A heart about the size of a fist contracts roughly 100,000 times a day, pushing blood out with each beat.
Pulmonary circulation runs from the right ventricle to the lungs and back to the left atrium; systemic circulation runs from the left ventricle around the body and back to the right atrium.
In the diagram, blue vessels carry CO₂-rich venous blood and red vessels carry O₂-rich arterial blood, and the color flips from blue to red as blood passes through the lungs.
The cross-sections underneath set an artery's thick wall, a capillary's single-cell wall and a vein's wide channel side by side.
A Pump That Never Rests
❤️ Why the Left Ventricle Has the Thickest Wall
①The heart has four chambers: right atrium, right ventricle, left atrium and left ventricle
②Atria receive blood; ventricles contract and send it out
③The left ventricle has to push blood all the way around the body, so its muscular wall is the thickest
④Valves between atria and ventricles, and between ventricles and arteries, keep blood moving one way
Two Circulation Loops
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Front-view diagram of the heart's four chambers with the pulmonary and systemic routes
Pulmonary circulation
Right ventricle → pulmonary artery → lungs → pulmonary veins → left atrium
In the lung capillaries blood gives up CO₂ and picks up O₂, so venous blood becomes arterial blood
Systemic circulation
Left ventricle → aorta → body → venae cavae → right atrium
In the body's capillaries blood hands O₂ and nutrients to cells and collects CO₂ and wastes, so arterial blood becomes venous blood
Three Kinds of Blood Vessels
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Cross-sections of an artery, a capillary and a vein
💡 Why Each Vessel Has a Different Wall
①Artery: a thick, elastic wall that withstands the high pressure from the ventricles
②Capillary: a wall only one cell thick, so substances pass between blood and tissue cells
③Vein: low pressure, so the wall is thin, and valves along the way stop blood from flowing backward
④Blood pressure falls from arteries to capillaries to veins, and blood moves slowest in the capillaries
Plasma carries dissolved nutrients, CO₂ and wastes; red blood cells carry O₂, white blood cells fight germs, and platelets help blood clot
Summary
Circulation at a Glance
Pulmonary: right ventricle → lungs → left atrium / Systemic: left ventricle → body → right atrium
The two atria contract together, then the two ventricles contract together, so both loops advance side by side, one beat at a time
🎯 Common Test Points
①Every loop starts in a ventricle and ends in an atrium
②The pulmonary artery carries venous blood and the pulmonary veins carry arterial blood
③Vessels are named by direction: away from the heart (artery) or toward it (vein)
④The left ventricle has the thickest wall
⑤Exchange of substances happens only in capillaries
Examples and Unit Review
Example 1
Blood leaves the right ventricle. Which path does it take to reach the left atrium, and how does the blood change on the way?
1
When the right ventricle contracts, blood travels through the pulmonary artery to the lungs.
2
In the capillaries around the alveoli it releases CO₂ and takes in O₂, then returns through the pulmonary veins to the left atrium.
▸ Right ventricle → pulmonary artery → lungs → pulmonary veins → left atrium; venous blood becomes arterial blood on the way
Pressing the Pulmonary button in the diagram leaves only this route lit.
Example 2
Does the pulmonary artery carry arterial or venous blood, and how can you tell?
1
Arteries and veins are named by whether the vessel carries blood away from the heart or toward it. The pulmonary artery leaves the right ventricle, so it is an artery.
2
The blood inside has just come back from the body through the right atrium and right ventricle, so it is CO₂-rich venous blood.
▸ Venous blood: the vessel is an artery, but the blood in it is venous
Vessel names (artery, vein) and blood types (arterial, venous) use different criteria. The pulmonary veins are the reverse case: they carry arterial blood.
Unit Test
Which statement about blood circulation is correct?
①Pulmonary circulation starts at the left ventricle
②Arterial blood flows in the pulmonary artery
③Systemic circulation starts at the left ventricle and returns to the right atrium
④Capillaries have thick walls that aid material exchange
⑤Red blood cells are in charge of immunity
▸ ③ Systemic circulation starts at the left ventricle and returns to the right atrium
1
Pulmonary circulation starts at the right ventricle (① wrong), and venous blood flows in the pulmonary artery (② wrong).
2
Capillaries have very thin walls that ease exchange (④ wrong), and white blood cells handle immunity while red cells carry O₂ (⑤ wrong); systemic circulation runs left ventricle to body to right atrium (③ correct).