Transport
Blood is the body’s main internal transport fluid, carrying respiratory gases, nutrients, wastes, hormones, and heat while defending against infection and sealing damaged vessels. The AAMC MCAT content outline tests blood composition, oxygen and carbon dioxide transport by hemoglobin, the ABO and Rh blood-group systems, and hemostasis (clotting). This is a high-yield area for the Bio/Biochem section.
Blood Composition
Blood is a specialized fluid connective tissue. A healthy adult has ~5 L. When spun down it separates into a fluid plasma (~55% by volume) and formed elements (~45%). The packed red-cell fraction is the hematocrit (~45% in men, ~40% in women).
The straw-colored fluid matrix — about 90% water. Its solutes:
- Plasma proteins (~7–8%): albumin (most abundant; maintains blood osmotic/oncotic pressure and transports fatty acids & drugs), globulins (transport proteins and antibodies/immunoglobulins), and fibrinogen (soluble clotting precursor). All made by the liver except antibody globulins (plasma cells).
- Electrolytes: Na+, K+, Cl−, HCO3−, Ca2+ — set osmotic balance and pH.
- Nutrients (glucose, amino acids, lipids as lipoproteins), wastes (urea, creatinine, bilirubin), hormones, and dissolved gases.
- Plasma vs Serum
- Plasma = the fluid of unclotted blood; contains all clotting factors, including fibrinogen. Serum = the fluid left after blood has clotted; it is plasma minus fibrinogen and the consumed clotting factors.
- Hematocrit
- The percentage of blood volume occupied by red blood cells. Raised in dehydration and polycythemia; lowered in anemia.
All formed elements arise from a common stem cell in red bone marrow (hematopoiesis).
- Erythrocytes (red blood cells, RBCs) — ~5 million/mm3. Biconcave and (in mammals) anucleate with no mitochondria, so they rely on anaerobic glycolysis and cannot consume the O2 they carry. Packed with hemoglobin. Lifespan ~120 days; old cells are broken down by the spleen and liver, and iron is recycled. RBC production (erythropoiesis) is driven by erythropoietin from the kidney in response to low O2.
- Leukocytes (white blood cells, WBCs) — ~4,000–11,000/mm3. Granulocytes (neutrophils, eosinophils, basophils) and agranulocytes (lymphocytes, monocytes). Defense and immunity; neutrophils are most numerous and phagocytose bacteria.
- Platelets (thrombocytes) — ~150,000–400,000/mm3. Anucleate cell fragments shed from megakaryocytes. Essential for hemostasis.
| Feature | Erythrocyte (RBC) | Leukocyte (WBC) | Platelet |
|---|---|---|---|
| Approx. count /mm3 | ~5,000,000 | ~4,000–11,000 | ~150,000–400,000 |
| Nucleus | Absent (mature) | Present | Absent (fragment) |
| Main role | O2/CO2 transport | Immune defense | Clotting (hemostasis) |
| Lifespan | ~120 days | Hours to years | ~8–10 days |
| Origin | Red bone marrow (hematopoietic stem cells) | ||
Gas Transport in Blood
The chief transport job of blood is moving O2 from lungs to tissues and CO2 back. (The mechanics of gas exchange at the alveoli and the shape of the dissociation curve are detailed in Respiration; here we focus on the carrier chemistry.)
Hemoglobin (Hb) is a tetramer — four globin chains (adult HbA = 2α + 2β), each cradling one heme group with a central Fe2+ that reversibly binds one O2. So one Hb carries up to four O2. About 98.5% of blood O2 is bound to Hb (oxyhemoglobin); only ~1.5% is dissolved in plasma. Binding is cooperative — each O2 bound raises the affinity for the next — giving the sigmoid (S-shaped) dissociation curve. Fetal hemoglobin (HbF, 2α + 2γ) has higher O2 affinity than HbA, letting the fetus pull O2 from the mother across the placenta.
CO2 travels from tissues to lungs in three forms:
- ~70% as bicarbonate (HCO3−). In the RBC, carbonic anhydrase speeds CO2 + H2O → H2CO3 → H+ + HCO3−. HCO3− leaves the cell for plasma in exchange for Cl− — the chloride shift — while H+ is buffered by hemoglobin.
- ~23% as carbaminohemoglobin — CO2 bound to the amino groups of globin (not the heme iron).
- ~7% dissolved in plasma.
Because H+ and CO2 lower Hb’s O2 affinity (Bohr effect) and O2 binding lowers CO2/H+ carriage (Haldane effect), gas loading and unloading reinforce each other at the tissues and lungs.
Blood Groups
Blood groups are defined by the antigens on the RBC surface and the matching antibodies in plasma. The two clinically critical systems are ABO and Rh.
Two possible antigens (A and B) on RBCs; the plasma carries pre-formed antibodies against whichever antigen is absent:
- Group A — A antigen; anti-B antibody in plasma.
- Group B — B antigen; anti-A antibody.
- Group AB — both antigens; no anti-A or anti-B antibody. Universal recipient.
- Group O — no A/B antigen; both anti-A and anti-B antibodies. Universal donor.
A mismatched transfusion triggers agglutination: recipient antibodies cross-link donor RBCs, causing clumping and hemolysis.
