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A liver sinusoid is a type of capillary known as a sinusoidal capillary, discontinuous capillary or sinusoid, that is similar to a fenestrated capillary, having discontinuous endothelium that serves as a location for mixing of the oxygen-rich blood from the hepatic artery and the nutrient-rich blood from the portal vein. [1]
Types of capillaries: (left) continuous with no big gaps, (center) fenestrated with small pores, and (right) sinusoidal (or 'discontinuous') with intercellular gaps. Blood capillaries are categorized into three types: continuous, fenestrated, and sinusoidal (also known as discontinuous).
In continuous capillaries the endothelial cells are tightly spaced, allowing only small molecules like ions or water to diffuse through the intercellular clefts (the gaps between the endothelial cells). In fenestrated and sinusoidal capillaries there is more space between the cells, allowing the diffusion of macro-molecules and some proteins.
Diffusion through the capillary walls depends on the permeability of the endothelial cells forming the capillary walls, which may be continuous, discontinuous, and fenestrated. [4] The Starling equation describes the roles of hydrostatic and osmotic pressures (the so-called Starling forces ) in the movement of fluid across capillary endothelium .
The peptides released at the median eminence enter the primary plexus capillaries. From there, they are transported to the anterior pituitary via hypophyseal portal veins to the secondary plexus. The secondary plexus is a network of fenestrated sinusoid capillaries that provide blood to the anterior pituitary.
The structure of the vasa vasorum varies with the size, function and location of the vessels. Cells need to be within a few cell-widths of a capillary to stay alive. In the largest vessels, the vasa vasorum penetrates the outer (tunica adventitia) layer and middle (tunica media) layer almost to the inner (tunica intima) layer.
This is called the precapillary sphincter. The precapillary sphincter has now also been found in the brain, where it regulates blood flow to the capillary bed. [3] The sphincter can open and close the entrance to the capillary, by which contraction causes blood flow in a capillary to change as vasomotion occurs. [4] [unreliable source?
It appears that different signaling pathways regulate the constriction of capillaries by pericytes and of arterioles by smooth muscle cells. [33] Recent studies on rats have found such a signaling pathway in which after spinal cord injury and induced hypoxia below the injury, there is excess activity of monoamine receptors on pericytes which ...