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Introduction

This chapter is the background knowledge that makes the rest of the exam coherent: how blood moves, what distinguishes arterial from venous and capillary blood, the vein map of the antecubital fossa, which tube each common test needs, and the word parts and abbreviations printed on every requisition. It is the smallest slice of the exam, but it is high-leverage — understanding why the median cubital is safest or why capillary blood differs from venous turns a dozen memorized rules into a handful of understood ones.

Do not over-invest here relative to its weight, but do not skip it either. A surprising number of "collection" and "specimen" questions are really anatomy questions in disguise: the vein you choose, the tube a test needs, the difference between plasma and serum. Learn this chapter and those questions become free points.

The circulatory system and the path of blood

The heart drives two circuits in series: the pulmonary circuit to the lungs and the systemic circuit to the rest of the body. Knowing the sequence of chambers and valves explains why the pulmonary vessels are the one exception to the usual oxygen rule. Blood vessels also have three wall layers, and the middle muscular layer is what makes arteries pulse.

• Four chambers, two circuits. Deoxygenated blood enters the right atrium, passes the tricuspid valve to the right ventricle, exits through the pulmonary valve to the lungs, returns to the left atrium, passes the mitral (bicuspid) valve to the left ventricle, and leaves through the aortic valve into the aorta.
• The pulmonary exception. Arteries usually carry oxygenated blood and veins carry deoxygenated blood — but the pulmonary artery carries deoxygenated blood to the lungs, and the pulmonary veins carry oxygenated blood back to the heart. "Artery = away from the heart" is the reliable definition, not "artery = oxygen-rich."
• Three vessel layers. Arteries and veins share a tunica intima (lining), a tunica media (smooth muscle and elastic tissue), and an outer tunica adventitia (externa). The media is thickest in arteries, which is why arteries are elastic and palpably pulsatile — and why an accidental arterial stick spurts.
• Veins have valves. Veins carry blood back to the heart at low pressure and rely on one-way valves and skeletal-muscle contraction to prevent backflow — which is why a tourniquet distends the veins below it, not above.
• Capillaries are the exchange site. Capillary walls are a single endothelial cell thick, which lets oxygen, carbon dioxide, nutrients, and waste diffuse between blood and tissue.
• Related systems. The circulatory system works with the lymphatic system (returns interstitial fluid, houses immune cells) and the hemostatic process that stops bleeding when a vessel is injured.

Key numbers & facts — circulation - Path: RA → tricuspid → RV → pulmonary valve → lungs → LA → mitral → LV → aortic valve → aorta. - Arteries carry blood AWAY from the heart (definition), veins carry it back — pulmonary vessels are the exception to the oxygen rule. - Layers: intima (lining) · media (muscle, thickest in arteries) · adventitia (outer). - Veins have valves and run at low pressure; tourniquet distends below it.

Worked example — the "wrong" pulmonary artery. A question claims "all arteries carry oxygen-rich blood." True or false? False — the pulmonary artery carries deoxygenated blood from the right ventricle to the lungs, and the pulmonary veins carry oxygenated blood back. The dependable rule is directional: an artery carries blood away from the heart, a vein carries it toward the heart, regardless of oxygen content. The exam plants the "arteries are always oxygenated" statement specifically to catch students who memorized the shortcut instead of the definition.

Common traps in this section

• "Arteries always carry oxygenated blood." The pulmonary artery does not. Use the directional definition.
• Mixing up the tricuspid (right) and mitral (left) valves. Tri = right.
• Forgetting veins have valves and arteries have the thick muscular media. That media is why arteries pulse.

Arterial, venous, and capillary blood, and what blood is made of

The three blood sources are not interchangeable, and the differences explain several collection rules. Whole blood is roughly 55% plasma and 45% formed elements, and the difference between plasma and serum is the single most reliable exam question in this section. Capillary blood is a mixture, which is why some analytes differ slightly from a venous draw.

