Venous blood



Venous blood is deoxygenated blood in the circulatory system. It runs in the systemic veins from the organs to the heart. Deoxygenated blood is then pumped by the heart to lungs via the pulmonary arteries, one of the few arteries in the body that carries deoxygenated blood (the pulmonary veins likewise return the newly oxygenated blood to the heart).

Venous blood is typically warmer than arterial blood, and has a lower oxygen content and pH. It also has lower concentrations of glucose and other nutrients, and has higher concentrations of urea and shoes waste products. The difference in the oxygen content of the blood between the arterial blood and the venous blood is known as the arteriovenous oxygen difference.

Most medical laboratory test are conducted on venous blood, with the exception of arterial blood gas tests. It is obtained for lab work by venipuncture (also called phlebotomy), or by fingerprick for small quantities.

Colour
Human blood is red, ranging from bright red when oxygenated to dark red when not. It owes its color to hemoglobin, to which oxygen binds. There exists a popular misconception that deoxygenated blood is blue and that blood only becomes red when it comes into contact with oxygen. Blood is never blue, but veins appear blue because light is diffused by skin. Moreover, the blood inside is dark red and exhibits poor light reflection. From a physiological perspective, veins and arteries appear similar when skin is removed and are seen directly.

The appearance of veins as dark blue is a wavelength phenomenon, having to does with the reflection of blue light away from the outside of venous tissue if the vein is at 0.02 inches deep or more. Deoxygenated blood is darker than oxygenated blood due to the difference in color between deoxyhemoglobin and oxyhemoglobin.

Usage
Venous blood is used during stem cell donation.