Composition and Functions of Blood

1. Plasma: the liquid matrix

Plasma is roughly 91–92% water; the rest is solutes. The three major protein groups (made mainly in the liver, except the antibodies) are:

• Albumin — most abundant (~60% of plasma proteins). It provides most of the colloid osmotic (oncotic) pressure that holds water in vessels, and transports bilirubin, fatty acids, hormones, and some drugs.
• Globulins — alpha and beta globulins transport lipids, metals, and hormones and include some clotting factors; gamma globulins are immunoglobulins (antibodies) made by plasma cells.
• Fibrinogen — the soluble precursor of fibrin, essential for clotting. Plasma with the clotting proteins removed is called serum.

Other solutes include electrolytes (Na⁺, K⁺, Ca²⁺, Cl⁻, HCO₃⁻), nutrients (glucose, amino acids, lipids), wastes (urea, creatinine, bilirubin), dissolved gases (O₂, CO₂), and hormones.

2. Erythrocytes: structure, function, and lifecycle

Erythrocytes are biconcave discs (~7.5 µm) with no nucleus and no mitochondria. The biconcave shape maximizes surface area for gas diffusion and lets the cell flex through narrow capillaries. Without mitochondria they use anaerobic glycolysis for ATP, so they never consume the oxygen they carry. Each cell holds ~280 million hemoglobin molecules; hemoglobin's four heme groups each bind one O₂, and hemoglobin also carries some CO₂ and buffers pH.

Erythropoiesis occurs in red bone marrow. A hematopoietic stem cell (hemocytoblast) becomes a proerythroblast, matures through erythroblast stages while filling with hemoglobin, ejects its nucleus to become a reticulocyte, and is released into the blood, finishing maturation in about 1–2 days. The process is driven by erythropoietin (EPO), a hormone released by the kidneys in response to low tissue oxygen (hypoxia). Building hemoglobin also requires iron, vitamin B₁₂, folate, and amino acids.

Iron metabolism: dietary iron is absorbed in the small intestine, carried in blood bound to transferrin, and stored in the liver, spleen, and bone marrow as ferritin (and hemosiderin). Most body iron is recycled from old red cells rather than newly absorbed.

Destruction and bilirubin: after ~120 days, aging red cells are engulfed by macrophages in the spleen and liver. Globin is broken into amino acids for reuse and iron is salvaged. The heme ring is opened to biliverdin (green), then reduced to bilirubin (yellow, lipid-soluble). Bilirubin travels to the liver bound to albumin, is made water-soluble by conjugation with glucuronic acid, and is secreted into bile. Gut bacteria convert it to urobilinogen; some is reabsorbed and excreted in urine, and most becomes stercobilin, the brown pigment of feces. If bilirubin accumulates, tissues turn yellow — jaundice.

3. Blood typing: ABO and Rh

Blood type depends on antigens (agglutinogens) on the red-cell surface and antibodies (agglutinins) in the plasma:

• Type A — A antigen, anti-B antibodies
• Type B — B antigen, anti-A antibodies
• Type AB — both A and B antigens, no antibodies (universal recipient)
• Type O — neither antigen, both anti-A and anti-B antibodies (universal donor)

These anti-A/anti-B antibodies are pre-formed (IgM). If a mismatched transfusion mixes an antigen with its matching antibody, red cells agglutinate (clump) and then hemolyze — a potentially fatal transfusion reaction, so blood is crossmatched before any transfusion.

The Rh system centers on the D antigen. Rh-positive (Rh⁺) people have it; Rh-negative (Rh⁻) people do not. Unlike ABO, anti-D antibodies are not pre-formed — they appear only after exposure (an Rh⁻ person given Rh⁺ blood, or an Rh⁻ mother carrying an Rh⁺ fetus). In a later Rh⁺ pregnancy, maternal anti-D IgG can cross the placenta and destroy fetal red cells (hemolytic disease of the newborn). Giving Rh⁻ mothers anti-D immune globulin (RhoGAM) after delivery prevents this sensitization.

How it works

1. Bone marrow stem cells mature into red cells while loading up on hemoglobin.
2. Kidneys release EPO whenever tissue oxygen drops, speeding up production.
3. Mature red cells circulate about 120 days delivering O₂ and picking up CO₂.
4. Aging cells are engulfed by spleen and liver macrophages; iron and amino acids are recycled.
5. Heme is converted to bilirubin, conjugated in the liver, and excreted via bile and stool.
6. Plasma proteins — albumin, globulins, fibrinogen — maintain fluid balance, immunity, and clotting readiness.
7. Before any transfusion, ABO and Rh types are matched to avoid agglutination.

Common confusions

Memory aids

"A Great Fluid" — Albumin, Globulins, Fibrinogen, the three plasma proteins. For blood types, remember "O gives, AB receives" — O has no antigens to be attacked, and AB has no antibodies to attack.

Quick review

Topic Recap

Blood is plasma plus formed elements. Plasma is mostly water carrying albumin (oncotic pressure and transport), globulins (transport and antibodies), and fibrinogen (clotting). Erythrocytes are anucleate biconcave disks that carry O₂ and CO₂, are made in bone marrow under EPO control, recycle iron through transferrin and ferritin, and end their lives with heme becoming bilirubin for excretion. ABO and Rh antigens on red cells define blood type and dictate transfusion safety.

Knowledge Check

1. Which plasma protein contributes the most to colloid osmotic pressure?
2. Why does a mature red cell not consume the oxygen it carries?
3. What hormone, released from which organ in response to what stimulus, drives erythropoiesis?
4. Trace heme from an aging red cell to the brown color of stool.
5. A person is blood type A, Rh-negative. Which antibodies would you expect in the plasma before any transfusion?

Answers and Rationales

1. Albumin — it is the most abundant plasma protein and the main determinant of oncotic pressure that keeps water in the circulation.
2. It has no mitochondria, so it uses only anaerobic glycolysis and never uses the O₂ bound to hemoglobin for its own energy needs.
3. Erythropoietin (EPO) from the kidneys, released in response to tissue hypoxia; it accelerates red-cell production and maturation.
4. Heme → biliverdin → bilirubin → (liver conjugation) → bile → intestine → urobilinogen → stercobilin in feces. Blockage at any step causes jaundice.
5. Anti-B antibodies (from ABO type A) but no anti-D initially — anti-D would form only after exposure to Rh⁺ blood or an Rh⁺ pregnancy.