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THE COMPOSITION OF BLOOD

  1. Blood is composed of cellular elements (i.e., red blood cells, white blood cells, and platelets) suspended in blood plasma.

    1. A person who weighs 70 kg has approximately 5 L of blood with about 2 L of cellular elements and 3 L of plasma.

  2. Plasma composition.

    1. Plasma is the part of the ECF contained within the cardiovascular system.

    2. It is approximately 92% water, 7% protein, and 1% small dissolved solutes (e.g., ions, urea, glucose, amino acids, and lipids).

    3. Plasma concentrations of ions and small molecules are similar to those in the interstitial fluid, due to the free exchange of water and small solutes across most blood capillaries (Table 3-1).

    4. Protein concentration is higher in plasma than interstitial fluid because most capillaries are impermeable to plasma proteins.

      1. The protein concentration gradient creates the colloid osmotic (oncotic) pressure gradient that opposes the filtration of plasma out of the capillaries (see Chapter 4).

      2. Albumin is the most abundant type of plasma protein and is the greatest contributor to the plasma oncotic pressure (Table 3-2).

      3. imageHypoalbuminemia has many causes, including nephrotic syndrome, liver failure, and severe malnutrition. Regardless of the cause, hypoalbuminemia can result in anasarca (generalized massive edema).image

  3. There are three main cell types in circulating blood (Table 3-3):

    1. Red blood cells (erythrocytes) are essential for the transport of O2 and CO2 in blood. There are approximately 5 × 1012 red blood cells per liter of blood.

    2. Platelets (thrombocytes) are small cellular fragments that play a key role in hemostasis. There are approximately 300 × 109 platelets per liter of blood.

    3. White blood cells (leukocytes) are the only fully functional nucleated cells in circulating blood. White cells play a defensive role in destroying infecting organisms and in the removal of damaged tissue. The total white cell count is approximately 5 × 109 cells per liter of blood.

Table 3-1Normal Plasma Composition
Table 3-2The Major Types of Plasma Proteins

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