Blood Composition, Gas Transport, Immune Function (3B)

SECTION 1

Historical Context & Motivation

The study of blood has a remarkably deep history, evolving from Galenic humoral theory into a rigorous molecular science that underpins modern hematology and immunology. Ancient physicians recognized that blood was essential for life, yet they lacked the tools to discern its complex cellular and biochemical composition. The invention of the microscope in the seventeenth century opened the door to identifying discrete cellular elements, while the development of quantitative chemistry in the nineteenth century enabled researchers to isolate hemoglobin and characterize its oxygen-binding properties. These discoveries set the stage for understanding how blood simultaneously serves as a transport medium, a buffering system, and a mobile immune surveillance network—concepts that are central to MCAT Foundational Concept 3B.

1658

Jan Swammerdam Observes Red Blood Cells

Using early microscopy, Swammerdam became the first to describe red blood cells (erythrocytes), establishing that blood contained discrete cellular elements rather than being a homogeneous humor.

1862

Felix Hoppe-Seyler Crystallizes Hemoglobin

Hoppe-Seyler isolated and crystallized hemoglobin from blood, demonstrating that a single protein was responsible for oxygen binding and for blood's characteristic red color, thereby opening the field of respiratory biochemistry.

1904

Christian Bohr Describes the Bohr Effect

Christian Bohr (father of physicist Niels Bohr) demonstrated that elevated CO₂ and decreased pH reduce hemoglobin's oxygen affinity, producing the Bohr effect—a principle that explains enhanced O₂ unloading at metabolically active tissues.

1960

Max Perutz Solves Hemoglobin's Crystal Structure

Using X-ray crystallography, Perutz revealed the quaternary structure of hemoglobin, explaining cooperative binding at the molecular level and earning the Nobel Prize in Chemistry.

1980s–Present

Modern Immunology and Hematopoiesis

Advances in monoclonal antibody technology, flow cytometry, and genomics have elucidated the lineage relationships among blood cells, the molecular basis of innate and adaptive immunity, and the role of cytokines in orchestrating immune responses.

The central question that this topic addresses is both integrative and mechanistic: How do the cellular and molecular constituents of blood coordinate gas exchange, pH buffering, and immune surveillance to maintain organismal homeostasis? Answering this question requires an understanding of plasma composition, the oxygen-hemoglobin dissociation curve, CO₂ transport mechanisms, and the differentiation and function of leukocytes—all of which converge on the MCAT under the umbrella of Foundational Concept 3B.

SECTION 2

Core Principles & Definitions

Blood is a specialized connective tissue that constitutes roughly 7–8% of body weight in the average adult, totaling approximately 5 liters. It can be separated into two broad fractions: plasma (≈55% by volume), which is the acellular, protein-rich fluid; and the formed elements (≈45%), which include erythrocytes, leukocytes, and platelets. The ratio of formed elements to total blood volume is quantified as the hematocrit. Understanding these basic compositional categories is prerequisite to grasping how blood performs its three principal roles: gas transport, immune defense, and hemostasis.

Blood Composition

Plasma contains water (≈92%), plasma proteins (albumin, globulins, fibrinogen), electrolytes, nutrients, and waste products. The formed elements—erythrocytes, leukocytes, and thrombocytes—arise from hematopoietic stem cells in the bone marrow and serve specialized functions.

Gas Transport

Oxygen is carried primarily bound to hemoglobin's four heme groups (≈98.5%), with a small fraction dissolved in plasma. Carbon dioxide is transported as bicarbonate (≈70%), carbaminohemoglobin (≈23%), and dissolved CO₂ (≈7%). The interconversion between CO₂ and HCO₃⁻ is catalyzed by carbonic anhydrase.

Oxygen-Hemoglobin Dissociation

Hemoglobin exhibits cooperative binding, yielding a sigmoidal O₂-dissociation curve. Allosteric modulators—pH, pCO₂, temperature, and 2,3-bisphosphoglycerate (2,3-BPG)—shift the curve to optimize O₂ loading in the lungs and unloading at tissues.

Innate Immunity

Non-specific defenses include physical barriers (skin, mucosae), chemical barriers (lysozyme, complement), and cellular components such as neutrophils, macrophages, natural killer cells, and dendritic cells. Pattern recognition receptors (e.g., Toll-like receptors) detect conserved pathogen-associated molecular patterns (PAMPs).

