Blood and Body Fluids
Learning Objectives
By the end of this page, you should be able to:
- Describe the composition of blood and the relative proportions of plasma, RBCs, WBCs, and platelets
- Explain the structure and function of red blood cells, white blood cells, and platelets
- Outline the coagulation cascade and distinguish the intrinsic, extrinsic, and common pathways
- Explain the ABO and Rh blood group systems and why mismatched transfusions cause hemolysis
- Differentiate the body fluid compartments (ICF, ECF, plasma, interstitial fluid) and their approximate volumes
- Classify common types of anemia by underlying mechanism
- Apply hematocrit and fluid balance concepts to clinical scenarios such as dehydration, hemorrhage, and edema
Quick Answer
Blood is a specialized connective tissue made of a liquid matrix (plasma) suspending three cellular elements: red blood cells (oxygen transport), white blood cells (immune defense), and platelets (hemostasis). It makes up about 7-8% of body weight, and roughly 45% of its volume is cells (the hematocrit) with the remaining 55% plasma. Blood plays a dual role in physiology: it is itself a fluid compartment, and it is also the vehicle that keeps every other body fluid compartment — intracellular, interstitial, and the specialized fluids like CSF and synovial fluid — supplied and balanced. Understanding blood composition and coagulation is essential for interpreting a CBC, understanding anemia, managing bleeding disorders, and reasoning about transfusion medicine — all high-yield clinical and exam topics.
Composition of Blood
Blood is unique among tissues because its "extracellular matrix" — plasma — is a liquid. When you spin a blood sample in a centrifuge, it separates into three visible layers: plasma on top (~55%), a thin white "buffy coat" of WBCs and platelets (~1%), and packed RBCs at the bottom (~45%). That bottom layer's percentage is exactly what a hematocrit measures.
Plasma
Plasma is roughly 92% water and 7% protein by weight, with the remainder made up of electrolytes, gases, hormones, nutrients, and waste products in transit.
- Albumin (~60% of plasma protein) — maintains oncotic (colloid osmotic) pressure, the force that keeps water inside vessels; also transports fatty acids, bilirubin, and many drugs
- Globulins — alpha and beta globulins transport lipids and metal ions; gamma globulins are antibodies (immunoglobulins)
- Fibrinogen — the soluble precursor of fibrin, the structural protein of a blood clot
Serum is simply plasma with the clotting factors (especially fibrinogen) removed — this distinction shows up constantly in lab medicine questions.
Red Blood Cells (Erythrocytes)
RBCs are biconcave, anucleate discs packed with hemoglobin. The biconcave shape maximizes surface area for gas exchange and lets the cell deform to squeeze through capillaries narrower than its own diameter. Because RBCs have no nucleus or mitochondria, they rely entirely on anaerobic glycolysis for energy — a design that keeps them from consuming the oxygen they are supposed to deliver.
Each hemoglobin molecule carries four heme groups, each capable of binding one oxygen molecule, so one RBC (containing ~270 million hemoglobin molecules) can carry over a billion oxygen molecules. RBCs live about 120 days before being removed by macrophages in the spleen and liver, where hemoglobin is broken down into bilirubin (excreted in bile) and iron (recycled).
White Blood Cells (Leukocytes)
WBCs are the mobile arm of the immune system. They are far less numerous than RBCs (about 1 WBC per 700 RBCs) but far more diverse:
- Neutrophils (~60%) — first responders to bacterial infection; phagocytic
- Lymphocytes (~30%) — B cells (antibody production) and T cells (cell-mediated immunity)
- Monocytes (~6%) — mature into tissue macrophages
- Eosinophils (~3%) — combat parasitic infections; mediate allergic responses
- Basophils (<1%) — release histamine and heparin, involved in allergic/inflammatory reactions
A useful mnemonic for relative abundance: "Never Let Monkeys Eat Bananas" (Neutrophils > Lymphocytes > Monocytes > Eosinophils > Basophils).
Platelets (Thrombocytes)
Platelets are cell fragments budded off from megakaryocytes in the bone marrow, not true cells. They circulate for 8-10 days and are the first responders to vessel injury, initiating the hemostatic plug before the coagulation cascade reinforces it with fibrin.
