15.2 Physiology Relevant to Laboratory Testing

AMT III.10.B is the matching sentence: apply basic knowledge of physiology. Physiology is what the organs do, which is why a number on a report moves and why a tube has a clock. A CMLA does not diagnose disease from a pattern. You collect the specimen that matches the physiologic question, keep preanalytic conditions honest, and leave high-complexity interpretation to qualified testing personnel.

Quick Answer: Labs monitor homeostasis. Hematopoiesis in marrow makes blood cells (the CBC). Hemostasis is platelets + coagulation factors + fibrin: PT is extrinsic and monitors warfarin; PTT is intrinsic and monitors unfractionated heparin. Kidneys filter creatinine and urea (BUN); UA reflects that filtrate. The liver metabolizes and synthesizes—LFTs. Insulin lowers glucose; fasting must be labeled honestly. ABG is arterial acid-base with special handling. Antibodies explain serology. Circadian rhythms explain timed draws.

Homeostasis and why laboratories exist

Homeostasis is the body's effort to keep the internal environment in a narrow range: pH, glucose, electrolytes, oxygen, temperature, and volume. Almost every chemistry panel, blood-gas order, and many hematology orders ask whether a variable has left that range. A basic metabolic panel (BMP) is a homeostasis snapshot: sodium, potassium, chloride, carbon dioxide (CO2) / bicarbonate, blood urea nitrogen (BUN), creatinine, and glucose (often with calcium). You do not interpret the pattern. You prevent preanalytic lies: hemolysis that falsely raises potassium, EDTA contamination that falsely raises potassium and lowers calcium, or a nonfasting patient labeled fasting.

Hematopoiesis

Hematopoiesis is blood-cell production, mainly in bone marrow. Stem cells become erythrocytes, leukocytes, and platelets (Chapter 11). A complete blood count (CBC) is the laboratory picture of that production on mixed ethylenediaminetetraacetic acid (EDTA) whole blood. Reticulocytes are young red cells just released from marrow. Anemia, leukocytosis, and thrombocytopenia are clinical words for too few red cells, a white-cell response, or too few platelets. You collect and mix the lavender-top tube; you do not independently sign out a high-complexity morphologic differential.

Hemostasis: plug, cascade, fibrin

Hemostasis stops bleeding in three CMLA-level pieces:

1. Platelets adhere and aggregate (primary hemostasis).
2. Plasma coagulation factors generate thrombin (secondary hemostasis).
3. Thrombin converts fibrinogen to fibrin, which stabilizes the plug.

Do not call a platelet a clotting factor, and do not call fibrin a red-cell protein on the CBC.

Pathway pairing—the same pairing as Chapter 11:

• The extrinsic pathway (tissue factor / factor VII into the common pathway) is what prothrombin time (PT, protime) times. PT/INR monitors warfarin (Coumadin).
• The intrinsic pathway (XII, XI, IX, VIII into the common pathway) is what partial thromboplastin time (PTT/aPTT) times. PTT monitors unfractionated heparin. Low-molecular-weight heparin is often followed with an anti-Xa assay, not treated as if PTT were always equivalent.

The specimen is light-blue 3.2% sodium citrate at a 9:1 blood-to-anticoagulant fill. Short draws falsely prolong times. Do not swap the medications, and do not use a green-top heparin chemistry tube as a PTT specimen.

Renal filtration: creatinine, BUN, and UA

The kidney's glomerulus filters blood. Creatinine (from muscle creatine) and urea (reported as BUN) are wastes the kidney should clear. When filtration falls, serum creatinine and BUN rise. Many reports include a calculated estimated glomerular filtration rate (eGFR); you do not compute it by hand on CMLA.

Urine is that filtrate after tubules have reabsorbed water, glucose, and other substances. That is why a UA glucose pad can be positive when plasma glucose exceeds the renal threshold, why protein or blood on a dipstick is not a chemistry BMP, and why a 24-hour urine measures excretion over time (Chapter 9). Hematuria, proteinuria, and pyuria are urinary findings. They do not replace a serum creatinine, and BUN/creatinine are not hepatic enzymes.

