BLOOD AND CIRCULATION 557 



them and increases the pressure gradient. The other factor is that the 

 contraction and relaxation of body muscles exert a "milking" action on 

 the veins. When the muscles contract, their bulging squeezes the veins 

 and forces the blood toward the heart, for the valves in the veins prevent 

 the blood from moving in any other direction. All of these factors 

 increase during exercise, Avhich makes for a more rapid return of blood, 

 and an increased cardiac output. 



The return of lymph is dependent upon similar forces. The tissue 

 fluid itself has a certain pressure derived from the flow of liquid out of 

 the capillaries. This establishes a pressure gradient in the lymphatics 

 that is made steeper by the negative intrathoracic pressure. The "milk- 

 ing" action of surrounding muscles, and, for lymphatics returning from 

 the intestine, the contraction of the villi, help considerably. Some lower 

 vertebrates have lymph "hearts"— specialized, pulsating segments of 

 lymphatic vessels. 



Questions 



1. How does the blood maintain a relatively constant pH despite its uptake of acid sub- 

 stances in the tissues? 



2. Describe the current theory of the mechanism of blood clotting. 



3. One of the adaptations to high altitude is an increase in the number of erythrocytes. 

 Of what advantage to the organism is this? 



4. Describe two ways that leukocytes protect the body from microorganisms. 



5. What factors would have to be taken into consideration in giving a blood transfusion 

 to an Rh negative woman who has had several Rh positive children? 



6. How did the transition from water to land alfect the pattern of the blood vessels and 

 the structure of the heart? \\ hat further changes have occurred during the evolution 

 to mammals? 



7. Define and give an example of a portal system. 



8. How does the circulation through the heart of a mammalian fetus differ from that in 

 an adult? 



9. ^Vhat prevents blood from flowing the wrong way in the heart? 



10. How does the heart adjust its rate and output per beat to the increased venous return 

 that occurs during increased body activity? 



11. Describe two functions of arteries in addition to their function of transportation. 



12. What forces are involved in the exchange of water and solutes between the capillaries 

 and tissue fluid? 



13. What factors supplement blood pressure in the return of venous blood? 



14. List the functions of the lymphatic system. Do all vertebrates have this system? 



Supplementary Reading 



The reader is referred again to the general references on anatomy and physiology 

 cited at the end of Chapter 25. Wiener's Blood Groups and Transfusions contains inter- 

 esting accounts of the disco\er)' of the blood groups and their applications to problems of 

 transfusion, anthropology-, disputed paternity and forensic medicine. Harvey's Anatomical 

 Studies on the Motion of'the Heart and Blood, originally published in 1628, was translated 

 and reprinted in 1931. His classic experiments established the circulation of the blood 

 and introduced the experimental method into biologic research. Krogh's Silliman lectures 

 on The Anatomy and Physiology of Capillaries contain excellent accounts of that portion 



