Blood is composed of solid and liquid components. The solid part contains white blood cells, red blood cells, and platelets — about 45 percent of the total blood volume. The remaining 55 percent is plasma, the liquid component. Though it makes up more than half of this vital fluid, it is often overlooked. Plasma plays many important roles, including carrying the solid blood components throughout the body.
About 92 percent of plasma is water. The remaining 8 percent is sugar, fat, salts, and various proteins, including albumin and immunoglobulins. In addition to carrying blood cells and platelets around the body, plasma also transports nutrients, antibodies, chemical messengers like hormones, clotting proteins, and waste products.
Plasma serves four main purposes. First, it maintains adequate blood pressure and blood volume. It also carries the essential proteins required for blood clotting and immunity, so they are readily available, and delivers electrolytes to the muscles. Finally, plasma helps maintain a proper pH balance to support cellular function.
Albumin makes up about 50 percent of the total protein content in plasma. It is made in the liver and serves several functions, the most important of which is providing osmotic pressure to prevent fluids in the vascular space from leaking into tissues. Albumin also carries many important hormones, including cortisol, testosterone, and thyroxine.
Globulins are the second most common proteins in plasma. There are three main subgroups of globulins. Alpha and beta globulins are produced in the liver and transport fat-soluble vitamins, lipids, and iron to the cells. Like albumin, they also play a role in providing osmotic pressure.
The third type of globulin is gamma globulins, better known as immunoglobulins or antibodies. Gamma globulins are involved in immunity. They are not made in the liver like alpha and beta globulins but by specialized plasma cells called leukocytes. Immunoglobulins make up about 38 percent of plasma protein and are the most prevalent protein after albumin.
Another protein carried in the plasma is fibrinogen, an essential component of blood clotting. Fibrinogen is made in the liver; it helps form blood clots and plays a role in wound healing. Fibrinogen is the least abundant protein in the plasma, making up only about 7 percent of total plasma proteins.
Plasma is usually yellow because of components such as bilirubin, hemoglobin, carotenoids, and iron. In rare cases, plasma can be dark brown or red, and this discoloration indicates hemolysis or broken blood cells. Plasma should also be transparent. If it is opaque, it is usually the result of hyperlipidemia or excess fats — triglycerides or cholesterol — in the blood.
Plasmapheresis is a treatment that involves separating the plasma. Blood travels from the body through a large catheter and then a machine that separates the plasma from the other blood components. The blood cells return to the body through another catheter, while another device treats the plasma to remove antibodies. Finally, the treated plasma is returned to the body.
Plasmapheresis treats multiple conditions, including autoimmune disorders like multiple sclerosis, Guillain-Barre syndrome, and systemic lupus erythematosus. Because the process removes circulating antibodies, the person may experience relief from the symptoms of these conditions, which are caused by an overactive immune system. Plasmapheresis does not treat the underlying causes of these conditions, though, so it only provides temporary relief.
In plasma donation, a machine separates the donor's plasma from the rest of their blood. This occurs during the donation process, and the blood cells and platelets are immediately returned to the donor, along with saline. The plasma is frozen to preserve its clotting factors. Donated plasma is used in emergency medicine, primarily for major traumas and burn patients. People with type AB blood are universal donors.
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