BLOOD AND CIRCULATION 549 



of a primitive tetrapod than in a fish. Blood in tlie dorsal aorta of a 

 frog has a mean pressure of 30 mm. Hg., twice that of the dogfish. This 

 makes for a greater efficiency of circulation, but this benefit is somewhat 

 offset by the fact that the blood delivered to the tissues is mixed to some 

 extent, and does not contain relatively as much oxygen as it did in a fish. 



Mammals. Higher tetrapods depend upon their lungs for external 

 respiration. Since no respiration occurs in the skin, there is no mixing 

 of aerated blood from the skin with blood from the body. The mixing in 

 the heart of arterial blood from the lungs with venous blood from the 

 body is lessened in reptiles by a partial division of the ventricle and by a 

 complex, tripartite division of the conus (Fig. 27.3). In birds and mam- 

 mals, there is no mixing at all, for the ventricle is completely divided. 

 Venous blood from the body enters the right atrium, into which the 

 primitive sinus venosus has become incorporated. Arterial blood from 

 the lungs enters the left atrium. The atria pass the blood on to the 

 right and left venJricles resjjectively. The ventricles have more muscular 

 walls than in lower vertebrates, and so can increase the blood pressure 

 considerably. The primitive conus arteriosus has become completely 

 divided, part contributing to the pulmonary artery leading from the 

 right ventricle to the lungs and the rest to the arch of the aorta leading 

 from the left ventricle to the body. 



The sixth pair of aortic arches form the major part of the mam- 

 malian pulmonary arteries, and the third pair contribute to the internal 

 carotid arteries. But it will be observed in Figure 27.3 that only the left 

 side of the fourth arch, known as the arch of the aorta, leads to the 

 dorsal aorta. The right fourth arch contributes to the right subclavian 

 artery to the shoulder and arm, but does not connect with the aorta. In 

 ])irds it is the right fourth arch that leads to the dorsal aorta and the 

 left fourth arch contributes to the left subclavian artery. 



The major change in the veins is the complete loss of a renal portal 

 system. Blood from the tail and posterior appendages enters a posterior 

 vena cava, which continues forward to the heart. It receives blood from 

 the kidneys but does not carry blood to them. An anterior vena cava 

 drains the head and arms. The hepatic portal system is still present. The 

 pattern of the major arteries and veins of man is shown in Figure 27.4. 



These evolutionary changes have resulted in a very efficient mam- 

 malian circulatory systern. Mammals have relatively more blood than 

 lower vertebrates, it is distributed under gieater pressure, and there is no 

 mixing of arterial and venous blood. Man, for example, has 7.6 ml. of 

 blood per 100 gm. of body weight compared with 2 ml. per 100 gm. in a 

 fish. The mean pressure in the dorsal aorta of man is about 100 mm. of 

 mercury. 



236. The Fetal Circulation 



The placenta of the mammalian fetus, rather than the digestive 

 tract, lungs and kidneys, is the site for exchange of materials. This, 

 together with the fact that the vessels in the lungs of the fetus are not 

 developed enough to handle the total volume of blood that is circulating 



