The Lymphatic System A Shadow Circulation Of The Blood

The last chapter or two have focused upon the circulation of the blood. However, the blood circulation is being followed! It is being followed by something like a shadow! It is being followed by the lymphatic (lim-FAT-ik) circulation, a major part of the lymphatic system.

As overviewed in Chapter 3, the lymphatic system is an organ system that consists of a collection of lymphatic organs and lymphatic vessels running between them. In a practical sense, the lymphatic system can be thought of as a shadow circulation of the blood circulation. The main reason is that the lymphatic capillaries run side-by-side (like a shadow) along the tiny blood capillaries (Figure 12.3).

The lymphatic capillaries are special in that they are dead-ended. Since they are closed at the far end, their fluid contents - the lymph - flows in one direction only, towards the heart. The lymph (which looks like clear spring water) is actually a filtrate (FILL-trait) - filtration product - of the blood, the plasma in the nearby blood capillaries. The blood pressure (BP) pushes outward, thereby filtering water, NaCl, and various foreign objects (such as dirt, bacteria, or cancer cells) out of the blood and into the lymphatic capillaries.

BLOOD CAPILLARY

Fig. 12.3 Lymph is formed as a shadow of the blood. BP = blood pressure.

BLOOD CAPILLARY

Fig. 12.3 Lymph is formed as a shadow of the blood. BP = blood pressure.

Since the lymph usually contains no erythrocytes (RBCs), it looks clear, rather than red-colored. But eventually, the lymphatic vessels drain their cleansed lymph into several major blood veins that flow into the top of the heart.

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