The circulatory system
Blood flows ceaselessly through vessels, bringing oxygen and nutrients to cells, taking away waste and carbon dioxide, and defending the body from germs. Blood consists of a liquid part, plasma, and cells. Erythrocytes (red cells) carry oxygen and make blood red. Leukocytes (white cells) destroy germs. Platelets help stop bleeding and close a wound. The heart is mostly muscle, about the size of your fist. It contracts rhythmically to push blood into the vessels and beats roughly 100,000 times a day. Arteries carry blood from the heart to the organs; their walls are thick because blood enters them under strong pressure. Veins return blood from the organs to the heart. In organs there are the tiniest vessels, capillaries, through which oxygen passes to the tissues. Blood rich in oxygen is bright red, blood poor in oxygen is darker. Veins look bluish through the skin, but the blood inside is still red.
Count your pulse at rest, then after 30 seconds of squats, and write both in a table. Explain the difference.