Hemostasis and the Coagulation Cascade
Hemostasis — stopping bleeding from a damaged vessel — happens in three overlapping phases:
- Vascular spasm — injured vessel smooth muscle contracts, reducing blood flow to the area
- Platelet plug formation — platelets adhere to exposed collagen (via von Willebrand factor), activate, and aggregate, forming a temporary plug
- Coagulation (secondary hemostasis) — a cascade of plasma clotting factors converts fibrinogen into a stable fibrin mesh that reinforces the platelet plug
The coagulation cascade has two initiating pathways that converge on a common pathway:
- Extrinsic pathway — triggered by tissue factor (Factor III) released from damaged tissue outside the vessel; fast, measured clinically by PT (prothrombin time)
- Intrinsic pathway — triggered by contact activation with exposed collagen inside the vessel; slower, measured clinically by PTT (partial thromboplastin time)
- Common pathway — both converge at Factor X, which converts prothrombin to thrombin, which converts fibrinogen to fibrin
Clinically: warfarin inhibits vitamin K-dependent factors (II, VII, IX, X) and prolongs PT/INR; heparin potentiates antithrombin III and prolongs PTT. Hemophilia A (Factor VIII deficiency) and hemophilia B (Factor IX deficiency) prolong PTT because they knock out the intrinsic pathway.
Blood Groups and Transfusion
The ABO System
Blood type is determined by antigens present on the RBC surface, and antibodies are automatically produced in plasma against whichever ABO antigen you lack — a phenomenon unique to the ABO system.
| Blood Type | RBC Antigen | Plasma Antibody | Can Donate To | Can Receive From |
|---|---|---|---|---|
| A | A | Anti-B | A, AB | A, O |
| B | B | Anti-A | B, AB | B, O |
| AB | A and B | None | AB only | AB, A, B, O (universal recipient) |
| O | None | Anti-A and Anti-B | O, A, B, AB (universal donor) | O only |
If a person with type A blood receives type B blood, their pre-existing anti-B antibodies immediately bind the transfused B antigens, triggering complement-mediated intravascular hemolysis — this is why cross-matching before transfusion is non-negotiable.
The Rh System
Rh(D) antigen presence defines Rh-positive vs. Rh-negative status. Unlike ABO, anti-Rh antibodies are not naturally present — they only develop after an Rh-negative person is exposed to Rh-positive blood (e.g., an Rh-negative mother carrying an Rh-positive fetus). This is why the first Rh-mismatched pregnancy is usually fine, but a second one risks hemolytic disease of the newborn (erythroblastosis fetalis) because the mother has since developed anti-Rh IgG antibodies that cross the placenta. This is prevented by giving Rh-negative mothers anti-D immunoglobulin (RhoGAM) during and after pregnancy.
Anemia: A Classification Framework
Anemia is a reduction in the oxygen-carrying capacity of blood, usually reflected by low hemoglobin or hematocrit. Classifying by RBC size (mean corpuscular volume, MCV) is the fastest way to generate a differential:
| Category | MCV | Common Causes |
|---|---|---|
| Microcytic | <80 fL | Iron deficiency, thalassemia, chronic disease (late) |
| Normocytic | 80-100 fL | Acute blood loss, hemolysis, chronic disease (early), anemia of renal failure |
| Macrocytic | >100 fL | Vitamin B12 deficiency, folate deficiency, alcohol use |
Iron deficiency anemia is the most common type worldwide and results from inadequate iron for hemoglobin synthesis — chronically small, pale (hypochromic) RBCs are the hallmark. B12/folate deficiency impairs DNA synthesis during erythropoiesis, producing fewer but abnormally large RBCs (megaloblastic anemia).
Types of Body Fluids
Total body water is about 60% of body weight in an average adult, distributed across compartments:
- Intracellular fluid (ICF): fluid inside cells; the largest single compartment, rich in potassium
- Extracellular fluid (ECF): everything outside cells; rich in sodium
- Interstitial fluid: bathes cells directly, the "middleman" between plasma and cells
- Plasma: the only ECF compartment that circulates and can be sampled by venipuncture
- Transcellular fluid: specialized, actively secreted fluids — CSF (cushions brain/spinal cord), synovial fluid (lubricates joints), pericardial fluid (reduces friction around the heart), pleural fluid (reduces friction during lung expansion)
Fluid moves between plasma and interstitial space at the capillary level based on the balance of hydrostatic pressure (pushing fluid out) and oncotic pressure from albumin (pulling fluid back in) — the Starling forces. When albumin drops (e.g., liver failure, nephrotic syndrome) or venous hydrostatic pressure rises (e.g., heart failure), fluid accumulates in the interstitium as edema.
