Physiology and Hygiene for Secondary Schools. Francis M. Walters
however, that appear to be in solution are not able to penetrate membranes, or take part in osmosis.
Kinds of Solutions in the Body.—The substances in solution in the body liquids are of two general kinds known as colloids and crystalloids. The crystalloids are able to pass through membranous partitions, while the colloids are not. An example of a colloid is found in the albumin of an egg, which is unable to penetrate the membrane which surrounds it. Examples of crystalloids are found in solutions of salt and sugar in water. The inability of a colloid to penetrate a membrane is due to the fact that it does not form a true solution. Its particles (molecules), instead of being completely separated, still cling together, forming little masses that are too large to penetrate the membrane. Since, however, it has the appearance, on being mixed with water, of being dissolved, it is called a colloidal solution. The crystalloid substance, on the other hand, completely separates in the water and forms a true solution—one which is able to penetrate the partition or membrane.
Osmosis not a Sufficient Cause.—The passage of materials through animal membranes, according to the principle of osmosis, is limited to crystalloid substances. But colloid substances are also known to pass through the various partitions of the body. An example of such is found in the proteids of the blood which, as a colloidal solution, pass through the capillary walls to become a part of the lymph. Perhaps[pg 074] the best explanation offered as yet for this passage is that the colloidal substances are changed by the cells lining the capillaries into substances that form true solutions and that after the passage they are changed back again to the colloidal condition.
Summary.—Between the cells and the capillaries is a liquid, known as the lymph, which is similar in composition and physical properties to the blood. It consists chiefly of escaped plasma. The vessels that contain it are connected with the system for the circulation of the blood. By adding new material to the lymph and withdrawing waste material from it, the blood keeps this liquid in a suitable condition for supplying the needs of the cells. Supplementing each other in all respects, the blood and the lymph together form the nutrient cell fluid of the body. The interchange of material between the blood and the lymph, and the lymph and the cells, takes place in part according to the principle of osmosis.
Exercises.—1. Explain the necessity for the lymph in the body.
2. Compare lymph and water with reference to density, color, and complexity of composition.
3. Compare lymph and blood with reference to color, composition, and movement through the body.
4. Show how blood pressure in the capillaries causes a flow of the lymph.
5. Show how contracting muscles cause the lymph to move. Compare with the effect of muscular contraction upon the blood in the veins.
6. Trace the lymph in its flow from the right hand to where it enters the blood; from the feet to where it enters the blood.
7. What conditions prevail at the cells to cause a movement of food and oxygen in one direction and of waste materials in the opposite direction?
8. What part does water play in the exchanges at the cells?
9. Show that the blood and the lymph together fulfill all the requirements of a nutrient cell fluid in the body.
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PRACTICAL WORK
To illustrate the Effect of Breathing upon the Flow of Lymph.—Tightly holding one end of a glass tube between the lips, let the other end extend into water in a tumbler on a table. In this position quickly inhale air through the nostrils, noting that with each inhalation there is a slight movement of the water up the tube. (No sucking action should be exerted by the mouth.) Apply to the movements in the large blood and lymph vessels entering the thoracic cavity.
To illustrate Osmosis.—1. Separate the shell from the lining membrane at one end of an egg, over an area about one inch in diameter. To do this without injuring the membrane, the shell must first be broken into small pieces and then picked off with a pair of forceps, or a small knife blade. Fit a small glass tube, eight or ten inches long, into the other end so that it will penetrate the membrane and pass down into the yolk. Securely fasten the tube to the shell by melting beeswax around it, and set the egg in a small tumbler partly filled with water. Examine in the course of half an hour. What evidence now exists that the water has passed through the membrane?
2. Tie over the large end of a "thistle tube" (used by chemists) a thin animal membrane, such as a piece of the pericardium or a strip of the membrane from around a sausage. Then fill the bulb and the lower end of the tube with a concentrated solution of some solid, such as sugar, salt, or copper sulphate. Suspend in a vessel of water so that the liquid which it contains is just on a level with the water in the vessel. Examine from time to time, looking for evidence of a movement in each direction through the membrane. Why should the movement of the water into the tube be greater than the movement in the opposite direction? (If the thistle tube has a very slender stem, it is better to fill the bulb before tying on the membrane. The opening in the stem may be plugged during the process of filling.)
Fig. 32—An osmosometer.
Note.—With a special piece of apparatus, known as an osmosometer, the principle of osmosis may be more easily illustrated than by the method in either of the above experiments (Fig. 32). This apparatus may be obtained from supply houses.
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CHAPTER VII - RESPIRATION
Through the movements of the blood and the lymph, materials entering the body are transported to the cells, and wastes formed at the cells are carried to the organs which remove them from the body. We are now to consider the passage of materials from outside the body to the cells and vice versa. One substance which the body constantly needs is oxygen, and one which it is constantly throwing off is carbon dioxide. Both of these are constituents of
The Atmosphere.—The atmosphere, or air, completely surrounds the earth as a kind of envelope, and comes in contact with everything upon its surface. It is composed chiefly of oxygen and nitrogen,29 but it also contains a small per cent of other substances, such as water-vapor, carbon dioxide, and argon. All of the regular constituents of the atmosphere are gases, and these, as compared with liquids and solids, are very light. Nevertheless the atmosphere has weight and, on this account, exerts pressure upon everything on the earth. At the sea level, its pressure is nearly fifteen pounds to the square inch. The atmosphere forms an essential part of one's physical environment and serves various purposes. The process[pg 077] by which gaseous materials are made to pass between the body and the atmosphere is known as
Respiration.—As usually defined, respiration, or breathing, consists of two simple processes—that of taking air into special contrivances in the body, called the lungs, and that of expelling air from the lungs. The first process is known as inspiration; the second as expiration. We must, however, distinguish between respiration by the lungs, called external respiration, and respiration by the cells, called internal respiration.
The purpose of respiration is indicated by the changes that take place in the air while it is in the lungs. Air entering the lungs in ordinary breathing parts with about five per cent of itself in the form of oxygen and receives about four and one half per cent of carbon dioxide, considerable water-vapor, and a small amount of other impurities. These changes suggest a twofold purpose for respiration:
1. To obtain from the atmosphere the supply of oxygen needed by the body.
2. To transfer to the atmosphere certain materials (wastes) which must be removed from the