Hello everyone! My name is Xiaohong, and I am an ordinary red blood cell. I travel around the world with my brothers and sisters through a long pipe every day. First, I pushed open a heavy "door" from the left atrium and entered a "house" with a thick wall. This is the left ventricle that I am very familiar with. That "door" is the atrioventricular valve that prevents blood flow. Then, I followed the big army to the main artery, passed some branches of the main artery, and continued to flow until I reached the "glass hall". It is called "glass hall" because its outer wall is very thin and there is only one layer of epithelial cells; It is also very narrow, and only red blood cells can pass through one by one. We call it capillary. When I reached the middle of the capillary, I felt weak all over. This is because red blood cells are rich in hemoglobin, which can combine with oxygen. When the red blood cells reach the capillaries, the oxygen in the red blood cells will exchange substances with tissue cells through the capillaries. Therefore, I will be "weak". Immediately, I was flowing again. I completed a "systemic circulation" through the superior and inferior vena cava to the right atrium and then to the right ventricle. Then, I had another gas exchange with my lungs through pulmonary artery and capillaries. I was amazing! Ha ha! I returned to the left atrium through the pulmonary vein and completed a "pulmonary circulation". What's the matter? Am I handsome? I haven't passed out after so many turns. Let me tell you a few more secrets: there is an arterial valve between the ventricle and the artery; There are venous valves in the veins of limbs; It is not necessarily venous blood that flows in veins; It is not necessarily arterial blood that flows in the artery. As for why, study for yourself!