high endothelial venule

Examples of high endothelial venule in the following topics:

• Venules

  • Many venules unite to form a vein.
  • Venule walls have three layers: an inner endothelium composed of squamous endothelial cells that act as a membrane, a middle layer of muscle and elastic tissue, and an outer layer of fibrous connective tissue.
  • In contrast to regular venules, high-endothelial venules (HEV) are specialized post-capillary venous swellings.
  • They are characterized by plump endothelial cells as opposed to the usual thinner endothelial cells found in regular venules.
  • Venules form when capillaries come together and converging venules form a vein.

• Capillaries

  • Capillaries, which form part of the micro-circulation, are the smallest of the body's blood vessels at between 5-10 μm in diameter with the endothelial vessel wall of only one cell thick.
  • Capillaries form a network through body tissues that connects arterioles and venules and facilitates the exchange of water, oxygen, carbon dioxide, and many other nutrients and waste substances between blood and surrounding tissues.
  • Continuous - Endothelial cells provide an uninterrupted lining, only allowing small molecules like water and ions to diffuse through tight junctions.
  • Fenestrated - Fenestrated capillaries have pores in the endothelial cells (60-80 nanometers in diameter) that are spanned by a diaphragm of radially-oriented fibrils.
  • When blood pressure increases, the arterioles that lead to the capillary bed are stretched and subsequently constrict to counteract the increased tendency for high pressure to increase blood flow.

• Arteries, Veins, and Capillaries

  • The capillaries converge again into venules that connect to minor veins, which connect to major veins that take blood high in carbon dioxide back to the heart.
  • The inner, tunica intima is a smooth, inner lining of endothelial cells that are in contact with the red blood cells.
  • Unlike veins and arteries, capillaries have only one tunic; this single layer of cells is the location of diffusion of oxygen and carbon dioxide between the endothelial cells and red blood cells, as well as the exchange site via endocytosis and exocytosis.

• Blood Vessel Structure

  • Capillaries in turn merge into venules, then into larger veins responsible for returning the blood to the heart.
  • The inner layer (tunica intima) is the thinnest layer, formed from a single continuous layer of endothelial cells and supported by a subendothelial layer of connective tissue and supportive cells.
  • In smaller arterioles or venules, this subendothelial layer consists of a single layer of cells, but can be much thicker in larger vessels such as the aorta.
  • Capillaries consist only of the thin endothelial layer of cells with an associated thin layer of connective tissue.

• Transcytosis

  • Substances are transported through the endothelial cells themselves within vesicles.
  • The substance to be transported is endocytosed by the endothelial cell into a lipid vesicle which moves through the cell and is then exocytosed to the other side.
  • Exploiting the body's own transport mechanism can help to overcome the high selectivity of this barrier, which blocks the uptake of most therapeutic antibodies into the brain and central nervous system.

• Blood-Brain Barrier

  • Modalities for drug delivery through the BBB entail its disruption by osmotic means, biochemically by the use of vasoactive substances, or by localized exposure to high-intensity focused ultrasound.
  • The BBB results from the selectivity of the tight junctions between endothelial cells in CNS vessels that restrict the passage of solutes.
  • Each of these transmembrane proteins is anchored into the endothelial cells by another protein complex.
  • This barrier also includes a thick basement membrane and astrocyte cell projections called astrocytic feet (forming the thin barrier called the glia limitans) that surround the endothelial cells of the BBB, providing biochemical support to those cells.
  • The BBB endothelial cells restrict the passage of substances from the bloodstream to a greater extent than endothelial cells in capillaries elsewhere in the body.

• Macrophages

  • Most macrophages express high levels of interferon-gamma, a mechanism through which antigen presentation and T cell activation is enhanced.
  • To enter a tissue, the monocyte in peripheral blood must adhere to the vessel wall, cross the endothelial cell barrier, and then migrate towards the stimulus; a process known as chemotaxis.
  • They respond to local stimuli by producing cytokines that make the endothelial cells more sticky (through the increased expression of cell adhesion molecules such as P-selectin) and so-called chemokines, that promote the directed migration of inflammatory cells.

• Platelets and Coagulation Factors

  • The inner surface of blood vessels is lined with a thin layer of cells (endothelial cells) that under normal situations produce chemical messengers that inhibit platelet activation.
  • When the endothelial layer is injured, collagen is exposed, releasing other factors to the bloodstream which attracts platelets to the wound site.
  • Non-physiological flow conditions (especially high values of shear stress) caused by arterial stenosis or artificial devices (e.g. mechanical heart valves or blood pumps) can also lead to platelet activation.

• Biological and Medical Applications

  • The capillaries connect to venules, into which the deoxygenated blood passes from the cells back into the blood.
  • The micro-circulation (arterioles, capillaries and venules) constitutes most of the area of the vascular system and is the site of the transfer of O2 into the cells.
  • Another important form of fluid movement is osmosis—the transport of water through a semipermeable membrane (shown in ) from a region of high concentration to a region of low concentration.

• Blood Flow Through the Body

  • With each rhythmic pump of the heart, blood is pushed under high pressure and velocity away from the heart, initially along the main artery, the aorta .
  • If all of the sphincters are closed, then the blood will flow directly from the arteriole to the venule through the thoroughfare channel.
  • After the blood has passed through the capillary beds, it enters the venules, veins, and finally the two main venae cavae (singular, vena cava) that take blood back to the heart.