Physiology · Temperature, exercise and metabolism
Physiology of the vital signs (including blood pressure measurement) !
- Measurement
- Exam priority: high
Temperature, pulse, breathing, blood pressure and oxygen saturation: five numbers that sum up how the body is doing right now.
Updated 2026-11-01
Also called: Vital signs, Blood pressure measurement
See it in 3D: Radial artery · Brachial artery
What it is
Vital signs are a quick readout of the body's big control loops: the hypothalamus (temperature), heart and vessels (pulse and pressure), brainstem (breathing) and gas exchange (SpO₂). Each is the output of a negative-feedback circuit. Knowing what drives each one lets you interpret a change instead of just charting it.
Key facts
- Rectal temperature runs ≈0.5 °C above oral, axillary ≈0.5 °C below.
- Pulse pressure (systolic − diastolic) reflects stroke volume and aortic stiffness: it widens in older adults and aortic regurgitation and narrows in tamponade and hypovolemic shock.
- Orthostatic hypotension: a fall of ≥20 mmHg systolic or ≥10 mmHg diastolic within 3 minutes of standing.
- Pulse oximetry compares red and infrared light absorption by pulsatile blood; it cannot tell carboxyhemoglobin from oxyhemoglobin.
How it works, step by step
- The cuff is inflated above systolic pressure: the brachial artery collapses and flow stops.
- As it deflates below systolic pressure, blood spurts through in turbulent jets: the first Korotkoff sound (phase I = systolic).
- While the artery opens and closes with each beat, the sounds continue.
- Once cuff pressure falls below diastolic, flow becomes laminar again and the sounds vanish (phase V = diastolic in adults).
- In some hypertensive patients the sounds stop for a stretch and return (auscultatory gap): palpating the systolic first avoids underestimating it.
Oxygen–hemoglobin dissociation curve
P50 26.8 mmHgArterial blood 97 %Mixed venous blood 75 %O₂ given up to the tissues 23 %
Normal curve. Tap a point (arterial, venous or P50) or move a control.
- Current curve
- Normal curve (pH 7.40 · 37 °C)
- Arterial blood (PO₂ 100)
- Mixed venous blood (PO₂ 40)
- O₂ given up to the tissues
Equations
- SBP
- systolic pressure
- DBP
- diastolic pressure
Diastole lasts longer than systole at normal rates; with tachycardia MAP approaches the simple average.
Resting adult reference values
| Parameter | Value | Note |
|---|---|---|
| Heart rate | 60–100/min | <60 bradycardia, >100 tachycardia; athletes may sit at 40–60 |
| Respiratory rate | 12–20/min | count 30–60 s without the patient noticing |
| Blood pressure | <120/<80 mmHg | ≥130/80 hypertension (ACC/AHA 2017); ≥140/90 in other guidelines |
| Oral temperature | ≈36.5–37.3 °C | circadian rhythm: lowest before dawn |
| SpO₂ | ≥95% at sea level | ≈90–94% can be normal at 2600 m |
| Mean arterial pressure | ≈70–100 mmHg | DBP + (SBP − DBP)/3 |
Approximate values for a healthy adult at rest; each laboratory sets its own ranges.
Clinical notes
- Sepsis: fever or hypothermia, tachycardia, tachypnea, then hypotension; RR ≥22 is one of the three qSOFA criteria.
- Hypovolemic shock: tachycardia and vasoconstriction first (cold skin), then a narrow pulse pressure and, late, hypotension.
- Cushing's triad (hypertension, bradycardia, irregular breathing) signals raised intracranial pressure.
- Pulsus paradoxus (systolic fall >10 mmHg on inspiration): cardiac tamponade, severe asthma.
Test yourself
Think of the answer, then tap the question to see it.
Why does a cuff that is too narrow for the arm give a falsely high blood pressure?
More cuff pressure is needed to transmit through more tissue and collapse the artery, so the reading overestimates true pressure. The bladder should encircle ≈80% of the arm.
A bleeding patient has BP 118/96 mmHg and HR 124/min. What does the 22 mmHg pulse pressure tell you?
Low stroke volume with compensatory vasoconstriction: systolic is held up and diastolic rises. This is compensated shock even though systolic looks normal.
Why can SpO₂ read 98% in carbon monoxide poisoning?
A two-wavelength oximeter reads carboxyhemoglobin as oxyhemoglobin. True O₂ content is low; CO-oximetry is needed.
Why can a healthy adult in Bogotá have an SpO₂ of 92%?
Barometric pressure (~560 mmHg) lowers inspired PO₂, leaving PaO₂ ≈60–70 mmHg, on the part of the curve where saturation starts to fall.
See it in 3D
The structures of Durgical's 3D anatomy where it happens.
Related topics
Related conditions
Educational physiology content; not a diagnostic or treatment guide and no substitute for a health professional's advice. Values are approximate references. No doses are given.
Sources: Vital signs: MedlinePlus Medical Encyclopedia (MedlinePlus, US National Library of Medicine, 2026); Blood pressure measurement: MedlinePlus Medical Encyclopedia (MedlinePlus, US National Library of Medicine, 2026); 20.2 Blood Flow, Blood Pressure, and Resistance (OpenStax (Rice University), 2022); 1.5 Homeostasis (OpenStax (Rice University), 2022); Hall JE. Guyton and Hall Textbook of Medical Physiology, 13th ed. (ch. 74). Reference book used to check facts; its text and figures are not reproduced (Elsevier, 2016).
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Representative model, for educational purposes only.