Components of Blood

Why This Matters

Blood isn't just a red fluid—it's a complex tissue that performs virtually every homeostatic function your body needs to survive. When you study blood components, you're really studying transport mechanisms, immune defense, gas exchange, and hemostasis all at once. These concepts connect directly to cardiovascular physiology, respiratory function, and immune responses, making blood one of the most interconnected topics you'll encounter.

Don't fall into the trap of memorizing isolated facts about plasma percentages or cell lifespans. You're being tested on how these components work together, why their structure matches their function, and what happens when something goes wrong. For every component, ask yourself: what job does this do, and how does its design make that job possible?

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The Liquid Matrix: Plasma and Serum

Blood's liquid portion serves as the universal transport medium—every substance moving through your bloodstream travels suspended or dissolved in this fluid. Understanding the difference between plasma and serum is a classic exam distinction.

Plasma

• Comprises 55% of blood volume and is 90% water—this high water content makes it an ideal solvent for transporting dissolved gases, nutrients, and wastes
• Contains three major protein groups: albumin (maintains osmotic pressure), globulins (immune function), and fibrinogen (clotting)
• Serves as the transport highway for hormones, electrolytes, and metabolic wastes—essentially everything cells need delivered or removed

Serum

• Plasma minus clotting factors—the liquid remaining after blood coagulates, making it ideal for diagnostic blood tests
• Contains antibodies and electrolytes but lacks fibrinogen, which was consumed during clot formation
• Used clinically for immunological testing because it preserves immune proteins without interference from clotting components

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Oxygen Carriers: Red Blood Cells and Hemoglobin

The entire purpose of your cardiovascular system centers on one critical task: delivering oxygen to tissues and removing carbon dioxide. Red blood cells are exquisitely designed for this single function.

Red Blood Cells (Erythrocytes)

• Lack a nucleus and organelles—this unusual feature maximizes space for hemoglobin and creates the flexible, biconcave shape needed to squeeze through capillaries
• Live approximately 120 days before being recycled by the spleen and liver, with iron being recovered and reused for new hemoglobin synthesis
• Most abundant blood cells at 4-6 million per microliter—this massive number reflects how critical oxygen delivery is to survival

Hemoglobin

• Each molecule carries four oxygen molecules thanks to four heme groups, each containing an iron atom that reversibly binds $O_2$
• Exhibits cooperative binding—once one oxygen binds, the remaining binding sites become more receptive, creating efficient loading in the lungs
• Also transports $CO_2$ (about 23%) and acts as a blood pH buffer by binding hydrogen ions—it's a multitasking protein

Hematocrit

• Percentage of blood volume occupied by RBCs—normal ranges are approximately 42-52% for males and 37-47% for females
• Elevated hematocrit can indicate dehydration, polycythemia, or adaptation to high altitude; low hematocrit typically signals anemia
• Directly affects blood viscosity—higher hematocrit means thicker blood, which increases cardiac workload

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Immune Defense: White Blood Cells

White blood cells are your body's mobile defense force—they can leave the bloodstream entirely to hunt pathogens in tissues. This ability, called diapedesis, distinguishes them from other blood cells.

White Blood Cells (Leukocytes)

• Five major types with distinct functions: neutrophils (bacterial infection), lymphocytes (adaptive immunity), monocytes (phagocytosis), eosinophils (parasites/allergies), and basophils (inflammation)
• Can migrate through capillary walls using diapedesis to reach infection sites—they're not confined to blood vessels like RBCs
• Elevated counts (leukocytosis) typically indicate infection or inflammation; depressed counts (leukopenia) suggest immune compromise or bone marrow disorders

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Hemostasis: Platelets and Clotting Proteins

When a blood vessel is damaged, your body must seal the breach quickly without clotting your entire circulation. This balance between clotting and anti-clotting is hemostasis, and it requires precise coordination between platelets and plasma proteins.

Platelets (Thrombocytes)

• Cell fragments from megakaryocytes—not true cells, but cytoplasmic pieces that retain the ability to aggregate and release chemicals
• Form the initial platelet plug by adhering to exposed collagen at injury sites and to each other, creating a temporary seal within seconds
• Release clotting factors and vasoconstrictors that amplify the response and recruit additional platelets—a positive feedback cascade

Fibrinogen

• Soluble plasma protein converted to fibrin by thrombin during the coagulation cascade—this conversion is the key step in forming a stable clot
• Fibrin threads create a mesh that reinforces the platelet plug and traps RBCs, transforming a temporary seal into a durable clot
• Measured clinically to assess bleeding disorders—low fibrinogen indicates clotting problems; elevated levels suggest inflammation or cardiovascular risk

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Blood Typing: Surface Antigens and Compatibility

Blood type isn't just trivia—it's a life-or-death consideration for transfusions and pregnancy. The immune system treats mismatched blood as a foreign invader, triggering potentially fatal reactions.

Blood Types (ABO System)

• Based on surface antigens: Type A has A antigens, Type B has B antigens, Type AB has both, and Type O has neither—your plasma contains antibodies against antigens you lack
• Type O is the universal donor (no antigens to trigger reactions); Type AB is the universal recipient (no antibodies to attack donor cells)
• Transfusion reactions occur when recipient antibodies attack donor RBCs, causing agglutination and hemolysis—this is why cross-matching is critical

Rh Factor

• Presence or absence of D antigen on RBC surfaces determines Rh-positive or Rh-negative status—about 85% of the population is Rh-positive
• Critical in pregnancy: an Rh-negative mother carrying an Rh-positive fetus can develop anti-Rh antibodies that attack subsequent Rh-positive pregnancies (hemolytic disease of the newborn)
• Prevented with RhoGAM injections that block maternal antibody formation—a clinical application of understanding blood antigen immunology

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Quick Reference Table

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Self-Check Questions

1. Which two blood components work together to form a stable clot, and what specific role does each play in hemostasis?

2. Compare and contrast plasma and serum—what is present in one but not the other, and why does this difference matter clinically?

3. Why do red blood cells lack a nucleus, and how does this structural feature relate to their primary function?

4. If a patient has Type A blood, which blood types can they safely receive, and what would happen if they received Type B blood?

5. An Rh-negative mother is pregnant with her second Rh-positive child. Explain why this pregnancy carries more risk than her first, and identify the underlying immunological mechanism.