^4 GENERAL CONCEPTS 



27. Circulation 



The metabolic processes of all cells require a constant supply of 

 food and oxygen and constant removal of wastes. In protozoans the 

 transport of substances is effected by the diffusion of the molecules, aided 

 generally by streaming movements of the cytoplasm itself. The flowing 

 of the cytoplasm from rear to front as an ameba moves, and the circular 

 movement of the cytoplasm in protozoa with a fixed shape (such as 

 paramecia) are examples of these (Fig. 5.2). Transport from cell to cell 

 in simple multicellular animals such as sponges, coelenterates and flat- 

 worms occurs by diffusion. This is aided in some animals by the stirring 

 of the body fluids brought about by the contraction of the muscles of 

 the body wall. Diffusion, you will recall, is the movement of molecules 

 from a region of high concentration to a region of lower concentration. 

 The rate of diffusion is directly proportional to the difference in con- 

 centration in the two regions and inversely proportional to the distance 

 separating them. From this we can see that an adequate supply of food 

 and oxygen can be maintained by diffusion alone only in a small animal; 

 in a larger animal the slower diffusion rate over the greater distance 

 would not suffice. Such animals must develop some system of internal 

 transport— some type of circulatory system. Not only absolute size, but 

 also the shape and the activity of an animal determine the need for a 

 circulatory system. 



The proboscis worms or Nemertea are the simplest living animals 

 to have a distinct circulatory system; it consists of a dorsal and two 

 lateral blood vessels which extend the whole length of the body and 

 are connected by transverse vessels. The earthworm has a more com- 

 plicated circulatory system: a dorsal vessel, in which blood flows 

 anteriorly, a ventral vessel and a subneural vessel in which blood flows 

 posteriorly, and five jDairs of pulsating tubes ("hearts") at the anterior 

 end which drive blood from the dorsal to the ventral vessel (Fig. 5.2). 

 In other segments of the body a network of vessels connecting dorsal and 

 ventral vessels ramifies through the body wall and the wall of the 

 intestine. The blood in these vessels does not flow regularly in one direc- 

 tion, but ebbs and flows as the vessels constrict and dilate. 



A typical circulatory system includes blood vessels and heart and 

 the fluid within them— the blood— which in turn is composed of a fluid— 

 plasma— and blood cells or corpuscles. Oxygen is carried in most cir- 

 culatory systems not simply dissolved in the plasma but in combination 

 with a heme protein pigment. The one found in the earthworm and 

 man is hemoglobin, a red, iron-containing pigment. The hemoglobin of 

 vertebrate blood is located in cells, the red blood cells. In many inverte- 

 brates, the hemoglobin or other pigment is dissolved in the plasma, and 

 whatever cells are present are colorless. The respiratory pigment of crab 

 blood is a different heme protein, blue-green hemocyanin, which con- 

 tains copper in place of iron. 



The circulatory system of the annelid worms and the vertebrates is 

 said to be "closed," i.e., the blood in the course of circulation remains 

 within blood vessels. In contrast, the circulatory system of arthropods 



