Blood Composition

9 min read Updated Mar 26, 2026

Blood is not just a red liquid. It is a complex tissue made up of cells suspended in a protein-rich fluid. An average adult has about 5 liters of blood, and every component has a specific job. Understanding blood composition is essential for MCAT questions on gas transport, immunity, clotting, and transfusion compatibility.

Diagram showing a centrifuged blood sample separated into three layers: plasma on top (55%), buffy coat of white blood cells and platelets in the middle, and packed red blood cells on the bottom (45%) — When blood is centrifuged, it separates into three layers: plasma (top), the buffy coat of WBCs and platelets (middle), and packed red blood cells (bottom). *Credit: OpenStax Anatomy and Physiology 2e, CC BY 4.0*

Plasma (~55% of Blood Volume)

Plasma is the liquid matrix of blood. It is about 90% water and 10% dissolved solutes:

Plasma proteins (~7%) - albumin (maintains oncotic pressure), globulins (antibodies and transport), fibrinogen (clotting precursor)
Electrolytes - Na+, K+, Ca2+, Cl-, HCO3-
Nutrients - glucose, amino acids, lipids
Waste products - urea, creatinine, bilirubin
Dissolved gases - O2, CO2
Hormones - transported to target organs

Serum is plasma minus the clotting factors (what you get after blood clots and the clot is removed).

Red Blood Cells (Erythrocytes)

RBCs are the most abundant cells in blood (~4.5-5.5 million per microliter). Their sole purpose is gas transport.

Wright-stained blood smear viewed through a microscope showing stained blood cells at high magnification — A Wright-stained blood smear viewed through a light microscope. This is the type of preparation used in clinical labs to count and identify blood cell types. *Credit: Pexels, free to use*

The proportion of blood volume occupied by RBCs is called the hematocrit (normally ~42-52% in males, ~37-47% in females). RBC production is stimulated by erythropoietin (EPO), a hormone released by the kidneys in response to low oxygen levels (hypoxia). This is why people living at high altitude develop higher hematocrit - their kidneys sense low O2 and ramp up RBC production.

Interactive 3D Hemoglobin. Rotate to see the four subunits (2 alpha, 2 beta) and the heme groups that bind oxygen. Credit: Nima via Sketchfab, CC BY

Key features of RBCs:

Biconcave disc shape - maximizes surface area for gas exchange and allows flexibility to squeeze through narrow capillaries
No nucleus or organelles - mature RBCs eject their nucleus during development, leaving more room for hemoglobin
Packed with hemoglobin - each RBC contains ~280 million hemoglobin molecules
No mitochondria - RBCs rely entirely on anaerobic glycolysis for their own energy (they cannot consume the oxygen they carry)
Lifespan: ~120 days - old RBCs are removed by macrophages in the spleen and liver

Hemoglobin is a tetrameric protein (two alpha and two beta subunits in adult hemoglobin, HbA), each containing a heme group with an iron (Fe2+) atom that binds one O2 molecule. Each hemoglobin molecule can carry 4 O2 molecules.

The Oxygen-Hemoglobin Dissociation Curve

Hemoglobin does not bind oxygen all-or-nothing. As each O2 binds, the hemoglobin subunits shift toward a higher-affinity state, making the next O2 easier to bind. This positive cooperativity produces the classic sigmoid (S-shaped) dissociation curve when % saturation is plotted against PO2.

Oxygen-hemoglobin dissociation curve showing sigmoid relationship between oxygen partial pressure and percent hemoglobin saturation, with right shift due to increased CO2, H+, temperature, and 2,3-BPG — The sigmoid O2-Hb dissociation curve. The plateau at high PO2 (lungs) ensures near-complete loading; the steep portion at low PO2 (tissues) ensures rapid unloading. Rightward shifts promote unloading; leftward shifts promote loading. *Credit: Wikimedia Commons, CC BY-SA 4.0*

Two regions of the curve matter:

Plateau (high PO2, ~100 mmHg in lungs): Hemoglobin stays ~97% saturated even if PO2 drops somewhat. Loading is protected.
Steep portion (low PO2, ~40 mmHg in tissues): Small drops in PO2 cause large drops in saturation. Unloading is maximized where O2 is needed.

Right Shift = More Unloading

A rightward shift means hemoglobin releases O2 more readily at any given PO2 (lower affinity). This happens in metabolically active tissue, which is exactly where extra O2 is needed.

