Serous fluid

In physiology, the term serous fluid or serosal fluid is used for various bodily fluids that are typically pale yellow and transparent, and of a benign nature, that fill the inside of body cavities. Serous fluid originates from serous glands, with secretions enriched with proteins and water. Serous fluid may also originate from mixed glands, which contain both mucous and serous cells. A common trait of serous fluids is their role in assisting digestion, excretion, and respiration.

In medical fields, especially cytopathology, serous fluid is a synonym for effusion fluids from various body cavities. There are many causes of effusions which include involvement of the cavity by cancer. Cytopathology evaluation is recommended to evaluate the causes of effusions in these cavities.

Examples
Saliva consists of mucus and serous fluid; the serous fluid contains the enzyme amylase important for the digestion of carbohydrates. Minor salivary glands of von Ebner present on the tongue secrete the amylase. The parotid gland produces purely serous saliva. The other major salivary glands produce mixed (serous and mucus) saliva.

Another type of serous fluid is secreted by the serous membranes (or serosa), two layered membranes which line the body cavities. The serous fluid between the two layers acts as a lubricant and reduces friction from muscle movement. This can be seen in the lungs. Cytopathologic evaluation is recommended to diagnose the causes of fluid accumulation which include involvement of the cavity by cancer.

Blood serum is the liquid part of blood remaining after clotting, and is therefore lacking in clotting factors and distinct from blood plasma.