Blood capillaries sinusoidal

Capillaries are the exchange vessels of the body. They have structural variations to allow different levels of metabolic exchange (of exogenous and endogenous substances) between blood and the surrounding tissues. The structure of the walls varies depending on their resident tissue. There are three major types of blood capillaries continuous fenestrated and sinusoidal (discontinuous) [1] ... 

Figure 5.1 Schematic illustration of the structure of the wall of different classes of blood capillaries. (1) Continuous capillary (as found in the general circulation). The endothelium is continuous with tight junctions between adjacent endothelial cells. The subendothehal basement membrane is also continuous. (2) Fenestrated capillary (as found in exocrine glands and the pancreas). The endothelium exhibits a series of fenestrae which are sealed by a membranous diaphragm. The subendothehal basement membrane is continuous. (3) Discontinuous (sinusoidal) capillary (as found in the liver, spleen and bone marrow). The overlying endothelium contains numerous gaps of varying size. The subendothehal basement is either absent (hver) or present as a fragmented interrupted structure (spleen, bone marrow)...

S. Minot coined the term sinusoid to describe the blood capillaries leading in a radial fashion to the central vein of the lobule. 

Fig. 3 Scanning electron microscope images of blood vessels in normal tissues and blood vessels of tnmor. Normal capillary of the pancreas (A), colon (intestinal villi) (B), and liver (sinusoid) (C), and enlarged image of blood capillary of normal liver (D) are shown. (E) Metastatic tumor nodule (area ) in the normal liver is shown. (F) Tumor vessels at capillary level (larger magnification) and showing rough surface, and early phase of extravasating vessels (shown by arrows). No leakage of polymer is seen in normal tissues (A-D), whereas tumor-selective extravasation of polymer (by EPR effect) is seen clearly in the tumor nodule (E)...

Particle size. Particles greater than 7 pm are larger than blood capillaries ( 6 pm) and become entrapped in the capillary beds of the lungs (which may have fatal effects). The majority of particles that pass the lung capillary bed accumulate in the elements of the RES (spleen, liver and bone marrow). The degree of splenic uptake increases with particle size. Removal of particles > 200 nm is due to a non-phagocytic process (physical filtration) in the spleen and phagocytosis (by Kupffer cells) by the liver. Particles < 200 nm decreases splenic uptake and the particles are cleared by the liver and bone marrow. Colloidal particles not cleared by the RES can potentially exit the blood circulation via the sinusoidal fenestration of the liver and bone marrow. 

The 3 10 cells in the liver—particularly the hepatocytes, which make up 90% of the cell mass—are the central location for the body s intermediary metabolism. They are in close contact with the blood, which enters the liver from the portal vein and the hepatic arteries, flows through capillary vessels known as sinusoids, and is collected again in the central veins of the hepatic lobes. Hepatocytes are particularly rich in endoplasmic reticulum, as they carry out intensive protein and lipid synthesis. The cytoplasm contains granules of insoluble glycogen. Between the hepatocytes, there are bile capillaries through which bile components are excreted. 

The liver contains an enormous number of hepatocytes that perform the various functions noted above. The hepatocytes are contained within minute units known as hepatic lobules, in which the cell layers (which are one or two cells thick) are in contact with networks of minute blood channels - the sinusoids - which ultimately join the venous capillaries. Capillaries carrying blood from the hepatic artery and the portal vein empty separately into the sinusoids. The walls of sinusoids and liver cells are incomplete, and blood is brought into direct contact with the hepatocytes. 

Occasionally toxic compounds can directly damage the hepatic sinusoids and capillaries. One such toxic compound is monocrotaline, a naturally occurring pyrrolozidine alkaloid, found in certain plants (Heliotropium, Senecio, and Crotolaria species). Monocrotaline (Fig. 7.7) is metabolized to a reactive metabolite, which is directly cytotoxic to the sinusoidal and endothelial cells, causing damage and occlusion of the lumen. The blood flow in the liver is therefore reduced and ischemic damage to the hepatocytes ensues. Centrilobular necrosis results, and the venous return to the liver is blocked. Hence, this is known as veno-occlusive disease and results in extensive alteration in hepatic vasculature and function. Chronic exposure causes cirrhosis. 

Blood entering the hepatic sinusoids carries many bacteria from the digestive tract. The phagocytic Kupffer cells interspersed among the typical endothelial cells lining this specialized capillary bed rapidly phagocytize more than 99% of bacteria and other foreign particles in the blood. 

Arterial blood reaches the pituitary gland via the superior hypophyseal artery, a branch of the internal carotid artery. Venous blood is supplied through a venous portal system that originates in the median eminence of the hypothalamus and ends in sinusoidal capillaries of the pituitary gland. This venous system is known as the hypothalamic-hypophyseal portal system. This system carries neurosecretory hormones from the hypothalamus to the adenohypophysis. These hypothalamic factors stimulate or inhibit the release of hormones from the adenohypophysis. Retrograde flow from the adenohypophysis to the median eminence of the hypothalamus is also believed to occur. With upstream flow, pituitary hormones can reach the hypothalamus and influence hypothalamic function through a short feedback loop. 

Kishi et al. (169)evaluated the acute toxicity of Lipiodol infusion into the hepatic arteries (HAD of beagles and found the influence of Lipiodol HAI to be dose dependent. The infused Lipiodol first passed through an arterioportal communication and distributed through the hepatic sinusoids to pulmonary capillaries and thence into systemic blood circulation. The circulation and embolization of oil droplets were found in the renal tubular cells of supracapsular cortex, the choroid plexus, the vascular endothelium, and the pancreatic duct epithelium, showing a process of intracellular collection of Lipiodol from the systemic blood circulation and of further metabolism-provoking cellular reactions. 

Soluble macromolecules of both natural and synthetic origins have been used as drug carriers. When compared with the particulate carriers, soluble macromolecules (i) encounter fewer barriers to their movement around the body and can enter into many organs by transport across capillary endothehum or in the liver by passage through the fenestration connecting the sinusoidal lumen to the space of Disse (ii) penetrate the cells by pinocytosis, which is a phenomenon universal to aU cells and which, unlike phagocytosis, does not require an external stimulus and (Hi) can be found in the blood many hours after their introduction (particulate carriers, in contrast, are rapidly cleared from the blood by the RES). The fate of soluble macromolecules in animals and humans, with special reference to the transfer of polymers from one body compartment to another, has been reviewed by Drobnik and Rypacek (67). 

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