Blood Physiology Explained Simply: From Plasma to ESR

Blood physiology feels like 10 chapters crammed into 1. Plasma, RBC, WBC, platelets, clotting, blood groups, spleen, jaundice, hemoglobin, ESR: ten topics, and every one seems to borrow words from another. Here's the good news. It's a single story. Follow 1 red cell from the bone marrow to the spleen and you'll meet half the chapter on the way. The other half is the defense team and the repair crew that travel in the same fluid.

Table of Contents

Blood Physiology Starts in a Test Tube: What Is Plasma?

Take a tube of blood with a little anticoagulant in it (a chemical that stops clotting) and spin it in a centrifuge for a few minutes. It sorts itself into 3 layers. The diagram below shows exactly what you'd see.

Figure 1: A centrifuged blood sample separates into plasma, a thin buffy coat, and packed red cells.

The red layer at the bottom holds the red blood cells. Their share of the total volume is the hematocrit (also called PCV, packed cell volume), roughly 45% in men and 40% in women, though textbooks quote slightly different ranges. The thin whitish line above it is the buffy coat: white cells and platelets, under 1% of the volume. The straw-colored liquid on top is our first topic.

Plasma is the straw-colored liquid part of blood, about 55% of its volume. It is roughly 90 to 93% water, with proteins (albumin, globulins, fibrinogen), electrolytes, nutrients, hormones, and waste dissolved in it. Its proteins, mainly albumin, keep water inside the vessels, and fibrinogen lets it clot.

An adult has about 5 liters of blood, roughly 7% of body weight in a 70 kg person. That blood does 4 jobs: it transports, it defends, it stops bleeding (hemostasis), and it holds pH (7.35 to 7.45) and temperature steady.

The proteins in plasma add up to about 7 g/dL, and they split into 3 families.

Plasma protein	Approx. level	Made by	Main job
Albumin	about 4.5 g/dL	Liver	Holds water in the vessels (colloid osmotic pressure); carries bilirubin, hormones, and drugs
Globulins	about 2.5 g/dL	Alpha and beta: mostly liver. Gamma: plasma cells	Transport (alpha, beta) and antibodies (gamma)
Fibrinogen	about 0.3 g/dL	Liver	Becomes fibrin, the mesh of a clot

Students mix up plasma and serum all the time, so settle it now. Plasma is the liquid of anticoagulated blood, with fibrinogen and the other clotting factors still inside. Serum is the liquid left after blood has clotted, once those factors have been used up. The single difference that separates them is fibrinogen.

Now the protein that earns the most exam marks. According to Guyton and Hall (14th ed., 2021), albumin supplies roughly 80% of the colloid osmotic pressure of plasma, the pulling force that draws water back into capillaries. Plasma holds more protein than the fluid between your cells, so water keeps getting pulled back in. That pull is about 25 to 28 mmHg.

What happens if albumin falls? Water loses its anchor and drifts out into the tissues. That's edema, and you'll meet it in liver disease (less albumin made), nephrotic syndrome (albumin lost in urine), and severe malnutrition.

Plasma carries the cargo, but the cell layer at the bottom of the tube does the heavy lifting. Open it up.

How Is a Red Cell Built, and What Does Hemoglobin Do Inside It?

A red blood cell (RBC, or erythrocyte) is a doughnut with a dimple instead of a hole, about 7.5 µm across. Men carry roughly 5 million per microliter of blood and women roughly 4.5 million. A single pinprick drop holds a couple of hundred million of them.

The shape does real work. The dimple (a biconcave disc) gives the cell a large surface for gas exchange and lets it fold through capillaries narrower than itself. A mature red cell also has no nucleus and no mitochondria, so it runs on glucose through anaerobic glycolysis. It never burns the oxygen it carries. Every molecule it picks up goes to somebody else.

So who builds these cells, and what tells the factory to speed up? The factory is the red bone marrow, mostly in the flat bones of an adult (sternum, ribs, vertebrae, pelvis). The signal is low oxygen.

1. Low oxygen reaches the kidney (altitude, bleeding, anemia, or lung disease can cause it).
2. Kidney cells release erythropoietin (EPO). Guyton and Hall put the kidney's share at about 90%, with the liver making the rest.
3. EPO pushes marrow stem cells toward the red-cell line, starting with the proerythroblast.
4. The cell matures through the erythroblast stages. Hemoglobin builds up, the nucleus shrinks, and it is finally pushed out.
5. The young cell enters the blood as a reticulocyte (it still carries a little RNA) and becomes a mature red cell within 1 to 2 days. The whole line takes roughly a week.

