Arteries, Veins and Lymphatic Vessels

Blood and lymphatic vessels are soft tissues with densities which exhibit nonlinear stress-strain relationships [1]. The walls of blood and lymphatic vessels show not only elastic [2, 3] or pseudoelastic [4] behavior, but also possess distinctive inelastic character [5, 6] as well, including viscosity, creep, stress relaxation and pressure-diameter hysteresis. The mechanical properties of these vessels depend largely on the constituents of their walls, especially the collagen, elastin, and vascular smooth muscle content. In general, the walls of blood and lymphatic vessels are anisotropic. Moreover, their properties are affected by age and disease state. This section presents the data concerning the characteristic dimensions of arterial tree and venous system the constituents and mechanical properties of the vessel walls. Water permeability or hydraulic conductivity of blood vessel walls have been also included, because this transport property of blood vessel wall is believed to be important both in nourishing the vessel walls and in affecting development of atherosclerosis [7-9]. 

The data are collected primarily from human tissue but animal results are also included in places for completeness. Among the three kinds of vessels, the arterial wall has been extensively investigated while studies of lymphatic vessels are very rare. 

Handbook of Biomaterial Properties. Edited by J. Black and G. Hastings. Published in 1998 by Chapman Hcill, London. ISBN 0 412 60330 6. 

---

Figure 9.17 Routes of parenteral medication, showing the tissues penetrated by intramuscular, intravenous, subcutaneous and intradermal injections the needles, with bevel up, penetrate the epidermis (cuticle) consisting of stratified epithelium with an outer horny layer, the corium (dermis or true skin) consisting of tough connective tissue, elastic fibres, lymphatic and blood vessels, and nerves, the subcutaneous tissue tela subcutanea) consisting of loose connective tissue containing blood and lymphatic vessels, nerves, and fat-forming cells, the fascia (a thin sheet of fibrous connective tissue), and the veins, arteries and muscle.

We now turn to an anatomical description of lymph nodes. The lymph node is surrounded by a thick, fibrous capsule and is subdivided into compartments by trabeculae. Inside the capsule is the subcapsular or marginal sinus, which forms the entry point of lymphatic fluid into the node, via the afferent vessel. The lymph node cortex, which lies beneath the subcapsular sinus, is the location of the primary and secondary lymphoid follicles. The primary follicles are comprised of B-lymphocytes. An immune response stimulates B-cells to replicate and differentiate, converting the primary follicle into a secondary follicle or germinal center, surrounded by a zone of small lymphocytes. The paracortex surrounds the germinal centers and primary follicles and contains mostly T-lymphocytes. The medulla is composed of medullary cords, consisting of macrophages and plasma cells, and medullary sinuses. The medullary vessels include the arteries and veins, and the afferent and efferent lymphatic vessels, respectively, deliver the lymphatic fluid into and out of the lymph node. 

Hepatocytes are the commonest cell type found in the liver, constituting about 70% of the total liver mass. The plasma membranes of these cells have three functional domains the sinusoidal domain, an intercellular domain with gap junctions that is the contact area between hepatocytes, and the canalicular domain, where many of the hepatic secretory functions are performed. The hepatocytes are arranged in single cell layers around sinusoids, which are vascular capillary vessels connected to the hepatic portal vein and hepatic artery the perisinusoidal space of Disse separates the endothelial cell from the hepatocytes. Fenestrations (or windows) in the cells lining the sinusoids allow the formation of hepatic lymph fluid and the movement of proteins into the space of Disse. The lymph leaves the liver through the lymphatic vessels, the lymph nodes, and the thoracic duct, although a small proportion leaves the liver through lymph vessels associated with the hepatic vein. 

The lobes of the liver are divided into smaller lobules with a roughly hexagonal arrangement of hepatocytes around a central vein. At the vertices of the lobules are bile ducts, terminal branches of the hepatic artery, and portal veins—termed the portal triad. Connective stromal tissues extend throughout the liver, providing support for cells and routes for blood vessels, lymphatic vessels, and bile ducts. The hepatocytes form groups of cells around small branches of the portal vein, hepatic arteriole, bile duct, lymph vessel, and nerves this functional unit is called an acinus (plural acini). The acini form part of a larger structure, which can be divided into three zones ... 

The rate of molecular movement by diffusion decreases dramatically with distance, and is generally inadequate for transport over distances greater than 100/rm (recall Table 4.8). The movement of molecules over distances greater than 100 jxro. occurs in specialized compartments in the body blood circulates through arteries and veins interstitial fluid collects in lymphatic vessels before returning to the blood cerebrospinal fluid (CSF) percolates through the central nervous system (CNS) in the brain ventricles and subarachnoid space. In these systems, molecules move primarily by bulk flow, or convection. 

Within the triangle in a medial-to-lateral position is the neurovascular bundle represented by the femoral sheath, which has three components the lateral one contains the femoral artery, the intermediate one the femoral vein, and the medial, and smallest, compartment is called the femoral canal. The neurovascular bundle encases efferent lymphatic vessels and a lymph node embedded in a... 

Mechanical properties of the arteries from human and various animals have been extensively studied. However, literature on lymphatic vessels is very scarce. The data on the circumferential elastic modulus of the lymphatic vessels obtained by Ohhashi et al [53, 54] seem to be too low considering that the lymphatics are originating from the veins. 

Within the villi, the chylomicron enters a lymphatic capillary called a lacteal, which merges into larger lymphatic vessels. It is transported via the lymphatic system and the thoracic duct up to a location near the heart (where the arteries and veins are larger). The thoracic duct empties the chylomicrons into the bloodstream via the left subclavian vein. At this point the chylomicrons can transport the triglycerides to where they are needed. 

Lymph nodes vary in size and are aggregates of lymphatic tissue. Kidney bean-shaped, each consists of a cortex containing germinal centers and an inner medulla. Reticuloendothelial ceils are located along trabecular connective tissue and act as a filtration system for particulate matter. An efferent lymph vessel exits at the hilum, along with a vein, and is accompanied by an artery. An afferent lymph vessel enters on the convex side through the capsule. 

The spleen is the largest of all lymphatic tissues and rests against the lower ribs (nine through eleven), the diaphragm, the stomach, and the left kidney in the left hypochondral region. A parenchyma of red and white pulp is surrounded by a capsule of fibrous connective tissue that contains efferent lymphatics, blood vessels, nerves, and some smooth muscle. The red pulp is a cordal and sinusoidal system concerned primarily with the production of blood products. The while pulp, which is lymphoid tissue, produces ihymus-dependent T lymphocytes and B lymphocytes and plasma cells. The latter components produce the humeral antibody component of the immune system, and the T-lymphocytes are involved in the cell-mediated arm of the system. The vasculature consists of the splenic artery, the largest branch from the celiac trunk, and the splenic vein, which unites with the mesenteric vein to form the portal vein. 

Figure 10.11 Diagram showing microarchitecture of vessels near HCC tissne. The hepatic artery branches off from feeding the tumor tissue to feeding other arteries as well as the sinusoid, the bile duct, and the portal vein. Blood flow through the tumor is drained more directly into the portal vein than into the sinusoids. A portion of serum in the sinusoids flows into Disse s space through which oil droplets is drained off as lymphatic flow.

Absorption of the fat-soluble vitamins A, D, E and K all exhibit significant, if not total, absorption via the lymphatic route. Some lipophilic drugs also enter by this route, at a rate dependent on the availability of lipid. Note that the lymphatic S5 tem avoids the first-pass effect. The lymph vessels, in contrast to the blood vessels (where arteries and veins form a circuit), begin from blind vessels from the central lacteals of the villi. 

---