The Rh (D) antigen is present (Rh+) in ~85% of people or absent (Rh−). Anti-Rh antibodies are not pre-formed — an Rh− person makes them only after exposure to Rh+ blood.
Hemolytic disease of the newborn (erythroblastosis fetalis) can arise when an Rh− mother carries an Rh+ fetus. At the first delivery fetal RBCs enter maternal blood and she makes anti-Rh IgG; in a later Rh+ pregnancy that IgG crosses the placenta and attacks fetal RBCs. Prevented by giving anti-D immunoglobulin (RhoGAM) to Rh− mothers.
| Group | Antigen on RBC | Antibody in plasma | 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 | All groups (universal recipient) |
| O | None | anti-A and anti-B | All groups (universal donor, esp. O−) | O only |
Hemostasis & Blood Clotting
Hemostasis stops bleeding from a damaged vessel in three overlapping steps:
- 1. Vascular spasm — the injured vessel constricts, reducing blood loss.
- 2. Platelet plug — platelets adhere to exposed collagen (via von Willebrand factor), activate, and aggregate into a temporary plug.
- 3. Coagulation cascade — a chain of plasma clotting factors converges on a common pathway. Prothrombin is converted to thrombin (by activated factor X, with Ca2+ and phospholipid); thrombin then converts soluble fibrinogen → insoluble fibrin, which forms a mesh that traps cells to make a stable clot.
The cascade is triggered two ways that merge at factor X:
- Intrinsic pathway — activated by contact with exposed collagen inside the vessel; all factors are already in the blood.
- Extrinsic pathway — activated by tissue factor (factor III) released from damaged tissue; faster.
- Common pathway — factor X→Xa → prothrombin→thrombin → fibrinogen→fibrin, cross-linked by factor XIII.
Ca2+ (factor IV) and vitamin K (needed by the liver to make prothrombin and factors VII, IX, X) are essential. Later, the enzyme plasmin digests fibrin to dissolve the clot (fibrinolysis).
- Hemophilia
- Inherited (usually X-linked) deficiency of a clotting factor — factor VIII (hemophilia A) or IX (hemophilia B) — causing prolonged bleeding.
- Thrombocytopenia
- Abnormally low platelet count → easy bruising and bleeding.
- Thrombosis / embolism
- A clot (thrombus) forming inappropriately inside an intact vessel; a fragment that breaks free and lodges elsewhere is an embolus.
Worked MCQs
Five MCQs covering the high-yield testing patterns for blood and transport.
Q1. Which plasma protein is chiefly responsible for maintaining the osmotic (oncotic) pressure of blood?
Albumin is the most abundant plasma protein and the main determinant of blood colloid osmotic (oncotic) pressure, drawing fluid back into capillaries. Low albumin causes edema. Fibrinogen is for clotting, globulins for transport/immunity, and hemoglobin is inside RBCs, not free in plasma.
Q2. Most carbon dioxide is transported in the blood as:
About 70% of CO2 is carried as bicarbonate. Carbonic anhydrase in the RBC converts CO2 + H2O to H2CO3, which dissociates to H+ + HCO3−; the bicarbonate leaves for plasma in exchange for Cl− (chloride shift). Carboxyhemoglobin is the toxic CO complex, not a normal CO2 carrier.
Q3. A person with blood group O− is correctly described as the:
O− RBCs display no A, B, or Rh(D) antigens, so they provoke no reaction in any recipient — the universal donor. However, O plasma contains both anti-A and anti-B antibodies, so an O person can safely receive only O blood. AB+ is the universal recipient.
Q4. Serum differs from plasma in that serum:
Serum is the fluid left after blood has clotted, so its fibrinogen has been converted to fibrin and trapped in the clot. Plasma (from unclotted blood) still contains fibrinogen and all clotting factors. Both are cell-free and both retain antibodies and ions.
Q5. In the final step of the coagulation cascade, thrombin catalyzes the conversion of:
Thrombin cleaves soluble fibrinogen into insoluble fibrin, which polymerizes into the clot mesh (stabilized by factor XIII). Prothrombin→thrombin is the step before this, driven by factor Xa. Plasmin later breaks the clot down (fibrinolysis).
Quick Recap
- Blood ≈ 55% plasma + 45% formed elements; ~5 L total; hematocrit ≈ RBC fraction.
- Plasma proteins: albumin (osmotic pressure), globulins (transport/antibodies), fibrinogen (clotting).
- Plasma has fibrinogen; serum does not.
- RBCs (anucleate, Hb-packed, ~120 days), WBCs (immunity), platelets (clotting) — all from red marrow.
- O2: ~98.5% carried on hemoglobin (4 heme, cooperative, sigmoid curve); HbF has higher affinity than HbA.
- CO2: ~70% bicarbonate (chloride shift), ~23% carbaminohemoglobin, ~7% dissolved.
- CO forms carboxyhemoglobin (~250× affinity) — lethal, blocks O2.
- ABO: A→anti-B, B→anti-A, AB universal recipient, O universal donor; Rh− makes anti-Rh only after exposure.
- Hemostasis: vascular spasm → platelet plug → cascade (prothrombin→thrombin→fibrinogen→fibrin); needs Ca2+ and vitamin K.