Climb to high altitude and each breath carries less oxygen, so EPO rises and the marrow pushes out more cells within days. That's the logic behind altitude training. Reticulocytes make up about 1% of red cells in a healthy person, so a high reticulocyte count tells you the marrow is working hard.

The marrow also needs raw material. Iron builds the heme part of hemoglobin. Vitamin B12 and folic acid are needed to copy DNA so cells can divide. B12 absorption also needs intrinsic factor, a protein from the stomach's parietal cells.

Common Mistake! Students treat every anemia as the same disease. Iron deficiency means too little hemoglobin gets built, so cells come out small and pale (microcytic, hypochromic, low MCV). B12 or folate deficiency means DNA can't be copied fast enough, so division stalls while the cytoplasm keeps growing, and cells come out big and immature (megaloblastic, high MCV). A normal MCV is about 80 to 100 fL. The differentiator: a hemoglobin shortage makes small cells, a DNA shortage makes big ones.

Now the cargo. Hemoglobin (Hb) is the red, iron-containing protein that grabs oxygen in the lungs and releases it in the tissues. How does 1 protein do both jobs? Through its structure. Adult hemoglobin (HbA) is built from 4 globin chains (2 alpha, 2 beta), and each chain wraps 1 heme group with an iron atom in the Fe²⁺ state at its center. That's 4 hemes and 4 irons, so each hemoglobin carries up to 4 oxygen molecules.

The fetus uses a different version. As StatPearls describes, fetal hemoglobin (HbF) has 2 alpha and 2 gamma chains, while HbA has 2 alpha and 2 beta. HbF holds oxygen more tightly than HbA, which helps the fetus draw oxygen across the placenta.

Normal hemoglobin runs about 14 to 18 g/dL in men and 12 to 16 g/dL in women, and newborns start higher. The World Health Organization (2011) defines anemia as hemoglobin below 13 g/dL in adult men and below 12 g/dL in non-pregnant women.

Here's what that hemoglobin buys you. Each gram binds about 1.34 mL of oxygen when fully saturated (Guyton and Hall). At 15 g/dL, that's about 20 mL of oxygen per 100 mL of blood. Oxygen dissolved in plasma alone would give only about 0.3 mL, so hemoglobin multiplies the carrying capacity roughly 60 times.

Now the part students find hardest: how does hemoglobin decide when to let go? Plot oxygen saturation against PO₂ and you get an S-shaped line, the oxygen-hemoglobin dissociation curve. The S appears because the 4 chains cooperate. Once the first oxygen binds, the molecule relaxes and the next ones bind more easily. In the lungs (PO₂ about 100 mmHg) you sit on the flat top, so hemoglobin is about 97% saturated. In resting tissues (PO₂ about 40 mmHg) you've slid down the steep part, and saturation falls to about 75%. The halfway point (P50) is around 26 mmHg.

The curve also moves, and the direction is exam gold.

Shift	What causes it	What hemoglobin does
Right	Raised CO2, raised acidity (Bohr effect), raised 2,3-DPG, exercise, raised temperature	Releases oxygen more easily
Left	The opposite conditions, plus fetal hemoglobin and carbon monoxide	Holds oxygen more tightly

A well-known memory aid is CADET, face right: CO₂, Acid, 2,3-DPG, Exercise, Temperature. Picture your quadriceps in a sprint. They pump out CO₂, acid, and heat, so the curve shifts right and the hemoglobin passing through hands over more oxygen exactly where it's needed. Hemoglobin grips oxygen tightly where oxygen is plentiful (the lungs) and lets go where the local chemistry says "I'm working hard."

Quick pause. Low oxygen raises EPO, EPO raises red-cell output, and hemoglobin inside those cells decides how much oxygen each liter of blood can carry. Hold that chain, because the next section starts where it ends.

Hemoglobin can also go wrong in several distinct ways, and each has its own signature.

