The question of whether plasma has a smell often arises from the distinctive, metallic odor people associate with whole blood. Plasma is the light, straw-colored liquid component of blood, making up about 55% of its total volume. It is important to distinguish this fluid from the cellular components, which include red blood cells, white blood cells, and platelets. This difference in composition holds the scientific answer to plasma’s inherent odor.
The Composition of Plasma and Odor
Plasma is nearly odorless because of its simple and non-volatile chemical makeup. The fluid is composed of 91 to 92% water, which is an odorless solvent. The remaining 8 to 9% consists primarily of dissolved solids, including proteins like albumin and fibrinogen, along with electrolytes such as sodium and potassium.
These dissolved substances do not easily evaporate into the air to be detected by the olfactory system. If plasma has any scent at all, it is described as a very faint, slightly briny or serum-like aroma, as it lacks the cells that contain the strong odor commonly attributed to blood.
The Source of Blood’s Metallic Scent
The strong, metallic scent associated with a wound or a nosebleed is chemically generated by the cellular fraction of blood, specifically the red blood cells. The odor is the product of a chemical reaction when iron-rich blood interacts with substances on the skin or in the air.
Hemoglobin, the protein inside red blood cells, contains iron atoms that catalyze a reaction upon exposure to skin lipids. This process breaks down the lipids and releases highly volatile organic compounds (VOCs). The primary compound responsible for the coppery smell is trans-4,5-epoxy-(E)-2-decenal, an aldehyde detectable at very low concentrations.
Another related molecule, oct-1-en-3-one, is also produced when the iron from hemoglobin reacts with oxidized skin oils. These volatile aldehydes and ketones create the metallic sensation that the brain interprets as the “smell of blood,” indicating the presence of red blood cells and their iron content.
When Plasma Carries Abnormal Odors
While plasma is odorless, its function as a carrier means that waste products can lead to abnormal odors in the body. Plasma transports metabolic byproducts to the kidneys and liver for excretion, and failure in these systems can cause an accumulation of compounds. These odors are carried within the plasma, not generated by it.
For instance, kidney failure leads to uremia, where waste products like urea build up in the plasma. This results in “uremic fetor,” an ammonia-like odor on the breath, as the body attempts to excrete the toxins through the lungs.
Similarly, uncontrolled diabetes can result in diabetic ketoacidosis, where the plasma carries a high concentration of ketone bodies. The buildup of the ketone acetone is then expelled through respiration, giving the breath a characteristic fruity or sweet odor.