), an important vitamin in calcium and phosphate metabolism. Vitamin D is acquired from dietary vitamin D3 and is also produced in the skin during exposure to ultraviolet light by conversion of 7-dehydrocholesterol. Unlike vitamin A, vitamin D is not stored in the liver but is distributed to skeletal muscles and adipose tissue. The liver plays an important role in vitamin D metabolism by converting vitamin D3 to 25-hydroxycholecalciferol, the predominant form of circulating vitamin D. Further chapter 18 Digestive System III: Liver, Gallbladder, and Pancreas LIVE R 629