Blood
High-Yield Summary
- Blood ≈ 55% plasma (liquid) + 45% cells by volume.
- Plasma's 3 major proteins: albumin (osmotic pressure), globulins (antibodies), fibrinogen (clotting).
- Red blood cells: no nucleus, packed with hemoglobin, biconcave disc shape, ~120-day lifespan. Hematocrit = % blood volume that's RBCs.
- White blood cells split into granulocytes (neutrophils, eosinophils, basophils) and agranulocytes (lymphocytes, monocytes).
- Platelets are megakaryocyte fragments that initiate clotting; production regulated by thrombopoietin.
- All blood cells arise from bone marrow stem cells via hematopoiesis.
- ABO type depends on A/B surface antigens; antibodies form against whichever antigens a person lacks. Rh status depends on the D antigen.
Key Terms
- Hematocrit
- Percentage of blood volume made up by red blood cells; clinical proxy for oxygen-carrying capacity.
- Hemoglobin
- Oxygen-binding protein packed into RBCs; each molecule carries up to 4 O₂.
- Megakaryocyte
- Large bone marrow cell that fragments to produce platelets.
- Thrombopoietin
- Hormone (made in liver/kidneys) that regulates platelet production.
- Hematopoiesis
- Process by which bone marrow stem cells give rise to RBCs, WBCs, and platelets.
White Blood Cell Types
- Neutrophils (granulocyte)
- Most abundant WBC type; first responders to infection, specialize in fighting bacteria.
- Eosinophils (granulocyte)
- Target parasites; play a role in mediating allergic responses.
- Basophils (granulocyte)
- Least common type; release histamine, drive inflammatory/allergic reactions.
- Lymphocytes (agranulocyte)
- B cells, T cells, NK cells — target specific pathogens, provide long-term immunity.
- Monocytes (agranulocyte)
- Circulate in blood, migrate into tissue and become macrophages.
ABO / Rh Blood Typing
- Type A: A antigens → anti-B antibodies. Type B: B antigens → anti-A antibodies.
- Type AB: both antigens → no antibodies (universal recipient, especially AB positive).
- Type O: no antigens → anti-A AND anti-B antibodies (O negative = universal donor).
- Rh factor = D antigen. Present → Rh-positive; absent → Rh-negative.
- Rh-negative mother + Rh-positive fetus: usually fine 1st pregnancy, but mother can develop anti-Rh antibodies risking hemolytic disease of the newborn in later pregnancies — prevented with Rho(D) immune globulin.
Common MCAT Trap
- Transfusion reactions come from the RECIPIENT's antibodies attacking donor RBC antigens — e.g. giving type A blood to a type B person triggers the recipient's anti-A antibodies.
- Rh incompatibility usually doesn't harm the FIRST pregnancy — sensitization happens at delivery, and antibodies threaten a LATER Rh-positive pregnancy.
- RBCs lack a nucleus (maximizes hemoglobin space) — don't confuse this with WBCs, which retain nuclei.
Quick Recall
What are blood's three major plasma proteins and their roles?
Which blood type is the universal donor, and why?
What cell fragments initiate blood clotting, and where do they come from?
How is hemolytic disease of the newborn prevented?