What is Circulation and How Do Blood Vessels Work?
Circulation is the continuous movement of blood through a closed network of vessels, driven by the heart, that delivers oxygen and nutrients to tissues and removes waste. Each vessel type — arteries, veins and capillaries — is structurally built for a specific job in that loop.
Arteries carry blood away from the heart under high pressure with thick muscular walls; veins return blood to the heart at low pressure using one-way valves; capillaries are single-cell-thick vessels where gas and nutrient exchange actually happens.
- •Carry blood away from the heart
- •Thick, elastic, muscular walls
- •High pressure, pulsatile flow
- •No valves (except at heart outlets)
- •Usually oxygen-rich (except pulmonary artery)
- •Carry blood back to the heart
- •Thin walls, larger lumen
- •Low pressure, steady flow
- •One-way valves prevent backflow
- •Usually oxygen-poor (except pulmonary vein)
Try it: interactive calculator
Step-by-step worked examples
A patient's blood pressure reads 120/80 mmHg. Calculate their mean arterial pressure (MAP).
MAP = DBP + 1/3(SBP − DBP) MAP = 80 + 1/3(120 − 80) MAP = 80 + 13.33 = 93.33 mmHg
A patient with hypertension has a blood pressure of 140/90 mmHg. Find their MAP.
MAP = 90 + 1/3(140 − 90) MAP = 90 + 1/3(50) MAP = 90 + 16.67 = 106.67 mmHg (elevated)
A patient in shock has a blood pressure of 90/60 mmHg. Is their MAP enough to perfuse vital organs (minimum ~60 mmHg needed)?
MAP = 60 + 1/3(90 − 60) MAP = 60 + 10 = 70 mmHg 70 mmHg is above the 60 mmHg minimum, so organ perfusion is still adequate, though it should be monitored closely
Flashcards
Quick quiz
Q1.Which vessel type has the thickest, most muscular walls to withstand high pressure?
Q2.What structural feature lets capillaries carry out gas and nutrient exchange?
Q3.A patient has a blood pressure of 130/85 mmHg. What is their MAP (to the nearest whole number)?
Q4.What structure prevents backflow of blood in the veins of the legs?
The full card deck, worked steps and AI-tutor support for “What is Circulation and How Do Blood Vessels Work?” are in Notek — study by hand before your exam.
Common mistakes
Arteries always carry oxygenated blood. — Correct: Arteries carry blood away from the heart — the pulmonary artery is an exception and carries deoxygenated blood to the lungs.
Veins have thicker walls than arteries because they hold more blood at once. — Correct: Veins actually have thinner walls; they carry blood at low pressure and rely on valves and skeletal muscle contraction, not wall thickness, to keep blood moving.
Blood pressure is roughly the same throughout the circulatory system. — Correct: Pressure drops progressively as blood moves from arteries → arterioles → capillaries → venules → veins.
Capillaries are where blood pressure and flow speed are highest. — Correct: Capillaries have the lowest pressure and slowest flow of any vessel type, which is exactly what allows time for exchange.
FAQ
What is mean arterial pressure (MAP)?
MAP is the average pressure driving blood through the arteries during one cardiac cycle, calculated as MAP = DBP + 1/3(SBP − DBP). A MAP above ~60 mmHg is generally needed to perfuse vital organs.
What is the difference between arteries and veins?
Arteries carry blood away from the heart under high pressure with thick muscular walls; veins carry blood back to the heart at low pressure and use valves to prevent backflow.
How does blood flow through the circulatory system?
Blood flows heart → arteries → arterioles → capillaries (exchange happens here) → venules → veins → back to the heart, with pressure dropping at each stage.
Why do veins have valves but arteries generally don't?
Veins move blood against gravity at low pressure, so valves are needed to stop backflow; arteries have high forward pressure from the heart, so backflow isn't a risk.