Читать книгу Compendium of Dr. Vodder's Manual Lymph Drainage - Renato Kasseroller - Страница 10

2 Anatomical, Histological and Pathological Fundamentals

2.1 The Vascular System

The blood's vascular system is made up of a venous and an arterial limb. The construction of this piping system varies depending on size and function.

The inner wall of all blood vessels is covered with an endothelial sheath. For the aorta and the large arteries, the next layer outward is the Elastica interna. In the middle, one finds a layer of smooth muscle cells. Outside of that is the Elastica externa, which, along with the Elastica interna, consists of elastic fibers and makes possible continuous blood flow by means of peristaltic action.

The vessels are connected to the surrounding connective tissue by the outermost layer, the Adventitia.

In the periphery, the arteries branch and rebranch down to arterioles. In the arterioles, the elastic tissue is less well developed; the smooth muscle, on the other hand, is relatively well developed. It is innervated both adrenergically as well as cholinergically. For the most part, this is responsible for the phenomenon of peripheral resistance. Many organs have, between the arterioles and the capillaries, what are called metarterioles, which do not possess an enclosed muscle layer. Branching on further, after the arterioles come the precapillaries and then the capillaries.

The inflow opening of the capillaries is surrounded by a smooth muscle called the capillary sphincter. When energy requirements are low, this muscle closes the capillaries, causing the blood to flow through an arteriovenous shunt (anastomosis) directly into the venous limb. The arterioles' vasoconstrictive fibers extend up to the sphincters, which, says Curri, are shaped like cushions.

See Fig. 6.

Capillary diameter in a relaxed state is 7 micrometers - which means that the erythrocytes have to be forced through, deforming them. These capillaries then flow into the venoles. The capillaries are also termed the terminal vessels. In some terminal vessel regions there are, as we have said, direct connections from metarterioles to venoles (capillary shunts).

Fig. 6: Vessel walls

These arteriovenous anastomoses have strong muscular walls and are richly innervated. Their function depends on the state of activity of the neighboring tissue. At rest, the capillaries are collapsed and the bloodstream flows through the anastomoses. The precapillary sphincter shuts off the capillaries until the arterial pressure exceeds the shutoff pressure. Capillary pressure amounts to 30 mm Hg at the arterial end and 15 mm Hg at the venous end.

The progressive narrowing of blood vessels from the aorta to the capillaries is - along with variations in vessel wall construction in the individual segments - responsible for peripheral resistance. The product of cardiac output and peripheral resistance gives the arterial blood pressure, which is normally about 120 to 80 mm Hg on the sphygmomanometer.

Because of their function, capillaries must, on the one hand, not leak, so that the blood flows smoothly; on the other hand, substances must be able to diffuse into the surrounding tissue. Capillary walls consist of a single layer of endothelial cells which are held together by a mortar substance (calcium proteinate). There are pores between the individual endothelial cells (30–70 A in diameter) through which the molecules can drift. The construction of the capillary walls varies depending on the containing organ: the basement membrane can be interrupted or continuous.

The basement membrane consists of densely interwoven reticulum fibrils. The space between the fibrils is filled with an amorphous ground substance. In many organs, this basement membrane consists of two sheets, between which one finds cells called pericytes, which are responsible for pinocytosis. Depending on the organ, there is a varying degree of fenestration between the endothelial cells. Thus, the structure of this capillary wall is yet another factor affecting filtration and diffusion. The permeability of the capillary wall is increased primarily by histamine and other substances produced by inflammatory conditions which have an effect on the vessels.

The blood vessels have vessels of their own, the Vasa vasorum, which see to their nutrient supply. Nerve supply is via the autonomic nervous system. The vascular musculature is supplied by the sympathetic nervous system, which, when stimulated, causes the vessels to constrict. Vasodilatation is effected by a reduction of the sympathetic tone.

In skeletal muscles, vasodilatation takes place via the parasympathetic network.

Capillaries can regenerate and branch out into scar tissue. The total capillary surface area amounts to roughly 6000 m² [65,000 sq. ft.].

The venous limb begins with the venoles, whose walls are not much thicker than those of the capillaries. They have very little musculature and are quite elastic. Venous constriction can take place via the adrenergic nervous system. Plasma protein pinocytosis takes place in the venoles just as in the venous capillaries. Even cells can get into the bloodstream via this route. The veins which follow the venoles are three-layered, but the muscle layer is much less developed than in the arteries.

Because of eversion of the inner layer, the veins have a valve system which prevents blood backflow (venous valves). In the large veins, the blood pressure drops to 5 mm Hg. Pressure regulation in the circulatory system is seen to, first, by the autonomic nervous system, second, by means of hormones such as angiotensin, cortisone, aldosterone or renin - and also by ions and the acid-base balance. The receptors for these are distributed throughout the entire organism.

The sum total diameter of all simultaneously open capillaries is 1000 times greater than the diameter of the aorta - which means that the velocity of the blood flow is correspondingly more rapid in the aorta. The vascular musculature is constantly changing in diameter and thus regulating circulation, in order to be able to respond to current demand.

See Fig. 7 on p. 48.

Fig. 7: Vascular tree

2.2 The Lymphatic System

The lymphatic system is not a circulatory system in the normal sense, but rather a one-way system which transports the lymph from the periphery to the core.

Its primary function is to transport products that cannot use the venous system. The lymphatic system removes excess fluid from the region of the terminal vessel system via its own set of intermediate stations and, at the end, introduces them into the venous arch of the circulatory system, thereby preventing any rise in interstitial pressure. If the backflow via the lymph vessels and venous system is too low to recompensate the interstitial pressure, then this leads to edema formation. The normal transport capacity of the lymphatic system is 1–4 liters [A quarts] per day. About 90 % of the filtered fluid is resorbed by the venous limb; the lymph vessels take up the remaining 10%. This relatively small amount is, however, quite significant because of its varied composition.

2.2.1 The Lymph Angion

The lymph vessels can be made X-ray visible by injecting them with a radiopaque medium. In an X-ray, they look like strings of pearls, where the string part represents the valves and the pearls the filled lymph angions. The lymph vessels are built up of valved segments, called lymph angions by Mislin. They should be regarded as an anatomical and functional unit. The valve determines the direction of flow and prevents reflux (backflow) of fluid. The lymph valve is a duplicate of the endothelial cell layer. All the lymph vessels have a basement membrane, which is more permeable than the basement membrane of the blood capillaries.

The lymph angion has both longitudinal and radial (ring-shaped) musculature. This layer contains numerous nerve endings with connections to the autonomic nervous system, with sensory-motor innervation. Filling pressure is registered by a receptor on the inner wall of the vessel and triggers a reflex contraction. In addition, there are receptors on the outside wall of the vessels which likewise react to a similar stimulus with a contraction reflex.

The lymph substance coming in from the periphery determines, via the filling pressure, the frequency of contraction and thus transport capacity. Elevated production of connective tissue fluid results in increased transport tempo. The outer receptors are affected by the longitudinal musculature, arterial pulse, thoracic pressure differentials due to respiration, intestinal peristalsis and external influences such as massage. [11]

The lymph angion musculature also has its own motoricity. Independently of external influences, it generates 5–7 action potentials per minute. The overall pulsation frequency of the lymphatic system varies between 1–30 per minute.