Type	What is different	Result
Carboxyhemoglobin	Carbon monoxide sits on the heme iron; its affinity is over 200 times that of oxygen	Oxygen can't bind, and the curve shifts left, so tissues starve despite a normal Hb count
Methemoglobin	Iron is in the Fe3+ (ferric) state	That hemoglobin can't carry oxygen
HbS (sickle)	Valine replaces glutamic acid at position 6 of the beta chain	Deoxygenated HbS polymerizes and the cell sickles
Thalassemia	Reduced or absent synthesis of one globin chain type; in alpha thalassemia, alpha-gene deletions on chromosome 16 (per StatPearls)	Less and smaller hemoglobin, with unbalanced chains

Every red cell works for about 120 days, then it retires. Who signs the papers?

Where Do Old Red Cells Go, and How Does That Lead to Jaundice?

Over 120 days a red cell makes well over 100,000 laps of the circulation. Its membrane stiffens and its enzymes run low. With no nucleus, it can't repair itself. So it needs a retirement office, and that's the spleen.

The spleen sits in the upper left abdomen, behind the stomach, about the size of a fist and roughly 150 g. Inside, it has 2 zones. The white pulp is lymphoid tissue that handles immune work. The red pulp is a spongy network of blood-filled channels, and that's where red cells get inspected. Blood there has to squeeze through narrow slits in the walls of the venous sinuses. A flexible, healthy cell slips through. A stiff or misshapen one gets stuck, and macrophages swallow it. Think of it like a turnstile with a narrow gap: flexible visitors pass, stiff ones get stopped.

Spleen function	What happens	Clinical link
Filter and recycle red cells	Red-pulp macrophages remove old or damaged cells and inclusions such as Howell-Jolly bodies	Howell-Jolly bodies appear on the blood film after splenectomy; an enlarged spleen is common in hemolysis
Immune defense	White pulp lymphocytes and macrophages respond to blood-borne microbes	After splenectomy, risk of severe infection by encapsulated bacteria (Streptococcus pneumoniae, Haemophilus influenzae, Neisseria meningitidis)
Platelet storage	About one-third of platelets pool in the spleen	An enlarged spleen can trap platelets and lower the blood count
Blood cell formation	The fetal spleen makes blood cells; adults can restart this in some marrow diseases	Called extramedullary hematopoiesis

Once a macrophage swallows the old cell, hemoglobin gets taken apart piece by piece, and every piece has a destination. Follow the flow in the diagram.

Figure 2: Fate of an old red cell, from spleen macrophage to bilirubin, then stool and urine pigments.

1. A macrophage in the spleen (the liver and bone marrow have them too) swallows the old red cell and splits hemoglobin into globin and heme.
2. Globin breaks into amino acids that get reused. Iron leaves the heme, rides the blood on transferrin, and is stored as ferritin or reused for new hemoglobin.
3. The rest of the heme becomes biliverdin (green) through the enzyme heme oxygenase, then unconjugated bilirubin (yellow-orange) through biliverdin reductase. Bilirubin won't dissolve in water, so it rides on albumin to the liver.
4. Inside a liver cell, the enzyme UDP-glucuronosyltransferase attaches glucuronic acid, creating conjugated bilirubin, which is water-soluble.
5. Conjugated bilirubin is secreted into bile and flows into the intestine.
6. Gut bacteria turn it into urobilinogen. Most becomes stercobilin, which colors stool brown. A little is reabsorbed (the enterohepatic circulation), and the kidney excretes a small amount as urobilin, which makes urine yellow.

Ganong's Review of Medical Physiology (26th ed., 2019) puts the daily bilirubin load at roughly 250 to 350 mg, most of it from old red cells. And 1 of those steps going wrong gives you jaundice: yellow staining of the skin, the sclera (white of the eyes), and mucous membranes when bilirubin builds up in blood. Normal total bilirubin is about 1 mg/dL or less, and yellowing usually becomes visible above roughly 2 to 3 mg/dL (textbook cut-offs differ a little). Sort the types by where the problem sits: before the liver (too much supply), inside the liver (processing fails), or after the liver (the exit is blocked).

