The Lymphatic System

The lymphatic system is a network of small organs (lymph nodes) and tubes (lymphatic vessels or simply "lymphatics") through which lymph—a fluid derived from interstitial fluid—flows. The lymphatic system is not technically part of the cardiovascular system, but it is described in this chapter because its vessels constitute a route for the movement of interstitial fluid to the cardiovascular system (Figure 14-51).

Present in the interstitium of virtually all organs and tissues are numerous lymphatic capillaries that are completely distinct from blood-vessel capillaries. Like the latter, they are tubes made of only a single layer of endothelial cells resting on a basement membrane, but they have large water-filled channels that are permeable to all interstitial-fluid constituents, including protein. The lymphatic capillaries are the first of the lymphatic vessels, for unlike the blood-vessel capillaries, no tubes flow into them.

Small amounts of interstitial fluid continuously enter the lymphatic capillaries by bulk flow (the precise mechanisms by which this occurs remain unclear). Now known as lymph, the fluid flows from the lymphatic capillaries into the next set of lymphatic vessels, which converge to form larger and larger lymphatic vessels. At various points, the lymph flows through lymph nodes, the function of which is described in Chapter 20. Ultimately, the entire network ends in two large lymphatic ducts that drain into the subclavian veins in the lower neck. Valves at these junctions permit only one-way flow from lymphatic ducts into the veins. Thus, the lymphatic vessels carry interstitial fluid to the cardiovascular system.

The movement of interstitial fluid to the cardiovascular system via the lymphatics is very important because, as noted earlier, the amount of fluid filtered out of all the blood-vessel capillaries (except those in the kidneys) exceeds that reabsorbed by approximately

Vander et al.: Human Physiology: The Mechanism of Body Function, Eighth Edition

Circulation CHAPTER FOURTEEN

Circulation CHAPTER FOURTEEN

Body Functions

FIGURE 14-50

Major factors determining peripheral venous pressure and, hence, venous return and stroke volume. The figure shows how venous pressure and stroke volume are increased. Reversing the arrows in the boxes would indicate how these can be decreased. The effects of increased inspiration on end-diastolic ventricular volume are actually quite complex, but for the sake of simplicity, they are shown only as increasing venous pressure.

FIGURE 14-50

Major factors determining peripheral venous pressure and, hence, venous return and stroke volume. The figure shows how venous pressure and stroke volume are increased. Reversing the arrows in the boxes would indicate how these can be decreased. The effects of increased inspiration on end-diastolic ventricular volume are actually quite complex, but for the sake of simplicity, they are shown only as increasing venous pressure.

4 L each day. This 4 L is returned to the blood via the lymphatic system. In the process, the small amounts of protein that leak out of blood-vessel capillaries into the interstitial fluid are also returned to the cardiovascular system.

Failure of the lymphatic system due, for example, to occlusion by infectious organisms (as in the disease elephantiasis) allows the accumulation of excessive interstitial fluid. The result can be massive swelling of the involved area. The accumulation of large amounts of interstitial fluid from whatever cause (others are described in the section on heart failure) is termed edema.

In addition to draining excess interstitial fluid, the lymphatic system provides the pathway by which fat absorbed from the gastrointestinal tract reaches the blood (Chapter 17). The lymphatics also, unfortunately, are often the route by which cancer cells spread from their area of origin to other parts of the body.

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Essentials of Human Physiology

Essentials of Human Physiology

This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.

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