Although the liver is the largest internal organ, it is, in a sense, only one to two cells thick. This is because the liver cells, or he-patocytes, form hepatic plates that are one to two cells thick. The plates are separated from each other by large capillary spaces called sinusoids (fig. 18.20).
The sinusoids have extremely large pores (called fenestrae) and, unlike other capillaries, lack a basement membrane. This makes the hepatic sinusoids much more permeable than other capillaries, even permitting the passage of plasma proteins with protein-bound nonpolar molecules, such as fat and cholesterol. The sinusoids also contain phagocytic Kupffer cells, which are part of the reticuloendothelial system (chapter 15). The fenestrae, lack of a basement membrane, and plate structure of the liver provide intimate contact between the hepatocytes and the content of the blood.
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