Blood: Composition and Functions

• Blood is a connective tissue
• Contains cells, cell fragments (formed elements), and extracellular fluid (plasma)
• Plasma is protein-rich
• Blood has no fibers except when it clots
• Blood accounts for 8% of body weight
• PH of blood is 7.35-7.45
• The average volume of blood is approximately 5-6 Liters in men and 4-5 Liters in women

Functions of Blood

• Transports nutrients and oxygen to the cells
• Transports waste material and carbon dioxide away from the cells
• Transports hormones and other regulatory substances
• Maintains homeostasis, and acts as a buffer to regulate pH
• Participates in coagulation
• Involved in thermoregulation

Plasma

• Acts as a medium for circulating cells and metabolic substances
• Plasma accounts for 55% of blood volume, while cells make up 45%
• Composition consists of 91.5% water, 7% proteins (albumin, globulins, fibrinogen), and 1.5% other solutes
• Transports nutrients, waste substances from tissue, and secretions from endocrine glands
• Serum is plasma without coagulation proteins
• Plasma maintains osmotic pressure and helps with immune response and blood clotting (coagulation)

Formed Elements

• Include red blood cells (erythrocytes), white blood cells (leukocytes), and platelets
• Produced by a process called hematopoiesis, which primarily occurs in red bone marrow

Examination of Blood Cells

• A blood smear is used to examine the number and morphology of different blood cells
• Special stains such as Gimsa stain and Wright’s stain are used

Erythrocytes (RBCs)

• Have no nucleus and no other organelles
• Have a biconcave disc shape which maximizes the cell's surface area and is important in gas transport and exchange
• Their flexibility allows them to pass through narrow blood vessels
• Contain hemoglobin, which transports oxygen and carbon dioxide
• Life span is 120 days
• 90% are broken down in spleen, bone marrow & liver
• Remaining 10% broken down in blood
• Have a flexible shape, and abnormal shapes can cause disease, like sickle cells

Platelets or Thrombocytes

• Formed by fragmentation of precursor cells called megakaryocytes
• Each megakaryocyte produces 3000-4000 platelets
• Have no nucleus
• Aggregate and release coagulation factors to stop the loss of blood from wounds and induce vascular spasm
• Life span is 7-9 days

Leukocytes or WBCs

• Nucleated and motile cells that fight pathogens and other substances in the body
• Total leukocyte Count (TLC) in adults is 5000-10000/ µl
• Life span varies from a few hours to many years
• Classified into two main groups: granulocytes and agranulocytes

Granulocytes

• Have specific granules in their cytoplasm
• Include neutrophils, eosinophils, and basophils

Neutrophils

• Account for 60-70% of WBCs
• Life span is 2-3 days
• Have a multilobed nucleus (3-5 lobes) connected by a strand of nuclear material
• Granules in the cytoplasm are pale and evenly distributed
• Function is phagocytosis of bacteria, viruses, toxins, and foreign cells
• When they die while fighting infections, they form pus (yellowish discharge)

Eosinophils

• Account for less than 5% of the WBCs
• Life span is up to 10 days
• Usually have a 2 lobed nucleus connected with a thin thread
• Cytoplasm is filled with red-orange, eosinophilic granules
• Granules contain digestive enzymes that are particularly effective against parasitic worms
• Nucleus is not obscured by the granules
• Attack parasites
• Phagocytize antigen-antibody complexes, but do not attack bacteria directly

Basophils

• Represent less than 1% of all leukocytes
• Rarely seen in blood smears
• Have a bilobed (U shaped) nucleus
• Have deeply stained bluish or basophilic granules in cytoplasm
• Granules obscure the nucleus
• Granules contain histamine, which causes vasodilation, and heparin, an anticoagulant
• Involved in hypersensitivity reactions and intensify the inflammatory response

Agranulocytes

• Have no granules in their cytoplasm
• Include lymphocytes and monocytes

Lymphocytes

• Account for 20-25% of leukocytes, and are the second most frequent leukocytes
• Life span varies from days to months, but can live for years
• Rounded cells vary in size (large, medium, small)
• Compact round nucleus occupies almost the whole cytoplasm
• Both the nucleus and cytoplasm stain bluish
• Main cells in the immune system
• T-lymphocytes attack virus, tumor cells, and transplanted tissue cells
• B-lymphocytes mature into plasma cells and produce antibodies, and act as memory B cells
• NK cells kill virus-infected cells and some tumor cells

Monocytes

• Largest of the leukocytes
• Life span is a few months
• Account for 3-8% of all leukocytes
• The nucleus is most often kidney bean shaped or horseshoe shaped
• Cytoplasm is abundant and light blue
• Leave the blood stream to become macrophages in tissues
• As a monocyte or macrophage, these cells are phagocytic and defend the body against bacteria and foreign matter