BLOOD AND CIRCULATION 545 



234. The Rh Factor 



A number of other inherited antigenic proteins may be present in 

 the blood. Most are rare and not apt to be involved in transfusions, but 

 one that is common is the Rh factor, so called because it was first dis- 

 covered in the rhesus monkey. About 87 per cent of North American 

 whites have this factor in their red cells and are said to be Rh positive. 

 The remaining 13 per cent do not have it, hence are Rh negative. If a 

 mother is Rh negative and the father Rh positive, the fetus may inherit 

 the factor from the father. In theory none of the fetal blood crosses the 

 placenta to enter the mother's blood, but there are usually small breaks 

 in the placenta that permit some mixing. Rh positive blood of the fetus, 

 on entering the mother, induces the formation of antibodies. This is a 

 slow process, and not enough are likely to be formed to cause trouble 

 in the first pregnancy. If a second fetus is also Rh positive, more Rh 

 positive blood enters the mother and more antibodies are formed in 

 the mother's blood. Some of these get back into the Rh positive blood 

 of the fetus and cause agglutination and hemolysis of the red blood 

 cells. This condition, erythroblastosis fetalis, may be fatal, or may result 

 in injury to the brain from the bile pigment (bilirubin) formed from 

 the hemoglobin released by the hemolysis of the red cells. A newborn 

 infant showing symptoms of it can be saved by extensive transfusions. 

 Ordinarily not enough Rh positive blood enters the mother to cause 

 any harm, but her blood contains the antibodies, and if she subsequently 

 needs a transfusion for any reason, Rh negative blood must be used. 



235. Patterns of Circulation 



Heart, arteries, capillaries and veins constitute the cardiovascular 

 system; the lymphatic vessels and nodes comprise the lymphatic system. 

 Most vertebrates have both, but primitive vertebrates such as cyclostomes 

 and cartilaginous fishes have no lymphatic system. These groups have 

 a lower blood pressure than other vertebrates, and their veins provide 

 adequate drainage for the tissues. A lymphatic system apparently evolved 

 as blood pressures became higher, and the veins could no longer drain a 

 sufficient amount of liquid from the tissues. Lymphatic vessels arise as 

 outgrowths from the veins, and, in general, they tend to parallel the 

 veins, and ultimately empty into them. Most are inconspicuous and are 

 seen only in special preparations. 



Primitive Fishes. The cardiovascular system has undergone some 

 striking changes during the evolution of vertebrates. Most of these are 

 correlated with the shift in the site of external respiration that occurred 

 during the transition from water to land, and with the development 

 of the efficient, high pressure circulatory system necessary for an active 

 terrestrial vertebrate. 



In a primitive, lungless fish (Fig. 27.2), all of the blood entering 

 the heart from the veins has a low oxygen and a high carbon dioxide 

 content, i.e., it is venous blood. The heart consists of a series of chambers 

 (a sinus venosus, a single atrium, a single ventricle and a conus arterio- 



