The circulatory system consists of a network of artery, veins and pumping organ called the heart.
The additional is the lymphatic system which is discussed separately.
The circulatory system essentially requires blood and heart.
The heart pumps the blood with a force of 120/80mmhg so that it can reach to peripheral organs.
Blood carries not only oxygen, co2 but also nutrition.
Hypoglycemia is the condition when the blood glucose level decreases. ( Brain may stop working if glucose don't reach within 6sec)
Hypoxia is the condition when the blood contains less of O2 and more of CO2. Cellular death occurs when prolonged hypoxia leading to ischemia and necrosis.