Serous fluid major salivary glands produce mixed (serous and mucus) saliva. Another type of serous fluid is secreted by the serous membranes (serosa), two-layered membranes which line the body cavities. Serous membrane fluid collects on microvilli on the outer layer and acts as a lubricant and reduces friction from muscle movement. This can be seen in the lungs, with the pleural cavity. Blood serum is the component of blood that is neither a blood cell, nor a clotting factor. Blood serum and blood plasma are similar, but serum does not contain any clotting factors, such as fibrinogen, prothrombin, thromboplastin and many others.
P-glycoprotein lumen, in liver cells where it pumps them into bile ducts, in the cells of the proximal tubule of the kidney where it pumps them into urinary filtrate (in the proximal tubule), and in the capillary endothelial cells composing the blood–brain barrier and blood-testis barrier, where it pumps them back into the capillaries. P-gp is a glycoprotein that in humans is encoded by the "ABCB1" gene. P-gp is a well-characterized ABC-transporter (which transports a wide variety of substrates across extra- and intracellular membranes) of the MDR/TAP subfamily. The normal excretion of xenobiotics back into the gut lumen by P-gp pharmacokinetically