Feature	Pre-hepatic (hemolytic)	Hepatic (hepatocellular)	Post-hepatic (obstructive)
Problem	Too much bilirubin made	Liver cells can't take up, conjugate, or secrete it	Bile flow blocked
Raised in blood	Unconjugated	Both forms	Conjugated
Urine bilirubin	Absent (acholuric)	Present	Present (dark urine)
Urine urobilinogen	Increased	Variable, often normal or reduced	Absent or reduced
Stool	Dark	Normal or pale	Pale, clay-colored
Examples	Hemolytic anemia, incompatible transfusion	Hepatitis, cirrhosis	Gallstone in the bile duct, tumor at the head of the pancreas

Here's where most students slip: they assume bilirubin in urine always means a kidney problem. Unconjugated bilirubin is stuck to albumin and water-insoluble, so the kidney can't filter it, and it never reaches urine. That's why hemolytic jaundice is called acholuric (without bile in urine). Conjugated bilirubin is water-soluble and spills into urine freely.

Exam Pointer The 3-type jaundice comparison is a frequent viva and long-answer question. If you remember only 2 rows, remember urine bilirubin and stool color: they separate the types fastest.

What if a gallstone blocks the bile duct? Conjugated bilirubin can't reach the gut, so no stercobilin forms and stool turns pale. The conjugated bilirubin backs up into the blood and spills into urine, turning it dark. Bile salts settle in the skin and cause itching. And here's a link most students miss: bile is needed to absorb fat-soluble vitamin K, and clotting factors II, VII, IX, and X depend on vitamin K, so a long-standing blockage can lead to bleeding. Hold that thought for the clotting section.

Clinical Note In newborns, red-cell breakdown is high and the liver's conjugating enzyme is still immature, so unconjugated bilirubin often rises around days 2 to 3 (physiological jaundice) and settles over the first 1 to 2 weeks. Very high unconjugated levels can cross the blood-brain barrier and damage the brain (kernicterus), which is why newborn bilirubin is watched closely and phototherapy, which converts bilirubin to water-soluble forms, is used in hospitals.

The spleen and liver clear out worn-out red cells. A different group of cells guards the body against invaders.

White Blood Cells: 5 Cell Types, 5 Different Jobs

White blood cells (WBCs, or leukocytes) are the defense team, and an adult has about 4,000 to 11,000 per microliter. They fall into 2 groups. Granulocytes (neutrophils, eosinophils, basophils) carry visible granules in their cytoplasm. Agranulocytes (lymphocytes, monocytes) do not.

The total count alone says little. The differential count, the percentage of each type, tells you who showed up. Here's the normal split.

Cell	Normal share	How you spot it	Main job	Count rises in
Neutrophil	50 to 70%	3 to 5 nuclear lobes, pale granules	First responder; engulfs bacteria	Bacterial infection
Lymphocyte	20 to 40%	Large round nucleus, thin rim of cytoplasm	Specific immunity: T cells, B cells, NK cells	Viral infection
Monocyte	2 to 8%	Largest WBC, kidney-shaped nucleus	Leaves blood and becomes a macrophage	Chronic infections
Eosinophil	1 to 4%	Bilobed nucleus, bright red-orange granules	Parasites and allergic reactions	Allergy, parasitic infection
Basophil	0 to 1%	Dark blue-purple granules hide the nucleus	Releases histamine and heparin	Allergic states, some marrow disorders

The memory hook is Never Let Monkeys Eat Bananas, which lists them from most to least abundant.

Picture an infection as an attack on a city. Neutrophils are the police: they arrive within hours and tackle anything that looks like bacteria. Monocytes become macrophages, the cleaners that stay in the tissue and swallow microbes and debris (up to about 100 bacteria each, according to Guyton and Hall). Lymphocytes play intelligence: slower to respond, but they identify the exact intruder and remember it.

How do these cells reach the battlefield? Damaged tissue releases chemical signals, and white cells follow the gradient (chemotaxis). They squeeze between the cells of the capillary wall (diapedesis) and then swallow the microbe (phagocytosis). A neutrophil can engulf roughly 3 to 20 bacteria before it's used up, and pus is largely dead neutrophils and debris. A neutrophil also spends only 4 to 8 hours in blood before moving into tissue. That's a short shift.

Here is the contrast to carry into the exam. Neutrophils are fast and general. Lymphocytes are slower and specific. Neutrophil counts jump in bacterial infections, while lymphocyte counts tend to rise in viral ones. Among lymphocytes, T cells mature in the thymus and run cell-mediated immunity, B cells turn into plasma cells that make antibodies (the gamma globulins from the plasma table), and NK cells kill virus-infected and abnormal cells without prior sensitization.

