Heart rate is universally related to body size: the bigger the animal, the slower its heart beats. Blood is up to 79% water.
Before World War I, almost no medical doctors were heart specialists. Electrocardiograph (ECG) was invented in 1902 by Dutch physiologist Willem Einthoven. It is still used to observe heart’s rate and rhythm. In 1929, German physician Werner Forssmann invented the procedure Cardiac catheterization. He first performed it on himself. On Earth, blood tends to pool in the legs because of gravity. In space, it stays in the chest and head. That’s why astronauts have puffy faces. Skin gets 13% of all blood supply. Microscopic veins are called venules. Veins are not blue. They appear that way as only blue light could penetrate skin and reflect back to your eyes without being absorbed by skin. Blood in veins is dark red because it holds very little oxygen. Heart actually can keep beating after being removed from the body provided that it continues to receive oxygen. Unlike most other cells, red blood cells have no nuclei and thus are able to carry more oxygen. However, they cannot divide. All blood vessels’ combined length is close to 100,000 km (60,000 miles). Capillaries alone contribute for 85% of this length. Veins, arteries and capillaries are the three types of blood vessels in the circulatory system. They transport blood containing oxygen, antibodies, nutrients, electrolytes and hormones. Blood vessels also transfer heat from blood to body organs. There are three main types of arteries: elastic (large), muscular (medium) and arterioles (small). Arterioles are less than half millimetre in diameter, barely visible to human eye. Blood vessels’ diameter is ranging from 25 mm (one inch) in aorta to only 6 μm in capillaries. Thickness of blood vessel walls is only one single cell in capillaries. Most veins and arteries can dilate or constrict in response to nervous and chemical stimuli. Adrenaline redirects blood to muscles and prepares the body for “fight or flight” response. Large arteries’ pale yellow colour is due to the elastic fibres, making up to 50% of arteries’ walls. Small arteries and arterioles determine blood pressure. Blood pressure is measured in arteries. First (higher) number is the systole phase, when your heart beats. Second is the diastole, when your heart is relaxed between beats.