Why It Matters Clinically
A complete blood count (CBC) with hematocrit, hemoglobin, and differential is one of the most frequently ordered tests in medicine, and interpreting it correctly requires understanding everything above: a low hematocrit could mean true anemia or simple dilution from fluid overload; a high hematocrit could mean polycythemia or dehydration-driven hemoconcentration. Coagulation studies (PT/INR, PTT) guide anticoagulant dosing and diagnose bleeding disorders. Blood typing and cross-matching prevent fatal transfusion reactions. Fluid compartment physiology underlies IV fluid selection (crystalloids expand ECF, colloids stay mostly in plasma) and explains why patients with heart failure or liver failure develop edema and ascites.
Key Terms
| Term | Definition | Related Concept |
|---|---|---|
| Hematocrit | Percentage of blood volume occupied by RBCs | Anemia, polycythemia, dehydration |
| Plasma | Liquid, acellular portion of blood (contains clotting factors) | Serum, oncotic pressure |
| Serum | Plasma minus clotting factors (obtained after clot formation) | Plasma, fibrinogen |
| Hemoglobin | Iron-containing protein in RBCs that binds and transports oxygen | Oxygen transport, anemia |
| Erythropoiesis | Production of red blood cells, stimulated by erythropoietin from the kidney | Anemia, renal failure |
| Coagulation cascade | Sequence of enzymatic activations of clotting factors ending in fibrin formation | Hemostasis, PT/PTT |
| Fibrinogen | Soluble plasma protein converted to fibrin during clotting | Coagulation, thrombin |
| ABO blood group | Classification based on A/B antigens on RBC surface | Transfusion, cross-matching |
| Rh factor | Antigen (D antigen) whose presence/absence defines Rh+/Rh- status | Hemolytic disease of the newborn |
| Oncotic pressure | Osmotic pressure exerted by plasma proteins (mainly albumin) that holds water in vessels | Starling forces, edema |
| Interstitial fluid | Fluid occupying the space between cells, outside blood vessels | ECF, edema |
| Edema | Abnormal accumulation of fluid in the interstitial space | Starling forces, heart failure |
Common Mistakes
Misconception: Blood type O is the "universal donor" because it has no antigens, so it is universally safe for anyone in any amount.
Why it's wrong: Type O RBCs lack A and B surface antigens, so they don't trigger a reaction from the recipient's anti-A/anti-B antibodies — this makes packed O-negative RBCs safe in emergencies. But type O plasma still contains both anti-A and anti-B antibodies, so whole blood or large-volume plasma-containing transfusions from an O donor can still cause a reaction in a non-O recipient.
Correct understanding: "Universal donor" applies specifically to packed red blood cells (antigens removed with the plasma), not to whole blood or plasma transfusions.
Misconception: A low hematocrit always means the patient doesn't have enough red blood cells.
Why it's wrong: Hematocrit is a ratio (RBC volume ÷ total blood volume), not an absolute cell count. Acute fluid overload (e.g., aggressive IV fluid resuscitation) dilutes the same number of RBCs into a larger plasma volume, dropping the hematocrit without any RBCs being lost or destroyed.
Correct understanding: Always interpret hematocrit alongside the clinical picture and fluid status — dilutional "anemia" is not true anemia and resolves once fluid balance normalizes.
Misconception: PT and PTT test "how well blood clots" in a generic, interchangeable way.
Why it's wrong: PT and PTT test two distinct pathways that share only the common pathway downstream. A prolonged PT with normal PTT points to extrinsic/Factor VII issues (e.g., early warfarin effect or vitamin K deficiency); a prolonged PTT with normal PT points to intrinsic pathway issues (e.g., hemophilia, heparin therapy, lupus anticoagulant).
Correct understanding: PT and PTT localize the defect to a specific part of the cascade — treat them as diagnostic tools that map to different factors, not as one generic "clotting test."
