Anatomy and Physiology 2e · The Cardiovascular System: Blood
An Overview of Blood
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In 30 seconds
Blood is the body's transport and defense system in liquid form. It is a connective tissue because its cells are suspended in a nonliving liquid matrix — the Plasma The liquid matrix of blood (~55% of whole blood; ~92% water) Full entry →. An adult typically carries about 5–6 liters (roughly 8% of body weight), pumped continuously to every tissue.
Blood does three big jobs: it transports oxygen, carbon dioxide, nutrients, hormones, wastes; it regulates body temperature, fluid balance, pH; and it protects — clotting seals leaks, white blood cells and antibodies fight infection. This overview covers blood's composition and sets up the topics that follow.
Why this matters
Blood is the most-sampled tissue in medicine. The complete blood count (CBC) — the routine test of red cells, white cells, and platelets — is ordered for nearly every hospitalized patient; interpreting it starts with the components. Understanding plasma proteins explains why severe protein loss causes swelling (low Albumin Most abundant plasma protein; maintains osmotic pressure, carries small molecules Full entry → means less osmotic pull, so fluid leaves vessels). Understanding components explains why anemia causes fatigue, low platelets cause easy bruising, and transfusion requires blood-type matching (covered later in this chapter). For exams, this topic is foundational and fact-dense: the plasma/formed-element percentages, the three plasma proteins, and which formed element does what.
The college version
Core Concepts
Composition: plasma and formed elements
Centrifuged whole blood separates into layers. The top layer, plasma, is the liquid matrix, about 55% of whole blood. The bottom layer, the Formed elements The cells and cell fragments of blood: RBCs, WBCs, platelets Full entry → (the blood cells), is about 45%. Between them sits the buffy coat — a thin whitish layer of white blood cells and platelets, less than 1% of the total. The red-cell fraction of whole blood is the Hematocrit The red-cell fraction of whole blood (commonly taught ~45%) Full entry →, commonly taught as averaging about 45% in healthy adults, varying by sex, age, and reference laboratory. These are reference concepts, not fixed values — always check the range on a lab report.
Plasma: the liquid matrix
Plasma is about 92% water; the rest is dissolved solutes, the most important being the plasma proteins (about 7–8% of plasma), made mostly by the liver:
- Albumin — the most abundant. It creates the colloid osmotic pressure that keeps water inside the vessels and carries small molecules such as bilirubin, fatty acids, and some drugs.
- Globulins Plasma proteins including antibodies (immunoglobulins) and transporters Full entry → — include the immunoglobulins (antibodies) from plasma cells, which defend against infection, plus transporters that carry lipids, iron, and hormones.
- Fibrinogen Plasma protein converted to fibrin during clotting Full entry → — the clotting protein; during clotting it is converted to fibrin, which forms the mesh of the clot.
Plasma also carries electrolytes, nutrients (glucose, amino acids, lipids), gases, hormones, and wastes destined for the kidneys. Serum Plasma with clotting factors removed Full entry → is plasma minus the clotting factors — what remains after blood has clotted.
The formed elements
- Erythrocytes Biconcave, nucleus-free red blood cells packed with hemoglobin Full entry → (red blood cells) — the most numerous formed element (commonly taught as about 4.5–5.5 million per microliter, varying by sex and range). Biconcave discs packed with Hemoglobin The iron-containing protein in RBCs that binds oxygen Full entry →, the iron-containing protein that carries oxygen; mature RBCs lack a nucleus or organelles, leaving more room for hemoglobin. Their job is oxygen and carbon dioxide transport.
- Leukocytes White blood cells; granulocytes and agranulocytes Full entry → (white blood cells) — the defenders (commonly taught as about 4,500–11,000 per microliter), which leave the bloodstream to fight infection in tissues. Granulocytes: neutrophils (most numerous; phagocytic first responders to bacteria), eosinophils (fight parasites; participate in allergy), basophils (release histamine and heparin, promoting inflammation). Agranulocytes: lymphocytes (adaptive immune cells — B cells make antibodies, T cells kill infected cells) and monocytes, which leave the blood to become macrophages.
- Platelets (thrombocytes) Cell fragments from megakaryocytes Full entry → — not whole cells but fragments of large marrow cells called megakaryocytes. They are the first responders of hemostasis: they stick to damaged vessel walls, aggregate, and help form the platelet plug that starts clotting.
Blood characteristics
Blood is slightly viscous and slightly alkaline — pH commonly taught as about 7.35–7.45 — at about 38°C (100.4°F). Its color reflects oxygen content: oxygen-rich (arterial) blood is bright red, oxygen-poor (venous) blood darker red. Venous blood is not blue — surface veins just look blue from light scattering through skin.
