Key Takeaways
- The circulatory system consists of the heart, blood vessels (arteries, veins, capillaries), and blood
- Arteries carry oxygenated blood AWAY from the heart (except pulmonary arteries); veins carry deoxygenated blood TOWARD the heart (except pulmonary veins)
- Blood consists of plasma (55%) and formed elements (45%) — red blood cells, white blood cells, and platelets
- The three layers of a blood vessel are: tunica intima (inner), tunica media (middle), and tunica adventitia (outer)
- Red blood cells (erythrocytes) carry oxygen via hemoglobin and have a lifespan of approximately 120 days
- White blood cells (leukocytes) fight infection — types include neutrophils, lymphocytes, monocytes, eosinophils, and basophils
- Platelets (thrombocytes) are essential for hemostasis (blood clotting) and are fragments of megakaryocytes
- The coagulation cascade involves intrinsic and extrinsic pathways that converge at the common pathway to form a fibrin clot
Anatomy & Physiology for Phlebotomists
A solid understanding of the circulatory system, blood composition, and hemostasis is essential for phlebotomy practice. This knowledge helps you select appropriate veins, understand why certain tests require specific tubes, and recognize complications.
The Circulatory System
Blood Vessels
| Vessel Type | Direction | Characteristics | Phlebotomy Relevance |
|---|---|---|---|
| Arteries | Away from heart | Thick-walled, muscular, pulsating, bright red blood | ABG collection only — NOT routine phlebotomy |
| Veins | Toward heart | Thinner-walled, contain valves, dark red blood, no pulse | Primary site for venipuncture |
| Capillaries | Connecting arteries to veins | Microscopic, one-cell-thick walls, gas exchange site | Site for capillary/dermal puncture |
Vessel Wall Layers (Tunica)
| Layer | Position | Function |
|---|---|---|
| Tunica intima | Inner layer | Smooth endothelial lining; contact with blood; initiates clotting cascade when damaged |
| Tunica media | Middle layer | Smooth muscle and elastic tissue; controls vessel diameter |
| Tunica adventitia | Outer layer | Connective tissue; provides structural support |
Clinical Relevance: When a phlebotomist punctures a vein, the needle passes through all three layers. Damage to the tunica intima activates the clotting cascade, which is why hemostasis (stopping bleeding) begins at the puncture site.
Blood Composition
Blood makes up approximately 7-8% of total body weight (about 5 liters in an average adult).
Plasma (55% of blood volume)
- Water (92%) — carrier for cells, nutrients, and waste
- Proteins (7%) — albumin (maintains osmotic pressure), globulins (antibodies), fibrinogen (clotting)
- Other solutes (1%) — electrolytes, nutrients, hormones, waste products, gases
Formed Elements (45% of blood volume)
| Component | Count | Function | Lifespan |
|---|---|---|---|
| Red Blood Cells (RBCs / Erythrocytes) | 4.5-5.5 million/mcL | Oxygen transport via hemoglobin | ~120 days |
| White Blood Cells (WBCs / Leukocytes) | 4,500-11,000/mcL | Immune defense — fight infection | Hours to years (varies by type) |
| Platelets (Thrombocytes) | 150,000-400,000/mcL | Hemostasis — blood clotting | ~8-10 days |
Hematocrit
The hematocrit (Hct) is the percentage of whole blood volume occupied by red blood cells:
- Normal male: 42-52%
- Normal female: 37-47%
- Measured by centrifuging blood in a capillary tube (microhematocrit)
- After centrifugation: RBCs on bottom, buffy coat (WBCs + platelets) in middle, plasma on top
White Blood Cell Differential
The WBC differential identifies the percentage of each type of white blood cell:
| WBC Type | Normal % | Primary Function |
|---|---|---|
| Neutrophils | 50-70% | First responders to bacterial infections; most abundant WBC |
| Lymphocytes | 20-40% | Immune response — T cells (cellular immunity) and B cells (antibodies) |
| Monocytes | 2-8% | Phagocytosis; become macrophages in tissues |
| Eosinophils | 1-4% | Allergic responses and parasitic infections |
| Basophils | 0.5-1% | Release histamine and heparin; allergic reactions |
Memory Aid: "Never Let Monkeys Eat Bananas" — Neutrophils, Lymphocytes, Monocytes, Eosinophils, Basophils (in order of normal percentage).
Hemostasis — The Clotting Process
Hemostasis is the body's process of stopping bleeding. It occurs in three stages:
Stage 1: Vascular Spasm
- Damaged blood vessel constricts (narrows) to reduce blood flow
- Immediate but temporary response
Stage 2: Platelet Plug Formation
- Platelets adhere to the damaged vessel wall (adhesion)
- Platelets aggregate (clump together) to form a temporary plug
- Platelets release chemicals that attract more platelets (positive feedback)
Stage 3: Coagulation Cascade
- A complex series of enzymatic reactions involving clotting factors (numbered I-XIII)
- Extrinsic pathway: Initiated by tissue factor (damaged tissue) — measured by PT/INR
- Intrinsic pathway: Initiated by contact with collagen (exposed vessel wall) — measured by PTT/aPTT
- Both pathways converge at the common pathway: Factor X -> Prothrombin -> Thrombin -> Fibrinogen -> Fibrin
- Fibrin threads form a mesh that stabilizes the platelet plug into a permanent clot
Anticoagulants and the Coagulation Cascade
| Anticoagulant | Mechanism | Found In |
|---|---|---|
| EDTA | Chelates calcium (Factor IV) | Lavender/pink tubes |
| Sodium citrate | Chelates calcium (Factor IV) | Light blue tubes |
| Heparin | Inhibits thrombin (Factor IIa) | Green tubes |
| Sodium fluoride/oxalate | Oxalate chelates calcium; fluoride inhibits glycolysis | Gray tubes |
Key Concept: Calcium (Factor IV) is essential for the coagulation cascade. Anticoagulants like EDTA, citrate, and oxalate prevent clotting by binding (chelating) calcium, removing it from the reaction. Heparin works differently — it directly inhibits thrombin.
What percentage of whole blood is composed of plasma?
Which type of white blood cell is the most abundant and first to respond to bacterial infections?
The extrinsic pathway of the coagulation cascade is monitored by which laboratory test?
Red blood cells have an average lifespan of approximately ___ days.
Type your answer below
Match each blood component to its primary function:
Match each item on the left with the correct item on the right
The normal platelet count range is:
Which anticoagulant works by inhibiting thrombin rather than chelating calcium?
Which of the following are functions of plasma? (Select all that apply)
Select all that apply
After centrifuging an anticoagulated lavender tube, the "buffy coat" layer between the plasma and RBCs contains:
The three layers of a blood vessel wall, from innermost to outermost, are:
Which protein in red blood cells is responsible for carrying oxygen?
The stage of hemostasis where the coagulation cascade produces a fibrin mesh to stabilize the platelet plug is: