A handful of numbers tell doctors more about your cardiovascular health than almost any other test. Understanding what they mean — not just whether they're "in range," but what the physiology behind them is — helps you make better decisions about your health.
Blood Pressure: The Two Numbers
Blood pressure is reported as two measurements in mmHg (millimetres of mercury):
- Systolic (top number): pressure in arteries during a heartbeat
- Diastolic (bottom number): pressure between beats, when the heart is filling
A reading of 120/80 mmHg means 120 mmHg systolic and 80 mmHg diastolic.
AHA Classification (2017)
| Category | Systolic | | Diastolic | |----------|----------|--|-----------| | Normal | < 120 | and | < 80 | | Elevated | 120–129 | and | < 80 | | Stage 1 High BP | 130–139 | or | 80–89 | | Stage 2 High BP | ≥ 140 | or | ≥ 90 | | Hypertensive Crisis | > 180 | and/or | > 120 |
Derived Measurements
Pulse pressure = Systolic − Diastolic. A normal pulse pressure is 40 mmHg. A widening pulse pressure (above 60) can indicate aortic stiffness; a narrowing pulse pressure (below 25) may signal heart failure or shock.
Mean arterial pressure (MAP) = Diastolic + (Pulse pressure / 3). This represents the average pressure driving blood to organs throughout the cardiac cycle. A MAP below 60 mmHg is generally considered insufficient for organ perfusion — this is the threshold used in critical care.
Why Single Readings Are Unreliable
Blood pressure follows a circadian rhythm — it naturally dips 10–20% during sleep and rises sharply in the morning (the "morning surge"). It also responds to stress, caffeine, exercise, temperature, and talking. A single reading at a doctor's office captures none of this context.
White-coat hypertension — elevated BP in clinical settings but normal at home — affects 15–30% of patients and carries far lower cardiovascular risk than sustained hypertension.
For meaningful results: measure twice, sitting after 5 minutes of rest, arm at heart level, no coffee or exercise in the preceding 30 minutes.
Heart Rate Zones
Your heart rate during exercise tells you which energy system is working hardest. The Karvonen formula uses your heart rate reserve (HRR) for personalised zones:
HRR = HRmax - Resting HR
Zone HR = Resting HR + (HRR × intensity %)
HRmax is estimated as 220 − age (though this varies considerably between individuals — the actual formula's standard deviation is ±10–12 bpm).
| Zone | % HRR | Energy system | Feels like | |------|-------|---------------|------------| | Z1 Recovery | 50–60% | Fat (aerobic) | Easy conversation | | Z2 Aerobic base | 60–70% | Fat (aerobic) | Can speak in sentences | | Z3 Aerobic | 70–80% | Mixed | Short phrases only | | Z4 Threshold | 80–90% | Carbohydrate (anaerobic) | Cannot hold conversation | | Z5 VO2max | 90–100% | Creatine phosphate | Maximum effort |
Why Zone 2 matters: Endurance athletes (and an increasing number of recreational exercisers following polarised training) spend 70–80% of their training in Zone 2. This builds mitochondrial density, fat-burning capacity, and aerobic base without the recovery cost of harder sessions.
Kidney Function: eGFR
The kidneys filter about 180 litres of blood per day. eGFR (estimated Glomerular Filtration Rate) measures how efficiently they do this, in mL of blood cleaned per minute per 1.73m² body surface area.
The CKD-EPI 2021 formula calculates eGFR from serum creatinine, age, and sex.
| CKD Stage | eGFR | Description | |-----------|------|-------------| | G1 | ≥ 90 | Normal or high | | G2 | 60–89 | Mildly reduced | | G3a | 45–59 | Mildly-moderately reduced | | G3b | 30–44 | Moderately-severely reduced | | G4 | 15–29 | Severely reduced | | G5 | < 15 | Kidney failure (requires dialysis/transplant) |
Creatinine clearance (Cockcroft-Gault) is a related but distinct calculation, used primarily for drug dosing rather than CKD staging. Many renally-cleared drugs (metformin, direct oral anticoagulants, some antibiotics) require dose adjustment when CrCl falls below 60 mL/min.
Lung Function: FEV1 and Spirometry
Spirometry measures how much air you can force out of your lungs and how fast. The two key values are:
- FVC (Forced Vital Capacity): total air expelled in one maximal breath-out
- FEV1 (Forced Expiratory Volume in 1 second): air expelled in the first second
The FEV1/FVC ratio is the diagnostic cornerstone:
- < 0.70: Obstructive pattern (COPD, asthma) — airflow is limited
- Normal FEV1/FVC with low FVC: Restrictive pattern (pulmonary fibrosis) — lung volume is reduced
COPD severity is staged by FEV1 as a percentage of the age- and sex-predicted value:
| GOLD Stage | FEV1 % predicted | |-----------|-----------------| | Mild (1) | ≥ 80% | | Moderate (2) | 50–79% | | Severe (3) | 30–49% | | Very severe (4) | < 30% |
Lung age is a useful communication tool: rather than telling a smoker their FEV1 is 72% of predicted, calculating that their lungs function like those of a 60-year-old when they're actually 42 is often more motivating.
Body Fat Percentage
BMI (Body Mass Index) is a ratio of weight to height squared. It's a useful population screening tool but has well-known limitations — it cannot distinguish muscle from fat, doesn't account for fat distribution, and has different risk thresholds for different ethnicities.
More informative alternatives:
US Navy method estimates body fat from circumference measurements:
- Males: neck and waist measurements + height
- Females: neck, waist, and hip measurements + height
| Category | Males | Females | |----------|-------|---------| | Essential fat | 2–5% | 10–13% | | Athletic | 6–13% | 14–20% | | Fitness | 14–17% | 21–24% | | Average | 18–25% | 25–31% | | Obese | > 25% | > 32% |
Waist-to-height ratio is increasingly considered the single best simple measure: keep your waist circumference less than half your height (ratio < 0.5). This works across ethnicities with the same threshold.
Body Roundness Index (BRI) is a newer measure that uses waist circumference and height to estimate central adiposity — a stronger predictor of cardiovascular risk than BMI.
Blood Glucose
Fasting blood glucose interpretation:
| Fasting glucose | Status | |----------------|--------| | < 5.6 mmol/L (< 100 mg/dL) | Normal | | 5.6–6.9 mmol/L (100–125 mg/dL) | Prediabetes | | ≥ 7.0 mmol/L (≥ 126 mg/dL) | Diabetes (if confirmed) |
HbA1c measures average blood glucose over the preceding 2–3 months (reflecting the lifespan of red blood cells). It's more informative than a single fasting reading:
- < 5.7%: Normal
- 5.7–6.4%: Prediabetes
- ≥ 6.5%: Diabetes
Making Sense of Your Results
No single number tells the full story. A blood pressure of 135/85 means something different in a 25-year-old athlete than a 70-year-old with diabetes and three other cardiovascular risk factors. The trend over time, the context of other measurements, and the clinical picture all matter.
That said, understanding what these numbers represent — not just whether they're "in range" but what the physiology behind them means — makes you a far more informed participant in your own healthcare.
Use our Blood Pressure Analyser, Heart Rate Zone Calculator, eGFR Calculator, FEV1 Calculator, and Body Fat Calculator to understand your own numbers.