The circulatory system
Identify and name the main parts of the human circulatory system and describe the functions of the heart, blood vessels, and blood
What a learner can do afterwards
- Name the main components: heart, arteries, veins, capillaries, blood
- Describe the heart as a pump that pushes blood around the body in a continuous loop
- Explain that blood carries oxygen and nutrients to cells and removes waste products like carbon dioxide
1 · Read
You have learned the names artery, vein, and capillary. These are the three main types of blood vessel, the tubes blood travels through. The circulatory system has three main parts: the heart, blood vessels, and blood. The heart is a muscular pump. It squeezes to push blood through vessels.
Arteries carry blood away from the heart. Veins carry blood toward it. In the body's tiny capillaries, oxygen and nutrients pass from blood through thin walls to nearby cells. Carbon dioxide, a waste gas made by cells, passes into the blood. Blood carries supplies to cells and takes waste away.
When Maya runs, her working muscles need more oxygen. Her heart beats faster and sends more blood to them. At body capillaries, blood delivers oxygen and nutrients and collects carbon dioxide from the cells. Veins return this blood to the heart. The heart then pumps it to the lungs, where carbon dioxide leaves the blood to be breathed out.
Follow the carbon dioxide: from muscle cells into blood, through the heart, then to the lungs.
Blood makes two linked loops. One goes from the heart to the lungs and back. In the lungs, blood loses carbon dioxide and gains oxygen. The other goes from the heart to the rest of the body and back. Each loop returns blood to the heart, ready to be pumped onward.
The lung loop and the body loop each begin and end at the heart. Blood keeps moving through both.
The heart pumps blood through lung and body loops. Arteries carry it away, veins bring it back, and capillaries let supplies and waste pass between blood and cells.
Original circulatory-system lesson, questions and flashcards written for this dataset.
Five original readings written for this dataset. Textbook records were consulted for facts; their prose and figures are not reproduced.
More to read
How food and air reach body cells
A sandwich stays in the digestive tract while it is broken down. Many of its nutrients then pass through the small intestine's wall into blood. The sandwich itself does not flow through blood vessels.
Oxygen takes another route. It enters blood in the lungs. The heart pumps blood carrying oxygen and nutrients toward body cells.
At body capillaries, these supplies pass toward nearby cells. Carbon dioxide moves from the cells into blood to be carried away. Digestion, breathing and circulation help with different parts of this journey.
Squeezing and filling
The heart needs blood inside it before it can pump blood out. Its muscle alternates between tightening and relaxing. Tightening pushes blood into arteries. Relaxing lets the heart fill again.
Veins return blood to the heart. The returning blood lets the heart keep pumping while you rest, sleep or move.
During exercise, working muscles need more oxygen. A faster heartbeat helps send more blood to them. The heart supplies the push; oxygen enters the blood in the lungs.
An artery is named by direction
Imagine tracing a vessel from the heart toward the lungs. Blood going along that route has not yet collected oxygen in the lungs. It is still travelling through arteries because it is moving away from the heart.
After the exchange in the lungs, blood has more oxygen. Veins carry it back toward the heart.
This example helps separate two questions: which way is the blood moving, and how much oxygen does it carry? Arteries lead away from the heart. Veins lead toward it. Their names describe direction, not oxygen content.
What travels in blood?
Blood contains a liquid called plasma. Dissolved nutrients travel in this liquid, along with blood cells and platelets.
Red blood cells carry oxygen. White blood cells help fight infection. Platelets are small pieces of cells. When a vessel is damaged, platelets help form a clot that reduces bleeding.
Blood also carries carbon dioxide toward the lungs and spreads heat around the body. Its different parts help with different jobs, so blood does more than transport oxygen.
Two routes meet at the heart
Choose the lungs as a starting point. Blood gains oxygen there and releases carbon dioxide. It returns to the left side of the heart, which pumps it toward the rest of the body.
At body capillaries, oxygen passes to nearby cells and carbon dioxide enters blood. Veins then return blood to the right side of the heart. That side pumps it toward the lungs.
The heart sits between the lung route and the body route. Blood passes through it twice during a journey through both loops. There is no single travel time that fits every person and every activity.
2 · Watch
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