Short answers, pulled from the story.
The circulatory system in vertebrates is a system of organs that includes the heart, blood vessels, and blood circulated throughout the body. It includes the cardiovascular system, made up of the heart and blood vessels, and it is closed, meaning the blood never leaves the network of vessels.
The circulatory system has two divisions, the pulmonary circulation and the systemic circulation. The pulmonary circuit takes deoxygenated blood from the right heart to the lungs and returns it to the left heart, while the systemic circuit delivers oxygenated blood from the left heart to the body and returns it to the right heart.
An average adult contains five to six quarts of blood, roughly 4.7 to 5.7 liters, which accounts for approximately 7% of their total body weight. The blood consists of plasma, red blood cells, white blood cells, and platelets.
In arthropods the open circulatory system has a heart that pumps a fluid called hemolymph, which bathes the organs directly and returns via the body cavity rather than through blood vessels. There is no distinction between blood and interstitial fluid, and the primary oxygen transporter is hemocyanin.
Marcello Malpighi discovered the capillary system connecting arteries and veins in 1661. This completed the work of William Harvey, who in 1628 argued for a continuous circulation through minute connections at the body's extremities but never identified them.
In 1242 Ibn al-Nafis described pulmonary circulation in greater and more accurate detail than his predecessors, rejecting the idea that the heart's septum had pores. He also predicted that small communications, or pores, must exist between the pulmonary artery and vein, more than 400 years before the discovery of the capillary system.