What is the Circulatory System?
The circulatory system moves blood, oxygen, nutrients, and waste throughout the body, powered by the heart's continuous pumping. Blood follows two loops — pulmonary circulation to the lungs and systemic circulation to the rest of the body — through four heart chambers and a network of vessels.
The circulatory system is the heart, blood vessels, and blood that transport oxygen and nutrients to cells and remove waste; the heart's output is measured as cardiac output, CO = HR × SV.
- 1.Right atrium — Receives deoxygenated blood returning from the body via the vena cava.
- 2.Right ventricle — Pumps deoxygenated blood into the pulmonary artery toward the lungs.
- 3.Lungs (pulmonary circulation) — Blood releases CO2 and picks up O2 in the lung capillaries.
- 4.Left atrium — Receives oxygenated blood from the lungs via the pulmonary veins.
- 5.Left ventricle — Pumps oxygenated blood into the aorta toward the whole body (systemic circulation).
- 6.Body tissues — Cells exchange O2 and nutrients for CO2 and waste at the capillaries.
- 7.Vena cava — Deoxygenated blood returns to the right atrium, restarting the cycle.
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Step-by-step worked examples
A resting adult has a heart rate of 72 bpm and a stroke volume of 70 mL. Find cardiac output.
CO = HR × SV CO = 72 × 70 CO = 5040 mL/min ≈ 5.04 L/min
During exercise, heart rate rises to 150 bpm and stroke volume to 100 mL. Find cardiac output.
CO = HR × SV CO = 150 × 100 CO = 15,000 mL/min = 15 L/min
A trained athlete at rest has HR = 60 bpm and SV = 50 mL. Find cardiac output.
CO = HR × SV CO = 60 × 50 CO = 3000 mL/min = 3 L/min
Flashcards
Quick quiz
Q1.If HR = 80 bpm and SV = 60 mL, what is the cardiac output?
Q2.Which heart chamber pumps oxygenated blood to the whole body?
Q3.Which blood vessel is the exception that carries deoxygenated blood away from the heart?
Q4.Where does gas exchange between blood and body cells occur?
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Common mistakes
Arteries always carry oxygenated blood. — Correct: Most arteries do, but the pulmonary artery carries deoxygenated blood from the heart to the lungs.
The heart has only two chambers. — Correct: The heart has four chambers: two atria (upper) and two ventricles (lower).
Veins always carry deoxygenated blood. — Correct: Most veins do, but the pulmonary vein carries oxygenated blood from the lungs to the heart.
Cardiac output only depends on heart rate. — Correct: Cardiac output depends on both heart rate AND stroke volume: CO = HR × SV.
FAQ
What is the formula for cardiac output?
CO = HR × SV, where HR is heart rate (beats/min) and SV is stroke volume (mL/beat).
What is the circulatory system?
The heart, blood vessels, and blood that together transport oxygen, nutrients, and waste throughout the body.
How to calculate cardiac output?
Multiply heart rate by stroke volume: CO = HR × SV, typically measured in mL/min or L/min.
What are examples of circulatory system function?
Resting cardiac output (~5 L/min), exercise cardiac output rising to 15+ L/min, and blood pressure regulation are all examples of circulatory function.