blood

Introduction & Top Questions
Blood components
- Plasma
- Blood cells
  - Red blood cells (erythrocytes)
    - Hemoglobin
    - Production of red blood cells (erythropoiesis)
    - Destruction of red blood cells
  - White blood cells (leukocytes)
    - Granulocytes
      Neutrophils
      Eosinophils
      Basophils
    - Monocytes
    - Lymphocytes
  - Platelets (thrombocytes)
Functions of blood
- Respiration
- Nutrition
- Excretion
- Immunity
- Temperature regulation
- Hemostasis
Laboratory examination of blood

References & Edit History Related Topics

Images & Videos

Observe a red blood cell as it travels from the heart to the lungs and other body tissues

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

Close-up of a medical blood bag filled with red blood, showing attached tubes and connectors against a blurred background.

Illustration of a blood vessel with a cutaway view showing red blood cells, white blood cells, platelets, and plasma, each labeled to highlight the main components of blood.

Close-up illustration of multiple human red blood cells with their characteristic disc shape, floating against a red background.

Diagram of the hemoglobin tetramer showing two alpha and two beta subunits, each in a different color, with four heme groups labeled, each containing a central iron atom for oxygen binding.

Learn about sickle cell anemia and blood behavior in sickle cell patients

Labeled illustration of a macrophage showing its internal organelles and the process of engulfing and digesting bacteria, with bacteria drawn outside and inside the cell as it undergoes phagocytosis.

At a Glance

blood summary

Quizzes

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

The Human Body

Related Questions

How are old red blood cells removed and recycled by the spleen and liver?
How are plasma and serum different?
How can incompatible ABO blood cause a dangerous transfusion reaction?
How do antibodies help prevent reinfection by the same microorganism?
How do granulocytes and monocytes remove bacteria and damaged cells?
How do low-salt and high-salt solutions affect red blood cells?
How do plasma proteins help keep water inside blood vessels?
How do plasma proteins transport substances such as iron and cholesterol?
How do red blood cells squeeze through capillaries narrower than themselves?
How do the lungs change the oxygen and carbon dioxide content of blood?
How do white blood cells protect the body from infectious organisms?
How does 2,3-DPG help red blood cells deliver oxygen to tissues?
How does blood carry hormones from endocrine glands to target tissues?
How does blood carry nutrients absorbed from the digestive tract?
How does blood help keep its pH near 7.4?
How does blood help maintain homeostasis in the human body?
How does blood help regulate body temperature?
How does blood remove carbon dioxide from body tissues?
How does hemoglobin carry oxygen from the lungs to body tissues?
How does insulin affect the level of glucose in the blood?
How does low serum albumin lead to edema or swelling?
How does pH affect hemoglobin's ability to release oxygen?
How does plasmin help remove blood clots after healing?
How does sweating work with blood circulation to cool the body?
How does the complement system damage invading microorganisms?
How does the hypothalamus control heat loss through blood vessels in the skin?
How is blood glucose kept within a safe range?
How is iron from old red blood cells reused to make new hemoglobin?
How long do human red blood cells usually live?
How much blood does an average adult human body contain?
How much of blood plasma is water?
What are the main components of human blood?
What does a hematocrit test measure in a blood sample?
What happens during hemolysis of red blood cells?
What is the Bohr effect in oxygen transport by blood?
What percentage of human blood is made up of red blood cells?
What substances are carried in blood plasma?
Where are new red blood cells made in the human body?
Which organs help maintain the chemical composition of blood?
Which red blood cell types can be donated to people with type O blood?
Why can an unusual hematocrit level be an early sign of disease?
Why do mature human red blood cells lack a nucleus?
Why do people living at high altitudes tend to have more red blood cells?
Why does blood plasma contain fibrinogen while serum does not?
Why does rising carbon dioxide in the blood create an urge to breathe?
Why does warm blood flow to the skin when the body needs to lose heat?
Why is Rh-positive blood usually avoided for Rh-negative recipients?
Why is a steady supply of oxygen in the blood especially important for the brain?
Why is blood circulation essential for keeping body cells alive?
Why must donor and recipient blood types be matched before a transfusion?
What are the four main types of tissue in the human body?
What are B cells?
What is a white blood cell?
What are the major classes of white blood cells?
What is a healthy white blood cell count?

Health & Medicine Anatomy & Physiology

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External Websites

Interesting Facts - Mosquitos May Prefer a Certain Blood Type
Thompson Rivers University - Human Biology - Blood
Johns Hopkins Medicine - Facts About Blood
National Center for Biotechnology Information - What does blood do?
MSD Manual - Consumer Version - Overview of Blood
Cleveland Clinic - Blood

Britannica Websites

Articles from Britannica Encyclopedias for elementary and high school students.

blood - Children's Encyclopedia (Ages 8-11)
blood - Student Encyclopedia (Ages 11 and up)

blood

biochemical fluid

Written by

Robert S. Schwartz

Professor of Medicine, Tufts University, Boston; former Chief, Division of Hematology-Oncology, New England Medical Center.

Robert S. SchwartzAll

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....

Britannica Editors

Last updated

Nov. 11, 2026 •History

blood, fluid that transports oxygen and nutrients to the cells and carries away carbon dioxide and other waste products. Technically, blood is a transport liquid pumped by the heart (or an equivalent structure) to all parts of the body, after which it is returned to the heart to repeat the process. Blood, however, is also type of a tissue, because it is a collection of similar specialized cells that serve particular functions. These cells are...

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