Nanosized Drug Delivery Vectors and the Reticuloendothelial System - Intracellular Delivery: Fundamentals and Applications - page 158

Fig. 2 Diagram of
the organs that make
up the reticuloen-
dothelial system

Thymus

Lungs

Liver
Spleen

Lymph nodes

Bone marrow

The RES is therefore largely responsible for the removal of nanomedicines from
the systemic circulation. The uptake of particles is largely size dependent.
Compared to circulating biological proteins, drug delivery particles are usually
large in size (Fig. 1 ), making them appear as foreign materials to the monophago-
cytic system. Opsonisation of drug delivery particles by plasma proteins and lipo-
proteins accelerates and enhances the recognition process. The pathophysiology of
several major organs of the RES (liver, spleen, lymph nodes and lungs) and how
this relates to particle uptake is therefore described in the following section.

2.1 The Liver

The liver is one of the most important metabolic organs in the body and is a part of the
digestive system. The visceral surface of the liver contains the porta hepatis, the site at
which vessels and ducts enter and leave the liver, including the portal vein, the hepatic
artery and the common hepatic, cystic and bile ducts. At any one time, approximately
20% of the bodies blood supply resides in this organ and the liver is a major site for
the removal of drugs absorbed into the blood from the gastrointestinal tract.