plasma

Meaning: Plasma is the straw-colored fluid that remains when blood cells are removed. It is mostly water. The dissolved solutes include albumin, globulins (such as antibodies), fibrinogen, electrolytes (Na⁺ is the main cation), glucose, hormones, urea and dissolved gases. Most CO₂ travels in plasma as bicarbonate, HCO₃⁻, which forms in red cells through carbonic anhydrase and then leaves them. Plasma carries all of these between organs. (In physics, plasma is also an ionized gas, a fourth state of matter.)

On the MCAT: A centrifuged, anticoagulated blood sample separates into three layers. Packed red cells are at the bottom, a thin buffy coat of white cells and platelets is in the middle, and plasma is on top. The red-cell layer gives the hematocrit. Plasma also appears in Starling-force questions. Capillary hydrostatic pressure pushes fluid out, and plasma oncotic pressure, mostly from albumin, pulls fluid in. If plasma albumin falls, oncotic pressure falls and fluid accumulates in the tissues (edema). Passages may also ask how water loss or ADH changes plasma volume and osmolarity.

Common confusion: Plasma is not serum. Serum is the fluid left after blood clots, so it lacks fibrinogen and the other clotting factors used up in the clot. Plasma is also not interstitial fluid, which is similar but has far less protein. Finally, plasma is not the plasma membrane, which is the phospholipid bilayer around a cell.