Blood Composition and Function

Blood Composition and Function

• Blood is a type of extracellular fluid, making up 8% of total body mass.
• Average male blood volume: 5-6L; average female: 4-5L.
• Blood is more viscous than water with a temperature maintained around 38ºC and pH range of 7.35-7.45.
• Blood's three main functions: transportation (oxygen, nutrients, hormones, heat, waste), regulation (pH, temperature, osmotic pressure), and protection (against blood loss and disease).

Blood Composition

• Blood has two main components:
  • Blood plasma (55%): 91.5% water; 8.5% solutes. The solutes include proteins (albumin, globulins, fibrinogen) and other substances like electrolytes, nutrients, hormones, and gases. Albumin maintains osmotic pressure; globulins include antibodies and transport proteins; and fibrinogen is involved in blood clotting.
  • Formed elements (45%): erythrocytes (red blood cells), leukocytes (white blood cells), and platelets.

Erythrocytes (Red Blood Cells)

• Most abundant formed element.
• Transport oxygen and carbon dioxide.
• Mature erythrocytes lack organelles, allowing for maximum hemoglobin storage.
• Hemoglobin (Hb) transports oxygen; accounts for ~1/3 of RBC weight.
• Contain carbonic anhydrase to convert CO₂ to carbonic acid, a critical blood buffer system (pH regulation).
• Biconcave shape for large surface area.
• Flexible to pass through capillaries.
• Life span: ~120 days; constant regeneration is needed. Destruction occurs in the liver, spleen, or red bone marrow.

Leukocytes (White Blood Cells)

• Five types with diverse roles in fighting infection and combating disease.
• Granular leukocytes (neutrophils, eosinophils, basophils): contain granules that stain differentially.
  • Neutrophils are phagocytic cells, with a multi-lobed nucleus, fighting bacterial infection.
  • Eosinophils fight parasitic infections. Their granules stain acidic, with a bi-lobed nucleus.
  • Basophils release histamine, a key player in inflammatory responses.
• Agranular leukocytes (lymphocytes, monocytes): lack visible granules in the cytoplasm.
  • Lymphocytes: primarily in lymphatic system, crucial for cell-mediated and humoral immunity.
    • B lymphocytes mature into plasma cells to secrete antibodies.
    • T lymphocytes and natural killer cells directly attack invading pathogens and infected cells.
  • Monocytes: develop into macrophages that phagocytose cellular debris and infection sites.

Platelets

• Thrombocytes: cell fragments involved in blood clotting.
• Formed from megakaryocytes in red bone marrow.
• Contain granules essential for blood clotting.

Hemostasis

• The process preventing hemorrhage (excessive bleeding):
  • Vascular spasm: smooth muscle contraction reduces blood loss.
  • Platelet plug formation: platelets adhere, activate, and aggregate to form a temporary clot.
  • Blood clotting (coagulation): formation of a stable fibrin clot.
    • Two pathways (intrinsic and extrinsic) converge to form a common pathway involving factors such as calcium ions and enzymes leading to fibrin formation.
• Regulation of clotting is crucial: clots can cause strokes and pulmonary embolisms.

Blood Cell Formation (Hemopoiesis)

• Occurs in red bone marrow.
• Starts with pluripotent stem cells.
• Two main lineages: myeloid and lymphoid.
• Myeloid stem cells develop into erythrocytes, leukocytes (granular and agranular), and platelets.
• Lymphoid stem cells develop into lymphocytes.
• Growth factors (EPO, TPO, cytokines) regulate the process.

Blood Groups

• ABO blood groups based on glycolipid antigens.
• Agglutinins (antibodies) recognize and bind foreign antigens, causing erythrocyte clumping (agglutination).
• Individuals make antibodies against foreign antigens.
• Blood transfusions must consider blood type compatibility to prevent agglutination.
• Rh factor (another antigen) can cause hemolytic disease of the newborn (HDN) if a mother's immune system attacks the fetus's blood.

Diseases/Disorders of Blood

• Anemia (low RBC count) has various causes.
• Sickle cell disease: genetic mutation causes abnormal hemoglobin, leading to deformed red blood cells. This trait provides resistance to malaria.
• Hemophilia: inherited bleeding disorder due to insufficient clotting factors.
• Polycythemia (high RBC count): too many red blood cells that lead to thickened blood, increasing the risk of blood clots.