Hepatic metabolism and why LFTs exist

The liver metabolizes drugs and bilirubin, synthesizes albumin and many clotting factors, and stores glycogen. Hepatic (liver) function tests (LFTs) exist because injury or obstruction changes those jobs (Chapter 10):

• Aspartate aminotransferase (AST) and alanine aminotransferase (ALT) leak from injured hepatocytes.
• Alkaline phosphatase (ALP) rises with biliary obstruction (and can rise from bone).
• Bilirubin rises when conjugation or excretion fails; protect some neonatal samples from light.
• Low albumin and a prolonged PT can reflect failed synthesis—physiology overlap with coagulation, not a reason to skip the light-blue tube.

A CMLA recognizes an hepatic panel, collects the standard operating procedure (SOP) tube, and does not diagnose hepatitis from a number.

Glucose regulation and fasting

Insulin (from pancreatic beta cells) moves glucose into cells and lowers plasma glucose. Glucagon raises it. After a meal, glucose rises; insulin should bring it down. Fasting (commonly 8–12 hours, water allowed) removes the meal so a glucose or many lipid results are comparable (Chapter 7). Never label a nonfasting patient as fasting. Hemoglobin A1c reflects average glucose over roughly two to three months—it is not a fasting glucose and not a urine dipstick.

An oral glucose tolerance test (OGTT) needs a fasting baseline, a glucose load, and timed draws. A random glucose from yesterday does not replace that protocol, and patients must not eat or smoke through the timed points.

Acid-base: why ABG is special handling

Blood pH is kept near 7.35–7.45 by lungs (CO2 elimination) and kidneys (bicarbonate). An arterial blood gas (ABG) measures pH, PaCO2, PaO2, and related values on arterial blood. That is why ABG is not the venous chemistry CO2 on a BMP. Handling is special: collect in the approved heparinized syringe (usually by personnel trained in arterial puncture), expel air bubbles, cap, mix, transport promptly on ice slurry when the SOP requires it, and often hand-carry—pneumatic-tube exclusions apply (Chapter 8). A CMLA does not independently perform arterial puncture as routine phlebotomy. If you receive an ABG syringe, protect the physiology in it: air bubbles change gases; delay lets cells consume oxygen; freezing the syringe or tubing it against policy ruins the result.

Immune response and serology

Innate immunity is immediate (barriers, neutrophils). Adaptive immunity is antigen-specific: B lymphocytes become plasma cells that secrete antibodies; T lymphocytes help or kill. Laboratory serology detects that binding—agglutination, a cassette line, or blood-bank hemolysis as a positive reaction (Chapter 12). IgM is often early; IgG appears later and crosses the placenta. You collect the matrix the instructions for use (IFU) name. You do not “see” immunity on a differential you are not authorized to sign out, and you do not treat a Strep A cassette as a chemistry glucose.

Circadian physiology and timed draws

Circadian rhythms are roughly 24-hour cycles driven by the body clock. Cortisol and iron are typically higher in the morning, which is why those orders often specify an a.m. window (Chapter 7). A therapeutic-drug trough is drawn immediately before the next scheduled dose—pharmacokinetics plus the clock, not “with morning labs.” Collect timed tests at the ordered time and write the actual time. Do not draw 16:00 cortisol and label it 08:00, and do not draw a trough two hours after a dose because you were already on the floor.

Physiologic process → common laboratory tests

Scenario

A patient on warfarin needs PT/INR, a fasting glucose, morning cortisol, and an ABG collected by respiratory therapy. You fill a light-blue citrate tube to the 9:1 line for PT (extrinsic pathway / warfarin), confirm the 8–12 hour fast before labeling glucose as fasting, draw cortisol in the ordered morning window, and ice and hand-carry the ABG syringe after air is expelled—without calling the femoral artery your backup venipuncture site.

Exam traps

• Swapping PT/warfarin (extrinsic) with PTT/unfractionated heparin (intrinsic).
• Calling BUN/creatinine hepatic tests or AST a kidney test.
• Labeling a fed patient as fasting.
• Treating venous chemistry CO2 as an ABG.
• Drawing a trough two hours after a dose, or an afternoon cortisol labeled as morning.

When CMLA shows a physiologic word, name the process, the test that monitors it, and the collection rule that keeps the number honest.