Circulatory system

Introduction
Main features of circulatory systems
- General features of circulation
- Body fluids
- Fluid compartments
Invertebrate circulatory systems
- Basic physicochemical considerations
- Animals without independent vascular systems
- Vascular systems
  - Blood
  - Hearts
- Acoelomates and pseudocoelomates
- Coelomates
  - Annelida
  - Echiura
  - Mollusca
  - Brachiopoda
  - Arthropoda
  - Echinodermata
  - Hemichordata
  - Chordata
The vertebrate circulatory system
- The basic vertebrate pattern
  - The plan
    - The heart
    - The blood vessels
  - Evolutionary trends
    - Circulation in agnathans
    - Circulation in jawed vertebrates
- Modifications among the vertebrate classes
  - Fishes
  - Amphibians
  - Reptiles
  - Birds
  - Mammals
- Embryonic development of the circulatory system
- Biodynamics of vertebrate circulation
  - Blood pressure and blood flow
  - Electrical activity
  - Control of heartbeat and circulation

References & Edit History Related Topics

Images, Videos & Interactives

Silhouette diagram of the human body highlighting the heart and major blood vessels, with labeled arteries in red and veins in blue to show the structure of the cardiovascular system.

Diagram showing the human circulatory system with arteries and veins, highlighting the heart, and a detailed cross-section of the heart illustrating blood flow pathways.

Diagram of the human circulatory system showing major arteries in red and veins in blue, with labeled pathways for arterial supply and venous drainage to organs including the heart, lungs, liver, kidneys, intestines, and lower limbs.

Gloved hands hold a blood sample tube above a rack of vials in a laboratory, illustrating blood analysis in human body science.

Illustration of a kidney in cross section showing blood vessels entering and exiting, urine formation, and a detailed callout of a nephron where blood is filtered and urine is produced.

The human heart and cardiovascular system explained

Understanding the human cardiovascular system

Learn about the oxygen-transporting function and molecular structure of the protein hemoglobin

At a Glance

circulatory system summary

Quizzes

3d illustration human heart. Adult Anatomy Aorta Black Blood Vessel Cardiovascular System Coronary Artery Coronary Sinus Front View Glowing Human Artery Human Heart Human Internal Organ Medical X-ray Myocardium

Human Organs

Superficial arteries and veins of face and scalp, cardiovascular system, human anatomy, (Netter replacement project - SSC)

The Human Body

Related Questions

How are arteries different from veins in the human circulatory system?
How did separate pulmonary and systemic circuits improve oxygen transport in birds and mammals?
How do arterioles control the amount of blood entering capillary networks?
How do blood pressure and blood vessel diameter affect blood flow?
How do capillaries connect arteries and veins?
How do capillaries exchange oxygen, nutrients, hormones, and wastes with body tissues?
How do electrical signals travel from the sinoatrial node through the heart?
How do heart valves prevent blood from flowing backward?
How do skeletal muscle contractions help return blood through veins?
How do the atria and ventricles work together to move blood through the heart?
How do the foramen ovale and ductus arteriosus function before birth?
How do the pulmonary and aortic valves control blood leaving the heart?
How do the right and left ventricles differ in the pressure they produce?
How do the three layers of a vein wall differ from those of an artery?
How do three-chambered hearts in amphibians differ from four-chambered hearts in mammals and birds?
How do valves in leg veins help blood move toward the heart?
How does a closed circulatory system differ from an open circulatory system?
How does blood flow change after a meal and during exercise?
How does blood gain oxygen and release carbon dioxide in the lungs?
How does breathing help blood return to the heart through veins?
How does changing arteriole diameter direct blood flow to different organs?
How does circulation in fish differ from circulation in mammals?
How does oxygen-rich blood travel from the lungs to the left atrium?
How does pulmonary circulation move blood between the heart and lungs?
How does the aorta distribute blood from the left ventricle throughout the body?
How does the circulatory system help maintain homeostasis in the body?
How does the endothelium help regulate movement between blood and body tissues?
How does the heart act as a muscular pump?
How does the human cardiovascular system remove carbon dioxide and other wastes from tissues?
How does the human cardiovascular system transport oxygen and nutrients to body tissues?
How does the mitral valve control blood flow on the left side of the heart?
How does the sinoatrial node set the rhythm of a mammalian heart?
How does the tricuspid valve control blood flow on the right side of the heart?
How does venous blood from the heart muscle return to the right atrium?
How is the pulse produced in the arteries?
What are the three layers in the wall of an artery?
What are the two major circuits of the human cardiovascular system?
What causes the first and second heart sounds?
What changes in circulation occur at birth when the lungs begin to exchange gases?
What does stroke volume measure in the heart?
What happens during systole and diastole in the cardiac cycle?
What role does the endothelium play inside blood vessels?
What route does blood take through the heart, lungs, and body during a complete circulation?
Which parts of the heart receive blood from the left anterior descending coronary artery?
Why are capillary walls only one cell thick?
Why do arteries have thick, elastic walls?
Why do pulmonary arteries carry oxygen-poor blood?
Why do pulmonary veins carry oxygen-rich blood?
Why do veins have thinner walls and larger internal spaces than arteries?
Why does pulmonary circulation operate at a lower pressure than systemic circulation?
Why does the heart need its own coronary blood supply?
Why does the left side of the heart pump blood to the body?
Why does the right side of the heart pump blood to the lungs?
What is the function of capillaries?
How do veins help move blood against gravity?
What is angiogenesis?
What are some common blood vessel-related conditions?
What is systemic circulation?
What happens at the capillary level in systemic circulation?
How is blood flow regulated in systemic circulation?
What are some disorders that affect systemic circulation?
Where does gas exchange occur in pulmonary circulation?
What are some disorders affecting pulmonary circulation?

Health & Medicine Anatomy & Physiology

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External Websites

Washington University School of Medicine in St. Louis - Department of Surgery - Demystifying the Circulatory System
Healthdirect - Circulatory system
UH Pressbooks - Biology - Overview of the Circulatory System
The Nemours Foundation - For Teens - Heart and Circulatory System
Open Library Publishing Platform - Neuroscience: Canadian 1st Edition - The Circulatory System
Healthline - What Makes Up Your Circulatory System and How Does It Work?
National Center for Biotechnology Information - Physiology, Pulmonary Circulatory System
Cleveland Clinic - Circulatory System
Live Science - 11 Surprising Facts About the Circulatory System

Britannica Websites

Articles from Britannica Encyclopedias for elementary and high school students.

circulatory system - Children's Encyclopedia (Ages 8-11)

Circulatory system

anatomy

Written by

Bernard Edward Matthews

Lecturer in Zoology, University of Edinburgh. Coauthor of Biology of Nematodes.

Bernard Edward MatthewsAll

Fact-checked by

Britannica Editors

Encyclopaedia Britannica's editors oversee subject areas in which they have extensive knowledge, whether from years of experience gained by working on that content or via study for an advanced degree....

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Last updated

Nov. 6, 2026 •History

Circulatory system, network of tissues, blood vessels, lymph vessels, and supporting components that transports nutrients, respiratory gases, and metabolic products throughout a living organism, permitting integration among the various tissues. The process of circulation includes the intake of metabolic materials, the conveyance of these materials throughout the organism, and the return of harmful by-products to the environment. Invertebrates have a great variety of liquids, cells, and modes of circulation. Many invertebrates have what is called...

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