Plasma is the fluid portion of the blood and consists of over 90% water. Water is the liquid carrier of plasma solutes (dissolved substances) and formed elements, in addition to being the solvent of all living systems. Plasma contains a great variety of solutes, such as nutrients, enzymes, hormones, antibodies, waste products, electrolytes, and respiratory gases. Table below lists the major types of solutes in plasma. Plasma solutes are constantly being added and removed, so the solutes are normally in a state of dynamic balance that is maintained by a variety of homeostatic mechanisms.
| Solute | Description |
| Albumins | Help transport hydrophobic substances, maintain osmotic pressure and pH of blood |
| Globulins | Alpha and beta types transport lipids: gamma type is antibodies |
| Fibrinogen | Soluble protein that is converted to insoluble fibrin during formation of blood clot |
| Nitrogenous wastes | Breakdown products of proteins, nucleic acids, and creatine phosphate |
| Nutrients | Amino acids, fatty acids, glycerol, vitamins, and glucose |
| Enzymes and hormones | Help regulate metabolic processes |
| Electrolytes | Help regulate blood pH. osmotic pressure, and the ionic balance between blood and interstitial fluid |
| Respiratory gases | Approximately 1.5% of the oxygen and 7% of the carbon dioxide transported by blood is dissolved in plasma |
Plasma Proteins
Plasma proteins are the most abundant solutes. They are not used as an energy source but remain in the plasma. Less than 1% of plasma proteins are enzymes and hormones. The three major groups of plasma proteins are albumin, globulins, and fibrinogen. Except for gamma globulins, plasma proteins are produced by the liver and are released into the blood.
Nitrogenous Wastes
Nitrogenous wastes are nitrogen-containing substances that include ammonia, urea, uric acid, and creatinine. Ammonia and urea are wastes produced during protein metabolism. Uric acid comes from the catabolism of nucleic acids. Creatinine is produced as a result of creatine phosphate breakdown in the muscle cells. These wastes are carried in the blood to the kidneys, where they are excreted into urine. Plasma levels of these wastes are commonly used as indicators of kidney health.
Electrolytes
Most of the plasma electrolytes are ions of inorganic compounds that are either absorbed from the intestine or released from body cells. The common electrolytes include sodium ions (Na+), potassium ions (K+), calcium ions (Ca2+), chloride ions (Cl–), bicarbonate ions (HCO3–), and phosphate ions (PO43-). Electrolytes help to maintain the osmotic pressure and pH of the blood, and a normal ionic balance between interstitial fluid and blood.
Medical Insight
High levels of blood cholesterol are associated with an increased risk of heart disease. Cholesterol occurs in the blood in combination with triglycerides and carrier proteins. These lipid-protein complexes are called lipoproteins. Considerable evidence links a high concentration of blood low-density lipoprotein (LDL), the so-called “bad” cholesterol, with heart disease. In contrast, high levels of blood high-density lipoprotein (HDL), the “good” cholesterol, reduce the risk of heart disease. Blood cholesterol levels result from a combination of heredity, diet, and exercise.
A total blood cholesterol level less than 200 mg/dl (milligrams per deciliter) is a desirable goal. A blood LDL concentration of 100 to 130 mg/dl is near optimal. Persons at risk of coronary artery disease, such as smokers and the elderly, should strive for an LDL level less than 100. Reducing the amount of saturated fats (red meat, milk products, and egg yolks) and trans fats (present in hydrogenated oils) in the diet can decrease the LDL level.
Desired HDL levels average 40 to 50 mg/dl in men and 50 to 60 mg/dl in women. HDL levels may be increased by exercise and maintaining a healthy weight.
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