Human Blood Characteristics and Composition

Blood Characteristics

• Blood is a sticky, opaque fluid that is heavier and thicker than water.
• Oxygen-rich blood is scarlet red, while oxygen-poor blood is duller red or purple.
• Blood has a metallic, salty taste.
• The pH of blood is slightly alkaline, ranging from 7.35 to 7.45.
• Blood temperature is slightly higher than body temperature, around 38°C or 100.4°F.

Blood Volume

• A healthy adult has approximately 5-6 liters, or about 6 quarts, of blood.
• Blood accounts for approximately 8% of body weight.

Plasma Composition

• Plasma is primarily made up of water (90%).
• It is a straw-colored fluid containing many dissolved substances, including nutrients, salts (electrolytes), respiratory gases, hormones, plasma proteins, and waste products.

Plasma Proteins

• Plasma proteins are the most abundant solutes in plasma, primarily produced by the liver.
• Albumin is an essential blood buffer and contributes to osmotic pressure.
• Clotting proteins help to stop blood loss when a blood vessel is injured.
• Antibodies protect the body from pathogens.

Blood Composition Variability

• The composition of blood fluctuates as cells exchange substances with the blood.
• The liver produces more proteins when levels decline.
• The respiratory and urinary systems regulate blood pH to normal levels when it becomes too acidic or alkaline.
• Plasma distributes body heat.

Formed Elements

• Blood contains:
  • Erythrocytes (RBCs)
  • Leukocytes (WBCs)
  • Platelets

Erythrocytes (Red Blood Cells)

• Their main function is to carry oxygen.
• They lack a nucleus, contain few organelles, and have no mitochondria.
• Erythrocytes are essentially bags of hemoglobin (Hb).
• They are shaped like biconcave discs.
• Normal blood contains around 5 million RBCs per cubic millimeter (mm3) of blood.

Hemoglobin

• Hemoglobin is an iron-bearing protein that binds oxygen.
• Each hemoglobin molecule can bind to 4 oxygen molecules.
• An individual erythrocyte has 250 million hemoglobin molecules.
• Normal blood contains 12-18 g of hemoglobin per 100 milliliters (ml) of blood.

Anemia

• A decrease in the blood's oxygen-carrying capacity due to:
  • Lower than usual RBC count
  • Abnormal or deficient hemoglobin content in RBCs
• Sickle cell anemia (SCA) is caused by abnormally shaped hemoglobin.

Granulocytes

• Neutrophils:

  • The most common type of WBC.
  • Have a multilobed nucleus.
  • Cytoplasm stains pink and contains fine granules.
  • They are phagocytes that function at active sites of infection.
  • Numbers increase during infection.
  • There are 3,000-7,000 neutrophils per mm3 of blood (40-70 percent of WBCs).

• Eosinophils:

  • Have a nucleus that stains blue-red.
  • Have brick-red cytoplasmic granules.
  • Function to kill parasitic worms and participate in allergy attacks.
  • There are 100-400 eosinophils per mm3 of blood (1-4 percent of WBCs).

• Basophils:

  • The rarest of the WBCs.
  • Possess large histamine-containing granules that stain dark blue.
  • Contain heparin (an anticoagulant).
  • There are 20-50 basophils per mm3 of blood (0-1 percent of WBCs).

Agranulocytes

• Lymphocytes:

  • Have a large, dark purple nucleus.
  • Slightly larger than RBCs.
  • Reside in lymphatic tissues.
  • Play a crucial role in immune responses.
  • There are 1,500-3,000 lymphocytes per mm3 of blood (20-45 percent of WBCs).

• Monocytes:

  • The largest of the white blood cells.
  • Have a unique U- or kidney-shaped nucleus.
  • Function as macrophages when they migrate into tissues.
  • Important in fighting chronic infections.
  • There are 100-700 monocytes per mm3 of blood (4-8 percent of WBCs).

Platelets

• Fragments of megakaryocytes (multinucleate cells).
• Essential for the clotting process.
• Normal platelet count is 300,000 platelets per mm3 of blood.

Regulation of Red Blood Cell Production

• Normal blood oxygen levels are maintained by a feedback mechanism that regulates the rate of RBC production.
• When oxygen-carrying ability decreases (due to low RBC count, decreased hemoglobin, or reduced oxygen availability), the kidneys (and liver to a lesser extent) release erythropoietin.
• Erythropoietin stimulates red bone marrow to produce more RBCs, thereby increasing the blood's oxygen-carrying capacity.