Blood is a living tissue that moves through vessels to connect every part of the body. It carries oxygen, nutrients, hormones, heat, wastes, and immune cells, so it is essential for homeostasis. Although blood looks like one red fluid, it is a mixture of plasma, red blood cells, white blood cells, and platelets.
Understanding these components helps explain transport, immunity, clotting, and many medical tests.
Key Facts
- Blood is about 55% plasma and 45% formed elements by volume.
- Plasma is mostly water and carries dissolved proteins, nutrients, hormones, gases, and wastes.
- Red blood cells contain hemoglobin, which binds oxygen in the lungs and releases it to tissues.
- White blood cells defend the body against pathogens and remove damaged cells.
- Platelets help form clots by sticking to damaged blood vessels and activating clotting proteins.
- Most blood cells are made by hematopoiesis in red bone marrow.
Vocabulary
- Plasma
- Plasma is the pale yellow liquid part of blood that carries cells, proteins, nutrients, hormones, and wastes.
- Red blood cell
- A red blood cell is a flexible cell packed with hemoglobin that transports oxygen and some carbon dioxide.
- White blood cell
- A white blood cell is an immune cell that helps detect, attack, and clean up pathogens or damaged tissue.
- Platelet
- A platelet is a small cell fragment that helps seal breaks in blood vessels and start clot formation.
- Bone marrow
- Bone marrow is soft tissue inside certain bones where most new blood cells are produced.
Common Mistakes to Avoid
- Saying plasma is just water is wrong because plasma also contains important proteins, salts, nutrients, hormones, and wastes.
- Thinking red blood cells fight infections is wrong because their main job is gas transport, while white blood cells handle immune defense.
- Calling platelets full cells is wrong because platelets are cell fragments that come from larger bone marrow cells called megakaryocytes.
- Assuming blood cells are made in the heart is wrong because most blood cell production occurs in red bone marrow, not in the pumping organ.
Practice Questions
- 1 A 5.0 L blood sample is 55% plasma by volume. How many liters of plasma does it contain?
- 2 If formed elements make up 45% of a 6.0 L blood volume, what volume is made of red blood cells, white blood cells, and platelets combined?
- 3 A patient has a low platelet count but normal red and white blood cell counts. Predict which blood function is most directly affected and explain why.