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# The layout of the circulation: vessel segments, pressures and how flow is measured

- Concept
- Exam priority: medium

Blood runs through two circuits in series. Each vessel type has one job, and most of the pressure is spent in the small arteries and arterioles.

Updated Nov 13, 2026

Also called: Functional organization of the circulation, Systemic vs pulmonary circuit

**See it in 3D:** [Ascending aorta](https://durgical.com/en/anatomy/ascending-aorta/) · [Abdominal aorta](https://durgical.com/en/anatomy/abdominal-aorta/) · [Pulmonary arteries](https://durgical.com/en/anatomy/right-pulmonary-artery/) · [Inferior vena cava](https://durgical.com/en/anatomy/inferior-vena-cava/) · [Superior vena cava](https://durgical.com/en/anatomy/superior-vena-cava/)

## Key points

- About 84% of blood volume is in the systemic circulation, roughly two-thirds of it in veins; the heart holds ≈ 7% and the lungs ≈ 9%.
- The biggest pressure fall is across small arteries and arterioles: from ≈ 85 to ≈ 30 mmHg at the capillary entrance.
- Pulmonary pressures are ≈ 1/6 of systemic (mean ≈ 15 vs ≈ 95 mmHg) for the same flow, so pulmonary resistance is ≈ 1/6 of systemic resistance.
- Velocity is inversely proportional to total cross-sectional area: fastest in the aorta, slowest in capillaries (≈ 0.3 mm/s), which gives time for exchange.
- Doppler flowmeters measure velocity from the frequency shift of reflected ultrasound; flow = velocity × cross-sectional area.

## What it is

The systemic and pulmonary circulations are connected in series, so in the steady state both carry the same flow (the cardiac output). Arteries carry blood under high pressure, arterioles set resistance, capillaries exchange, venules collect and veins store most of the blood. The largest pressure drop occurs across the arterioles, which is why they control both local flow and arterial pressure.

## Key facts

- Pressure units: 1 mmHg ≈ 1.36 cmH₂O. Venous and CSF pressures are often given in cmH₂O.
- Fluid-filled catheters with an electronic transducer record fast pressure changes; a long, compliant or bubbled line damps the trace and lowers the systolic peak.
- Electromagnetic flowmeters (a magnet around a vessel) measure flow from the voltage induced by moving blood; they are used in research, not at the bedside.
- Doppler is the clinical method: echo-Doppler for cardiac and valve flow, handheld Doppler for limb pulses and the ankle–brachial index.

## Equations

**Ohm's law of flow**Q = ΔP / R

**Q**
flow (mL/s or L/min)

**ΔP**
pressure difference across the bed (mmHg)

**R**
resistance

For the whole systemic bed ΔP ≈ mean arterial pressure − right atrial pressure.

**Peripheral resistance unit**1 PRU = 1 mmHg / (1 mL/s)

**PRU**
peripheral resistance unit

100 mmHg ÷ 100 mL/s ≈ 1 PRU for the systemic circulation. In CGS units multiply mmHg·min/L by 80 to get dyn·s/cm⁵.

**Velocity and area**v = Q / A

**v**
mean velocity

**A**
total cross-sectional area

Capillary area is ≈ 1000 times the aortic area, so velocity there is ≈ 1/1000.

## Reference values (resting adult)

| Parameter | Value | Note |
| --- | --- | --- |
| Mean aortic pressure | ≈ 95–100 mmHg | |
| Arterial pressure (systolic/diastolic) | ≈ 120/80 mmHg | |
| Capillary pressure (arterial → venous end) | ≈ 35 → 10 mmHg | systemic |
| Right atrial pressure | ≈ 0 mmHg | |
| Pulmonary artery pressure | ≈ 25/8 mmHg, mean ≈ 16 mmHg | |
| Pulmonary capillary pressure | ≈ 7 mmHg | |
| Total peripheral resistance | ≈ 1 PRU ≈ 1000–1400 dyn·s/cm⁵ | |
| Pulmonary vascular resistance | ≈ 0.14 PRU ≈ 100–250 dyn·s/cm⁵ | |

Approximate values for a healthy adult at rest; each laboratory sets its own ranges.

