(e) From that portion of the gut where materials are absorbed through
the walls into the capillaries, the blood receives a great variety of substances. While
most of these substances are useful, some may be harmful to the body. The blood
carrying these substances is carried directly to the liver by the hepatic portal venous
system. This blood is specially treated and conditioned in the liver before it is returned
to the general circulation by way of the hepatic veins.
Section IV. THE HUMAN LYMPHATIC SYSTEM
Between the cells of the body are spaces filled with fluid. This is the interstitial
(or tissue) fluid, often referred to as intercellular fluid. There are continuous exchanges
between the intracellular fluid, the interstitial fluid, and the plasma of the blood. The
lymphatic system returns to the bloodstream the excess interstitial fluid, which includes
proteins and fluid derived from the blood.
STRUCTURES OF THE HUMAN LYMPHATIC SYSTEM
See Figure 2-7 for an illustration of the human lymphatic system.
a. Lymphatic Capillaries. Lymphatic capillaries are located in the interstitial
spaces. Here, they absorb the excess fluids.
b. Lymph Vessels. A tributary system of vessels collects these excess fluids,
now called lymph. Like veins, lymphatic vessels are supplied with valves to help
maintain a flow of lymph in one direction only. The lymphatic vessels, to a greater or
lesser extent, parallel the venous vessels along the way. The major lymph vessel in the
human body is called the thoracic duct. The thoracic duct passes from the abdomen up
through the thorax and into the root of the neck in front of the vertebral column. The
thoracic duct empties into the junction of the left subclavian and jugular veins.
c. Lymph Nodes. Along the way, lymphatic vessels are interrupted by special
structures known as lymph nodes. These lymph nodes serve as special filters for the
lymph fluid passing through.
d. Tonsils. Tonsils are special collections of lymphoid tissue, very similar to a
group of lymph nodes. These are protective structures and are located primarily at the
entrances of the respiratory and digestive systems.