17.3 Physiology for Phlebotomy

Physiology That Explains Every Tube You Pick

Domain VIII-C connects living systems to collection decisions. If you understand what blood is made of, how clotting starts and stops bleeding, and how blood circulates, order-of-draw rules and additive choices stop feeling like arbitrary color codes. They become consequences of physiology.

Blood Composition: Whole Blood, Plasma, and Serum

Whole blood is the specimen as it leaves the vessel: roughly 55% liquid (plasma) and 45% formed elements (erythrocytes, leukocytes, thrombocytes/platelets) — percentages vary by patient. Formed elements are suspended in plasma, which is mostly water with proteins (including fibrinogen and other clotting factors), electrolytes, hormones, nutrients, and waste products.

Plasma is obtained when blood is drawn into an anticoagulant tube and centrifuged. Because clotting was prevented, fibrinogen and clotting factors remain in the liquid. Plasma is required for coagulation testing (citrate) and used for many chemistry and immunoassay methods (heparin plasma), as well as EDTA plasma in selected tests.

Serum is the liquid remaining after blood clots and the clot retracts/is centrifuged away. During clotting, fibrinogen converts to fibrin and is consumed in the clot, so serum lacks fibrinogen. Many traditional chemistry assays are validated on serum (red/gold SST tubes). Clot activators and gel separators speed processing but still depend on an intact clotting pathway — anticoagulant contamination can leave fibrin strands that ruin the specimen.

Exam trap: saying "plasma and serum are the same liquid" is wrong. Both are liquid fractions, but clotting history differs — and that difference decides whether coagulation tests or certain chemistry methods are valid.

Formed Elements at a Glance

• Erythrocytes (RBCs) — Carry oxygen via hemoglobin; most numerous cell type; hemolysis releases intracellular potassium and enzymes
• Leukocytes (WBCs) — Immune defense; five types (neutrophils, lymphocytes, monocytes, eosinophils, basophils); counted on CBC
• Thrombocytes (platelets) — Cell fragments essential for hemostasis; low count (thrombocytopenia) increases bleeding risk after venipuncture

Where Blood Cells Come From

Hematopoiesis is the production of blood cells, and after infancy it happens in the red bone marrow of the flat and irregular bones — sternum, ribs, pelvis, vertebrae, and the proximal femur and humerus. Every formed element descends from a single pluripotent hematopoietic stem cell, which commits to either a myeloid line (red cells, platelets, granulocytes, monocytes) or a lymphoid line (B, T, and NK lymphocytes). Red cell production is driven by erythropoietin from the kidney in response to low tissue oxygen, and platelet production by thrombopoietin from the liver. This is why renal failure causes anemia, and why a CBC is often the first test a phlebotomist collects on a patient being worked up for fatigue.

Red cells survive about 120 days; platelets about 8–10 days; most circulating neutrophils only hours. Those lifespans explain why a transfused patient's counts change quickly while an HbA1c — reflecting the red cell's whole lifespan — reports roughly the previous 2–3 months of glucose control.

Adult Reference Intervals Worth Recognizing

Reference intervals vary by laboratory, method, age, and sex — always report against the performing laboratory's own ranges. These representative adult values are the ones RPT items use to signal "normal" versus "alarming":

Notice how narrow the potassium window is. That is exactly why a hemolyzed specimen — which spills intracellular potassium into the plasma — can manufacture a false critical value and trigger treatment the patient never needed.

ABO and Rh: The Physiology Behind Bedside Labeling

Red cell membranes carry inherited antigens; plasma carries antibodies against the antigens a person lacks. That reciprocal relationship is what makes a mislabeled blood bank tube lethal.

Rh (D) antigen is separate: Rh-positive people carry D, Rh-negative people do not. Unlike ABO, anti-D antibodies are not naturally occurring — an Rh-negative person makes them only after exposure through transfusion or pregnancy. That is the basis of hemolytic disease of the fetus and newborn, and the reason Rh-negative pregnant patients receive Rh immune globulin and have their specimens handled with blood bank identification rigor.

Because ABO antibodies are naturally occurring and act immediately, transfusing ABO-incompatible red cells causes an acute hemolytic transfusion reaction within minutes. The failure almost never happens in the laboratory — it happens at the bedside, when the wrong patient's blood ends up in a correctly labeled tube. This is the physiological reason Section 7.2's labeling discipline is non-negotiable.

Hemostasis Cascade at a High Level

Hemostasis is the physiologic process that stops bleeding after vessel injury. Phlebotomy creates controlled injury; understanding hemostasis explains why we apply pressure, why patients on anticoagulants bruise/bleed longer, and why coagulation tubes are so technique-sensitive.

