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What is blood plasma?

Plasma is the liquid portion of blood in which red and white blood cells and platelets are suspended. Plasma, which is 92 percent water, constitutes 55 percent of blood volume. Plasma contains albumin (the chief protein constituent), fibrinogen (responsible, in part, for the clotting of blood), and globulins (including antibodies). Plasma serves a variety of functions, from maintaining a satisfactory blood pressure and volume to supplying critical proteins for blood clotting and immunity. It also serves as the medium for the exchange of vital minerals, such as sodium and potassium and helps to maintain a proper pH (acid-base) balance in the body, which is critical to cell function.

Plasma is a clear, yellowish fluid, although sometimes it can appear milky after a very fatty meal or when there is a high level of lipids in the blood. Plasma is 90-percent water but that remaining 10 percent of plasma is essential for life. Plasma is circulated throughout the body, moving into tissues and cells where it is needed. It helps keep fluid in the blood vessels, helps with muscle contraction and helps maintain pH balance.

Blood pressure (or hydrostatic pressure) pushes fluid out of blood vessels. Balancing this is oncotic pressure (caused by proteins dissolved in the blood), which helps keep fluid inside the blood vessels.

A large part of the dissolved materials in plasma are proteins. Protein molecules, which are responsible of oncotic pressure, are larger than water molecules and, therefore, tend to stay in blood vessels. The proteins have a difficult time fitting through the pores in capillaries, so they have a higher concentration in blood vessels. Proteins attract water to keep their concentration in blood vessels similar to fluid outside the blood vessels. This helps the body maintain a constant volume of blood.

Beyond proteins, electrolytes are a large category of substances dissolved in plasma. Electrolytes include:

  • Sodium
  • Potassium
  • Chloride
  • Bicarbonate
  • Calcium
  • Magnesium

These chemicals are essential in many bodily functions including fluid balance, muscle contraction (including the heart), blood clotting, nerve conduction and pH balance.

Also found dissolved in plasma are carbohydrates (glucose), hormones, cholesterol and vitamins.

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Important: This content reflects information from various individuals and organizations and may offer alternative or opposing points of view. It should not be used for medical advice, diagnosis or treatment. As always, you should consult with your healthcare provider about your specific health needs.