18.1 An Overview of Blood

Blood is the vital fluid that keeps our bodies running. It's a complex mixture of cells, proteins, and nutrients that performs numerous essential functions. From delivering oxygen to fighting off infections, blood is the unsung hero of our circulatory system.

Understanding blood composition is key to grasping how our bodies maintain balance. We'll explore the different components of blood, their roles, and how they work together to keep us alive and healthy. Let's dive into the fascinating world of this life-sustaining fluid.

Blood Composition and Functions

Functions of blood

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• Transport
  • Carries oxygen from lungs to tissues via red blood cells (erythrocytes)
  • Moves carbon dioxide from tissues to lungs for exhalation
  • Delivers nutrients from digestive system to cells throughout the body
  • Transports waste products from cells to excretory organs for removal
  • Distributes hormones from endocrine glands to target tissues to regulate physiological processes
• Defense
  • White blood cells (leukocytes) protect against pathogens and foreign substances
    • Engulf and destroy bacteria and cellular debris through phagocytosis
    • Produce antibodies targeting specific antigens to neutralize threats
  • Platelets (thrombocytes) and clotting factors prevent blood loss through hemostasis by forming clots
• Homeostasis
  • Buffers pH to maintain blood pH between 7.35-7.45, ensuring optimal cellular function
  • Distributes heat and enables evaporative cooling to regulate body temperature
  • Maintains osmotic balance through plasma proteins and electrolytes, preventing cell shrinkage or swelling

Components and proportions of blood

• Plasma (55% of blood volume)
  • Liquid matrix of blood containing water, proteins, electrolytes, nutrients, and dissolved gases
• Formed elements (45% of blood volume)
  • Erythrocytes (red blood cells)
    • Comprise 99% of formed elements
    • Contain hemoglobin, enabling efficient oxygen transport throughout the body
  • Leukocytes (white blood cells)
    • Make up 1% of formed elements
    • Play crucial roles in immune defense against infections and foreign substances
    • Include neutrophils, lymphocytes, monocytes, eosinophils, and basophils, each with specific functions
  • Thrombocytes (platelets)
    • Cell fragments essential for blood clotting (coagulation) and hemostasis to prevent blood loss
• Hematocrit
  • The percentage of blood volume occupied by red blood cells, typically around 45% in healthy adults

Unique properties of blood

• Viscosity
  • Blood is more viscous than water due to suspended formed elements and dissolved plasma proteins
  • Viscosity influences blood flow and resistance within blood vessels, affecting circulation
• Specific gravity
  • Blood has a specific gravity of 1.060-1.065, higher than water (1.000) due to dissolved solutes and cells
• Osmolarity
  • Blood osmolarity ranges from 280-300 mOsm/L, maintained by plasma proteins, electrolytes, and hormonal regulation
• pH
  • Normal blood pH range is 7.35-7.45 (slightly alkaline), tightly regulated by buffer systems, respiratory, and renal functions

Key elements in blood plasma

• Water (91-92% of plasma)
  • Serves as the solvent for dissolved substances, enabling their transport and distribution
• Plasma proteins (7-8% of plasma)
  • Albumin
    • The most abundant plasma protein, maintains osmotic pressure and transports hormones and fatty acids
  • Globulins
    • Include alpha, beta, and gamma globulins, involved in immune function and transport of lipids and fat-soluble vitamins (A, D, E, K)
  • Fibrinogen
    • Converted to fibrin during blood clotting, forming the structural framework of clots
• Electrolytes
  • Include sodium ($Na^+$), potassium ($K^+$), calcium ($Ca^{2+}$), magnesium ($Mg^{2+}$), chloride ($Cl^-$), and bicarbonate ($HCO_3^-$)
  • Maintain osmotic balance and support various physiological functions (nerve conduction, muscle contraction, pH buffering)
• Nutrients, waste products, and dissolved gases
  • Glucose, amino acids, lipids (cholesterol, triglycerides), urea, creatinine, oxygen ($O_2$), and carbon dioxide ($CO_2$) are transported in plasma

Blood Formation and Regulation

• Bone marrow
  • The primary site of blood cell production (hematopoiesis) in adults
• Hematopoietic stem cells
  • Multipotent cells in bone marrow that give rise to all blood cell types
• Blood transfusion
  • The process of transferring blood or blood components from a donor to a recipient, used to treat various medical conditions
• Blood doping
  • The practice of artificially increasing red blood cell count to enhance athletic performance, which is considered unethical and banned in sports

Blood Analysis

• Hemocytometer
  • A specialized counting chamber used to determine the concentration of blood cells in a sample