1. 37.1 The Circulatory System

2. Myocardium Thick middle muscle layer of the heart; pumps blood through the circulatory system.

3. Atrium Upper chamber of the heart that receives and holds blood that is about to enter the ventricle.

4. Ventricle Lower chamber of the heart that pumps blood out of the heart.

5. Pulmonary Circulation Pathway of circulation between the heart and the lungs.

6. Systemic Circulation Pathway of circulation between the heart and the rest of the body except the lungs.

7. Valve Flap of connective tissue between an atrium and a ventricle, or in a vein, that prevents backflow of blood.

8. Pacemaker Small group of cardiac muscle cells in the right atrium that “set the pace” for the heart as a whole; also known as the sinoatrial node.

9. Aorta Large blood vessel in mammals through which blood travels after it leaves the left ventricle.

10. Artery Large blood vessel that carries blood from the heart to the tissues of the body.

11. Capillary Smallest blood vessel; brings nutrients and oxygen to the tissues and absorbs carbon dioxide and waste products.

12. Vein In plants, a cluster of vascular tissue in leaves; in animals, a blood vessel that returns blood to the heart.

13. Atherosclerosis Condition in which fatty deposits called plaque build up on the inner walls of the arteries.

14. Key Concept The human circulatory system consists of the heart, a series of blood vessels, and the blood that flows through them.

15. Key Concept As blood flows through the circulatory system, it moves through three types of blood vessels—arteries, capillaries, and veins.