• Arterial versus venous blood. Arterial blood is bright red, oxygen-rich, under high pressure, and uniform throughout the body; venous blood is darker, carries carbon dioxide and waste, flows under low pressure, and varies slightly by drainage bed.
• Capillary blood is a mixture (CLSI GP42). A dermal puncture yields a blend of arterialized capillary blood, venous blood, and interstitial fluid, so glucose runs slightly higher and potassium, total protein, and calcium slightly lower than in a venous specimen.
• Plasma versus serum — the classic. Plasma is the liquid part of anticoagulated blood and still contains fibrinogen and the clotting factors. Serum is the liquid left after blood clots, so it has no fibrinogen. Because plasma has not given up its fibrinogen to a clot, plasma tubes yield more fluid volume than serum tubes of the same size.
• Composition of whole blood. About 55% plasma (roughly 90% water plus proteins, electrolytes, and waste) and about 45% formed elements. After centrifugation, the thin buffy coat between the layers holds the white cells and platelets.
• The formed elements. Erythrocytes (red cells) carry oxygen on hemoglobin and live about 120 days; leukocytes (white cells) defend against infection in five types — neutrophils, lymphocytes, monocytes, eosinophils, basophils; thrombocytes (platelets) are cell fragments that begin clot formation.
• Hemostasis in four phases. Injury triggers vascular constriction, then a platelet plug, then the coagulation cascade that converts fibrinogen to fibrin, and finally fibrinolysis that dissolves the clot as the vessel heals. PT reflects the extrinsic pathway; aPTT reflects the intrinsic pathway.

Key numbers & facts — blood composition - Plasma = anticoagulated, HAS fibrinogen; serum = clotted, NO fibrinogen. Plasma yields more volume. - Whole blood ≈ 55% plasma / 45% formed elements; buffy coat = WBCs + platelets. - Capillary vs venous: glucose slightly higher; K⁺, protein, calcium slightly lower. - RBC lifespan ≈ 120 days; 5 WBCs = neutrophils, lymphocytes, monocytes, eosinophils, basophils. - PT → extrinsic pathway; aPTT → intrinsic pathway.

Worked example — plasma or serum? A tube is spun and the technologist notes it still contains fibrinogen and yielded a large volume of liquid. Plasma or serum? Plasma — it retained its fibrinogen and clotting factors, which means the blood was anticoagulated (e.g., a heparin or EDTA tube) rather than allowed to clot. Serum comes from a clotted tube and has no fibrinogen, and it yields less liquid because the fibrinogen was consumed forming the clot. This single distinction — anticoagulated-and-fibrinogen-present versus clotted-and-fibrinogen-gone — is the most dependable point in the whole chapter.

Common traps in this section

• Saying serum contains fibrinogen. It does not — the clot used it up. Plasma has it.
• Expecting equal volume from serum and plasma tubes. Plasma yields more.
• Assuming capillary and venous values are identical. Glucose runs a bit higher, potassium/protein/calcium a bit lower.
• Swapping PT and aPTT pathways. PT = extrinsic, aPTT = intrinsic.

Antecubital vein anatomy

The antecubital fossa is the shallow triangular depression in front of the elbow where the superficial veins converge, and it is where most venipunctures happen. Two common surface patterns are described — the H pattern and the M pattern — and both include the same three named veins. The order of preference among them, from Chapter 1, is a safety ranking, not a convenience ranking.

• H pattern and M pattern. In the H pattern — seen in roughly seven of ten people — the median cubital connects the cephalic and basilic veins at an angle that looks like an H. In the M pattern, the median cephalic and median basilic branch upward from the median antebrachial vein in an M shape.
• Median cubital vein (CLSI GP41). Central, largest, most superficial, best anchored, and farthest from the brachial artery and median nerve — the easiest and safest choice.
• Cephalic vein. Runs along the lateral (thumb) side and continues up the outer upper arm. Often the only palpable option in patients with more subcutaneous tissue, but it tends to roll.
• Basilic vein. Runs along the medial (little-finger) side, directly above the brachial artery and median nerve — the last choice, and the vein most often implicated in nerve injury and accidental arterial puncture.
• What a good vein feels like. A suitable vein feels spongy, bouncy, and resilient and refills when depressed. A hard, cord-like vessel is usually sclerosed or thrombosed; a pulsating vessel is an artery and must not be entered.
• Nearby structures to respect. The brachial artery, median nerve, and tendons lie deep to the basilic vein, and the median and ulnar nerves run near the underside of the wrist — which is why that site is never used.

