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[Physiology](https://durgical.com/en/physiology/) · [Temperature, exercise and metabolism](https://durgical.com/en/physiology/#s-integrative)

# 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](https://durgical.com/en/anatomy/radial-artery/) · [Brachial artery](https://durgical.com/en/anatomy/brachial-artery/)

## Key points

- Resting adult: HR 60–100/min, RR 12–20/min, BP
- 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 mmHg **Arterial 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

Saturation (SpO₂) is the height of the curve: try «CO poisoning» to see why the oximeter misleads.

## Equations

**Mean arterial pressure**MAP ≈ DBP + (SBP − DBP)/3

**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 | 100 tachycardia; athletes may sit at 40–60 |
| Respiratory rate | 12–20/min | count 30–60 s without the patient noticing |
| Blood pressure |
- 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.

## Memory aid

«The pulse runs before the pressure falls»: in shock, tachycardia warns you before the cuff does.

## 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.

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## See it in 3D

The structures of Durgical's 3D anatomy where it happens.

- [Radial artery](https://durgical.com/en/anatomy/radial-artery/)
- [Brachial artery](https://durgical.com/en/anatomy/brachial-artery/)

## Related topics

- [Body temperature regulation](https://durgical.com/en/physiology/body-temperature-regulation/)
- [Fever](https://durgical.com/en/physiology/fever/)
- [High altitude and acclimatization](https://durgical.com/en/physiology/high-altitude-and-acclimatization/)

## Related conditions

- [Sepsis and septic shock](https://durgical.com/en/conditions/sepsis-and-septic-shock/)
- [Hypovolaemic shock](https://durgical.com/en/conditions/hypovolaemic-shock/)
- [Arterial hypertension](https://durgical.com/en/conditions/arterial-hypertension/)

[← All of physiology](https://durgical.com/en/physiology/)

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](https://medlineplus.gov/ency/article/002341.htm) (MedlinePlus, US National Library of Medicine, 2026); [Blood pressure measurement: MedlinePlus Medical Encyclopedia](https://medlineplus.gov/ency/article/007490.htm) (MedlinePlus, US National Library of Medicine, 2026); [20.2 Blood Flow, Blood Pressure, and Resistance](https://openstax.org/books/anatomy-and-physiology-2e/pages/20-2-blood-flow-blood-pressure-and-resistance) (OpenStax (Rice University), 2022); [1.5 Homeostasis](https://openstax.org/books/anatomy-and-physiology-2e/pages/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.

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License: CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). Cite as: DurgiPedia (durgical.com), https://durgical.com/en/physiology/physiology-of-the-vital-signs/