Venous blood is deoxygenated blood

Blood

Blood is a specialized bodily fluid in animals that delivers necessary substances such as nutrients and oxygen to the cells and transports metabolic waste products away from those same cells....

 in the circulatory system

Circulatory system

The circulatory system is an organ system that passes nutrients , gases, hormones, blood cells, etc...

. 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

Pulmonary artery

The pulmonary arteries carry deoxygenated blood from the heart to the lungs. They are the only arteries that carry deoxygenated blood....

, one of the few arteries in the body that carries deoxygenated blood (the pulmonary vein

Pulmonary vein

The pulmonary veins are large blood vessels that carry blood from the lungs to the left atrium of the heart. In humans there are four pulmonary veins, two from each lung...

s likewise return the newly oxygenated blood to the heart).

Venous blood is typically warmer than arterial blood

Arterial blood

Arterial blood is the oxygenated blood in the circulatory system found in the lungs, the left chambers of the heart, and in the arteries. It is bright red in color, while venous blood is dark red in color...

, and has a lower oxygen

Oxygen

Oxygen is the element with atomic number 8 and represented by the symbol O. Its name derives from the Greek roots ὀξύς and -γενής , because at the time of naming, it was mistakenly thought that all acids required oxygen in their composition...

 content and pH

PH

In chemistry, pH is a measure of the acidity or basicity of an aqueous solution. Pure water is said to be neutral, with a pH close to 7.0 at . Solutions with a pH less than 7 are said to be acidic and solutions with a pH greater than 7 are basic or alkaline...

. It also has lower concentrations of glucose

Glucose

Glucose is a simple sugar and an important carbohydrate in biology. Cells use it as the primary source of energy and a metabolic intermediate...

 and other nutrients, and has higher concentrations of urea

Urea

Urea or carbamide is an organic compound with the chemical formula CO2. The molecule has two —NH2 groups joined by a carbonyl functional group....

 and other 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

Arteriovenous oxygen difference

The arteriovenous oxygen difference, or a-vO2 diff, is the difference in the oxygen content of the blood between the arterial blood and the venous blood. It is an indication of how much oxygen is removed from the blood in capillaries as the blood circulates in the body...

.

Most medical

Medicine

Medicine is the science and art of healing. It encompasses a variety of health care practices evolved to maintain and restore health by the prevention and treatment of illness....

 laboratory tests are conducted on venous blood, with the exception of arterial

Artery

Arteries are blood vessels that carry blood away from the heart. This blood is normally oxygenated, exceptions made for the pulmonary and umbilical arteries....

 blood gas tests. It is obtained for lab work by venipuncture

Venipuncture

In medicine, venepuncture, venopuncture or venipuncture is the process of obtaining intravenous access for the purpose of intravenous therapy or for blood sampling of venous blood. This procedure is performed by medical laboratory scientists, medical practitioners, some EMTs, paramedics,...

 (also called phlebotomy), or by fingerprick

Fingerprick

Fingersticks and heelpricks are small superficial wounds that provide venous blood for some blood tests. Various methods are used to open the wound, which produces no more than a few drops of blood...

 for small quantities.

Colour

Venous blood is not blue as it is often depicted in many medical diagrams.

Veins often look blue when seen through the skin, but this is due to Rayleigh scattering

Rayleigh scattering

Rayleigh scattering, named after the British physicist Lord Rayleigh, is the elastic scattering of light or other electromagnetic radiation by particles much smaller than the wavelength of the light. The particles may be individual atoms or molecules. It can occur when light travels through...

 – venous blood itself is actually a dark red color (but looks purple through the opaque skin).

The appearance of veins as dark blue is a wavelength phenomenon, having to do 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.