Adaptive Immunity

Antigen-specific responses are mediated by B lymphocytes (humoral immunity) and T lymphocytes (cell-mediated immunity). Clonal selection, affinity maturation, and immunological memory enable faster, stronger secondary responses.

✦ KEY TAKEAWAY

Think of blood as a highly sophisticated logistics network. Plasma is the highway, carrying dissolved nutrients and wastes. Erythrocytes are specialized tanker trucks optimized for gas cargo. Leukocytes are mobile security patrols that identify and neutralize threats, while platelets serve as the rapid-response repair crew that patches damaged vessels. The efficiency of the entire system depends on each component functioning within tightly regulated parameters—much as a city's infrastructure collapses if any one utility fails.

SECTION 3

Visual Explanation — Blood Composition & Gas Transport

This diagram illustrates the two major compartments of blood—plasma and formed elements—along with the principal mechanisms of O₂ and CO₂ transport. Note the proportional distribution of each gas transport mode and the allosteric modulators that govern oxygen-hemoglobin affinity.

The diagram above captures the essential architecture of blood as a transport medium. In the upper panels, the distinction between plasma and formed elements is drawn with quantitative markers that frequently appear on the MCAT. Erythrocytes dominate the formed element fraction by sheer number (≈5 million per microliter), reflecting their singular dedication to gas exchange. Their biconcave disc morphology maximizes surface-area-to-volume ratio, facilitating rapid diffusion of O₂ and CO₂ across the membrane. In the lower panel, the three mechanisms of CO₂ transport are shown alongside their relative contributions. The chloride shift (also called the Hamburger phenomenon) is the compensatory exchange of Cl⁻ into erythrocytes as HCO₃⁻ is exported into plasma via the band 3 protein (AE1), maintaining electroneutrality. This mechanism couples CO₂ transport to the bicarbonate buffer system and is a high-yield MCAT integration point.

SECTION 4

Mathematical Framework — Gas Transport Equations

Several quantitative relationships govern gas transport in blood and are essential for the MCAT. The oxygen content of blood depends on both hemoglobin-bound and dissolved fractions, and the relationship between partial pressure of oxygen and hemoglobin saturation is described by the sigmoidal dissociation curve. Meanwhile, the bicarbonate buffer system is governed by the Henderson-Hasselbalch equation, which links blood pH to the ratio of bicarbonate to dissolved CO₂.

OXYGEN CONTENT OF BLOOD

CaO₂ = (1.34 × [Hb] × SaO₂) + (0.003 × PaO₂)

CaO₂ = arterial O₂ content (mL O₂/dL blood); 1.34 = Hüfner's constant (mL O₂/g Hb); [Hb] = hemoglobin concentration (g/dL); SaO₂ = fractional arterial O₂ saturation; 0.003 = solubility coefficient of O₂ in plasma (mL O₂/dL per mmHg); PaO₂ = partial pressure of O₂ (mmHg). The first term dominates under physiological conditions.

BICARBONATE BUFFER EQUILIBRIUM

CO₂ + H₂O ⇌ H₂CO₃ ⇌ H⁺ + HCO₃⁻

Carbonic anhydrase (CA) in erythrocytes catalyzes the first equilibrium with extraordinary speed (k_cat ≈ 10⁶ s⁻¹). The second equilibrium is spontaneous. The net effect: CO₂ produced at tissues is rapidly converted to HCO₃⁻, which is the primary form in which CO₂ is transported in plasma.

HENDERSON-HASSELBALCH EQUATION (BLOOD PH)

pH = 6.1 + log([HCO₃⁻] / (0.03 × PCO₂))

6.1 = pK_a of carbonic acid; [HCO₃⁻] = bicarbonate concentration (mEq/L; normal ≈ 24); 0.03 = solubility of CO₂ (mEq/L per mmHg); PCO₂ = partial pressure of CO₂ (mmHg; normal ≈ 40). Normal blood pH of 7.4 corresponds to [HCO₃⁻]/(0.03 × PCO₂) = 24/1.2 = 20:1.

