Human Anatomy: Blood – Cells, Plasma, Circulation, and More

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Illustration of blood

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Blood is a constantly circulating fluid providing the body with nutrition, oxygen, and waste removal. Blood is mostly liquid, with numerous cells and proteins suspended in it, making blood “thicker” than pure water. The average person has about 5 liters (more than a gallon) of blood.

A liquid called plasma makes up about half of the content of blood. Plasma contains proteins that help blood to clot, transport substances through the blood, and perform other functions. Blood plasma also contains glucose and other dissolved nutrients.

About half of blood volume is composed of blood cells:

• Red blood cells, which carry oxygen to the tissues
• White blood cells, which fight infections
• Platelets, smaller cells that help blood to clot

Blood is conducted through blood vessels (arteries and veins). Blood is prevented from clotting in the blood vessels by their smoothness, and the finely tuned balance of clotting factors.

Blood Conditions

  • Hemorrhage (https://www.webmd.com/a-to-z-guides/complete-blood-count-cbc
  • Blood smear: Drops of blood are smeared across a microscope slide, to be examined by an expert in a lab. Leukemia, anemia, malaria, and numerous other blood conditions can be identified with a blood smear.
  • Bone marrow biopsy: A thick needle is inserted into a large bone (usually in the hip), and bone marrow is drawn out for tests. Bone marrow biopsy can identify blood conditions that simple blood tests cannot.

Blood Treatments

  • Erythropoietin: A hormone produced by the kidney that stimulates red blood cell production. A manufactured form of erythropoietin can be given to improve the symptoms of anemia.
  • Bloodletting: In people with problems caused by too much blood (such as from hemochromatosis or polycythemia), occasional controlled removal of blood may be necessary.