484 Animal Biology 



Blood 



Blood is a liquid tissue, sometimes classified with the connective tis- 

 sues, sometimes separately. It consists of clear, straw-colored plasma in 

 which are suspended the red blood corpuscles (erythrocytes), the various 

 types of white blood corpuscles (leucocytes), and the blood platelets, 

 the latter assisting in blood clot formation. Blood forms about one- 

 thirteenth of the total body weight and in an average man totals about 

 6 liters (over 6 quarts) . Arterial blood is bright red, while venous blood 

 is dark red, depending upon the amount of oxygen present. Blood is 

 somewhat viscous and slightly heavier than water. Blood is slightly alka- 

 line (pH of 7.35). 



Erythrocytes (e -rith' ro site) (Gr. erythros, red; kytos, cell) constitute 

 about 50 per cent of the volume of blood. When mature they are with- 

 out a nucleus and consist of a supporting framework known as the 

 stroma (Gr. stroma, bedding) and hemoglobin (he mo -glo' bin) (Gr. 

 haima, blood; globos, sphere). Hemoglobin consists of a protein and 

 an iron-containing compound, the latter being responsible for the chemi- 

 cal affinity for oxygen. When hemoglobin carries oxygen, it is known as 

 oxyhemoglobin and liberates its contained oxygen where needed. Anemia 

 (an e' me ah) (Gr. an, deficient; aima, blood) is a condition in which 

 there is a decrease in the number of erythrocytes or in the amount of 

 hemoglobin or in both. These conditions may occur from impaired 

 blood formation or increased destruction of erythrocytes or both. When 

 blood escapes from an injured blood vessel, it is known as a hemorrhage 

 (hem'oraj) (L. haemorrhagia, blood, to break). The following corre- 

 lated measures are taken when a hemorrhage occurs : ( 1 ) clotting of 

 blood at the site of the injury, (2) decrease in the general blood pres- 

 sure, (3) contraction of the small vessels of the skin, muscles, and intes- 

 tines in order to supply the vital parts of the body, (4) increase the 

 blood volume by the contraction of the spleen which normally contains 

 a large quantity of blood, and (5) passage of water and salts from the 

 tissues into the capillaries because of increased osmotic pressure. 



Leucocytes (lu'kosite) (Gr. leukos, white; kytos, cell) because of 

 their amoeboid movements are able to escape from the blood vessels and 

 penetrate into the body tissues. Leucocytes may be classified as ( 1 ) 

 granulocytes (granular leucocytes) with distinguishing granules in the 

 cytoplasm and (2) agranulocytes (nongranular leucocytes) without 

 granules in the cytoplasm. Because of the variations in the lobes of the 

 nuclei of the granulocytes, the latter are sometimes referred to as poly- 