A count above 11,000 is leukocytosis, and below 4,000 is leukopenia. Neither names a disease, only a direction. White cells answer infection. A different emergency, a cut, calls in the smallest cells in the blood.

Platelets and Clotting: How Does a Cut Stop Bleeding?

Platelets (thrombocytes) are cell fragments, 2 to 4 µm across, pinched off from giant marrow cells called megakaryocytes under the influence of the hormone thrombopoietin. The normal count is about 150,000 to 400,000 per microliter. They have no nucleus, live roughly 8 to 10 days, and about one-third of them sit pooled in the spleen at any moment.

What happens in the first seconds after a cut? Hemostasis (stopping bleeding) runs as 4 overlapping steps.

1. Vascular spasm: the torn vessel's smooth muscle contracts and cuts blood flow.
2. Platelet plug: platelets stick, activate, and clump together.
3. Coagulation: a fibrin mesh reinforces the plug into a stable clot.
4. Retraction and repair: the clot tightens, the vessel heals, and the enzyme plasmin later dissolves the clot (fibrinolysis).

The plug itself forms in 3 moves. Adhesion: von Willebrand factor (vWF) glues platelets to exposed collagen through the GPIb receptor. Activation: stuck platelets change shape, grow spiky arms, and release ADP, thromboxane A₂, and serotonin, which call in more platelets. Aggregation: fibrinogen bridges neighboring platelets through GP IIb/IIIa receptors. Aspirin blocks thromboxane A₂ formation, which is why it's studied as an antiplatelet drug. Think of it like first aid on a leaking pipe: tape holds the leak at once (the platelet plug), and epoxy makes the repair permanent (the fibrin mesh).

The epoxy comes from coagulation: turning soluble fibrinogen into an insoluble fibrin mesh. The clotting factors circulate as inactive precursors and are numbered with Roman numerals. When one switches on, it gets an "a" (X becomes Xa), and each activated factor switches on many copies of the next, so a tiny trigger becomes a big response.

Factor	Common name	Role
I	Fibrinogen	Becomes fibrin
II	Prothrombin	Becomes thrombin (vitamin K dependent)
III	Tissue factor (thromboplastin)	Starts the extrinsic pathway; comes from damaged tissue, not plasma
IV	Calcium	Needed at many steps
V	Proaccelerin (labile factor)	Cofactor for Xa in the common pathway
VII	Proconvertin (stable factor)	Extrinsic pathway (vitamin K dependent)
VIII	Antihemophilic factor	Cofactor for IXa; missing in hemophilia A
IX	Christmas factor	Intrinsic pathway; missing in hemophilia B (vitamin K dependent)
X	Stuart-Prower factor	Start of the common pathway (vitamin K dependent)
XI	Plasma thromboplastin antecedent	Intrinsic pathway
XII	Hageman factor	Contact activation; starts the intrinsic pathway in the test tube
XIII	Fibrin-stabilizing factor	Cross-links fibrin into a firm clot

Common Mistake! There is no factor VI (it turned out to be activated factor V, so the number was retired). Another trap: students tag factor VIII as vitamin K dependent because it sits near the others. Only II, VII, IX, and X depend on vitamin K, and the memory hook is 1972, which spells 10, 9, 7, 2.

The classic cascade has 2 starting routes that merge. The extrinsic pathway begins when injured tissue exposes tissue factor, which binds factor VII and activates factor X within seconds. The intrinsic pathway begins with contact activation of XII, then runs through XI and IX (with VIII as helper) to activate X more slowly, in minutes. Both routes meet at factor X. From there, the common pathway takes over: Xa with Va and calcium converts prothrombin to thrombin, thrombin turns fibrinogen into fibrin, and factor XIII cross-links the fibrin. Thrombin also activates V, VIII, XI, and XIII, so it speeds up its own production.

Figure 3: The clotting cascade, with extrinsic and intrinsic pathways merging at factor X.

Quick pause. Platelet plug first, fibrin mesh second. Extrinsic is fast and starts outside the blood, intrinsic is slower and starts inside it, and both meet at X. Hold that picture, because the next part is about how you'd test it.