HILL EQUATION (COOPERATIVE BINDING)

Y = (PO₂)ⁿ / (P₅₀ⁿ + (PO₂)ⁿ)

Y = fractional saturation of hemoglobin; PO₂ = partial pressure of oxygen; P₅₀ = PO₂ at which Hb is 50% saturated (≈26 mmHg for adult Hb under normal conditions); n = Hill coefficient (≈2.8 for Hb, reflecting positive cooperativity; n = 1 indicates no cooperativity, as in myoglobin). A higher Hill coefficient produces a steeper sigmoidal curve.

⚕ MCAT Integration Point

The Henderson-Hasselbalch equation connects respiratory physiology to acid-base chemistry. Respiratory acidosis arises from hypoventilation (↑ PCO₂ → ↓ pH), while respiratory alkalosis arises from hyperventilation (↓ PCO₂ → ↑ pH). Metabolic acidosis (↓ [HCO₃⁻]) and metabolic alkalosis (↑ [HCO₃⁻]) shift the ratio from the numerator side. Always identify whether the primary disturbance is respiratory (change in PCO₂) or metabolic (change in HCO₃⁻), then determine if compensation has occurred.

SECTION 5

Detailed Breakdown — Leukocytes and Immune Function

The immune function of blood resides predominantly in the leukocytes (white blood cells), which are derived from a common hematopoietic stem cell (HSC) in the bone marrow. Leukocytes bifurcate into two major lineages: the myeloid lineage (which produces granulocytes, monocytes/macrophages, and dendritic cells) and the lymphoid lineage (which generates T cells, B cells, and natural killer cells). The mnemonic "Never Let Monkeys Eat Bananas" captures the order of leukocyte abundance from highest to lowest: Neutrophils (60–70%), Lymphocytes (20–25%), Monocytes (3–8%), Eosinophils (1–4%), and Basophils (<1%).

This diagram traces hematopoiesis from the hematopoietic stem cell (HSC) through myeloid and lymphoid progenitors to their mature effector cells. The lower panel contrasts innate and adaptive immunity in terms of speed, specificity, and memory—distinctions that are heavily tested on the MCAT.

Summary of leukocyte types, relative abundance, functions, and distinguishing features

Leukocyte	% of WBCs	Key Functions	Granule Contents / Markers
Neutrophil	60–70%	Phagocytosis, respiratory burst (ROS), NETs; first responder to bacterial infection	Myeloperoxidase, defensins, lysozyme; multilobed nucleus
Lymphocyte	20–25%	B cells: antibody secretion; T helper: cytokine regulation; Cytotoxic T: kill virus-infected/cancer cells; NK: kill MHC I-deficient cells	CD markers (CD4, CD8, CD19/20); perforin, granzymes (CTL/NK)
Monocyte	3–8%	Differentiate into macrophages (tissue) or dendritic cells; phagocytosis, antigen presentation via MHC II	Largest WBC; kidney-shaped nucleus; CD14
Eosinophil	1–4%	Defense against parasites (helminths); modulate allergic inflammation	Major basic protein, eosinophil peroxidase; bilobed nucleus; eosin-staining granules
Basophil	<1%	Release histamine, heparin; mediate IgE-dependent allergic responses; functionally similar to tissue mast cells	Histamine, heparin; large blue-staining granules obscure nucleus

SECTION 6

Worked Example — Oxygen Content and pH Calculation

The following worked example integrates the oxygen content equation with the Henderson-Hasselbalch equation, simulating the type of integrative reasoning required on MCAT passages involving arterial blood gas (ABG) data.

Calculating Arterial O₂ Content and Blood pH from ABG Values

Step 1 — Identify Given ValuesA patient presents with the following arterial blood gas (ABG) results: [Hb] = 15 g/dL, SaO₂ = 0.97, PaO₂ = 95 mmHg, [HCO₃⁻] = 24 mEq/L, and PCO₂ = 40 mmHg. We are asked to determine the arterial oxygen content (CaO₂) and confirm the expected blood pH.

Step 2 — Calculate Hb-Bound O₂Using the oxygen content equation: Hb-bound O₂ = 1.34 × [Hb] × SaO₂ = 1.34 × 15 × 0.97.

Hb-bound O₂ = 19.50 mL O₂/dL

Step 3 — Calculate Dissolved O₂Dissolved O₂ = 0.003 × PaO₂ = 0.003 × 95.

