Physiology and Hygiene for Secondary Schools. Francis M. Walters
conditions for the development of white corpuscles (page 29). They also separate harmful germs and poisonous wastes from the lymph, thereby preventing their entrance into the blood.
Relations of the Lymph, the Blood, and the Cells.—While the blood is necessary as a carrying, or transporting, agent in the body, the lymph is necessary for transferring materials from the blood to the cells and vice versa. Serving as a physiological "go between," or medium of exchange, the lymph enables the blood to minister to the[pg 069] needs of the cells. But the lymph and the blood, everything considered, can hardly be looked upon as two separate and distinct liquids. Not only do they supplement each other in their work and possess striking similarities, but each is made in its movements to pass into the vessels occupied by the other, so that they are constantly mixing and mingling. For these and other reasons, they are more properly regarded as two divisions of a single liquid—one which, by adapting itself to different purposes,27 supplies all the conditions of a nutrient fluid for the cells.
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 lymphatics 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. (See Figs. 28 and 30.)
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[pg 070] 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 (Fig. 29). 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.
Fig. 29—Diagram to show how the muscles pump lymph. A. Relaxed muscle beside which is a lymphatic tube. B. Same muscle in state of contraction.
3. The Inspiratory 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 (Chapter VII), 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. (See Practical Work.)
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[pg 071] 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 (page 54), 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.28 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 (Fig. 30).
Fig. 30—Diagram showing general movement of lymph from the place of relatively high pressure at the lymph spaces to the place of relatively low pressure in the thoracic cavity.
Lymph Movements at the Cells.—The double movement of the lymph from the capillaries toward the cells[pg 072] 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.
Fig. 31—Vessel with an upright membranous partition for illustrating osmosis.
Osmosis.—The term "osmosis" is used to designate the passage of liquids through some partition which separates them. Thus, 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 (Fig. 31). 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 (see Practical Work) 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.
Osmosis at the Cells.—In the body osmosis takes place between the[pg 073] blood and the lymph and between the lymph and the cells, the movements being through the capillary walls and the membranes inclosing the cells (Fig. 27). Oxygen and food materials, which are found in great abundance in the blood, are less abundant in the lymph and still less abundant in the cells. According to the principle of osmosis, the main flow of oxygen and food is from the capillaries toward the cells. On the other hand, the wastes are most abundant in the cells where they are formed, less abundant in the lymph, and least abundant in the blood. Hence the wastes flow from the cells toward the capillaries.
Solutions.—Neither the blood plasma nor the lymph, as already shown, are simple liquids; but they consist of water and different substances dissolved in the water. They belong to a class of substances called solutions. The chief point of interest about substances in solution is that they are very finely divided and that their little particles are free to move about in the liquid that contains them. Both the motion and the finely divided condition of the dissolved substances are necessary to the process of