40.2 Components of the Blood

Components of Blood

Blood is a connective tissue made up of cells and cell fragments suspended in a liquid matrix called plasma. Its components handle oxygen transport, immune defense, clotting, and the delivery of nutrients and signaling molecules throughout the body. This section covers the four main components: red blood cells, white blood cells, platelets, and plasma.

Components and Functions of Blood

Red blood cells (erythrocytes) are the most abundant cells in blood. They contain hemoglobin, an iron-containing protein that binds $O_2$ in the lungs and releases it in tissues. Hemoglobin also carries some $CO_2$ back to the lungs for exhalation (though most $CO_2$ travels dissolved in plasma as bicarbonate, $HCO_3^-$). Their biconcave disc shape increases surface area relative to volume, making gas exchange more efficient.

White blood cells (leukocytes) defend the body against pathogens and foreign substances. There are five main types, each with a distinct role:

• Neutrophils — the most common WBC; phagocytize (engulf and destroy) bacteria
• Eosinophils — combat parasitic infections and modulate allergic responses
• Basophils — release histamine during allergic and inflammatory reactions
• Monocytes — phagocytize pathogens and present antigens to other immune cells; they mature into macrophages in tissues
• Lymphocytes — include B cells (produce antibodies for humoral immunity) and T cells (carry out cell-mediated immunity, including cytotoxic killing of infected cells)

Platelets (thrombocytes) are small, disc-shaped cell fragments produced from large cells called megakaryocytes in the bone marrow. They don't have a nucleus. When a blood vessel is damaged, platelets adhere to the site, release clotting factors from their granules, and aggregate to form a platelet plug. This initiates the coagulation cascade that stops bleeding.

Plasma is the liquid portion of blood, making up roughly 55% of total blood volume. It's mostly water (~90–92%) with dissolved proteins, ions, nutrients, hormones, and waste products. Plasma serves as the transport medium for everything blood carries.

Structure and Roles of Blood Cells

Red blood cells are highly specialized for gas transport. They lack a nucleus and most organelles, which frees up internal space for hemoglobin. Each RBC contains about 200–300 million hemoglobin molecules. Their flexible membrane lets them squeeze through capillaries narrower than their own diameter, ensuring oxygen reaches every tissue.

White blood cells are nucleated, larger than RBCs, and far less numerous (roughly 1 WBC for every 700 RBCs). Their shapes vary by type: neutrophils have multi-lobed nuclei, while lymphocytes are smaller and more spherical. Functionally, they split into two broad categories:

• Phagocytic cells (neutrophils, monocytes/macrophages) — engulf and digest pathogens directly
• Lymphocytes — mediate specific immune responses. B cells produce antibodies targeting particular antigens, while cytotoxic T cells destroy infected or abnormal host cells

Platelets contain granules loaded with clotting factors (like fibrinogen and thrombin), platelet-activating factors, and vasoconstrictors that narrow damaged vessels to reduce blood flow. The clotting process, called hemostasis, follows a general sequence:

1. A blood vessel is damaged, exposing collagen in the vessel wall.
2. Platelets adhere to the exposed collagen.
3. Adhered platelets activate and release the contents of their granules.
4. Released signals recruit more platelets, which aggregate into a platelet plug.
5. The coagulation cascade converts fibrinogen into fibrin threads that reinforce the plug into a stable clot.

Composition and Importance of Plasma

Plasma composition breaks down as follows:

• Water (90–92%) — the solvent that carries all dissolved substances
• Proteins (7–8%) — three major groups:
  • Albumin — the most abundant plasma protein; maintains oncotic pressure (the osmotic pressure that keeps fluid from leaking out of blood vessels into tissues)
  • Globulins (α, β, γ) — transport lipids and metals, and γ-globulins (immunoglobulins) function as antibodies
  • Fibrinogen — converted to fibrin during clotting
• Ions — $Na^+$ (osmotic balance), $K^+$ (membrane potential), $Ca^{2+}$ (clotting and nerve signaling), $Cl^-$ and $HCO_3^-$ (pH buffering)
• Nutrients — glucose, amino acids, and lipids transported to tissues
• Regulatory molecules — hormones like insulin, glucagon, and ADH; enzymes used as clinical markers

Plasma functions go beyond simple transport:

• Acts as the delivery system for nutrients, wastes, and hormones between organs
• Maintains osmotic pressure and pH balance of extracellular fluid
• Carries clotting factors (fibrinogen, prothrombin) essential for coagulation
• Distributes heat throughout the body as part of thermoregulation

Blood Production and Circulation

All blood cells originate in the bone marrow through a process called hematopoiesis. A single type of stem cell, the hematopoietic stem cell, gives rise to every blood cell lineage: red blood cells, all five types of white blood cells, and the megakaryocytes that produce platelets.

Blood circulates through three types of vessels: arteries (carry blood away from the heart), veins (return blood to the heart), and capillaries (thin-walled exchange vessels where gases, nutrients, and wastes move between blood and tissues).

Blood types (A, B, AB, O) are determined by specific antigens on the surface of red blood cells. Type A has A antigens, type B has B antigens, type AB has both, and type O has neither. This matters for transfusions because mismatched blood triggers an immune reaction against the foreign antigens.