Labs split the cascade in half. Prothrombin time (PT) checks the extrinsic and common pathways (VII, X, V, II, fibrinogen) and is reported as the INR to monitor warfarin. Activated partial thromboplastin time (aPTT) checks the intrinsic and common pathways (XII, XI, IX, VIII, X, V, II, fibrinogen) and monitors heparin. PT normally runs about 11 to 15 seconds, though ranges vary by lab. Hemophilia A (factor VIII) and hemophilia B (factor IX) are X-linked recessive, so boys are mainly affected. They give a long aPTT with a normal PT and a normal platelet count, and the bleeding often goes into joints (hemarthrosis), which is why you'll meet hemophilia in rehabilitation.

Now the classic-versus-current divide. Your exam expects the 2-pathway cascade, and the cascade model comes from the 1960s. Reviews such as Davie, Fujikawa, and Kisiel (1991) describe how clotting is initiated, maintained, and regulated. Current evidence refines this: in the body, tissue factor with VIIa is the main trigger, and the intrinsic factors (XI, IX, VIII) amplify the response rather than run as an independent route. That explains an odd clinical fact. People lacking factor XII don't bleed, while people lacking VIII or IX do.

Going deeper: the cell-based model of coagulation

Modern teaching describes 3 overlapping phases on cell surfaces. In initiation, tissue-factor-bearing cells generate a small spark of thrombin. In amplification, that thrombin activates platelets and factors V, VIII, and XI. In propagation, the IXa-VIIIa complex and the Xa-Va complex assemble on the platelet surface and produce a large burst of thrombin. The classic cascade is the shorthand for this, and it still matches how the PT and aPTT tests work.

The body also applies the brakes. Antithrombin inactivates thrombin and Xa, and heparin works by boosting it. Protein C and protein S inactivate factors Va and VIII, as StatPearls summarizes. Warfarin blocks vitamin K recycling, so factors II, VII, IX, and X are made in a nonfunctional form, and it takes days to act because factors already in plasma must fade first. In the lab, citrate, oxalate, and EDTA bind calcium (factor IV) and keep a sample liquid. Once the wound heals, plasminogen is converted to plasmin (by tissue plasminogen activator), and plasmin chops the fibrin mesh apart.

Clotting depends on proteins your body makes. A different set of proteins, written on the red-cell surface, decides who can safely receive whose blood.

Blood Groups: Why Can the Wrong Match Be Dangerous?

In 1901, Karl Landsteiner mixed red cells and serum from different people and noticed that some mixes clumped (agglutination) while others stayed smooth. That clumping reveals 2 things: antigens (markers on the red-cell surface, also called agglutinogens) and antibodies (proteins in plasma, also called agglutinins). The rule is simple: you carry antibodies against the antigens you lack.

Blood group	Antigen on red cell	Antibodies in plasma	Can donate red cells to	Can receive red cells from
A	A	Anti-B	A, AB	A, O
B	B	Anti-A	B, AB	B, O
AB	A and B	None	AB	A, B, AB, O
O	None	Anti-A and anti-B	A, B, AB, O	O

Why do these antibodies exist without any prior transfusion? Guyton and Hall explain that foods and gut bacteria carry A-like and B-like molecules, so in the first months of life the immune system makes IgM antibodies against whichever type the baby's own cells lack. The genes for A and B are codominant, while O is recessive, and the gene sits on chromosome 9.

See it in action. Give group A blood to a group B patient and the patient's anti-A antibodies meet the donor's A antigens. The donor cells clump and then burst (hemolysis). The free hemoglobin and clumps cause fever, a fall in blood pressure, and kidney injury. Notice the jaundice story returning here: hemolysis means too much bilirubin made.

Now a second system. Another antigen, D, defines the Rh system: D present means Rh-positive, absent means Rh-negative. Here's the contrast that separates it from ABO. ABO antibodies appear naturally in everyone, while anti-D antibodies appear only after exposure, through a transfusion or a pregnancy.

That difference creates hemolytic disease of the newborn (erythroblastosis fetalis). An Rh-negative mother carries an Rh-positive baby. At delivery, a few fetal cells enter her blood and she makes anti-D (an IgG, small enough to cross the placenta). The first baby usually escapes. A later Rh-positive baby doesn't, because her IgG crosses the placenta and attacks the baby's red cells. The result is anemia and jaundice, sometimes severe. Giving the mother anti-D immunoglobulin after sensitizing events prevents her from making her own.

