Lymph capillaries

Pulmonary lymphatics are critical to clearing dust particles which have reached the lung fluid. 

Using scanning electron microscopy of casts by a resin cast into the pulmonary blood vasculature of oedematous lungs, Schraufnagel (1992) categorised four forms of pulmonary lymphatics  

1) Prelymphatics, tissue planes with a characteristic structure that connect to reservoir and conduit lymphatics  

2) reservoir lymphatics, flat structures with textured surfaces that can have ribbon-like components with small lateral buds  

3) sacculo-tubular lymphatics, saccular structures that wrap around blood vessels, bronchi, and bronchioles, and also have tubular components  

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Except for intravenous administration, all routes of drug administration require that the drug be transported from the site of administration into the systemic circulation. A drug is said to be absorbed only when it has entered the blood or lymph capillaries. The transport of drugs across membranes entails one or more of... 

The plasma membrane, a phospholipid bilayer in which cholesterol and protein molecules are embedded. The bottom layer, which faces the cytoplasm, has a slightly different phospholipid composition from that of the top layer, which faces the external medium. While phospholipid molecules can readily exchange laterally within their own layer, random exchange across the bilayer is rare. Both globular and helical kinds of protein traverse the bilayer. Cholesterol molecules tend to keep the tails of the phospholipids relatively fixed and orderly in the regions closest to the hydrophilic heads the parts of the tails closer to the core of the membrane move about freely. This model is not believed to apply to blood or lymph capillaries. (Reprinted with permission from Bretscher MS. The molecules of the cell membrane. Sci Am 1985 253 104. Copyright 1985 by Scientific American, Inc. All rights reserved.)... 

Q1 The mucosa is a mucous membrane which forms the innermost layer of the intestine. In the small intestine the mucosal surface area is increased greatly by folds and by villi, finger-like projections containing a core with a lymph capillary (lacteal) and blood vessels. Villi are covered by absorptive columnar epithelial cells whose luminal surface is further increased by microvilli (brush border) on which digestive enzymes and transport mechanisms for inorganic ions are located. 

The liver forms more lymph than any other organ of the body (0.4-0.6 mg/kg BW/min). Lymph capillaries take up lymph from Disse s space (J. Disse, 1890) and thereafter from Mall s space (F.P. Mall, 1906), which lies between the limiting plate and the portal connective tissue. Disse s space is also considered to be the main source of lymph. In addition, lymph capillaries commence in the adventitia of sublobular veins and run close to the hepatic veins as far as the paracaval lymph nodes. Lymph vessels possess valves which permit the lymph to flow only in one direction. Lymphatic vessels are present in... 

A highly complex network of arteries, arterioles, and capillaries penetrates the dermis from below and extends up to the surface of, but not actually into, the epidermis. A matching venous system siphons the blood and returns it to the central circulation. Blood flow through the vasculature is linked to the production and movement of lymph through a complementary dermal lymphatic system. The dermis is laced with tactile, thermal, and pain sensors. 

The lymphatic system of the skin extends up and into the papillary layers of the dermis. A dense, flat meshwork of lymphatic capillaries is found here [11]. Lymph passes into a deeper network at the lower boundary of the dermis. Serum, macrophages, and lymphocytes readily negotiate through the skin s lymphatic and vascular networks. 

Similar to blood capillaries, the lymphatic capillaries consist of a single layer of endothelial cells joined together by intercellular junctions. The diameter of small pores is 12 nm, whereas large pores range between 50 and 70 nm. The rate of formation of lymph depends on the hydrostatic pressure of blood and the... 

The lymphatic capillaries are close-ended vessels in close proximity to blood capillaries and, like blood capillaries, lymphatic capillaries are composed of a single layer of endothelial cells. However, large gaps in between these cells allow not only fluid, but also proteins and particulate matter to enter the lymphatic capillaries quite readily. Once the fluid has entered these capillaries, it is referred to as lymph. Not surprisingly, the composition of this fluid is similar to that of the interstitial fluid. 

Lymphatic capillaries join together to form larger lymphatic vessels that have valves within them to ensure the one-way flow of lymph. The lymph is moved along by two mechanisms. Automatic, rhythmic waves of contraction of the smooth muscle in the walls of these vessels are the primary mechanism by which lymph is propelled through the system. Second, the contraction of skeletal muscles causes compression of lymphatic vessels. As in the veins, this pumping action of the surrounding skeletal muscles contributes to movement of the lymph. Ultimately, the lymph is returned to the blood when it empties into the subclavian and jugular veins near the heart. 

Two different circulatory systems, the bronchial and the pulmonary, supply the lungs with blood [133], The bronchial circulation is a part of the systemic circulation and is under high pressure. It receives about 1% of the cardiac output and supplies the conducting airways, pulmonary blood vessels and lymph nodes [133], It is important for the distribution of systemically administered drugs to the airways and to the absorption of inhaled drugs from the airways [18]. The pulmonary circulation comprises an extensive low-pressure vascular bed, which receives the entire cardiac output. It perfuses the alveolar capillaries to secure efficient gas exchange and supplies nutrients to the alveolar walls. Anastomoses between bronchial and pulmonary arterial circulations have been found in the walls of medium-sized bronchi and bronchioles [18, 65, 67],... 

A single artery enters the lymph node at the hilum. After branching, the vessels pass through the medulla within the medullary cords and into the inner and outer cortex. The vessels branch into capillaries around the follicles which provide oxygen, fuels and precursor molecules for the proliferating cells. 

Fig. 3.4 The structure of the pulmonary pleura 1, mesothelial layer 2, submesothelial layer 3, external elastic layer 4, interstitial layer 5, internal elastic layer B, blood capillary net E, elastic fiber net L, lymph vessel net. 

Fig. 3.5 The lymphatic flow system in the human lung. (A) Direction of efferent flow of lymph circulation and location of lymph nodes. Note the large size of the nodes at the mediastinum. (B) The capillary net of lymph vessels—periarterial, peri-venial and peribronchial—and the collecting lymph ducts and nodes.

The arteriolar dilation produced by kinins causes an increase in pressure and flow in the capillary bed, thus favoring efflux of fluid from blood to tissues. This effect may be facilitated by increased capillary permeability resulting from contraction of endothelial cells and widening of intercellular junctions, and by increased venous pressure secondary to constriction of veins. As a result of these changes, water and solutes pass from the blood to the extracellular fluid, lymph flow increases, and edema may result. 

Because of its large size (200-800 pm), the chylomicron cannot permeate the blood capillaries, and as a result, is absorbed into a porous mesenteric lymph vessel called lacteal, and travels with the lymph until drainage into the systemic blood circulation. 

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