Blood Composition and Plasma Constituents

Blood Composition

• Blood is a fluid connective tissue, constantly circulating throughout the body, propelled by the heart's pumping action.
• Blood transports oxygen, nutrients, hormones, heat, antibodies, immune cells, clotting factors, and waste products.
• Blood is composed of a clear, straw-colored, watery fluid called plasma, with various blood cells suspended within it.
• Plasma makes up approximately 55% of total blood volume.
• Cellular components (blood cells) account for the remaining 45% of blood volume.

Plasma Constituents

• The main constituent of plasma is water (90-92%).
• Dissolved and suspended substances in plasma include:
  • Plasma proteins (approximately 7%)
  • Inorganic salts (electrolytes)
  • Nutrients
  • Waste products
  • Hormones
  • Gases

Plasma Proteins

• Plasma proteins, primarily albumin and fibrinogen, are too large to escape through capillary pores.
• They are formed in the liver
• Function:
  • Creating osmotic pressure within blood
  • Maintaining plasma viscosity (thickness)
  • Reduction in plasma protein levels leads to fluid shifts into tissues (edema).

Specific Plasma Proteins (Albumin)

• Albumin is the most abundant plasma protein
• Function:
  • Maintaining normal blood osmotic pressure
  • Acting as a carrier for fatty acids, some drugs, and steroid hormones.

Specific Plasma Proteins (Globulins)

• Most globulins are produced in the liver, with some originating in lymphoid tissue.
• Function:
  • Antibodies (immunoglobulins), binding to and neutralizing foreign materials (e.g., microorganisms).
  • Transporting hormones and mineral salts (e.g., thyroglobulin carries thyroxine).
  • Inhibiting certain proteolytic enzymes.

Clotting Factors

• Essential for blood clotting.
• Serum is plasma from which clotting factors have been removed.
• Fibrinogen, synthesized in the liver, is an essential clotting factor.

Inorganic (Mineral) Salts

• Involved in various bodily activities, including muscle contractions, nerve impulse transmission, secretion formation, and acid-base balance maintenance.
• Blood's alkalinity is measured by pH, typically ranging from 7.35 to 7.45.

Nutrients

• Food is broken down into small molecules (e.g., monosaccharides, amino acids, fatty acids) in the alimentary tract and absorbed.
• Along with mineral salts, nutrients supply energy, heat, and materials for repair and replacement, and are essential for blood component and body secretion synthesis.

Waste Products

• Urea, creatinine, and uric acid are waste products of protein metabolism.
• These waste products are formed in the liver and transported to kidneys for excretion.
• Carbon dioxide, a waste product of tissue metabolism, is transported to the lungs for excretion.

Hormones

• Hormones are chemical messengers synthesized by endocrine glands.
• They are secreted into the blood and transported to their target tissues/organs.

Gases (Oxygen)

• Oxygen is not very soluble in water; only a small amount can be transported dissolved in plasma.
• Haemoglobin in red blood cells is crucial for oxygen transport (98% of oxygen in blood).
• Haemoglobin also binds some carbon dioxide.
• Most carbon dioxide is converted into bicarbonate ions in red blood cells, then transported in the plasma.

Cellular Content of Blood

• Stem cells in bone marrow give rise to blood cells.
• These are called pluripotent hematopoietic stem cells (PHSC).
• Committed stem cells form more specific cell types
• Erythrocytes (red blood cells)
• Leukocytes (white blood cells)
• Platelets (thrombocytes)

Erythrocytes

• Most abundant blood cells (99% of blood cells)

• Develop from stem cells in about 7 days (erythropoiesis)

• Released into bloodstream as reticulocytes, maturing over a few days

• Contain hemoglobin for oxygen transport

• Biconcave discs for increased surface area

• No nucleus for efficient oxygen transport

• Live an average of 120 days

• Oxygen transport mechanism:

  • Oxygen binds reversibly to hemoglobin to form oxyhemoglobin.

• Low oxygen levels drive further oxygen release.

• Temperature and pH also influence oxygen release rates.

• The various stages in erythropoiesis include: - proerythroblast - early normoblast - intermediate normoblast - late normoblast - reticulocyte - mature erythrocyte

Erythrocyte Destruction

• Life span of erythrocytes is approximately 120 days
• Breakdown handled by phagocytic reticuloendothelial cells in spleen, bone marrow, and liver.
• Hemoglobin breakdown yields iron and bilirubin.
• Iron is recycled and bilirubin is excreted as bile.

White Blood Cells (Leukocytes)

• White blood cells are colorless and nucleated.
• Play vital roles in body defense mechanisms.
• Classified into granulocytes (with granules) and agranulocytes (without granules)

Granulocytes

• Neutrophils: First line of defense against microorganisms, using enzymes (proteases, myeloperoxidases, elastases, metalloproteinases), and antimicrobial peptides (defensins).
• Eosinophils: Primarily act against parasites; their granules contain substances that become cytotoxic when released.
• Basophils: Crucial for inflammatory and hypersensitivity reactions (e.g., allergies). Their granules release histamine and heparin (important chemicals).
• Mast cells: Large tissue cells resembling basophils, present in bone marrow and blood vessels, play important roles in allergic and inflammatory reactions. Mast cells secrete heparin, histamine, serotonin, and hydrolytic enzymes.

Agranulocytes

• Monocytes: Mobile and phagocytic (capable of engulfing foreign material). Produce several important factors. They mature into tissue macrophages
• Lymphocytes: Classified into T-lymphocytes (cellular immunity) and B-lymphocytes (humoral immunity).

Platelets

• Thrombocytes; small, colorless, non-nucleated cell fragments originating from megakaryocytes.
• Essential for hemostasis (arrest of bleeding)
• Involved in these three main properties:
  • Adhesion
  • Aggregation
  • Agglutination
• Essential Roles:
  • blood clot formation; this process involves intrinsic prothrombin activator formation.
  • blood clot retraction, involves the proteins actin, myosin, and thrombosthenin. The contractile proteins facilitate clot retraction
  • prevention of blood loss (hemostasis) through:
    • constriction of blood vessels.
    • sealing damage in blood vessels.
    • formation of a temporary plug.
  • repair of ruptured blood vessels, using platelet-derived growth factor-PDGF formed in cytoplasm
  • defense mechanism through agglutination, encircling and killing foreign material through phagocytosis.

Haemostasis

• Definition: Arrest of bleeding or stoppage of bleeding

• Stages:

  1. Vasoconstriction
  2. Platelet plug formation
  3. Coagulation of blood
  4. Thrombolysis

Blood Grouping

• Red blood cell membranes have antigens; these can trigger immune responses if incompatible.
• ABO system classifies blood types A, B, AB, and O based on the presence/absence of A and B antigens.
• Rh factor (another antigen) leads to either Rh-positive or Rh-negative blood types
• Essential factor in blood transfusion: ensuring only compatible blood is used.

Cross-matching

• Essential step in blood transfusion.
• Involves mixing recipient serum with donor red blood cells.
• Agglutination indicates incompatibility; incompatible blood is not used in transfusion.