Sinusoids

Sinusoids are small, irregular, vascular spaces which are closely surrounded by the parenchyma of the organ. They replace capillaries in certain organs, like liver, spleen, bone marrow, suprarenal glands, parathyroid glands, carotid body, etc.

Liver Sinusoids 

(Source: Eug/Wikipedia)

Characteristic Features of sinusoids

These differ from capillaries in the following respects;

• Their lumen is wider (up to 30 micron) and irregular.
• Their walls are thinner and may be incomplete. They are lined by endothelium in which the phagocytic cells (macrophages) are often distributed. The adventitial support is absent.
• These may connect arteriole with venule (spleen, bone marrow), or venule with venule (liver).