The Bohr effect is the specific case of ↑CO2 and ↑H+ lowering hemoglobin’s O2 affinity. Active muscle produces both, so the curve shifts right exactly where O2 needs to be delivered. A leftward shift (↓CO2, ↓H+, ↓temp, ↓2,3-BPG) means tighter O2 binding and less unloading.

Fetal Hemoglobin

Fetal hemoglobin (HbF) has two alpha and two gamma subunits instead of two alpha, two beta. The gamma subunits bind 2,3-BPG more weakly, so HbF has a higher O2 affinity than adult HbA. Its dissociation curve sits to the left of the adult curve, letting the fetus pull O2 off maternal hemoglobin at the placenta.

White Blood Cells (Leukocytes)

WBCs defend the body against infection. They are far less numerous than RBCs (~5,000-10,000 per microliter) but are larger and nucleated. The specific roles of neutrophils, macrophages, and other frontline defenders are covered in innate immune cells.

Cell Type	Function	Key Facts
Neutrophils	First responders; phagocytose bacteria	Most abundant WBC; multilobed nucleus; short-lived; pus = dead neutrophils
Lymphocytes	Adaptive immunity (B cells, T cells, NK cells)	Second most abundant; small cells with large nucleus
Monocytes	Become macrophages in tissue; phagocytosis + antigen presentation	Largest WBC; kidney-shaped nucleus
Eosinophils	Combat parasites; modulate allergic response	Bilobed nucleus; red-staining granules
Basophils	Release histamine and heparin; involved in allergic reactions	Rarest WBC; large purple/blue granules; similar to mast cells

Platelets (Thrombocytes)

Platelets are not true cells - they are small, anucleate cell fragments (~150,000-400,000 per microliter) produced by megakaryocytes in the bone marrow. Platelet production is stimulated by thrombopoietin, a hormone from the liver and kidneys.

Platelets are essential for hemostasis (stopping bleeding). When a blood vessel is damaged, platelets adhere to exposed collagen, activate, change shape, and aggregate to form a temporary platelet plug. They also release chemicals that initiate the coagulation cascade.

Blood Types: ABO System

Blood type is determined by antigens on the surface of RBCs and antibodies in the plasma.

Diagram showing the four ABO blood types with their corresponding surface antigens on red blood cells and antibodies in the plasma — The ABO blood group system. Each blood type has specific antigens on RBCs and antibodies in the plasma against the antigens it lacks. *Credit: OpenStax Anatomy and Physiology 2e, CC BY 4.0*

Blood Type	Antigens on RBCs	Antibodies in Plasma	Can Donate To	Can Receive From
A	A antigen	Anti-B	A, AB	A, O
B	B antigen	Anti-A	B, AB	B, O
AB	Both A and B	Neither (universal recipient)	AB only	A, B, AB, O
O	Neither	Both anti-A and anti-B (universal donor)	A, B, AB, O	O only

The Rh Factor

The Rh system adds a second layer of compatibility. If you have the Rh antigen (Rh D protein) on your RBCs, you are Rh-positive (+). If not, you are Rh-negative (-).

Rh incompatibility in pregnancy: An Rh-negative mother carrying an Rh-positive fetus can develop anti-Rh antibodies if fetal blood enters her circulation (usually during delivery). These antibodies are harmless to the first baby but can attack the RBCs of subsequent Rh-positive fetuses, causing hemolytic disease of the newborn. Prevention: the mother receives Rh immunoglobulin injections to destroy any fetal Rh+ cells before her immune system can mount a response.

True universal donor: O-negative (no A, B, or Rh antigens). True universal recipient: AB-positive (no antibodies against any common antigens).

Why do mature red blood cells lack a nucleus and mitochondria? What is the advantage and disadvantage of this design?

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Advantage: More space for hemoglobin, maximizing oxygen-carrying capacity. Disadvantage: RBCs cannot repair themselves, synthesize new proteins, or undergo cell division. They rely on anaerobic glycolysis for energy (no mitochondria = no aerobic respiration). They wear out after ~120 days and must be replaced by new cells from the bone marrow.

A type B-negative patient urgently needs blood. No B-negative blood is available. What other blood type(s) can be safely transfused?

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O-negative. Type O blood has no A or B antigens (so the patient's anti-A antibodies will not react), and Rh-negative means no Rh antigen. O-negative is the universal donor. O-positive could also work in a life-threatening emergency (the Rh antigen is less immediately dangerous than ABO mismatches), but O-negative is the safest choice.