Dissolved O₂ = 0.285 mL O₂/dL

Step 4 — Sum for Total CaO₂CaO₂ = 19.50 + 0.285 ≈ 19.8 mL O₂/dL. Notice that the dissolved fraction contributes less than 1.5% of total oxygen content, reinforcing why hemoglobin is indispensable.

CaO₂ ≈ 19.8 mL O₂/dL (normal range: 16–22 mL O₂/dL)

Step 5 — Calculate Blood pHUsing Henderson-Hasselbalch: pH = 6.1 + log([HCO₃⁻]/(0.03 × PCO₂)) = 6.1 + log(24/(0.03 × 40)) = 6.1 + log(24/1.2) = 6.1 + log(20) = 6.1 + 1.30.

pH = 7.40 — consistent with normal arterial pH

Step 6 — Interpret ResultsThe patient's CaO₂ of 19.8 mL O₂/dL and pH of 7.40 are both within normal physiological ranges, indicating adequate oxygenation and acid-base balance. If [Hb] were reduced to 10 g/dL (anemia), CaO₂ would fall to approximately 13.3 mL O₂/dL even if SaO₂ and PaO₂ remained normal, illustrating why anemia impairs O₂ delivery without necessarily lowering PaO₂ or SaO₂.

SECTION 7

Hemoglobin vs. Myoglobin & Fetal Hemoglobin

A high-yield MCAT comparison involves distinguishing adult hemoglobin (HbA), fetal hemoglobin (HbF), and myoglobin (Mb). These three oxygen-binding proteins share the globin fold and heme prosthetic group, yet their quaternary structures and allosteric properties differ dramatically, producing distinct dissociation curves that reflect their physiological niches.

Comparison of oxygen-binding proteins relevant to MCAT

Feature	Hemoglobin A (HbA)	Fetal Hemoglobin (HbF)	Myoglobin (Mb)
Subunit composition	α₂β₂ (tetramer)	α₂γ₂ (tetramer)	Monomer (single polypeptide)
O₂-binding curve	Sigmoidal (cooperative)	Sigmoidal, left-shifted vs. HbA	Hyperbolic (no cooperativity)
P₅₀	≈26 mmHg	≈19 mmHg (higher affinity)	≈2.8 mmHg (very high affinity)
2,3-BPG sensitivity	Yes — binds β-chain cavity	Reduced — γ-chains bind BPG weakly	No (monomer, no allosteric pocket)
Bohr effect	Present	Reduced	Minimal
Physiological role	O₂ transport in blood; load at lungs, unload at tissues	Extract O₂ from maternal HbA across placenta	Intracellular O₂ storage in muscle; buffer against ischemia

✦ KEY TAKEAWAY

The relationship among HbA, HbF, and myoglobin is analogous to a relay race with runners of different speeds at different legs. HbA is the versatile middle-leg runner that picks up O₂ where partial pressure is high (lungs) and hands it off where partial pressure is low (tissues). HbF is the stronger grabber that can pull the baton (O₂) from HbA at the placenta because its higher affinity ensures net O₂ transfer from mother to fetus. Myoglobin is the anchor runner that never lets go of the baton until tissue PO₂ drops to critically low levels, serving as an intracellular O₂ reservoir in metabolically active muscle. Each protein's dissociation curve reflects its role in this cascade.

SECTION 8

Connection to Advanced Immunology & Clinical Correlations

While the MCAT does not require memorization of specific diseases, it does expect you to apply immunological principles to novel experimental scenarios. Understanding the interplay between MHC class I and class II presentation, clonal selection, and the effector arms of the immune response provides the framework for interpreting passage-based questions about autoimmunity, transplant rejection, and vaccine design. Additionally, concepts from this lesson bridge directly to Foundational Concept 3C (electrolyte balance and kidney function) and Foundational Concept 5 (molecular biology of immune receptor generation via V(D)J recombination).