Simplified sequence:

1. Vascular phase — Smooth muscle in the vessel wall contracts (vasospasm), reducing blood flow from the injury.
2. Platelet phase — Platelets adhere to exposed collagen, activate, and aggregate into a temporary platelet plug.
3. Coagulation phase (cascade) — Clotting factors activate in pathways traditionally called intrinsic, extrinsic, and common, culminating in thrombin converting fibrinogen to fibrin, which stabilizes the plug into a durable clot.
4. Fibrinolysis — Later, plasmin breaks down fibrin so the vessel can be remodeled and flow restored.

You do not need to memorize every Roman-numeral factor for the RPT, but you must know that PT/INR historically screens the extrinsic/common pathways and aPTT the intrinsic/common pathways — and both usually require a properly filled sodium citrate tube (light blue) with the correct blood-to-additive ratio. Underfilling leaves relative excess citrate, which binds too much calcium and falsely prolongs clotting times.

After venipuncture, external pressure assists the vascular and platelet phases while fibrin forms. If pressure is inadequate, blood escapes into tissue → hematoma. If the patient is thrombocytopenic or therapeutically anticoagulated, hemostasis is impaired → longer hold times and careful site checks.

Circulatory Flow Relevant to Collection

Trace a drop of blood through systemic circulation:

Body veins → right atrium → right ventricle → pulmonary arteries → lung capillaries (O₂ in, CO₂ out) → pulmonary veins → left atrium → left ventricle → aorta → systemic arteries → arterioles → capillaries → venules → veins → vena cavae → right atrium.

Routine arm venipuncture samples blood in systemic veins returning toward the right heart. Capillary puncture samples the capillary exchange bed fed by arterioles — hence values can differ slightly from pure venous draws. Arterial punctures sample blood leaving the left heart before tissue exchange — specialized testing (ABGs), not routine RPT scope in most settings.

Tourniquet application temporarily slows venous return, engorging distal veins for easier access. Leave it on too long (generally avoid beyond about one minute before the draw) and hemoconcentration / analyte shifts can occur — physiology again, not trivia.

Why Clotting and Anticoagulants Matter for Tubes

Tube additives manipulate hemostasis on purpose:

Order of draw exists because additive carryover changes physiology inside the next tube. Clot-activator contamination into a citrate tube can falsely activate clotting and skew PT/aPTT. EDTA or heparin carried into a serum tube can prevent a complete clot. EDTA into a green or chemistry specimen can falsely lower calcium and affect enzymes. Blood cultures come first to protect microbiology sterility.

Inversion mixes additive with blood so anticoagulation or clot activation distributes evenly — without shaking so hard you hemolyze red cells. Hemolysis spills intracellular potassium and enzymes into plasma/serum, falsely elevating K⁺ and wrecking specimen integrity — cellular physiology leaking into the wrong compartment.

Clinical Scenarios That Test VIII-C Thinking

• Need PT/INR → citrate plasma; fill to the line; invert gently; draw citrate early in sequence.
• Need CBC → EDTA whole blood; avoid clots in the lavender tube (mix promptly).
• Need comprehensive metabolic panel on serum → allow full clot in red/gold before centrifugation; do not contaminate with EDTA.
• Fingerstick glucose → dermal capillary blood; wipe first drop if policy requires; do not milk the site.

Linking All of Domain VIII

Terminology names the processes (hemo-, thromb-, hypo-/hyper-, anticoagulant). Anatomy locates the targets (veins, capillaries, chambers). Physiology explains why the liquid you send is plasma or serum, why pressure stops bleeding, and why a wrong additive falsifies results. Together they are only 5% of the blueprint — but they unlock safer practice across Obtaining Blood Samples and Specimen Processing items that dominate the rest of the RPT.

Therapeutic Anticoagulation and Collection

Patients taking warfarin, direct oral anticoagulants (DOACs), or heparin have impaired hemostasis. Physiology explains what you observe at the bedside:

• Prolonged bleeding after needle withdrawal — hold pressure longer than usual
• Larger hematomas from the same technique that would be minor in a normal patient
• PT/INR monitoring on warfarin — the very test that requires perfect citrate technique
• aPTT monitoring on unfractionated heparin — another citrate-dependent assay

Never assume a patient on anticoagulants needs a different tube type — they need the same tubes with better post-puncture care. The physiology of their medication affects hemostasis after the draw, not which additive the laboratory requires.

Preanalytical Physiology Summary