Key numbers & facts — antecubital anatomy - Three veins: median cubital (1st) · cephalic (2nd, lateral/thumb) · basilic (last, medial/little-finger). - Basilic overlies the brachial artery + median nerve → highest risk, last choice. - H pattern (~70%): median cubital bridges cephalic and basilic. M pattern: median cephalic + median basilic. - Good vein = spongy, bouncy, refills. Hard/cord-like = sclerosed. Pulsating = artery, do not enter.

Worked example — feel before you commit. You palpate a prominent vessel in the antecubital fossa and feel a distinct pulse under your finger. Do you draw from it? No — a pulsating vessel is an artery, not a vein, and entering it risks a serious arterial puncture and a difficult bleed. A vein feels spongy and bouncy and refills when you press and release; it does not pulse. This is why the whole discipline is "palpate, don't look": the pulse tells you what your eyes cannot. Reposition to a non-pulsatile, resilient vein — ideally the median cubital.

Common traps in this section

• Drawing from a pulsating vessel. That's an artery. Veins don't pulse.
• Choosing the basilic for convenience. It sits over the artery and nerve — last resort.
• Mistaking a hard, cord-like vein for a good one. That's sclerosed; find a resilient vein.

Common laboratory tests and the tubes they require

Matching a test to its tube is one of the highest-yield skills on the exam, because it shows up in order-of-draw items, rejection items, and department items alike. Group tests by department first, then attach the additive. When in doubt, reason from what the test needs: intact cells → EDTA, clotting studies → citrate, chemistry → serum or heparin plasma.

• Hematology → lavender EDTA. CBC with differential, hemoglobin and hematocrit, reticulocyte count, ESR, and hemoglobin A1c. (Some labs use a dedicated black tube for ESR.)
• Coagulation → light-blue sodium citrate (CLSI H21). PT/INR, aPTT, fibrinogen, D-dimer, and factor assays — filled completely to preserve the 9-to-1 ratio.
• Chemistry → gold/red serum or green lithium heparin. Basic and comprehensive metabolic panels, liver and lipid panels, thyroid studies, cardiac markers, and therapeutic drug levels — serum or heparin plasma depending on the platform.
• Glucose and alcohol → gray sodium fluoride. Glucose, glucose-tolerance samples, lactate, and blood alcohol. The fluoride keeps red cells from consuming glucose while the specimen waits.
• Blood bank and microbiology. Type/screen and crossmatch use pink or lavender EDTA with strict identification (AABB). Blood cultures use aerobic and anaerobic bottles or yellow SPS tubes, drawn first and with the strictest skin antisepsis.
• Trace elements and lead → royal blue (metal-free). Trace-metal studies use royal blue tubes manufactured to be metal-free, supplied with EDTA or with no additive depending on the analyte; lead levels typically use a royal blue or tan EDTA tube.

Key numbers & facts — test-to-tube - Lavender EDTA: CBC, ESR, retic, HbA1c; blood bank (also pink). - Light blue citrate: PT/INR, aPTT, fibrinogen, D-dimer — fill full. - Gold/red serum or green heparin: chemistry, liver/lipid/thyroid, cardiac markers, drug levels. - Gray fluoride/oxalate: glucose, GTT, lactate, blood alcohol. - Yellow SPS / culture bottles: blood cultures (drawn first, strict antisepsis). - Royal blue (metal-free): trace elements, lead.

Worked example — reason from the department. An order reads "CBC, PT/INR, and BMP." Which three tubes, and in what order? A CBC is hematology → lavender EDTA; a PT/INR is coagulation → light-blue citrate; a BMP is chemistry → gold/red serum (or green heparin). Now apply the order of draw: light blue (citrate) first, then serum, then lavender (EDTA) last. You just used both this chapter (test-to-tube) and Chapter 1 (order of draw) in one question — which is exactly how the exam bundles them.

Common traps in this section

• Putting a coagulation test in EDTA. Coag goes in citrate (light blue); EDTA is for CBC.
• Using an ordinary tube for trace metals or lead. Those need metal-free royal blue.
• Forgetting blood cultures come first and need the strictest antisepsis.