n this lesson, we transition from the history and basics of phlebotomy to the biological foundations of the craft. Understanding how blood circulates and where it lives is essential for safe and effective collection.
1. The Circulatory System Overview
The primary focus of phlebotomy is the circulatory system, specifically the systemic circuit. It consists of the heart, blood vessels, and blood.
-
Arteries: Carry oxygenated blood away from the heart. They have thick, muscular walls and a palpable pulse. (Phlebotomists generally avoid these).
-
Veins: Carry deoxygenated blood toward the heart. They have thinner walls and contain valves to prevent backflow.
-
Capillaries: Microscopic vessels where the exchange of gas and nutrients occurs. This is the site for “dermal punctures” (finger sticks).
2. Primary Vein Selection (The Antecubital Fossa)
The most common site for venipuncture is the antecubital fossa, the area of the arm anterior to the elbow. There are three main veins used, prioritized in this order:
-
Median Cubital Vein: The “first choice.” It is typically large, well-anchored, and the least painful to puncture.
-
Cephalic Vein: The “second choice.” Located on the outer (thumb) side of the arm. It is often harder to palpate but is a good alternative for obese patients.
-
Basilic Vein: The “third choice.” Located on the inner (pinky) side of the arm. It is close to the median cutaneous nerve and the brachial artery; caution is required to avoid injury.
3. Composition of Blood
To understand why we use different colored tubes, you must know what is inside the blood. If a sample is centrifuged (spun), it separates into three layers:
| Layer | Percentage | Description |
| Plasma | ~55% | The liquid portion containing water, proteins, and clotting factors (if an anticoagulant is used). |
| Buffy Coat | <1% | A thin middle layer containing White Blood Cells (Leukocytes) and Platelets (Thrombocytes). |
| Erythrocytes | ~45% | Red Blood Cells that carry oxygen. |
4. Key Terminology
-
Hemostasis: The process by which the body stops bleeding after an injury (crucial for post-puncture care).
-
Serum: The liquid portion of blood that remains after clotting has occurred (does not contain fibrinogen).
-
Plasma: The liquid portion of blood kept from clotting by an anticoagulant.
5. Essential Safety: Palpation
In this stage of training, you begin learning to palpate (feel) rather than look. A good vein should feel “bouncy” or “spongy.”
Note: Never select a site based on sight alone. A visible vein might be superficial or “rolling,” while a deep, palpable vein is often the most stable.