PRINCIPLES OF PHYSIOLOGY 89 



respiratory surface (external respiration) (Fig. 5.3). The respiratory sur- 

 face for many animals is simply the skin, or perhaps the lining of the 

 mouth. Fishes, many amphibia, molluscs, crustaceans and some worms 

 have developed gills— fine filaments of tissues containing blood channels, 

 and covered with an epithelium. Gases diffuse from the surrounding 

 water through the thin, moist membrane to the blood vessels. The 

 amount of dissolved oxygen in sea water is relatively constant, but the 

 amount in fresh water ponds may fluctuate widely. 



Insects and certain other arthropods have openings, called spiracles, 

 in each segment of the body through which air passes, via a system of 

 branched air ducts, called tracheae, to all of the internal organs. The 

 ducts end in microscopic, fluid-filled tracheoles; oxygen and carbon 

 dioxide pass by diffusion through the walls of the tracheoles to the 

 adjacent tissue cells. The larger insects can pump air through the tra- 

 cheae by contraction of muscles in the abdominal walls. This is an efficient 

 system for gas exchange in animals of the size of insects, for the oxygen 

 reaches the tissue cells and carbon dioxide is removed by diffusion alone; 

 no energy need be expended, as in the vertebrates, in maintaining a 

 rapid flow of blood to keep the body cells supplied with oxygen. 



The higher vertebrates have developed lungs for external respira- 

 tion. These are hollow spaces, usually greatly subdivided into thousands 

 of small hollow pockets (alveoli), kejn moist with w^ater, and richly 

 supplied with blood vessels. The walls of the alveoli are very thin and 

 supplied with a rich bed of capillaries. The network of elastic fibers 

 between the alveoli supports them and makes the lung very pliable. The 

 arrangement of the lung alveoli, as pockets, tends to minimize the loss 

 of water and thus keeps the alveolar surface moist. 



However different respiratory surfaces may appear morphologically, 

 they are essentially similar in consisting of a thin, moist membrane 

 richly supplied with blood vessels separating body fluid and external 

 environment. There is no evidence for the hypothesis that the cells of 

 the lung do work and actively secrete oxygen into the blood stream. It 

 can be calculated that diffusion is rapid enough to supply the oxygen 

 required. Oxygen molecules move from the air to the cells within the 

 body along a steep diffusion gradient, from a region of high concentra- 

 tion to a region of lower concentration. The partial pressure of oxygen 

 in air is about 150 mm. Hg, and that of the air in the lungs is about 105 

 mm. Hg. The oxygen tension of blood going to the tissues is about 

 100 mm. Hg and that of blood returning from tissues to lungs is about 

 40 mm. Hg. The oxygen tension in tissues may vary from to 40 

 mm. Hg. 



Aleans of Obtaining Oxygen. Air contains about 210 ml. of oxygen 

 per liter. Fresh pond water has dissolved in it about 7 ml., and sea water 

 about 5 ml., of oxygen per liter. An air-breathing animal has an obvious 

 advantage over a water-breathing one with respect to oxygen supply, 

 for the solubility of oxygen in water is low and its rate of diffusion is 

 much less in water than in air. To overcome this handicap, animals 

 breathing water usually have some mechanism to pass a fresh supply of 



