one that “leaks” from or seeps out of the blood (arteries) plus the fluid that fills the spaces between the cells (interstitial fluid). This excess fluid, about three liters per day, is collected and drains, then, into lymphatic capillaries, where it becomes lymph; it ultimately enters into the venous blood through the subclavian veins. In truth, the lymphatic system is a second circulatory system, carrying lymph instead of blood, and traveling through various lymphatic vessels, which resemble capillaries and veins, on its way back to the heart. Its chief function is to help drain the excess fluid from the interstitial area and to pick up proteins that may have leaked out of the capillaries.
ORIGIN AND HISTORY OF LYMPH
We exist within our bodies in a wet world, water comprising 70-80 percent of our being. Flowing between all the cells, interstitial fluid bathes these cells with life-giving substances. Its clear, colorless liquid carries such microscopic particles as white blood cells, proteins, and other substances vital to the life of the cell. Blood capillaries also exude fluid into tissues, but not all this fluid returns to the blood, thus creating an accumulation, or excess, of tissue fluid. Collecting this excess fluid from between the cells and eventually returning it to the body’s circulation is the purpose for this vast lymphatic network, with its miles of tubes and channels of interconnected vessels. When too much of the interstitial fluid accumulates, the lymphatic vessels, as stated previously, channel it through a series of closed-end tubules. Now it is called lymph, a word derived from the Latin lympha, meaning “clear water” or “spring water.”
Figure 3
Drainage of lymphatic fluid
Figure 4
Lymphatic capillaries and vessel wall musculature.
According to Webster’s New World Dictionary (3rd college edition), the word lymph is also influenced etymologically by the Greek nymphē, our English word nymph. In Roman and Greek mythology, nymphs were a group of minor nature goddesses, usually represented as young and beautiful, who inhabited rivers, springs, seas, or lakes: that is, places of clear water. The word limpid, with similar origin, means “perfectly clear; transparent; not cloudy or turbid.”
Owing to its transparent quality, lymph is difficult to see during dissections, so its discovery as part of an integrated system in the body developed slowly over time and arrived late on the medical scene.
Some ancient civilizations—such as China, India, and Egypt—had elementary notions of a “white blood,” possibly referring to the milky, intestinal lymph fluid that follows digestion of a fatty meal. Traditional Chinese medicine speaks of a water element connected with the bladder and kidney meridians; it differentiated between liquids of the body and blood. Ancient India’s Ayurvedic medicine had knowledge of “interstitial liquid,” which represented one of the seven systems of the body. According to the ancient Egyptians, the heart contained liquid; vessels existed that transported organic substances throughout the body.
The Greek physician Hippocrates (c. 460-c. 377 B.C.), often referred to as the Father of Medicine, noted and recognized a milky-white material being drained from the intestines and conjectured that this substance resulted from digested fatty material. This was later shown to be an accurate assessment. He was the first to use the word chyle (a milky fluid formed in the small intestine, composed of lymph and emulsified fats) and also listed a “lymphatic (phlegmatic) temperament” as one of the four main temperaments of the human being. Other ancient physicians did not clearly differentiate blood from lymph, yet they traced the lymph channels in the same geographic pattern as veins (vessels which return blood back to the heart). The actual pathway of these lymph vessels flowing into the bloodstream was not properly mapped until centuries later.
Aristotle (384-322 B.C.) spoke about structures with transparent fluid, while Herophilus (335-280 B.C.) noted the presence of mesenteric lymph nodes and “milky veins” (lacteals). During the course of the centuries following further observations and speculations, there was a surge in the seventeenth century that offered more insights and clarity into the nature of the lymphatic system. Just a few years before William Harvey (1578-1657) presented the physiology of the cardiovascular system in his published works (1628), Gasparo Asselli (1581-1626) noted the “white and milky veins” of a dog in 1622, the first documented discovery of the lymphatic vessels. In 1653 Johann Vesling (1598-1649) followed with the first illustrations of human lymphatics, but it was Olof Rudbeck (1630-1708), a Swedish anatomist and “Renaissance man,” who first recognized the lymphatic system as a complete system and as a part of the circulation. Using the ligature technique, he dissected more than four hundred animals to substantiate his ideas. Other scientists followed and built upon these discoveries and observations with their own theories of the flow of lymph and its function and role in the human body.
THE FLOW OF LYMPH
If the lymphatic system, as mentioned near the beginning of this chapter, does not have a muscular pumping organ (like the circulatory system does with the heart), how does the fluid get passed along through the various vessels and ducts? As an accessory route, this system does serve a unique transport function in that it returns tissue fluid, proteins, fats, and other substances to the general circulation. Yet there are differences from the true circulation of blood (as seen in the makeup of our cardiovascular system) to the flow of lymph. Unlike vessels in the blood vascular system, lymphatic vessels form only half a circuit, that is, they do not form a closed ring: there is not a continuous pathway with a “beginning” and an “end,” like the route of blood in its flow to and from the heart. Lymphatic vessels begin blindly in the intercellular spaces of the soft tissues of the body, collect the excess fluid there, finally draining it into the blood vascular venous system, and returning it to the heart.
The two systems, though, closely parallel each other and work in conjunction with each other. In the skin, lymphatic vessels lie in the subcutaneous tissue (under the skin) and generally follow veins, while lymphatic vessels of the viscera (organs) generally follow arteries, forming plexuses (networks) around them. To understand more clearly how the lymph flows throughout the body, it is helpful to examine the composition and structure of lymphatic capillaries, which constitute the beginning of lymphatic vessels.
All lymphatic vessels originate as lymphatic capillaries (also known as initial lymph vessels), tiny structures located in the spaces between the cells. Resembling a vast network of vessels, they form a fine mesh covering most of the body. Because their diameters are larger than blood capillaries, large substances that cannot be absorbed into a blood capillary (such as proteins) can be removed from the interstitial spaces and eventually returned to the blood.
Lymphatic capillaries have a unique structure that permits interstitial fluid to flow into them but not out. The ends of the endothelial cells that make up the inner walls of the capillary overlap like roof tiles, called flap valves, permitting the influx of interstitial fluid. (See Fig. 4.) This process constitutes essentially the formation of lymph fluid.
Just as blood capillaries converge to form venules, then veins, lymphatic capillaries also unite to form increasingly larger tubes (again like the branches and twigs of a tree). Though these larger lymphatic vessels resemble veins structurally, they have thinner walls and more valves, which allow the lymph to move in one direction
