Blood Cell Components

Why This Matters

Blood isn't just a red liquid—it's a complex tissue with specialized cells that keep you alive every second. In Honors Anatomy & Physiology, you're being tested on how structure determines function at the cellular level, how the body maintains homeostasis through coordinated systems, and how immune responses protect against threats. Understanding blood components means understanding gas exchange, hemostasis, innate vs. adaptive immunity, and transport mechanisms—all major exam themes.

Don't just memorize that red blood cells carry oxygen. Know why their biconcave shape matters, how different white blood cells divide labor in immune defense, and what makes plasma the perfect transport medium. Each component illustrates a core physiological principle, and that's exactly what FRQs will ask you to explain.

---

Oxygen Transport and Gas Exchange

The body's trillion-plus cells need constant oxygen delivery and carbon dioxide removal. This system relies on specialized structures optimized for maximum efficiency in gas binding and release.

Red Blood Cells (Erythrocytes)

• Biconcave disc shape—increases surface area-to-volume ratio for faster $O_2$ and $CO_2$ diffusion across the membrane
• Lack nucleus and organelles—maximizes internal space for ~280 million hemoglobin molecules per cell
• 120-day lifespan—old RBCs are recycled by the spleen and liver; iron is recovered for new hemoglobin synthesis

Hemoglobin

• Quaternary protein structure—four polypeptide chains (2 alpha, 2 beta), each with an iron-containing heme group that binds $O_2$
• Cooperative binding—when one heme binds oxygen, the others bind more easily, creating the oxygen-hemoglobin dissociation curve
• Buffering capacity—binds $H^+$ ions to help regulate blood pH, working alongside the bicarbonate buffer system

---

Hemostasis and Wound Repair

When blood vessels are damaged, the body must stop bleeding quickly without forming dangerous clots elsewhere. Hemostasis involves a precise cascade of cellular and chemical events.

Platelets (Thrombocytes)

• Cell fragments, not true cells—derived from megakaryocytes in bone marrow; lack a nucleus but contain granules with clotting factors
• Platelet plug formation—adhere to exposed collagen at wound sites, become activated, and aggregate to form a temporary seal
• Chemical signaling—release ADP, thromboxane, and serotonin to recruit more platelets and initiate the coagulation cascade

---

Innate Immune Response: First Responders

The innate immune system provides immediate, non-specific defense. These cells recognize general patterns on pathogens and respond within minutes to hours.

Neutrophils

• Most abundant WBC (50-70%)—short-lived but rapidly produced; you make ~100 billion daily
• Phagocytosis specialists—engulf and destroy bacteria using lysosomes filled with digestive enzymes and reactive oxygen species
• First to arrive—migrate from blood to infection sites via chemotaxis, following chemical signals from damaged tissue

Monocytes

• Largest WBC—circulate in blood for 1-3 days before entering tissues and differentiating
• Transform into macrophages—become long-lived tissue phagocytes that also clean up dead cells and debris
• Antigen-presenting cells (APCs)—display pathogen fragments on MHC II molecules to activate T cells, bridging innate and adaptive immunity

Eosinophils

• Parasite defense—release cytotoxic granules containing major basic protein that damages large parasites too big to phagocytose
• Allergic response role—accumulate in tissues during allergic reactions; contribute to inflammation in asthma
• Modulators—help regulate inflammation by breaking down histamine and other inflammatory mediators

Basophils

• Rarest WBC (<1%)—small numbers but powerful effects through chemical release
• Histamine and heparin release—promote vasodilation and increased capillary permeability, enhancing immune cell access to tissues
• Allergic reaction mediators—work alongside mast cells in type I hypersensitivity responses; IgE antibodies trigger degranulation

---

Adaptive Immune Response: Targeted Defense

Unlike innate immunity, adaptive immunity is specific and creates memory. Lymphocytes recognize unique antigens and mount tailored responses that improve with repeated exposure.

Lymphocytes

• B cells and T cells—B cells produce antibodies (humoral immunity); T cells directly destroy infected cells or coordinate responses (cell-mediated immunity)
• Antigen specificity—each lymphocyte recognizes only one specific antigen through unique surface receptors
• Immunological memory—after initial exposure, memory cells persist for years, enabling faster and stronger secondary responses

---

Transport Medium and Homeostasis

Blood cells need a liquid environment to travel through vessels and reach tissues. Plasma provides this medium while also transporting dissolved substances essential for homeostasis.

Plasma

• 55% of blood volume—the liquid matrix; remaining 45% is the hematocrit (mostly RBCs)
• 91-92% water—dissolves electrolytes ($Na^+$, $K^+$, $Ca^{2+}$), glucose, hormones, and waste products like urea
• Plasma proteins—albumin maintains osmotic pressure; globulins include antibodies; fibrinogen converts to fibrin during clotting

---

White Blood Cell Classification

Understanding how WBCs are categorized helps you predict their functions and recognize them on lab exams.

---

Quick Reference Table

---

Self-Check Questions

1. Which two blood components lack a nucleus, and what functional advantage does this provide for each?

2. Compare and contrast the roles of neutrophils and lymphocytes in immune defense. Which represents innate immunity and which represents adaptive immunity?

3. A patient has a parasitic infection. Which WBC type would you expect to see elevated on a blood test, and what mechanism does this cell use to combat parasites?

4. Explain how monocytes serve as a bridge between innate and adaptive immunity. What do they become when they leave the bloodstream?

5. If an FRQ asks you to trace the path of oxygen from the lungs to a muscle cell, which blood components would you discuss and what structural features make each effective?