Relate blood components and major organ systems to the common lab tests that check them.
Why it matters
Clinical laboratories run a large share of the tests that inform medical decisions. A medical laboratory technologist does not diagnose patients, but they must understand what each test measures and why a result is plausible or not. Biochemists and biological technicians use the same physiology when they work with serum, plasma or tissue. This lesson connects the body's systems to the tests you will hear about most.
Blood: the most tested tissue
Blood is about 55% plasma and 45% cells (the cell fraction is the haematocrit, typically around 40–50%).
- Plasma is mostly water carrying proteins (albumin, antibodies, clotting factors), glucose, electrolytes, hormones and waste such as urea.
- Serum is plasma with the clotting factors used up: what is left after blood clots in the tube. Many chemistry tests use serum; coagulation tests need plasma from a tube with an anticoagulant.
The cells:
| Cell | Job | Lab connection |
|---|---|---|
| Red blood cells (erythrocytes) | carry oxygen with haemoglobin; no nucleus; live about 120 days | RBC count, haemoglobin and haematocrit; low values suggest anaemia |
| White blood cells (leukocytes) | defend against infection (next lesson) | WBC count and differential (neutrophils, lymphocytes, monocytes, eosinophils, basophils) |
| Platelets (thrombocytes) | cell fragments that start clotting | platelet count; low counts raise bleeding risk |
These numbers come together in the complete blood count (CBC), one of the most ordered tests anywhere. A raised neutrophil count often points to bacterial infection; a raised lymphocyte count is common in viral infections. Those are patterns for clinicians to interpret, not rules, but knowing them helps a technologist spot an implausible result.
Organ systems and their tests
Cardiovascular system. The heart pumps blood through arteries, capillaries and veins. When heart muscle is damaged, cells release troponin into the blood, so a troponin test helps diagnose heart attacks. Cholesterol tests (a lipid panel) assess long-term risk.
Kidneys (urinary system). Kidneys filter the blood, remove waste and balance water and electrolytes. Creatinine and urea (BUN) rise when kidney filtration falls. Labs often report an estimated glomerular filtration rate (eGFR) calculated from creatinine. Urinalysis checks urine for protein, glucose, blood and cells.
Liver (digestive system). The liver processes nutrients and drugs and makes proteins such as albumin and clotting factors. Liver function tests include the enzymes ALT and AST (released when liver cells are damaged), bilirubin (raised in jaundice) and albumin.
Endocrine system. Glands release hormones into the blood. The pancreas makes insulin, which lowers blood glucose. Glucose and HbA1c (glucose stuck to haemoglobin, reflecting about the last three months, since red cells live about 120 days) are used to diagnose and monitor diabetes. The thyroid is checked with TSH.
Electrolytes. Sodium, potassium, chloride and bicarbonate keep nerves, muscles and fluid balance working. Remember the sodium-potassium pump from module 1: if a blood sample sits too long before separation, potassium leaks out of the cells and the result can be falsely high. Haemolysis (burst red cells) in the tube has the same effect.
Worked example: a basic metabolic panel
A basic metabolic panel (BMP) bundles glucose, calcium, sodium, potassium, chloride, bicarbonate, urea and creatinine. Imagine a result with high potassium and a lab note: "specimen haemolysed". Because red cells contain far more potassium than plasma, the burst cells have released potassium into the sample. The technologist flags the result and requests a new sample rather than reporting a value that might send a clinician chasing a problem that is not there. Knowing the physiology turns a number into a judgement.
Reference ranges
Every test result is compared with a reference range: the values found in most healthy people (often the middle 95%). Ranges differ between labs, methods, ages and sexes, so always use the range printed on the report. A value slightly outside the range is not automatically abnormal, and a value inside it does not rule out disease.
The careers behind the tests
Running and verifying these tests is regulated work. In the US, medical laboratory scientists usually complete a NAACLS-accredited program with clinical rotations and earn ASCP Board of Certification (MLS) certification; some states, such as California, New York and Florida, also require a licence. This course teaches background knowledge only. Official sources: NAACLS, https://www.naacls.org and ASCP Board of Certification, https://www.ascp.org/boc (Checked on 2026-10-23.)
Common mistakes
- Using serum and plasma as if they were the same. Serum has no clotting factors.
- Reading a result without its reference range.
- Ignoring pre-analytical problems such as haemolysis, delays or the wrong tube.
- Confusing the CBC with chemistry panels. The CBC counts cells; panels measure dissolved substances.
Quick recap
- Blood is plasma plus red cells, white cells and platelets; the CBC counts them.
- Troponin for the heart, creatinine and eGFR for the kidneys, ALT/AST and bilirubin for the liver, glucose and HbA1c for diabetes.
- Electrolytes reflect fluid balance; haemolysis can falsely raise potassium.
- Always interpret results against the lab's own reference range.