Now the classic-versus-current note. Your exam expects "O negative is the universal donor and AB positive the universal recipient," because O negative red cells carry no A, B, or D antigens, so the recipient's antibodies have nothing to attack. Practice refines this: other antigen systems exist (Kell, Duffy, Kidd), so hospitals prefer type-matched blood and cross-match every transfusion, keeping O negative mainly for emergencies when there's no time to test.

Exam Pointer Two classics: why the first Rh-incompatible pregnancy usually escapes harm, and why O negative is the "universal donor" for red cells. Practice the 4-group table until you can rebuild it from the rule alone: you carry antibodies against the antigens you lack.

Blood groups sit on the red cell's surface. One more test, a simple 1-hour one, reads the red cells and the plasma together.

ESR: What Does the Speed of Settling Tell You?

Let a tube of anticoagulated blood stand upright and the red cells slowly sink, leaving clear plasma above. The distance they fall in the first hour, in mm, is the erythrocyte sedimentation rate (ESR). We're back at the spun tube from the start, except gravity does the work now.

1. Aggregation (roughly the first 10 minutes): red cells stack like coins into rouleaux.
2. Sedimentation (roughly the next 40 minutes): the heavier stacks fall at a steady speed.
3. Packing (roughly the last 10 minutes): cells pile at the bottom and settling slows.

Why do rouleaux form? Red cells carry a negative surface charge, so they repel each other (the zeta potential). Fibrinogen and globulins coat the cells and reduce that repulsion, so the cells stick face to face. A single big stack falls faster than the same cells falling one by one. That's the whole mechanism.

The Westergren method mixes blood with sodium citrate in a 4:1 ratio, fills a 200 mm vertical tube, and reads the cell column after 1 hour. The Wintrobe method uses a shorter 100 mm tube with EDTA blood. Typical Westergren upper limits are roughly 15 mm/hr in men and 20 mm/hr in women, and they rise with age. Textbooks and labs quote slightly different cut-offs.

Condition	ESR	Why
Infection, inflammation (TB, rheumatoid arthritis)	Raised	More fibrinogen and globulins, so more rouleaux
Anemia	Raised	Fewer cells, so less crowding and faster stacks
Pregnancy, older age, female sex	Raised	Higher fibrinogen or a higher baseline
Polycythemia	Lowered	A crowd of cells hinders stacking and sinking
Sickle cell disease, spherocytosis	Lowered	Abnormal shapes can't form rouleaux
Low fibrinogen	Lowered	Less protein to cut the repulsion between cells

ESR is a nonspecific test. A raised value says something inflammatory may be going on, never which disease it is. CRP generally rises and falls faster than ESR, so the 2 tests answer slightly different questions.

That's blood physiology in 1 tube. ESR reads the plasma proteins and the behavior of the red cells together, so you've now met every layer of the tube you started with.

Quick Recap: High-Yield Points

• Blood physiology in brief: plasma (about 55%), buffy coat (WBCs and platelets, under 1%), red cells (hematocrit about 40 to 45%).
• Serum = plasma minus fibrinogen and used-up clotting factors. Albumin gives about 80% of the colloid osmotic pressure; low albumin causes edema.
• RBC: biconcave, no nucleus, lives 120 days. Low oxygen raises EPO (about 90% kidney). Iron deficiency makes small cells; B12 or folate deficiency makes big ones.
• Hb: 2 alpha + 2 beta chains, 4 hemes, 1.34 mL O₂ per g. Right shift (CADET): CO₂, Acid, 2,3-DPG, Exercise, Temperature.
• Spleen: filters old RBCs, immune defense, stores about one-third of platelets. Splenectomy raises the risk of infection by encapsulated bacteria.
• Bilirubin path: heme to biliverdin to unconjugated bilirubin (on albumin), liver conjugation, bile, urobilinogen, stercobilin (stool) and urobilin (urine).
• Jaundice: pre-hepatic (unconjugated up, acholuric), hepatic (both up), obstructive (conjugated up, pale stool, dark urine).
• WBC: 4,000 to 11,000 per microliter. Never Let Monkeys Eat Bananas: neutrophil, lymphocyte, monocyte, eosinophil, basophil.
• Platelets 150,000 to 400,000 per microliter. Hemostasis: spasm, plug, coagulation, retraction and repair. Vitamin K factors: II, VII, IX, X (1972). PT checks extrinsic, aPTT checks intrinsic.
• ABO: you carry antibodies against the antigens you lack. Anti-D forms only after exposure, which explains hemolytic disease of the newborn.
• ESR: rouleaux formation drives it. Raised by fibrinogen, globulins, and anemia; lowered by polycythemia and abnormal red-cell shapes.

