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Transport, exchange & defense · Topic 13 of 18

Blood: Components & Blood Types

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Anatomy · blood

Blood: Components & Blood Types

Blood is plasma plus red cells (carry O₂), white cells (defend) and platelets (clot); antigens on red cells set the ABO and Rh type.

01 · CompositionPlasma ~55% · formed elements ~45%
02 · Red cellsRBCs carry O₂ on hemoglobin
03 · TransfusionO− universal donor · AB+ universal recipient
01

What it is

Blood = plasma (~55%) + formed elements: RBCs, WBCs, platelets must know
02

Parts & jobs

RBCs (erythrocytes) carry O₂ on hemoglobin; no nucleus when mature must know
WBCs (leukocytes) defend the body; they have a nucleus and leave vessels must know
Platelets: cell fragments (from megakaryocytes) that plug leaks and clot must know
Plasma ≈ 92% water; proteins: albumin, globulins, fibrinogen
Neutrophils: most common WBC (50–70%); fast responders that engulf bacteria
Lymphocytes (20–30%): B cells, T cells and NK cells
Monocytes: largest WBC; become macrophages in tissues
Eosinophils (2–4%): fight parasitic worms; rise in allergies
Basophils (<1%): rarest WBC; release histamine and heparin
03

Step by step

All blood cells are made in red bone marrow (hematopoiesis)
Hemostasis: vascular spasm → platelet plug → coagulation (clot)
04

Know by heart

Blood pH 7.35–7.45; adult volume about 4–6 L (~8% of body weight)
05

The rules

ABO type = antigens on RBCs: A, B, both (AB) or neither (O) must know
WBC order: neutrophils > lymphocytes > monocytes > eosinophils > basophils
Universal donor: O− · universal recipient: AB+
Plasma holds antibodies to the ABO antigens you lack (A → anti-B; O → both)
Rh+ = has the D antigen; Rh− makes anti-Rh only after exposure
06

Red cells or white cells?

Commonly confused
Red blood cells (RBCs)White blood cells (WBCs)
Main jobCarry O₂ on hemoglobinDefend against pathogens and abnormal cells
Nucleus (mature cell)NoneYes
How manyMost numerous formed elementAbout 5,000–10,000 per µL
Where they workInside blood vesselsLeave vessels to work in tissues
07

More detail

Show 8 more facts
Hematocrit = percent of a blood sample that is red cells (~45%)
Hemoglobin: 4 globin chains + 4 iron hemes → up to 4 O₂
RBCs live ~120 days; macrophages (spleen, liver) recycle them
Low O₂ → kidneys release EPO → marrow makes more RBCs
Granular: neutrophil, eosinophil, basophil · agranular: lymphocyte, monocyte
Clotting factors → thrombin → fibrinogen becomes a fibrin mesh
Mismatched transfusion → agglutination (clumping) and hemolysis
Rh− parent + Rh+ baby: the risk falls on the next Rh+ pregnancy
Pearl

Never Let Monkeys Eat Bananas: neutrophils, lymphocytes, monocytes, eosinophils, basophils, from most to least common.

Sources · OpenStax Anatomy & Physiology 2e §18.1 An Overview of Blood · OpenStax Anatomy & Physiology 2e §18.2 Production of the Formed Elements · OpenStax Anatomy & Physiology 2e §18.3 Erythrocytes · OpenStax Anatomy & Physiology 2e §18.4 Leukocytes and Platelets · OpenStax Anatomy & Physiology 2e §18.5 Hemostasis · OpenStax Anatomy & Physiology 2e §18.6 Blood Typing · OpenStax Biology 2e §40.2 Components of the Blood (plasma 55%, O− donor, AB+ recipient) · Union Test Prep — HESI A2 Anatomy & Physiology study guide, page 5 (blood, blood types) · NurseHub — HESI A2 Anatomy & Physiology review course (lesson list: The Blood, Blood Types, Hemostasis) · Built from the verified NurseSavvy HESI A2 fact map (facts reviewed 2026-10-16). HESI is a trademark of Elsevier; NurseSavvy is not affiliated with Elsevier.

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