sinusoidal capillary

**Capillary types (high-yield comparison):** - **Continuous**: tight junctions, intact basement membrane. Found in muscle, skin, lung, and CNS (the BBB, with astrocyte foot processes). Permit only small molecules and water. - **Fenestrated**: pores covered by a diaphragm, with an intact basement membrane. Found in the kidney glomerulus (no diaphragm, with a negatively charged basement membrane), intestinal villi, and endocrine glands. They allow rapid fluid and solute exchange. - **Sinusoidal (discontinuous)**: wide gaps between endothelial cells, a discontinuous or absent basement membrane, and large fenestrations without diaphragms. They allow passage of large proteins and cells.

**Locations:** - **Liver**: Kupffer cells (macrophages) line the sinusoids. The space of Disse lies between the endothelium and hepatocytes. Hepatic stellate (Ito) cells in the space of Disse store vitamin A and drive fibrosis (cirrhosis) when activated. Gaps allow albumin and clotting factors to enter the blood and lipoproteins to be exchanged. - **Spleen**: red pulp sinusoids filter old or abnormal RBCs. Cells must squeeze through the interendothelial slits. Hereditary spherocytosis cells get trapped, causing extravascular hemolysis. - **Bone marrow**: allows newly formed blood cells to enter the circulation. - **Endocrine organs and others**: adrenal, pituitary, and some lymphoid tissue.

**Clinical correlations:** - **Portal hypertension and cirrhosis**: Sinusoidal fibrosis and capillarization of the sinusoids (a basement membrane forms and the fenestrations are lost) impair exchange. This leads to ascites, varices, and splenomegaly. - **Extramedullary hematopoiesis**: occurs in the liver and spleen sinusoids when bone marrow fails (e.g., myelofibrosis). - **Peliosis hepatis**: blood-filled cystic spaces from sinusoidal dilation, associated with anabolic steroids, OCPs, and Bartonella henselae in HIV patients (bacillary peliosis). - **Budd-Chiari syndrome and sinusoidal obstruction syndrome (veno-occlusive disease)**: hepatic venous outflow obstruction, or sinusoidal endothelial injury (e.g., after HSCT with high-dose chemotherapy or pyrrolizidine alkaloids), causes sinusoidal congestion, painful hepatomegaly, ascites, and centrilobular necrosis. - **Metastasis**: the leaky, slow-flow sinusoids of the liver and bone marrow are common sites for hematogenous metastases. - **Sickle cell disease and the spleen**: sickled cells obstruct splenic sinusoids, causing infarcts and autosplenectomy. This leads to susceptibility to encapsulated organisms.

**Mnemonic**: Sinusoidal = "Spleen, Sinusoids, Stuff passes (even cells)": liver, spleen, bone marrow.