Human blood in veins is not blue. In fact, blood is always red due to hemoglobin. Deoxygenated blood has a deep red color, and oxygenated blood has a light cherry-red color. The misconception probably arises for two reasons:

1. Veins appear blue because light, penetrating the skin, is absorbed and reflected back to the eye. Since only the higher energy wavelengths can do this (lower energy wavelengths just don't have the *oomph*), only higher energy wavelengths are seen. And higher energy wavelengths are what we call "blue." In an experiment, glass tubes were filled with blood and immersed in milk, milk having a similar ratio of fat, proteins, and water in emulsion as skin. At a certain depth, the tubes appeared blue.

2. From the way diagrams use colors to show the difference between veins (usually shown in blue) and arteries (usually shown in red).

There are three varieties of blood vessels: arteries, veins, and capillaries. During blood circulation, the arteries carry blood away from the heart. The capillaries connect the arteries to veins. Finally, the veins carry the blood back to the heart. If you took all of the blood vessels out of an average child, and laid them out in one line, the line would be over 60,000 miles long! An adult's vessels would be closer to 100,000 miles long!

When we measure blood pressure, we use the blood flowing through the arteries because it has a higher pressure than the blood in the veins. Your blood pressure is measured using two numbers. The first number, which is higher, is taken when the heart beats during the systole phase. The second number is taken when the heart relaxes during the diastole phase. Those two numbers stand for millimeters. A column of mercury rises and falls with the beat of the heart. The height of the column is measured in millimeters. Normal blood pressure ranges from 110 to 150 millimeters (as the heart beats) over 60 to 80 millimeters (as the heart relaxes). It is normal for your blood pressure to increase when you are exercising and to decrease when you are sleeping. If your blood pressure stays too high or too low, however, you may be at risk of heart disease.