Blood Overview and Plasma Composition

Blood Overview

• Blood is a specialized connective tissue, composed of blood cells (erythrocytes, leukocytes, and platelets) and a liquid matrix called plasma.
• It is a viscous, slightly alkaline liquid that circulates throughout the circulatory system.
• Blood transports substances, including nutrients from the digestive system to the rest of the body, waste products, hormones, respiratory gases (O2 and CO2), and immune cells, for disposal.
• It also helps regulate body temperature and osmotic balance of tissues.

Plasma

• Plasma is the liquid portion of blood, making up approximately 54% of blood volume.
• It is a yellowish liquid in which cells, platelets, organic compounds, and electrolytes are dissolved or suspended.
• Plasma is primarily composed of water (90%).
• Other components include proteins (9%), nutrients, and respiratory gases (1%).
• Plasma proteins maintain the osmotic pressure of blood and help maintain normal blood volume.
• Plasma leaves capillaries and enters tissues as tissue fluid (with similar composition to plasma).

Erythrocytes (Red Blood Cells)

• Erythrocytes are the most abundant formed elements in blood, with approximately 5 million per microliter.
• They lack a nucleus and organelles but contain numerous enzymes for ATP production.
• Their lifespan is about 120 days, after which they are destroyed in the liver, spleen, or bone marrow.
• Their biconcave disc shape provides a large surface area for gas exchange (transport of O2 and CO2).
• The shape is maintained by an actin cytoskeleton, requiring energy (ATP) for its function.

Transport of O2 and CO2

• Erythrocytes contain hemoglobin, a protein with iron-containing heme groups, which gives blood its red color and facilitates oxygen transport.
• In the lungs, where oxygen is abundant, hemoglobin binds to oxygen to form oxyhemoglobin.
• In tissues, where oxygen is low and carbon dioxide is high, hemoglobin releases oxygen and binds to carbon dioxide, forming carbaminohemoglobin.

Leukocytes (White Blood Cells)

• Leukocytes are larger than erythrocytes but less numerous.
• They have nuclei and organelles and are not typically functional within the blood, but use it as a transport system to reach tissues where they perform their immune functions.
• Leukocytes are classified into two main groups: granulocytes (polymorphonuclear cells) and agranulocytes.

Granulocytes

• Granulocytes contain specific granules in their cytoplasm for particular immune functions.
• Types of granulocytes:
  • Neutrophils (most common): actively phagocytic, engulf bacteria and other microorganisms, contain enzymes with antibacterial functions.
  • Eosinophils: have a bilobed nucleus, contain major basic protein to fight parasites, have receptors for IgG, IgE, and complement.
  • Basophils: S-shaped nucleus masked by granules, contain heparin, histamine, eosinophil chemotactic factor, proteases, and peroxidase.

Agranulocytes

• Agranulocytes do not contain specific granules in their cytoplasm.
• Types of agranulocytes
  • Monocytes: largest blood cell, kidney-shaped nucleus, remain in the blood for a short time and migrate to tissues where they differentiate into macrophages (for phagocytosis and immune function).
  • Lymphocytes: small leukocytes, round nucleus, scarce cytoplasm, with different types (T cells, B cells, and NK cells) for various immune functions:
    • T cells are formed in the bone marrow, migrate to the thymus to mature and acquire immunocompetence.
    • B cells and NK cells are formed in the bone marrow and migrate to tissues to perform their role.
• All cells types have lysosomes as non-specific granules.

Platelets

• Platelets are the smallest formed elements in blood, disc-shaped fragments of megakaryocytes.
• They lack nuclei but contain granules that are important for coagulation, and other functions.
• Granules contain a variety of substances including coagulation factors, serotonin, ATP, ADP, and lysosomes.

Hemostasis

• Platelets are essential for hemostasis (blood clotting), stopping bleeding.
• The process involves vascular injury, platelet adhesion, aggregation, formation of a platelet plug, activation of the coagulation cascade.
• This process leads to clot formation and eventual thrombus removal when the vessel is repaired.

Hematopoiesis

• Hematopoiesis is the continuous, physiological process of blood cell formation.
• All blood cells originate from pluripotent stem cells in the bone marrow (and in the liver and spleen in early development).
• These cells differentiate into various specialized blood cells throughout the body.

Immune System

• The immune system is responsible for defending the body against pathogens.
• It involves both innate (rapid non-specific response) and adaptive(slower specific with immunological memory response).
• Innate components: cells (macrophages, granulocytes, lymphocytes, mast cells), and molecules (complement).
• Adaptive components: mainly B and T lymphocytes.