Common Confusions
| Do Not Confuse | With | Difference |
|---|---|---|
| Plasma | Serum | Plasma includes clotting factors; serum is what remains after clotting removes them |
| Platelets | Whole cells | Platelets are fragments pinched off from megakaryocytes, not complete cells |
| Venous blood color | Blue | Oxygen-poor blood is darker red; veins look blue from light scattering — blood is never blue inside the body |
| Hematocrit | Hemoglobin | Hematocrit is the packed red-cell fraction of whole blood; hemoglobin is the oxygen-carrying protein inside those cells |
| Neutrophils | Lymphocytes | Neutrophils are first-responder phagocytes against bacteria; lymphocytes (B and T cells) run the slower, specific adaptive response |
| Blood is "just a liquid" | Blood is a connective tissue | Blood's cells are suspended in an extracellular matrix (plasma), the defining feature of connective tissue |
| RBCs are typical cells | Mature RBCs are specialized | Mature erythrocytes lack a nucleus and most organelles, maximizing space for hemoglobin — they cannot divide |
Eli explains
The same idea, in plain words
Explain it like I’m 10
Blood is like a delivery-and-protection river inside you. The liquid part (plasma) is the water, carrying food and trash. The red cells are delivery trucks carrying oxygen. The white cells are police and repair crews that fight germs. The platelets are like sandbags that plug leaks when you get a cut.
Worked example
A nurse spins a blood sample in a centrifuge; the tube shows three layers, each telling a story. The clear yellowish top layer — plasma — holds water, electrolytes, glucose, hormones, and proteins; a milky look may indicate excess lipids (a flag for review, not diagnosis). The thin gray-white buffy coat sits atop the red layer; a thick one suggests a high white cell count, common during infection. The deep red bottom layer is the erythrocytes — its height is the hematocrit. If the red layer is thin relative to plasma, the person may have too few red cells (anemia) and likely feels fatigued from poor oxygen delivery. Left to stand without anticoagulant, the blood clots: fibrinogen converts to fibrin, and the separated fluid is now serum — plasma minus clotting factors — what many chemistry tests actually measure.
Key takeaways
- Composition: plasma ~55% (mostly water + plasma proteins), formed elements ~45% (RBCs), buffy coat <1% (WBCs + platelets); hematocrit ≈ RBC fraction, commonly taught ~45%.
- Plasma proteins: albumin (osmotic pressure/transport), globulins (antibodies/transport), fibrinogen (clotting → fibrin). Serum = plasma minus clotting factors.
- Erythrocytes: most numerous; biconcave, no nucleus; carry oxygen via hemoglobin.
- Leukocytes: granulocytes (neutrophils, eosinophils, basophils) and agranulocytes (lymphocytes, monocytes → macrophages); immune defense.
- Platelets: fragments of megakaryocytes; start hemostasis by forming the platelet plug.
- Functions: transport, regulation (pH, temperature, fluid balance), protection (clotting + immunity).
- Characteristics: slightly alkaline (pH ~7.35–7.45), ~38°C; venous blood is darker red, not blue.
- Blood is a connective tissue — cells suspended in a liquid extracellular matrix (plasma).
Check yourself
6 review questions from the chapter. Try each one, then open the answer.
What are the three layers after centrifuging whole blood, and roughly what percentage is plasma?
Show answer
Top: plasma (~55% of whole blood). Middle: buffy coat of white blood cells and platelets (<1%). Bottom: packed erythrocytes (~45%; the hematocrit).
Name the three plasma proteins and one job of each.
Show answer
Albumin maintains osmotic pressure (keeping water in vessels) and transports small molecules; globulins include antibodies and transport proteins; fibrinogen is converted to fibrin to form clots.
Which formed element is a cell fragment, and what is its main role?
Show answer
Platelets (thrombocytes) are fragments of megakaryocytes; they stick to damaged vessel walls and form the platelet plug that begins hemostasis.
What is the difference between plasma and serum?
Show answer
Plasma is the liquid portion of unclotted blood and includes clotting factors; serum is what remains after clotting — plasma minus those factors.
List the five leukocyte types and their two groups.
Show answer
Granulocytes: neutrophils, eosinophils, basophils. Agranulocytes: lymphocytes and monocytes (which become macrophages in tissues).
Why is blood classified as a connective tissue?
Show answer
Blood is a connective tissue because its cells (formed elements) are suspended in an extracellular matrix — the liquid plasma.
Study tools & related lessonsKey vocabulary · Related
Key vocabulary
- Plasma
- The liquid matrix of blood (~55% of whole blood; ~92% water)
- Formed elements
- The cells and cell fragments of blood: RBCs, WBCs, platelets
- Hematocrit
- The red-cell fraction of whole blood (commonly taught ~45%)
- Albumin
- Most abundant plasma protein; maintains osmotic pressure, carries small molecules
- Globulins
- Plasma proteins including antibodies (immunoglobulins) and transporters
- Fibrinogen
- Plasma protein converted to fibrin during clotting
- Serum
- Plasma with clotting factors removed
- Erythrocytes
- Biconcave, nucleus-free red blood cells packed with hemoglobin
- Leukocytes
- White blood cells; granulocytes and agranulocytes
- Platelets (thrombocytes)
- Cell fragments from megakaryocytes
- Hemoglobin
- The iron-containing protein in RBCs that binds oxygen
Sources & references
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