Blood is a fluid tissue that carries oxygen, nutrients, hormones, heat and immune cells around the body. An adult has about 5 liters, roughly 7% of body weight.
Plasma
About 55% of blood is plasma, a straw-colored fluid that is mostly water. It carries proteins such as albumin, antibodies and clotting factors, plus salts, glucose and waste.
Red blood cells
Red cells make up almost all of the other 45%, a share called the hematocrit. They carry oxygen bound to hemoglobin.
White blood cells
White cells fight infection. Neutrophils attack bacteria, lymphocytes make antibodies and kill infected cells, and monocytes, eosinophils and basophils handle other immune jobs.
Platelets
Platelets are small cell fragments that plug holes in blood vessels and start clotting. Too few cause easy bruising and bleeding.
| Component | Main job | Normal adult value |
|---|---|---|
| Hemoglobin | Carries oxygen | About 13.5 to 17.5 g/dL in men, 12 to 15.5 g/dL in women |
| Hematocrit | Share of blood that is red cells | About 41 to 50% in men, 36 to 44% in women |
| White cells | Fight infection | 4 to 11 × 10⁹/L |
| Platelets | Clotting | 150 to 400 × 10⁹/L |
At the bedside
Fill the right tubes in the right order and avoid hemolysis, which falsely raises potassium. When reading a blood count, look at hemoglobin, white cells and platelets first.
Related guides
Written by Making Synapses. Topic reference: OpenStax, Anatomy and Physiology 2e (no text adapted).