MHC Class I vs. Class II antigen presentation pathways

Feature	MHC Class I	MHC Class II
Distribution	All nucleated cells (not mature RBCs)	Antigen-presenting cells (APCs): macrophages, dendritic cells, B cells
Peptide source	Endogenous (cytosolic) — proteasome-processed	Exogenous (endosomal/lysosomal) — phagocytosed antigens
Recognized by	CD8⁺ cytotoxic T lymphocytes (CTLs)	CD4⁺ helper T cells (Tₕ)
Structure	α chain (3 domains) + β₂-microglobulin	α chain (2 domains) + β chain (2 domains)
Functional outcome	CTL-mediated killing via perforin/granzymes → apoptosis of target	Tₕ activation → cytokine release → B cell activation, macrophage activation

Looking forward, advanced immunology courses build on these MCAT foundations with topics such as T cell receptor (TCR) signaling cascades, the molecular basis of positive and negative thymic selection, somatic hypermutation and class switching in B cells, and the regulatory T cell (T_reg) networks that prevent autoimmunity. Similarly, the gas transport equations presented here are extended in pulmonary physiology to include ventilation-perfusion (V/Q) matching, alveolar gas equations, and diffusion capacity measurements. Mastering the fundamental principles in this lesson provides the scaffold upon which these advanced topics are built.

⚠ Clinical Connection

Carbon monoxide (CO) binds hemoglobin with approximately 200–250 times greater affinity than O₂, forming carboxyhemoglobin (HbCO). This left-shifts the O₂-Hb dissociation curve for the remaining unaffected subunits, impairing both O₂ loading and unloading. Critically, pulse oximetry cannot distinguish HbCO from HbO₂, so SaO₂ readings appear falsely normal in CO poisoning — a fact that MCAT passages may exploit.

SECTION 9

Practice Problems

PROBLEM 1 — CONCEPTUAL

A patient with chronic obstructive pulmonary disease (COPD) exhibits chronically elevated PCO₂. Predict the direction of the shift in the oxygen-hemoglobin dissociation curve and explain the physiological advantage and disadvantage of this shift.

PROBLEM 2 — BASIC CALCULATION

A patient has [Hb] = 12 g/dL, SaO₂ = 0.95, and PaO₂ = 85 mmHg. Calculate the arterial oxygen content (CaO₂). Is this value normal, and what condition might explain a reduced [Hb]?

PROBLEM 3 — INTERMEDIATE

An arterial blood gas shows pH = 7.30, PCO₂ = 30 mmHg, and [HCO₃⁻] = 14 mEq/L. Classify this acid-base disturbance and identify whether compensation is occurring.

PROBLEM 4 — APPLIED

A researcher generates mice that lack the enzyme carbonic anhydrase in their erythrocytes. Predict the effects on (a) CO₂ transport from tissues to lungs, (b) arterial blood pH, and (c) the oxygen-hemoglobin dissociation curve at peripheral tissues. Explain the mechanistic basis for each prediction.

PROBLEM 5 — CRITICAL THINKING

Fetal hemoglobin (HbF) has a lower P₅₀ than adult hemoglobin (HbA) due to reduced 2,3-BPG binding by γ-subunits. A pharmaceutical company proposes treating sickle cell disease by reactivating the γ-globin gene to increase HbF production in adults. Analyze this strategy: What would be the predicted effects on (a) oxygen affinity, (b) O₂ delivery to tissues under exercise conditions, and (c) the polymerization of sickle hemoglobin (HbS)? What trade-offs might limit the therapeutic benefit?

SUMMARY

Lesson Summary

Blood is a connective tissue comprising plasma (≈55%) and formed elements (≈45%). Oxygen is transported primarily by hemoglobin (≈98.5%), which exhibits cooperative binding (sigmoidal curve, Hill coefficient ≈ 2.8). The Bohr effect ensures enhanced O₂ unloading at metabolically active tissues (right shift with ↑ PCO₂, ↓ pH, ↑ temperature, ↑ 2,3-BPG). Carbon dioxide travels mainly as bicarbonate (≈70%) via the carbonic anhydrase reaction and the chloride shift. The Henderson-Hasselbalch equation (pH = 6.1 + log([HCO₃⁻] / 0.03 × PCO₂)) links respiratory and metabolic contributors to blood pH.

Immune function in blood is executed by leukocytes arising from myeloid (neutrophils, monocytes, eosinophils, basophils) and lymphoid (T cells, B cells, NK cells) lineages. Innate immunity provides rapid, non-specific defense through phagocytosis, complement, and inflammation, while adaptive immunity offers antigen-specific, memory-generating responses via MHC I/CD8⁺ CTL and MHC II/CD4⁺ Tₕ pathways. Integrating these systems—gas transport, buffering, and immunity—is essential for mastering MCAT Foundational Concept 3B.