## Vessel segments and their job

| Segment | Main job | Key feature |
| --- | --- | --- |
| Aorta and large arteries | Carry blood fast under high pressure | Elastic walls store energy in systole (Windkessel) |
| Small arteries and arterioles | Control resistance and local flow | Thick smooth muscle; ≈ half of the total pressure drop |
| Capillaries | Exchange of fluid, gases and solutes | One endothelial layer; huge total area; slow flow |
| Venules | Collect blood from capillaries | Thin walls; part of exchange and inflammation (leukocyte exit) |
| Veins | Return blood and act as a reservoir | Low pressure, high compliance; hold ≈ 64% of blood |

## Clinical notes

- Arteriolar disease or constriction raises total peripheral resistance and therefore arterial pressure (arterial hypertension).
- Pulmonary hypertension appears when the low-resistance pulmonary bed loses vessels or constricts: mean pulmonary pressure rises above ≈ 20 mmHg.
- Peripheral artery disease: Doppler shows a damped, low-velocity signal distal to a narrowed segment and a low ankle–brachial index.

## Memory aid

Arteries pipe, arterioles tap, capillaries swap, veins store.

## Test yourself

Think of the answer, then tap the question to see it.

## In which segment of the systemic circulation is the largest fall in pressure?

Across the small arteries and arterioles, which have the highest resistance; pressure falls from ≈ 85 to ≈ 30 mmHg before the capillaries.

## Cardiac output is 5 L/min, mean arterial pressure 93 mmHg and right atrial pressure 3 mmHg. What is the systemic vascular resistance in mmHg·min/L?

R = ΔP/Q = (93 − 3)/5 = 18 mmHg·min/L (× 80 ≈ 1440 dyn·s/cm⁵).

## Why is pulmonary vascular resistance about one-sixth of systemic resistance?

Both circuits carry the same flow, but the pulmonary pressure drop is ≈ 1/6 of the systemic drop (≈ 10–14 vs ≈ 90 mmHg), because pulmonary vessels are short, wide and thin-walled.

## Why is blood velocity lowest in the capillaries even though each capillary is the narrowest vessel?

Velocity depends on the total cross-sectional area of the segment; the capillaries in parallel have by far the largest total area, so velocity there is the lowest.

## What does a Doppler flowmeter actually measure?

The frequency shift of ultrasound reflected by moving red cells, which gives blood velocity; flow is obtained by multiplying velocity by the vessel's cross-sectional area.

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## Connections

## See it in 3D 5

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

- [Ascending aorta](https://durgical.com/en/anatomy/ascending-aorta/)
- [Abdominal aorta](https://durgical.com/en/anatomy/abdominal-aorta/)
- [Pulmonary arteries](https://durgical.com/en/anatomy/right-pulmonary-artery/)
- [Inferior vena cava](https://durgical.com/en/anatomy/inferior-vena-cava/)
- [Superior vena cava](https://durgical.com/en/anatomy/superior-vena-cava/)

## Related topics 4

- [Flow, pressure and resistance: Poiseuille and turbulent flow](https://durgical.com/en/physiology/flow-pressure-and-resistance-poiseuille-and-turbulent-flow/)
- [Vascular compliance and capacitance](https://durgical.com/en/physiology/vascular-compliance-and-capacitance/)
- [The pulmonary circulation and hypoxic vasoconstriction](https://durgical.com/en/physiology/the-pulmonary-circulation-and-hypoxic-vasoconstriction/)
- [Systolic, diastolic, mean and pulse pressure](https://durgical.com/en/physiology/systolic-diastolic-mean-and-pulse-pressure/)

## Related conditions 3

- [Arterial hypertension](https://durgical.com/en/conditions/arterial-hypertension/)
- [Pulmonary hypertension and cor pulmonale](https://durgical.com/en/conditions/pulmonary-hypertension-and-cor-pulmonale/)
- [Peripheral artery disease](https://durgical.com/en/conditions/peripheral-artery-disease/)

[← 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: [Anatomy, Thorax, Heart Great Vessels](https://www.ncbi.nlm.nih.gov/books/NBK547680/) (StatPearls (NCBI Bookshelf), 2024); [Peripheral Arterial Disease](https://www.ncbi.nlm.nih.gov/books/NBK430745/) (StatPearls (NCBI Bookshelf), 2024); [Doppler ultrasound exam of an arm or leg: MedlinePlus Medical Encyclopedia](https://medlineplus.gov/ency/article/003775.htm) (MedlinePlus, US National Library of Medicine, 2026).

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