BtOOD AND CIRCULATION 55 } 



lungs to return to the left atrium and mix with blood from the posterior 

 vena cava. Most of the blood in the pulmonary artery goes through 

 another by-pass, the ductus arteriosus, to the dorsal aorta. The ductus 

 arteriosus represents the dorsal part of the left sixth aortic arch (Fig. 

 27.3). Since the ductus arteriosus enters the aorta after the arteries to the 

 head have been given off, the head receives the blood with the highest 

 oxygen content. After the entrance of the ductus arteriosus, the blood 

 in the aorta is highly mixed. This is the blood that is distributed to 

 the rest of the body and, by way of umbilical arteries, to the placenta. 



As the lungs develop during fetal life, more and more blood is sent 

 through their capillary bed, because the foramen ovale becomes rela- 

 tively smaller and less blood by-passes the lungs via this route. The 

 return of blood from the lungs to the left atrium is consequently grad- 

 ually increased, which increases the blood pressure in the left atrium. 

 The increased pressure in the left atrium keeps the flap guarding the 

 foramen ovale closed a greater fraction of the time and decreases 

 the amount of blood entering from the right atrium. These changes 

 insure a normal development of the pulmonary circulation and make 

 the transition from the fetal to the adult pattern less abrupt. At birth, the 

 placenta is expelled, carbon dioxide accumulates in the blood and stim- 

 ulates the respiratory center. Concurrently, the ductus arteriosus con- 

 tracts. More blood goes through the now functioning lungs, pressure 

 increases further in the left atrium, and the flap in the foramen ovale 

 is held shut. The adult pattern is now established. As time goes on, the 

 flap in the foramen ovale grows against the interatrial wall, the lumen 

 of the ductus arteriosus is occluded by the rapid proliferation of its 

 lining cells, and most portions of the umbilical vessels within the infant 

 atrophy. The failure of any of these changes to occur at birth results 

 in poor oxygenation of the blood, producing a condition known as 

 "blue baby." 



237. Flow of Blood and Lymph 



The Heart. The heart (Fig. 27.6) is the pump that builds up the 

 pressure gradient necessary for the blood and lymph to flow. It lies 

 within a division of the coelom, the pericardial cavity, which contains 

 some tissue fluid that lubricates it and facilitates its movements. It is 

 covered with a smooth coelomic epithelium, the visceral pericardium, 

 and is lined by the simple squamous epithelium, the endothelium, which 

 lines all parts of the circulatory system. The rest of its wall is composed 

 of cardiac muscle, which is unique in that its fibers branch and anas- 

 tomose profusely without cell membranes at their ends (Fig. 3.13). The 

 musculature of the atria is separate from that of the ventricles, but each 

 may be regarded as a syncytium, that is, a single multinucleated cell. 

 Each responds as a unit. Any stimulus that is strong enough to elicit a 

 response will elicit a total response. Thus the atria and ventricles follow 

 the "all-or-none" law that applies to individual motor units of skeletal 

 muscle. 



During a heart cycle, the atria and ventricles contract and relax in 



