阅读理解： Your blood makes a round trip through your body in less than a minute, thousands of times a day．It travels through tubes called blood vessels．There are three main kinds of blood vessels－arteries, veins, and capillaries(毛细血管)．When blood is pumped out of your heart it goes into your largest arteries．These branch into smaller arteries, which branch into still smaller ones．The blood flows from the smallest arteries into your capillaries． Capillaries are bridges between your arteries and your veins．They are the tiniest blood vessels, so small that you cannot see them without a microscope．Blood travels from the capillaries into tiny veins．These lead to larger and still larger veins．Finally, the largest veins take the blood back to your heart． Blood feeds your cells, cleans them, and works to keep them healthy．It carries food and oxygen to every cell of your body．The food and oxygen get into your cells by passing through the thin walls of your capillaries． Blood cleans all your cells by picking up wastes from them．The wastes pass from your cells, through the capillary walls, into your blood．The wastes called urea and uric acid are carried by the blood to your body and urine．The waste gas called carbon dioxide is carried by the blood to your lungs．It leaves your body when you breathe out． Blood also protects you．It has special cells in it that fight germs．The cells in your blood that fight germs are called“white cells”．These are like an army for your body．They kill harmful germs that get into your blood．When a large number of germs enter your body, the number of white cell“soldiers”grows．A lot of blood moves to the area where the germs are．The white cell soldiers attack the germs and kill them．The used white cells and dead germs form the thick yellow liquid called pus．If the pus is inside a sore on your skin, it may leak out． | 
