The Lymph and Its Movement through the Body : Part 2

Physiology and Hygiene for Secondary Schools
by Francis M. Walters

(Page 7 of 25)

Movements of the Lymph. - As compared with the blood, the lymph must be classed as a quiet liquid. But, as already suggested, it has certain movements which are necessary to the purposes which it serves. A careful study shows it to have three well-defined movements as follows:

1. A movement from the capillaries toward the cells.

2. A movement from the cells toward the capillaries.

3. A movement of the entire body of lymph from the lymph spaces into the lymphatic and along these channels to the ducts through which it enters the blood.

By the first movement the cells receive their nourishment. By the second and third movements the lymph, more or less laden with impurities, is returned to the blood stream.

Causes of the Lymph Movements. - Let us consider first the movement through the lymph tubes. No pump, like the heart, is known to be connected with these tubes and to supply the pressure necessary for moving the lymph. There are, however, several forces that indirectly aid in its flow. The most important of these are as follows:

1. Blood Pressure at the Capillaries. - The Plasma which is forced through the capillary walls by pressure from the heart makes room for itself by pushing a portion of the lymph out of the lymph spaces. This in turn presses upon the lymph in the tubes which it enters. In this way pressure from the heart is transmitted to the lymph, forcing it to move.

2. Variable Pressure on the Walls of the Lymph Vessels. - Pressure exerted on the sides of the lymph tubes by contracting muscles tends to close them up and to push the lymph past the valves, which, by closing, prevent its return. Pressure at the surface of the body, provided that it is variable, also forces the lymph along. The valves in the lymph vessels serve the same purpose as those in the veins.

3. The Respiratory Force. - When the thoracic cavity is enlarged in breathing, the unbalanced atmospheric pressure is exerted from all directions towards the thoracic space. This not only causes the air to flow into the lungs, but also causes a movement of the blood and lymph in such of their tubes as enter this cavity. It will be noted that both of the large lymph ducts terminate where their contents may be influenced by the respiratory movements.

Where the Lymph enters the Blood. - The fact that the lymph is poured into the blood at but two places, and these very close to each other, requires a word of explanation. As a matter of fact, it is impossible for the lymph to flow into blood vessels at most places on account of the blood pressure. This would force the blood into the lymph vessels, instead of allowing the lymph to enter the blood. The lymph can enter only at some place where the blood pressure is less than the pressure that moves the lymph. Such a place is found in the thoracic cavity. As already pointed out, the blood pressure in the veins entering this cavity becomes, with each expansion of the chest, negative, i.e., less than the pressure of the atmosphere on the outside of the body. This, as we have seen, aids in the flow of the blood into the right auricle. It also aids in the passage of lymph into the blood vessels. The lymph is said to be "sucked in," which means that it is forced in by the unbalanced pressure of the atmosphere. Some advantage is also gained by the lymph duct's entering the subclavian vein on the upper side and at its union with the jugular vein. Everything considered, it is found that the lymph flows into the blood vessels where it can be "drawn in" by the movements of breathing and where it meets with no opposition from the blood stream itself.

Lymph Movements at the Cells. - The double movement of the lymph from the capillaries toward the cells and from the cells toward the capillaries is not entirely understood. Blood pressure in the capillaries undoubtedly has much to do in forcing the Plasma through the capillary walls, but this tends to prevent the movement of the lymph in the opposite direction. Movements between the blood and the lymph are known to take place in part according to a general principle, known as osmosis, or dialysis.

Osmosis. - The term "osmosis" is used to designate the passage of liquids through some partition which separates them. therefore, if a vessel with an upright membranous partition be filled on the one side with pure water and on the other with water containing salt, an exchange of materials will take place through the membrane until the same proportion of salt exists on the two sides. The cause of osmosis is the motion of the molecules, or minute particles, that make up the liquid substance. If the partition were not present, this motion would simply cause a mixing of the liquids.

Conditions under which Osmosis occurs. - Osmosis may be shown by suitable experiments to take place under the following conditions:

1. The liquids on the two sides of the partition must be unlike either in density or in composition. Since the effect of the movement is to reduce the liquids to the same condition, a difference in density causes the flow to be greater from the less dense toward the denser liquid, than in the opposite direction; while a difference in composition causes the substances in solution to move from the place of greater abundance toward places of less abundance.

2. The liquids must be capable of wetting, or penetrating, the partition. If but one of the liquids penetrates the partition, the flow will be in but one direction.

3. The liquids on the two sides of the partition must readily mix with each other.

D.C. Heath and Co. - Publishers
Original copyright 1909