Comparison and Connections
| Feature | Red Blood Cells | White Blood Cells | Platelets |
|---|---|---|---|
| Origin | Bone marrow (erythropoiesis) | Bone marrow and lymphoid tissue | Megakaryocyte fragments |
| Nucleus | Absent (mature form) | Present | Absent (cell fragments) |
| Primary function | Oxygen/CO2 transport | Immune defense | Hemostasis |
| Lifespan | ~120 days | Hours to years (type-dependent) | 8-10 days |
| Regulator | Erythropoietin (kidney) | Cytokines, colony-stimulating factors | Thrombopoietin (liver) |
| Key clinical marker | Hemoglobin, hematocrit, MCV | Total WBC count, differential | Platelet count |
Practice Questions
Recall
-
What are the three cellular/formed elements of blood, and what percentage of total blood volume do RBCs typically occupy? Answer guidance: RBCs, WBCs, and platelets. RBCs occupy roughly 45% of blood volume (the hematocrit).
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Name the two initiating pathways of the coagulation cascade and the lab test associated with each. Answer guidance: Extrinsic pathway (tissue factor) — measured by PT. Intrinsic pathway (contact activation) — measured by PTT. Both converge on the common pathway (Factor X onward).
Understanding
-
Explain why a person with type AB blood is called the "universal recipient" but can only donate to other AB individuals. Answer guidance: AB RBCs carry both A and B antigens but the plasma has neither anti-A nor anti-B antibody, so an AB person can receive A, B, AB, or O blood without an antibody-antigen reaction against the incoming cells. But AB RBCs carry both antigens, so if given to an A, B, or O recipient, that recipient's existing antibodies would attack the AB cells — hence AB can only donate to AB.
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Why does Rh hemolytic disease of the newborn typically spare the first pregnancy but threaten subsequent ones? Answer guidance: Anti-Rh antibodies are not naturally present. Sensitization occurs when fetal Rh-positive cells enter maternal circulation (often at delivery), after which the mother produces anti-Rh IgG. These antibodies can cross the placenta in a subsequent pregnancy and attack a second Rh-positive fetus's RBCs.
Application
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A patient with cirrhosis has low serum albumin and develops significant peripheral edema and ascites. Using Starling forces, explain the mechanism. Answer guidance: Albumin maintains plasma oncotic pressure, which pulls fluid back into capillaries. With low albumin, oncotic pressure drops, so hydrostatic pressure pushing fluid out of capillaries is no longer opposed effectively, and fluid accumulates in the interstitium (edema) and peritoneal cavity (ascites).
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A patient on warfarin presents with a markedly elevated PT/INR but normal PTT. What does this tell you, and why would you expect PTT to eventually be affected too if warfarin dosing continued unchecked? Answer guidance: Warfarin inhibits vitamin K-dependent factors II, VII, IX, and X. Factor VII has the shortest half-life, so it drops first, prolonging PT before PTT. With continued inhibition, factors II, IX, and X (shared with the intrinsic/common pathway) also fall, eventually prolonging PTT as well.
Analysis
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A patient has a hemoglobin of 9 g/dL (low) with an MCV of 70 fL (microcytic). Contrast this with a second patient who has the same hemoglobin but an MCV of 110 fL (macrocytic). What does the MCV difference tell you about the likely underlying mechanism? Answer guidance: Microcytic anemia (low MCV) suggests a problem with hemoglobin synthesis — most commonly iron deficiency or thalassemia — producing small, hemoglobin-poor cells. Macrocytic anemia (high MCV) suggests impaired DNA synthesis during cell division — B12 or folate deficiency — producing fewer, abnormally large cells because cell division lags behind cytoplasmic growth. Same hemoglobin value, completely different mechanism and workup.
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Compare what happens physiologically when a healthy person donates 500 mL of blood versus when a patient loses 500 mL of blood acutely from a GI bleed with ongoing plasma volume loss (e.g., third-spacing). Why might their hematocrit values look similar initially despite very different clinical severity? Answer guidance: In a controlled donation, plasma volume is quickly restored (within 24-48 hours) via fluid shifts, so hematocrit changes minimally in the short term. In acute hemorrhage, both RBCs and plasma are lost proportionally at first, so the hematocrit may initially look deceptively normal — it only starts to fall as the body compensates by shifting interstitial fluid into the vasculature over hours, diluting the remaining RBCs. This is a classic exam trap: an unchanged hematocrit immediately after acute hemorrhage does not rule out significant blood loss.
