3.1 Blood Components and Their Functions

Blood Components and Homeostasis

Blood is a specialized connective tissue that serves as the body's primary transport and defense system. Understanding its components and their functions is foundational to the rest of the cardiovascular unit, since blood composition directly affects circulation, immunity, and hemostasis.

Composition of Blood

Blood consists of formed elements suspended in a liquid matrix called plasma. The formed elements are:

• Erythrocytes (red blood cells)
• Leukocytes (white blood cells)
• Thrombocytes (platelets)

The hematocrit is the percentage of blood volume occupied by erythrocytes, typically 38–48% in adults. Values outside this range can signal conditions like anemia (low hematocrit) or polycythemia (high hematocrit).

Functions of Blood in Maintaining Homeostasis

Blood does far more than just move oxygen around. Its homeostatic roles include:

• Gas transport: Erythrocytes carry $O_2$ to tissues and $CO_2$ back to the lungs.
• Defense: Leukocytes identify and destroy pathogens and foreign substances.
• Hemostasis: Thrombocytes initiate clotting to stop bleeding.
• Regulation: Blood helps maintain body temperature, pH (buffering systems), and osmotic pressure.
• Distribution: Transports nutrients (glucose), hormones (insulin), and waste products (urea) throughout the body.

Blood viscosity depends on the concentration of formed elements and plasma proteins. Higher viscosity increases resistance to flow, which raises blood pressure. This is why conditions that change hematocrit or plasma protein levels have cardiovascular consequences.

Structure and Function of Blood Cells

Erythrocytes (Red Blood Cells)

Erythrocytes are biconcave, anucleate (no nucleus) cells packed with hemoglobin, an iron-containing protein that binds $O_2$ and $CO_2$ for gas transport. Their biconcave shape isn't random: it maximizes surface area relative to volume, which makes gas exchange across the cell membrane more efficient.

• Lifespan: approximately 120 days
• Old or damaged erythrocytes are removed from circulation by macrophages in the liver and spleen
• Because they lack a nucleus and organelles, erythrocytes cannot divide or repair themselves

Leukocytes (White Blood Cells)

Leukocytes are nucleated cells responsible for immune defense. They're classified into two broad groups based on whether their cytoplasm contains visible granules when stained:

• Granulocytes: neutrophils, eosinophils, basophils
• Agranulocytes: lymphocytes, monocytes

Leukocyte lifespans vary widely, from a few hours (neutrophils during active infection) to several days or longer, depending on cell type and activity level.

Thrombocytes (Platelets)

Thrombocytes are small, anucleate cell fragments derived from large precursor cells called megakaryocytes in the bone marrow. They contain granules loaded with clotting factors and other substances that initiate the clotting cascade and promote hemostasis.

• Lifespan: 7–10 days

Hematopoiesis

All formed elements originate from hematopoietic stem cells in the red bone marrow. Hematopoiesis is the process of blood cell formation, and it's tightly regulated by growth factors and cytokines. For example:

• Erythropoietin (EPO), released by the kidneys in response to low $O_2$ levels, stimulates erythrocyte production.
• Interleukins and other cytokines regulate leukocyte and platelet production.

This means the body can ramp up production of specific cell types when demand increases, such as producing more neutrophils during a bacterial infection.

Composition and Functions of Blood Plasma

Composition of Plasma

Plasma is the liquid portion of blood, making up about 55% of total blood volume. Its composition:

• Water: 90–92%
• Proteins: 7–8%
• Other dissolved substances: electrolytes (sodium, potassium), nutrients (amino acids), hormones (cortisol), and waste products (creatinine)

Plasma Proteins and Their Functions

There are three major categories of plasma proteins, each with distinct roles:

Albumin is the most abundant plasma protein. It maintains colloid osmotic pressure, which keeps fluid from leaking out of capillaries into tissues. Albumin also acts as a carrier molecule, transporting hormones, fatty acids, and bilirubin through the blood.

Globulins come in three subtypes:

• Alpha and beta globulins transport lipids (cholesterol) and fat-soluble vitamins (vitamin D).
• Gamma globulins (immunoglobulins) are antibodies produced by B lymphocytes. These are the proteins that target specific pathogens during an immune response.

Fibrinogen is a soluble protein that becomes critical during clotting. When activated, it converts to insoluble fibrin, forming a mesh-like network that stabilizes a blood clot. Without fibrinogen, clots can't hold together.

Electrolytes and Osmotic Balance

Plasma contains dissolved electrolytes essential for osmotic balance, pH regulation, and normal cell function:

• Sodium ($Na^+$) and chloride ($Cl^-$) are the main extracellular electrolytes.
• Potassium ($K^+$) is the main intracellular electrolyte.
• Calcium ($Ca^{2+}$) is involved in muscle contraction, nerve signaling, and blood clotting.

The balance between these ions inside and outside cells is what drives nerve impulses, muscle contractions, and fluid distribution between compartments.

Leukocyte Types and Immune Functions

Granulocytes

Neutrophils are the most abundant white blood cells overall and the most common granulocytes. They're phagocytic, meaning they engulf and destroy bacteria (such as Staphylococcus aureus) and fungi (such as Candida albicans). Neutrophils are the first responders to sites of infection and are the hallmark cells of acute inflammation.

Eosinophils specialize in combating parasitic infections, particularly helminthic (worm) infections. They also play a role in allergic reactions by releasing enzymes that neutralize histamine and other inflammatory mediators.

Basophils are the least abundant granulocytes. They release histamine and other chemicals that drive inflammatory reactions, especially during allergic responses (including anaphylaxis) and parasitic infections.

Agranulocytes

Lymphocytes are the primary cells of the adaptive immune system, meaning they mount targeted responses against specific threats:

• B cells produce antibodies that bind to specific antigens (bacterial toxins, viral proteins).
• T cells come in two major subtypes:
  • Helper T cells (CD4+) coordinate the immune response by secreting cytokines like interleukin-2 that activate other immune cells.
  • Cytotoxic T cells (CD8+) directly kill virus-infected or abnormal (cancerous) cells.

Monocytes are large phagocytic cells that circulate in the blood before migrating into tissues, where they differentiate into macrophages or dendritic cells. They engulf pathogens, cellular debris, and foreign substances. Critically, they also serve as antigen-presenting cells, displaying fragments of what they've consumed to T cells, which bridges the innate and adaptive immune responses.

Natural Killer (NK) Cells

NK cells are a type of lymphocyte, but they belong to the innate (non-specific) immune system rather than the adaptive system. They provide rapid defense against virus-infected and tumor cells by inducing apoptosis (programmed cell death) in target cells. Unlike cytotoxic T cells, NK cells don't need prior sensitization to a specific antigen, which is what makes their response so fast.