300 THE BLOOD 



a closed system in whieh the fluid passes through tubes capable 

 of rhythmic contractions. 



{d) The vertebrates have a closed vascular system, with the 

 advantages of ease of control and freedom from constant adapta- 

 tion. It is the most economic system of transport known. 



When the sea animals crawled on to land and became breathers 

 of air, they included a certain proportion of the sea-water in their 

 vessels. By the alteration in surface tension caused by the 

 exchange of a protoplasm-water interface for a protoplasm-air 

 interface their open coelomic system automatically closed (cf. 

 camel's hair brush experiment, Part II.). The vertebrate has, 

 therefore, a fluid in its vessels having a composition similar to that 

 of the sea from which originally it came (see salts of plasma, 

 p. 310). This is a very pretty theory. It cannot be considered 

 as proved any more than the hypothesis of evolution, but in the 

 same sense both fit in with certain facts. 



2. Function. 



{a) The blood-stream conveys materials for building, repair and 

 renewal of tissues, as well as oxygen, water and potential energy 

 to all parts of the organism. 



{b) It removes the waste products of activity including carbon- 

 dioxide, which would paralyse function if allowed to accumulate. 



(c) The carriage of chemical substances (hormones) from the 

 organs in which they are produced in order to influence the activity 

 of other organs may be considered as the co-ordinative action of 

 the circulation. 



{d) The movement of blood aids in the regulation of the tem- 

 perature of the body (Chap. XXXII.). 



{e) It plays a very important part in the defence of the organism 

 against parasites, etc. 



(/) The preserv'ation of the H-ion concentration of the body 

 is principally a function of the circulating fluid (Chap. XXXI.). 



{g) It maintains the water and salt content of the body at a 

 certain level. 



3. Composition. 



Since the function of the blood is to act as common carrier 

 to all the parts of the body, it has to convey food material from 

 the digestive organs and oxygen from the lungs to the tissues. 

 From these it receives in exchange their waste products, viz. 

 results of nitrogenous metabolism (urea, etc.), COg and HgO, and 

 carries them away to the excretory organs, kidneys, lungs, skin, 

 etc., by which they are eliminated. It is therefore evident that the 