FAQ
Why doesn't blood clot inside healthy, uninjured blood vessels?
The intact vascular endothelium actively prevents clotting: it produces prostacyclin and nitric oxide (inhibit platelet activation), expresses thrombomodulin and heparin-like molecules (inhibit thrombin), and keeps tissue factor sequestered away from the bloodstream. Coagulation is triggered specifically by endothelial damage exposing collagen and tissue factor — it is a localized emergency response, not a default state.
What's the actual difference between plasma and serum, and why does it matter clinically?
Plasma is collected with an anticoagulant so clotting factors, including fibrinogen, remain in solution. Serum is what's left after blood clots and the clot (containing fibrinogen and cells) is removed. Many lab tests specify plasma vs. serum because certain analytes (like fibrinogen itself, or factors consumed during clotting) can only be measured in plasma.
Why do doctors care about MCV when working up anemia?
MCV (mean corpuscular volume) is the fastest way to narrow a broad differential for anemia down to a short list. Microcytic points toward iron/hemoglobin synthesis problems, macrocytic points toward DNA synthesis problems (B12/folate), and normocytic points toward acute loss, hemolysis, or early chronic disease. It's a triage step before ordering more specific (and more expensive) tests like iron studies or B12 levels.
Is edema always caused by a heart problem?
No. Edema results whenever the Starling forces are disrupted — this can happen from increased capillary hydrostatic pressure (heart failure, venous insufficiency), decreased oncotic pressure (liver failure, nephrotic syndrome, malnutrition), increased capillary permeability (inflammation, allergic reaction), or lymphatic obstruction (lymphedema after lymph node removal). Localizing the cause requires looking at the distribution (unilateral vs. bilateral) and associated findings, not just the presence of swelling.
Why does the body need so many different types of body fluid, rather than just blood?
Each fluid compartment is specialized for a particular job that plasma can't do directly. CSF cushions the brain and provides a chemically stable environment isolated from systemic blood by the blood-brain barrier. Synovial fluid lubricates joint surfaces and reduces friction. Pleural and pericardial fluids allow the lungs and heart to move smoothly against surrounding tissue without adhering. Interstitial fluid is the actual medium that touches cells — plasma never contacts tissue cells directly; nutrients and waste must cross into and out of the interstitium first.
Quick Revision
- Blood = plasma (~55%) + RBCs (~45%, the hematocrit) + WBCs and platelets (~1%, buffy coat)
- Plasma proteins: albumin (oncotic pressure), globulins (transport, antibodies), fibrinogen (clotting)
- RBCs are anucleate, biconcave, live ~120 days, rely on anaerobic glycolysis, carry hemoglobin
- WBC abundance order: Neutrophils > Lymphocytes > Monocytes > Eosinophils > Basophils
- Platelets are megakaryocyte fragments that form the initial hemostatic plug
- Coagulation: extrinsic pathway → PT; intrinsic pathway → PTT; both converge at Factor X → thrombin → fibrin
- ABO system: antibodies exist automatically against the antigen you lack; O = universal RBC donor, AB = universal recipient
- Rh sensitization requires prior exposure; first Rh-mismatched pregnancy is usually safe, later ones are not (prevent with RhoGAM)
- Anemia classification by MCV: microcytic (iron deficiency, thalassemia), normocytic (acute loss, hemolysis), macrocytic (B12/folate deficiency)
- Total body water ≈ 60% of body weight: 2/3 ICF, 1/3 ECF (split into interstitial fluid ~75% and plasma ~25%)
- Starling forces (hydrostatic vs. oncotic pressure) govern fluid movement between plasma and interstitium; imbalance causes edema
- Transcellular fluids (CSF, synovial, pleural, pericardial) are specialized, actively secreted fluids serving local structural/protective roles
Related Topics
Prerequisites: Introduction to Physiology, basic cell biology, general chemistry (osmosis, pH)
Related Topics: Cardiovascular System Physiology, Renal Physiology and fluid balance, Immunology (WBC function), Hematology-Oncology
Next Topics: Cardiovascular System Physiology, Respiratory System Physiology, Renal Physiology
This page is for educational purposes. Always verify with current clinical guidelines.