126 PRINCIPLES OF ANIMAL BIOLOGY 



in the pulmonary circulation, and for the bulk of the blood there is only 

 one in the systemic course. There are, however, certain exceptions. 

 The blood which traverses the stomach, intestines, pancreas, and spleen 

 collects into a vein (Fig. 99 'por) leading to the liver; in the liver it passes 

 through a second set of capillaries, then enters the large vein returning 

 to the heart. A circuit beginning and ending in capillaries is known as 

 a 'portal system, and that going from the abdominal viscera to the liver is 

 the hepatic portal system. Fishes and amphibia have a portal system 

 leading to the kidneys also, but that is lacking in man and mammals in 

 general. 



It has been estimated that about IY2 pei" cent of the weight of the 

 human body is blood. From the amount ejected from the heart at 

 each beat, it may be calculated that the speed of the blood is such that 

 an entire circulation, both systemic and pulmonary, requires on the 

 average only about 23 seconds. 



Composition of the Blood. — The blood consists of a liquid known as 

 the plasma and a number of kinds of cells or cell derivatives. The 



A BCD 



Fig. 100. — Formed elements of human blood. A, red corpuscle; B, C, two forms of white 



cell; D, platelets. 



plasma floats the cells, and in addition carries a number of kinds of 

 substances in solution. Among these substances are some temporary 

 ones such as the products of digestion (glucose, amino acids, neutral fats, 

 glycerol, fatty acids), waste materials (urea, uric acid), the respiratory 

 gases (oxygen and carbon dioxide), hormones (secretin and others), and 

 various enzymes, which are introduced and removed at certain places in 

 the system. Other substances are permanent. Of these, proteins make 

 up about 7 per cent of the weight of the plasma; one of the proteins is 

 fibrinogen which features prominently in the clotting of the blood. 

 Inorganic salts are about 1 per cent of the weight of the plasma; an 

 important one is a bicarbonate which carries carbon dioxide in its negative 

 ions (HCOs"). Finally, there are antibodies which the tissues of the 

 body have produced in reaction to and protection against foreign proteins, 

 including disease-producing organisms. 



The visible objects in the blood are of three general kinds: (1) red 

 cells, (2) white cells, and (3) platelets (Fig. 100). The red cells are flat 

 disks, circular in form and thin in the center in man and most of the 

 other mammals, but elliptical in other vertebrates. There are about 25 

 trillion (25 million million) red cells in an average human being. The 



