The four vital signs and their normal ranges
A vital sign is a quick body measurement that shows if basic life functions are working. There are four main ones.
- Body temperature: about 36.1–37.2 °C in the mouth. Above 38 °C is a fever.
- Pulse (heart rate): 60–100 beats per minute for a resting adult. Athletes can be lower.
- Breathing (respiratory) rate: 12–20 breaths per minute at rest.
- Blood pressure: written as two numbers, like 115/75 mmHg. The top number (systolic) is the push when the heart beats; the bottom (diastolic) is the pressure between beats. Normal is below about 120/80.
Doctors also use biochemical indicators from blood tests. Examples of normal adult ranges: fasting blood glucose about 4–6 mmol/L (70–100 mg/dL), haemoglobin about 12–17 g/dL, oxygen saturation (SpO₂) 95–100 %.
Children have higher pulse and breathing rates than adults, so ranges depend on age.
Blood under the microscope
A drop of blood is spread thin on a glass slide, stained and viewed with a light microscope (start at low power, then 400×).
- Red blood cells: most numerous, pink discs with a pale centre and no nucleus. They carry oxygen with haemoglobin.
- White blood cells: fewer and larger, with a dark purple nucleus. They fight infection.
- Platelets: tiny purple fragments. They help blood clot.
A complete blood count (CBC) counts these. Too few red cells suggests anaemia; too many white cells can mean infection.
Safe lab practice
Use only prepared slides in school. Wear gloves and never share lancets, because blood can carry disease.
Medical devices and how they collect data
- Digital thermometer: a sensor whose resistance changes with heat.
- Stethoscope: a disc picks up sound from the heart and lungs and tubes carry it to the ears. Used for pulse, heartbeat and breathing sounds.
- Blood pressure monitor (sphygmomanometer): a cuff squeezes the arm, then slowly lets go. A sensor detects when blood starts to flow (systolic) and when flow is smooth (diastolic).
- Pulse oximeter: shines red and infrared light through a finger. Oxygen-rich blood absorbs light differently, so it gives SpO₂ and pulse.
- ECG (electrocardiogram): stickers on the skin record the heart's tiny electrical signals as a wave.
- Glucometer: a test strip measures sugar in a drop of blood.
Key words: diagnosis (finding the illness), non-invasive (no cutting or needle), invasive (enters the body), therapy (treatment), prognosis (likely outcome).
Imaging and therapy technologies
- X-ray: X-rays pass through soft tissue but are absorbed by bone, so bones look white. Fast and cheap; small radiation dose.
- CT scan: many X-ray images from all angles are joined by a computer into slices. Very detailed, but a higher dose.
- Ultrasound: high-pitched sound echoes off organs. No radiation, so it is used in pregnancy.
- MRI: a strong magnet and radio waves make hydrogen in the body give signals. Superb for brain and joints; no ionising radiation, but slow, noisy and costly, and no metal allowed.
- Endoscopy: a thin tube with a camera looks inside the gut.
Treatments include medicines, surgery, radiation therapy for cancer, dialysis for kidneys and pacemakers for the heart. Alternative and complementary therapies (such as acupuncture, yoga, Ayurveda or herbal remedies) are used by many people; good practice is to check the scientific evidence for each claim and tell your doctor what you use.
Costs, benefits and new advances
To judge a technology, list its benefits (finds disease early, saves lives, less pain) and its costs (price, radiation or side-effects, need for trained staff, waiting lists, energy use, waste). An MRI machine costs millions of dollars, so small towns may not have one; an X-ray is cheaper and more widely available.
Recent advances: AI that reads scans, wearable watches that track heart rhythm, telemedicine video visits, robot-assisted surgery, 3D-printed implants, rapid test kits and mRNA vaccines. Each raises questions about privacy, cost and fair access.
Try it at home
Count your resting pulse at the wrist for 30 seconds and double it. Jog on the spot for one minute and count again. Is it still in the 60–100 range? Use the slider in step 6 to check.
Key formulas and definitions
- Pulse (beats/min) = beats counted in 15 s × 4
- Blood pressure = systolic / diastolic (mmHg), normal below about 120/80
- °F = °C × 9/5 + 32
- Mean arterial pressure ≈ diastolic + (systolic − diastolic) ÷ 3
Worked examples
1. A nurse counts 19 beats in 15 seconds. What is the pulse, and is it normal for an adult at rest?
Pulse = 19 × 4 = 76 beats per minute. That is inside 60–100, so it is normal.
2. A patient's temperature is 101.3 °F. Convert to °C. Is it a fever?
°C = (101.3 − 32) × 5/9 = 69.3 × 5/9 = 38.5 °C. Above 38 °C, so yes, a fever.
3. Blood pressure is 130/85 mmHg. Find the mean arterial pressure and say if the reading is in the normal range.
MAP ≈ 85 + (130 − 85) ÷ 3 = 85 + 15 = 100 mmHg. Both numbers are above 120/80, so it is raised (elevated) and should be rechecked.
4. A pregnant woman needs a scan of her baby. Which imaging method is best and why?
Ultrasound. It uses sound echoes, has no ionising radiation, is cheap and shows movement in real time.
Common mistakes
- Thinking oxygen saturation (SpO₂) is a vital sign of the main four. The classic four are temperature, pulse, breathing rate and blood pressure; SpO₂ is often added as a fifth.
- Reading blood pressure as one number. It is always two: systolic over diastolic.
- Saying MRI uses X-rays. MRI uses a magnet and radio waves; CT uses X-rays.
- Using adult ranges for children. A child's normal pulse and breathing rate are higher.