Domain 7.2Phlebotomy

Blood Physiology And Equipment

Last updated June 16, 2026

Overview

Understanding the components of blood is the foundation for selecting the correct equipment. As a medical assistant, you must know what is inside the blood to understand why we use specific tubes and additives. This guide integrates blood physiology with the practical selection of needles, tubes, and systems.

1. The Composition of Whole Blood

When a blood sample is collected, it is referred to as whole blood. Whole blood consists of two main parts: a liquid matrix and the cells suspended within it. After centrifugation (spinning at high speed), these components separate into distinct layers.

Plasma vs. Serum: A High-Yield Distinction

This is one of the most frequently tested concepts in phlebotomy. This biological distinction drives your choice of collection tube.

Feature Plasma Serum
Source The liquid portion of anticoagulated blood (blood that was prevented from clotting). The liquid portion of clotted blood.
Clotting Factors Contains clotting factors, such as fibrinogen. Does not contain clotting factors; they were used up to form the clot.
How to Obtain Collect blood in a tube with an anticoagulant (e.g., Lavender, Light Blue, Green top tubes) and centrifuge it. Collect blood in a tube with no anticoagulant or a clot activator (e.g., Red or Gold/SST top tubes), allow it to clot for 30-60 mins, then centrifuge.

🧠 Memory Aid:

  • Plasma has Proteins for clotting because an anticoagulant Prevents the clot from forming.
  • Serum is what's left Separated after the clot forms.

The Formed Elements (Cellular Components)

The formed elements make up about 45% of blood volume and include cells and cell fragments.

  1. Erythrocytes (Red Blood Cells - RBCs)
    • Function: Transport oxygen via the protein hemoglobin.
    • Characteristics: Most numerous blood cells; give blood its red color.
  2. Leukocytes (White Blood Cells - WBCs)
    • Function: The "soldiers" of the immune system; destroy pathogens.
  3. Thrombocytes (Platelets)
    • Function: Essential for coagulation (blood clotting).
    • Characteristics: Cell fragments that form a plug to stop bleeding.

🧠 Memory Aid: A thrombus is a blood clot. A thrombocyte is the cell fragment that helps form it.

2. Blood Collection Tubes: The Color Code System

The color of the tube's stopper tells you what additive is inside. This additive determines if you will get Plasma or Serum (based on the definitions above).

Key Concepts

  • Anticoagulant: A substance that prevents blood from clotting. Yields Plasma.
  • Clot Activator: A substance that promotes clotting. Yields Serum.

High-Yield Blood Tube Chart

Stopper Color Additive Specimen Yielded Common Tests Memory Aid / Key Takeaway
Yellow Yellow SPS or ACD Whole Blood Blood cultures, DNA Yellow for Cultures: SPS is for microbiology (cultures).
Light Blue Light Blue Sodium Citrate Plasma Coagulation (PT, PTT) Light Blue for Bleeding Time: Tests how well blood clots.
Red Red None or Clot Activator Serum Chemistry, Serology Red = Ready to Clot. The classic tube for serum.
Gold/Tiger Gold/Tiger Clot Activator & SST Gel Serum Chemistry (CMP, BMP) SST gel forms a barrier between serum and cells.
Green Green Sodium or Lithium Heparin Plasma "STAT" Chemistry Go Green for STAT! No clotting time needed.
Lavender Lavender EDTA Whole Blood/Plasma Hematology (CBC) L for Lavender, L for Lots of Cells. Best for cell morphology.
Gray Gray Sodium Fluoride & Potassium Oxalate Plasma Glucose, Alcohol Gray for Glucose. Stops cells from eating glucose.

3. Blood Collection Systems & Needles

There are three primary systems for venipuncture. Your choice depends on the patient's vein condition.

A. Evacuated Tube System (ETS)

  • Components: Multi-Sample Needle (with rubber sheath), Tube Holder, Evacuated Tube.
  • Key Feature: The rubber sheath prevents leakage when switching tubes, allowing for multiple tube collections.
  • Best For: Healthy adults with good veins.

B. Needle and Syringe System

  • How it Works: Manual control of vacuum pressure via the plunger.
  • Best For: Small, fragile, or collapsing veins (geriatric/oncology). It prevents the vein from collapsing under high vacuum.

C. Winged Infusion Set ("Butterfly Needle")

  • Best For: Small, superficial veins (hand/foot), pediatric, and geriatric patients.
  • Handling Technique: Always hold the needle by its wings (like a butterfly) during insertion. This provides the most precise control over the angle and depth.
  • Angle: Allows for a very shallow (10-15°) angle.

Needle Gauges

Rule: The larger the number, the smaller the needle.

  • 21-23 Gauge: Standard for routine venipuncture.
  • 24-25 Gauge: Small (Pediatric/Hand).
  • 14-18 Gauge: Blood donation (too large for labs).

4. Blood Types & Pathogens

The ABO and Rh System

  • Universal Red Cell Donor: O- (Has no A, B, or Rh antigens to trigger a reaction).
  • Universal Recipient: AB+ (Has no antibodies to attack donor blood).
  • Most Common Blood Type: O+ (approx. 1 in 3 people).

Blood-Borne Pathogens (BBPs)

  • Hepatitis B Virus (HBV): The primary blood-borne hazard for healthcare workers. The HBV vaccine is OSHA-mandated for employees.
  • Hepatitis C (HCV): No vaccine available.
  • HIV: Causes AIDS; attacks the immune system.

Standard Precautions: Treat all blood and body fluids as potentially infectious. Wear gloves and use safety needles.

5. Final Key Takeaways

  • Plasma vs. Serum: Plasma contains clotting factors (fibrinogen); Serum lacks them. Plasma requires anticoagulant; Serum requires clotting.
  • Match the System: ETS for healthy veins; Syringe for collapsing veins; Butterfly for small/hand veins (hold by the wings).
  • Needle Gauge: Standard is 21-23 gauge.
  • Tube Match: Lavender = CBC (EDTA); Light Blue = PT/PTT (Citrate); Gray = Glucose; Green = STAT.
  • Safety: Hepatitis B is the main occupational risk; get vaccinated.