Blood is a specialized connective tissue. Its complex chemical composition allows it to perform numerous functions, including transporting substances, regulating temperature and pH levels, and protecting the body from foreign invaders and blood loss. Understanding the specific chemical components of blood provides insight into how this dynamic fluid supports life.
Plasma The Liquid Foundation
Plasma is the pale yellow liquid component of blood, constituting approximately 55% of the total volume. It is primarily water (about 92%), serving as the solvent for dissolved solids. These solids, making up the remaining 8%, include proteins, electrolytes, nutrients, hormones, and metabolic waste products.
Plasma proteins are the largest proportion of dissolved solids, led by albumin. Albumin maintains the blood’s osmotic pressure, regulating water movement between blood and tissues. Globulins function in transport, binding to lipids and metal ions, and include immunoglobulins (antibodies) for defense.
Fibrinogen is a plasma protein precursor to fibrin, which is involved in blood clot formation. Dissolved electrolytes, such as sodium and potassium ions, help maintain the blood’s pH balance and osmotic equilibrium. Plasma also carries nutrients like glucose and amino acids, delivering them to cells for energy. Metabolic byproducts, such as urea, are dissolved for transport to the kidneys for excretion. Hormones also travel through the plasma to regulate distant target organs.
Red Blood Cells Oxygen Transport
Erythrocytes (red blood cells) are the most numerous formed elements, making up about 45% of the total blood volume. Their primary function is the efficient transport of respiratory gases, enabled by their chemical makeup. Mature erythrocytes lack a nucleus and mitochondria, maximizing space for their main component: hemoglobin.
Hemoglobin constitutes roughly 97% of the cell’s dry weight. This metalloprotein has a complex quaternary structure composed of four polypeptide chains (two alpha and two beta). Each chain is associated with an iron-containing heme group, which is the specific site where an oxygen molecule reversibly binds.
Hemoglobin carries up to four oxygen molecules from the lungs to the tissues. Oxygen binding results in bright red oxyhemoglobin, while its release creates darker deoxyhemoglobin. Erythrocytes also transport carbon dioxide; about 20% binds to the globin portion of hemoglobin, while the rest is dissolved in plasma or converted to bicarbonate ions. The erythrocyte’s biconcave disc shape increases its surface area, facilitating rapid gas diffusion across the cell membrane. The specialized design of the red blood cell ensures the delivery of oxygen required for cellular respiration.
White Blood Cells Immune System Response
Leukocytes (white blood cells) are the least numerous formed elements, responsible for the body’s immune defense system. Unlike erythrocytes, leukocytes possess a nucleus and can move independently, allowing them to migrate into tissues to combat threats. Their protective functions rely on diverse chemical processes and signaling molecules.
Granulocytes
Leukocytes are broadly categorized into granulocytes and agranulocytes, each with specialized roles in innate and adaptive immunity.
- Neutrophils: The most common type, they are first responders that engulf foreign particles and bacteria through phagocytosis.
- Eosinophils: Combat parasitic infections and modulate allergic reactions by releasing toxic chemicals from their granules.
- Basophils: The rarest type, they mediate inflammation by releasing chemical mediators like histamine, which causes local vasodilation and increased vascular permeability.
Agranulocytes
Agranulocytes are central to adaptive immunity and long-term defense.
- Monocytes: Leave the circulation and transform into macrophages in the tissues. Macrophages act as powerful, long-term phagocytic cells, breaking down pathogens and cellular debris.
- Lymphocytes: The central components of adaptive immunity, providing specific, long-lasting protection. B lymphocytes mature into plasma cells that produce specific antibodies. T lymphocytes directly attack infected or cancerous cells and coordinate the immune response through the release of chemical signals known as cytokines.
Platelets and Coagulation
Platelets (thrombocytes) are small, anucleated cell fragments derived from megakaryocytes. Their primary function is hemostasis: stopping bleeding at a site of vascular injury.
When a blood vessel is injured, platelets adhere to exposed collagen fibers and become activated. Activated platelets release chemical factors, such as ADP and serotonin, which attract and aggregate more platelets, forming a temporary plug. This plug is stabilized by the coagulation cascade, a rapid sequence of enzyme activations. The cascade activates thrombin, which converts soluble fibrinogen into insoluble fibrin strands. These fibrin strands weave a mesh that traps platelets and red blood cells, creating a stable blood clot.