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Human Body Study

Blood

The fluid the cardiovascular system circulates — classed as a connective tissue, despite being liquid. An adult has about 5 liters.

The closed-loop circulation A four-chambered heart at the centre, with a pulmonary loop to the lungs above and a systemic loop to a body capillary bed below; red vessels carry oxygen-rich blood, blue vessels oxygen-poor. A separate inset shows blood at a vessel cross-section. Each label links to the article for that part. Blood RA LA RV LV Heart Arteries Capillaries Veins The closed-loop circulation — schematic, not to scale.

Components

Section titled “Components”
  • Plasma — the straw-colored fluid (mostly water) that carries everything else.
  • Red blood cells — carry oxygen using the protein hemoglobin.
  • White blood cells — defend against infection.
  • Platelets — cell fragments that trigger clotting.

What it does

Section titled “What it does”
  • Transport — oxygen, nutrients, hormones, heat, and waste.
  • Defense — white cells and antibodies.
  • Clotting — seals damaged vessels.

Key idea

Section titled “Key idea”

Blood is the body’s universal delivery and courier service; its cells are made in the bone marrow and constantly renewed.