Chapter 5 of 510% of exam
Anatomy, Physiology, and Terminology
The background knowledge that makes the rest of the exam make sense: 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 used on every requisition.
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 pulmonary vessels are the 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 valve to the left ventricle, and leaves through the aortic valve into the aorta.
The pulmonary exception
Arteries usually carry oxygenated blood and veins deoxygenated blood, but the pulmonary artery carries deoxygenated blood to the lungs and the pulmonary veins carry oxygenated blood back to the heart.
Three vessel layers
Arteries and veins share a tunica intima lining, a tunica media of smooth muscle and elastic tissue, and an outer tunica adventitia. The media is thickest in arteries, which is why they are elastic and palpably pulsatile.
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 the tourniquet distends veins below it rather than above.
Capillaries are the exchange site
Capillary walls are a single endothelial cell thick, which allows oxygen, carbon dioxide, nutrients, and waste to diffuse between blood and tissue.
Related body systems
The circulatory system works alongside the lymphatic system, which returns interstitial fluid and houses immune cells, and the hemostatic process that stops bleeding when a vessel is injured.
Arterial, venous, and capillary blood, and what blood is made of
The three blood sources are not interchangeable, and knowing the differences explains several collection rules. Whole blood is roughly 55 percent plasma and 45 percent 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 in composition 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
A dermal puncture yields a blend of arterialized capillary blood, venous blood, and interstitial fluid, so glucose tends to run slightly higher and potassium, total protein, and calcium slightly lower than in a venous specimen.
CLSI GP42Plasma versus serum
Plasma is the liquid portion of anticoagulated blood and still contains fibrinogen and the clotting factors; serum is the liquid left after blood clots, so it has no fibrinogen. Plasma tubes therefore yield more fluid volume than serum tubes of the same size.
Composition of whole blood
Whole blood is about 55 percent plasma, which is roughly 90 percent water plus proteins, electrolytes, and waste, and about 45 percent formed elements. After centrifugation the thin buffy coat between the layers holds the white cells and platelets.
The formed elements
Erythrocytes carry oxygen on hemoglobin and live about 120 days, leukocytes provide immune defense in five types (neutrophils, lymphocytes, monocytes, eosinophils, basophils), and thrombocytes 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 and aPTT the intrinsic pathway.
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 occur. 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 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
Located centrally, largest, most superficial, best anchored by surrounding tissue, and farthest from the brachial artery and median nerve, which makes it both the easiest to enter and the safest choice.
CLSI GP41Cephalic vein
Runs along the lateral, thumb side of the arm and continues up the outer upper arm. It is often the only palpable option in patients with more subcutaneous tissue but tends to roll under the needle.
Basilic vein
Runs along the medial, little-finger side and sits directly above the brachial artery and median nerve, so it is the last choice and is the vein most often implicated in nerve injury and accidental arterial puncture.
CLSI GP41What a good vein feels like
A suitable vein feels spongy, bouncy, and resilient and refills when depressed. A hard, cordlike, or nonresilient vessel is usually sclerosed or thrombosed, and a pulsating vessel is an artery and must not be entered.
Nearby structures to respect
The brachial artery, median nerve, and tendons of the antecubital fossa 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.
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 appears 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 go in EDTA, clotting studies go in citrate, and chemistry goes in serum or heparin plasma.
Hematology
CBC with differential, hemoglobin and hematocrit, reticulocyte count, ESR, and hemoglobin A1c are collected in lavender EDTA; some laboratories use a dedicated black tube for ESR.
Coagulation
PT with INR, aPTT, fibrinogen, D-dimer, and factor assays are collected in light blue sodium citrate, filled completely to preserve the nine-to-one ratio.
CLSI H21Chemistry
Basic and comprehensive metabolic panels, liver and lipid panels, thyroid studies, cardiac markers, and therapeutic drug levels are collected in gold or red serum tubes or in green lithium heparin tubes depending on the platform.
Glucose and alcohol
Glucose, glucose tolerance samples, lactate, and blood alcohol go in gray sodium fluoride tubes; the fluoride prevents red cells from consuming glucose while the specimen waits.
Blood bank and microbiology
Type and screen and crossmatch use pink or lavender EDTA with strict identification requirements, and blood cultures use aerobic and anaerobic bottles or yellow SPS tubes collected first and with the strictest skin antisepsis.
AABB Standards for Blood Banks and Transfusion ServicesTrace elements and lead
Trace metal studies use royal blue tubes manufactured to be metal free, supplied with EDTA or with no additive depending on the analyte, and lead levels typically use a royal blue or tan EDTA tube.
Medical terminology and abbreviations
Medical words break into a prefix, a root or combining form, and a suffix, and decoding them from the suffix backward usually reveals the meaning. Requisitions are written almost entirely in abbreviations, so the exam expects fluency with the common ones. Directional and positional terms appear in questions about patient placement.
Roots worth memorizing
Phleb- and ven- mean vein, arteri- means artery, angi- and vas- mean vessel, hem- and hemat- mean blood, cardi- means heart, erythr- means red, leuk- means white, and thromb- means clot.
Suffixes that describe conditions
The suffix -emia means a blood condition, -penia means deficiency, -osis means abnormal increase or condition, -tomy means cutting into, -ology means the study of, and -stasis means stopping or controlling.
Prefixes of degree and position
Hyper- means above normal and hypo- below normal, inter- means between and intra- means within, peri- means around, and anti- means against. Hyperglycemia and hypokalemia are built entirely from these parts.
Directional terms
Proximal is closer to the trunk and distal farther from it, medial is toward the midline and lateral away from it, anterior is front and posterior is back, and superficial is near the surface while deep is farther in.
Patient positions
Supine is lying face up, prone is lying face down, and Fowler position is sitting up with the head of the bed raised. Supine is the safest position for a patient with a fainting history.
Abbreviations on requisitions
Common examples include CBC, BMP and CMP, PT with INR, aPTT, ESR, BUN, HbA1c, FBS, GTT, ABG, C and S for culture and sensitivity, TDM for therapeutic drug monitoring, POCT for point-of-care testing, STAT for immediately, and NPO for nothing by mouth.
Test your knowledge
Practice questions on Anatomy, Physiology, and Terminology