What is the threshold for cyanosis

health : Why do lips turn blue?

Air is a better source of oxygen than water. One liter of air contains an average of 200 milliliters of oxygen, while one liter of water only contains seven milliliters. When you breathe, oxygen from the air enters your blood through your lungs. It's a special juice. In contrast to water, it contains substances that can bind oxygen: protein molecules with copper or iron atoms in the center.

In lobsters or squids, copper-containing hemocyanin takes over the oxygen transport. You have blue blood. In humans, the iron-containing hemoglobin colors the blood bright red when it absorbs oxygen. It reaches the body tissue via arteries and capillaries, where the hemoglobin releases the oxygen again: it deoxygenates.

At the same time, the blood absorbs carbon dioxide. In tissues with an active metabolism, where a lot of carbon dioxide passes into the blood, the blood becomes acidic, which accelerates the release of oxygen. The returning venous blood contains more deoxygenated hemoglobin and is clearly darker to bluish in color.

Usually the blood flows quickly back to the lungs. If it stays longer in one place, it releases more oxygen. This happens, for example, when you bathe in cool water for a long time. "The body tightens the blood vessels when it is cold," says the hematologist Wolf-Dieter Ludwig from the Berlin Charité. "This slows the blood flow, the blood loses more oxygen." From a threshold of five grams of deoxygenated hemoglobin per 100 milliliters of blood, this shows up as a blue color.

The lips are colored most clearly. The skin is thin here, the blue blood shows through, especially in fair-skinned types whose lips are not interspersed with dark pigments. In general, the veins are more prominent in light skin - hence the idea of ​​“blue-blooded” people: In aristocratic circles it was considered elegant not to expose oneself to the sun.

A blue coloration (cyanosis) also occurs in diseases when the heart is no longer pumping vigorously enough and the blood lingers in the capillaries for longer. In extreme cases, things stand still: corpses always have blue lips. On the other hand, people who suffer from anemia do not get blue lips when bathing. If you have little hemoglobin in your blood anyway, the deoxygenated hemoglobin does not easily cross the threshold to blue color either.

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