Blood Physiology FAQs: Quick Answers for Students

What is the difference between plasma and serum?

Plasma is the liquid part of blood when clotting is prevented, so it still holds fibrinogen and the other clotting factors. Serum is the liquid left after blood has clotted, so fibrinogen and most other clotting factors have been used up. Here's the one-line test: if fibrinogen is still in the liquid, it's plasma. Plasma needs an anticoagulant tube (citrate, EDTA, or heparin), while serum comes from a plain tube that's allowed to clot.

Why do red blood cells have no nucleus?

Losing the nucleus frees space for more hemoglobin, and the dimpled shape lets the cell bend through narrow capillaries. The trade-off is that the cell can't make new proteins or repair itself, so it wears out in about 120 days. Without mitochondria, it also runs on glucose by anaerobic glycolysis, which means it never uses up the oxygen it carries.

What are the 3 types of jaundice and how do you tell them apart?

Pre-hepatic jaundice comes from too much bilirubin being made, as in hemolysis, so unconjugated bilirubin rises and urine stays free of bilirubin. Hepatic jaundice comes from liver cells failing to process it, so both forms rise. Obstructive jaundice comes from blocked bile flow, so conjugated bilirubin rises, urine turns dark, and stools turn pale. The urine and stool findings separate the types fastest.

Which clotting factors depend on vitamin K?

Factors II, VII, IX, and X, plus proteins C and S, need vitamin K to be made in working form in the liver. A handy memory hook is 1972, which is 10, 9, 7, and 2. This matters because warfarin blocks vitamin K recycling, and because bile blockage or fat malabsorption can lower vitamin K and prolong the PT.

Why is O negative called the universal donor?

O negative red cells carry no A, B, or D antigens on their surface, so a recipient's anti-A, anti-B, or anti-D antibodies have nothing to attack. That's why emergency teams reach for it when there's no time to test. In routine practice, though, matched blood is preferred, and cross-matching is still done because other antigen systems exist.

Why does ESR rise in infection and fall in polycythemia?

Infection raises fibrinogen and globulins, which reduce the negative charge keeping red cells apart, so the cells stack into rouleaux and settle faster. In polycythemia, the crowd of red cells hinders stacking and settling, and the ESR drops. Anemia works the other way: fewer cells means less crowding, so the ESR tends to rise.

What does the spleen do, and can you live without it?

The spleen filters old red cells, stores about a third of the platelets, and uses its white pulp to mount immune responses against blood-borne microbes. You can live without it, because the liver and bone marrow take over the red-cell clean-up. The cost is a higher risk of serious infection, especially from encapsulated bacteria, so people without a spleen are typically vaccinated.

This article is written for educational purposes for health-science students. It is not medical advice and never replaces clinical training, professional judgment, or your institution's curriculum. Real clinical decisions always require proper patient assessment under qualified supervision.

References

1. Hall JE, Hall ME. Guyton and Hall Textbook of Medical Physiology. 14th ed. Elsevier; 2021.
2. Barrett KE, Barman SM, Brooks HL, Yuan JXJ. Ganong's Review of Medical Physiology. 26th ed. McGraw Hill; 2019.
3. Sembulingam K, Sembulingam P. Essentials of Medical Physiology. Jaypee Brothers Medical Publishers.
4. Davie EW, Fujikawa K, Kisiel W. "The coagulation cascade: initiation, maintenance, and regulation." Biochemistry. 1991;30(43):10363-10370. doi:10.1021/bi00107a001. PMID: 1931959.
5. StatPearls Publishing. "Physiology, Coagulation Pathways." StatPearls. NCBI Bookshelf.
6. StatPearls Publishing. "Fetal Hemoglobin." StatPearls. NCBI Bookshelf.
7. World Health Organization. "Haemoglobin concentrations for the diagnosis of anaemia and assessment of severity." Vitamin and Mineral Nutrition Information System. 2011.