Blood is the lifeblood of our bodies, carrying oxygen, nutrients, and more. This section dives into its components: red blood cells, white blood cells, and platelets, each with unique roles in keeping us healthy.

We'll explore how these blood components work together to maintain balance in our bodies. From fighting infections to clotting wounds, blood's diverse functions are crucial for our survival and well-being.

Blood Components and Homeostasis

Composition of Blood

• Blood is a connective tissue composed of formed elements suspended in a liquid matrix called plasma
  • Formed elements include erythrocytes (red blood cells), leukocytes (white blood cells), and thrombocytes (platelets)
• The hematocrit is the percentage of blood volume occupied by erythrocytes, typically ranging from 38-48% in adults
  • Deviations from this range may indicate various health conditions

Functions of Blood in Maintaining Homeostasis

• Erythrocytes transport oxygen and carbon dioxide
• Leukocytes defend against pathogens and foreign substances
• Thrombocytes play a crucial role in blood clotting and hemostasis
• Blood helps maintain homeostasis by regulating body temperature, pH, and osmotic pressure
  • Transports nutrients (glucose), hormones (insulin), and waste products (urea) throughout the body
• Blood viscosity, determined by the concentration of formed elements and plasma proteins, affects blood flow and pressure within the cardiovascular system

Structure and Function of Blood Cells

Erythrocytes (Red Blood Cells)

• Biconcave, anucleate cells containing hemoglobin, an iron-containing protein that binds to oxygen and carbon dioxide for transport
  • Unique shape provides a large surface area for efficient gas exchange
• Lifespan of erythrocytes is approximately 120 days
  • Removed from circulation by the liver and spleen after this period

Leukocytes (White Blood Cells)

• Nucleated cells that defend the body against infections and foreign substances
• Classified as granulocytes (neutrophils, eosinophils, and basophils) or agranulocytes (lymphocytes and monocytes)
• Leukocytes have shorter lifespans, ranging from a few hours to several days, depending on the specific cell type and function

Thrombocytes (Platelets)

• Small, anucleate cell fragments derived from megakaryocytes in the bone marrow
• Contain granules that release clotting factors and other substances to initiate blood clotting and promote hemostasis
• Thrombocytes have a lifespan of 7-10 days

Hematopoiesis

• Erythrocytes, leukocytes, and thrombocytes originate from hematopoietic stem cells in the bone marrow
• Hematopoiesis is the process of blood cell formation, regulated by various growth factors (erythropoietin) and cytokines (interleukins)

Composition and Functions of Blood Plasma

Composition of Plasma

• Blood plasma is the liquid component of blood, consisting primarily of water (90-92%), proteins (7-8%), and other dissolved substances
  • Dissolved substances include electrolytes (sodium, potassium), nutrients (amino acids), hormones (cortisol), and waste products (creatinine)

Plasma Proteins and Their Functions

• Albumin, the most abundant plasma protein
  • Helps maintain osmotic pressure and serves as a carrier for various molecules (hormones, fatty acids, bilirubin)
• Globulins, including alpha, beta, and gamma globulins
  • Involved in immune function and transport of lipids (cholesterol) and fat-soluble vitamins (vitamin D)
  • Gamma globulins, or immunoglobulins, are antibodies produced by B lymphocytes
• Fibrinogen, a soluble plasma protein
  • Converted to insoluble fibrin during blood clotting, forming a mesh-like network to stabilize the clot

Electrolytes and Osmotic Balance

• Plasma contains electrolytes, such as sodium, potassium, calcium, and chloride ions
• Electrolytes are essential for maintaining osmotic balance, pH, and proper functioning of cells and tissues
  • Sodium and chloride are the main extracellular electrolytes, while potassium is the main intracellular electrolyte

Leukocyte Types and Immune Functions

Granulocytes

• Neutrophils, the most abundant granulocytes
  • Phagocytic cells that engulf and destroy bacteria (Staphylococcus aureus) and fungi (Candida albicans)
  • First responders to infection and crucial in acute inflammation
• Eosinophils target parasitic infections (helminthic worms) and play a role in allergic reactions
  • Release enzymes that neutralize histamine and other inflammatory mediators
• Basophils, the least abundant granulocytes
  • Release histamine and other substances involved in inflammatory reactions, particularly in allergic responses (anaphylaxis) and parasitic infections

Agranulocytes

• Lymphocytes, including B cells and T cells, are the primary cells of the adaptive immune system
  • B cells produce antibodies that target specific antigens (bacterial toxins, viral proteins)
  • T cells directly attack infected or abnormal cells and regulate the immune response
    • Helper T cells (CD4+) coordinate the immune response by secreting cytokines (interleukin-2) that activate other immune cells
    • Cytotoxic T cells (CD8+) directly kill infected (virus-infected) or abnormal (cancerous) cells
• Monocytes are large, phagocytic cells that migrate into tissues and differentiate into macrophages and dendritic cells
  • Engulf pathogens (bacteria), cellular debris, and foreign substances (silica particles)
  • Present antigens to T cells to initiate the adaptive immune response

Natural Killer (NK) Cells

• Type of lymphocyte that provides rapid, non-specific defense against virus-infected and tumor cells
• Induce apoptosis (programmed cell death) in target cells without prior sensitization