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Plasma Proteins

  • Plasma proteins are diverse and are carried in blood plasma; major categories include albumin, the globulins (alpha, beta, gamma), and apoproteins.

  • Some plasma proteins also include complement proteins that play a role in innate immunity; this will be revisited later in the course.

  • Albumin is the most abundant plasma protein and has a major effect on osmosis (movement of water between compartments). It is mostly synthesized by the liver.

  • Globulins are divided into alpha, beta, and gamma classes:

    • Alpha and beta globulins often function as transport proteins and include apoproteins for lipoproteins.

    • Gamma globulins are antibodies produced by lymphocytes (a type of white blood cell). They are the immune proteins that form Y-shaped antibodies.

  • Apolipoproteins (apo A, apo B) are protein components of lipoproteins; HDL and LDL are lipoprotein particles that combine lipid and protein components. HDL is the “good” lipoprotein, LDL is the relatively “less good” one.

  • Complement proteins participate in the immune system (innate immunity) and are part of the plasma protein landscape.

  • The liver is the primary source of most plasma proteins; gamma globulins (antibodies) are produced by lymphocytes, not the liver.

  • If you separate plasma proteins by electrophoresis, proteins separate into layers due to charge and size: albumin moves toward the positive charge most readily, followed by alpha, beta, and then gamma globulins. The densitometer trace shows peaks corresponding to each fraction; higher peak height indicates greater abundance of that protein:

    • Albumin: most abundant, moves closest to the positive charge

    • Beta globulins: second most abundant, not as close to the positive charge as albumin

    • Alpha globulins: several subtypes

    • Gamma globulins: antibodies

  • The takeaway: gamma globulins are antibodies produced by lymphocytes; the other plasma proteins are primarily liver-derived and carry out transportation, osmoregulation, and other functions.

Functions and Roles of Key Plasma Proteins

  • Albumin: major determinant of plasma oncotic (osmotic) pressure; contributes to fluid balance across capillaries.

  • Beta globulins: act as transport proteins for metal ions and for cholesterol (lipid transport roles).

  • Gamma globulins: antibodies (immunoglobulins) that target pathogens; produced by lymphocytes.

  • Apolipoproteins (apo A, apo B): components of lipoproteins that determine lipid transport and metabolism.

  • Complement proteins: part of innate immunity; function in immune defense and inflammation (to be studied in a dedicated chapter).

  • The liver also produces many clotting proteins and other plasma components (see clotting proteins below).

Lipids, Lipoproteins, and Associated Proteins

  • Lipoproteins are complexes of lipids and proteins that transport lipids (cholesterol, triglycerides) in blood.

  • HDL (high-density lipoprotein) and LDL (low-density lipoprotein) are types of lipoproteins with differing cholesterol content and health associations; HDL is considered the good lipoprotein, LDL less favorable.

  • Apolipoproteins (apo A, apo B) are the protein components of these lipoproteins and help to determine their density and function.

  • Cholesterol can be transported by these lipoproteins; LDL and HDL are density-based classifications that reflect their lipid-to-protein ratios.

Non-Protein Nitrogen Substances (NPNs) in Plasma

  • NPNs are small, nitrogen-containing substances not made of protein, carried dissolved in plasma.

  • Common NPNs include urea, creatinine, uric acid, and ammonium salts; these reflect protein metabolism and nitrogen balance:

    • Urea: produced by the liver from protein breakdown (deamination).

    • Creatinine: produced in muscle during ATP use; reflects muscle metabolism.

    • Uric acid: produced from amino acid metabolism in various tissues; part of urine.

    • Ammonium salts: dissolve in plasma; produced in protein metabolism.

  • Blood tests often measure NPNs (e.g., urea and creatinine) to assess metabolic and renal function.

Nutrients and Other Solutes in Plasma

  • Plasma contains a variety of nutrients and building blocks:

    • Carbohydrates: broken down to glucose (monosaccharides) and sometimes disaccharides like sucrose.

    • Proteins: digested into amino acids; proteins from diet (animal or plant) share the same 20 amino acids used to build human proteins.

    • Lipids: broken down into fatty acids and glycerol; triglycerides are composed of fatty acids and glycerol.

    • Cholesterol: a lipid that can be taken in diet or synthesized endogenously.

    • Nucleic acids: nucleotides from nucleic acids present in diet; cells in the body use nucleotides to synthesize DNA/RNA; this includes de novo synthesis and dietary sources.

    • Vitamins: some vitamins must be obtained from the diet; some can be synthesized by the liver.

  • Vitamins and minerals (electrolytes) are dissolved in plasma and are essential cofactors and building blocks for biological processes.

  • Nucleotides, amino acids, nucleic acids, vitamins, and minerals are all present in plasma as components necessary for DNA, RNA, and cellular function.

Minerals and Electrolytes in Plasma

  • Minerals exist as charged ions in plasma:

    • Cations: Na^+, K^+, Ca^{2+}, Mg^{2+}

    • Anions: Cl^-, phosphate (PO4^{3-} or HPO4^{2-} depending on pH), bicarbonate (HCO3^-), sulfate (SO4^{2-})

  • These electrolytes are critical for:

    • Acid-base balance

    • Osmotic pressure maintenance

    • Nerve impulse transmission and muscle contraction

  • Understanding and memorizing common ions is important for physiology and clinical exams.

Blood Components and Core Concepts

  • Oxygen and nitrogen transport:

    • Oxygen is carried mostly inside erythrocytes (RBCs) via hemoglobin; a small amount of oxygen is dissolved in plasma.

    • Nitrogen from the air is dissolved in blood but not utilized directly by the body; nitrogen is recovered from dietary proteins and metabolized amino acids; the body cannot use nitrogen gas directly to make proteins.

  • Hormones and enzymes in plasma:

    • Hormones (e.g., thyroxine, aldosterone, growth hormone, insulin) are transported by blood to distant targets; binding to receptors triggers biological effects; endocrine system works with the nervous system to maintain homeostasis.

    • Enzymes are carried in plasma as proteins; many are synthesized by the liver or other tissues and circulate to catalyze reactions.

Red Blood Cells (Erythrocytes)

  • Primary function: carry oxygen throughout the body; a small fraction of CO2 is carried inside RBCs, but most CO2 is carried in plasma.

  • Normal RBC values:

    • Roughly 4.7 to 6.0 million RBCs per microliter of blood in males; slightly lower in females.

    • A microliter is a tiny volume; there are about 20 microliters in a drop of water.

  • Hormonal regulation of RBC production:

    • Testosterone increases RBC production; estrogen suppresses RBC production.

  • Structure and morphology:

    • Mature RBCs are biconcave discs (donut-like shape) with a central thinning, which increases surface area for gas diffusion.

    • Diameter is about 7.5 μm7.5 \, \mu\text{m} and it is a very small cell.

    • Mature RBCs are anucleate and lack most organelles; they originate from precursor cells that have a nucleus and genes for hemoglobin and carbonic anhydrase.

    • The cell membrane surrounds a cytoplasm rich in hemoglobin; there are few organelles beyond that.

  • Internal composition and key proteins:

    • Hemoglobin: the main protein inside RBCs; carries oxygen and carbon dioxide.

    • Carbonic anhydrase: enzyme that catalyzes the reaction between CO2 and water to form carbonic acid; important for CO2 transport. The reaction can be written as
      CO<em>2+H</em>2O→H<em>2CO</em>3\mathrm{CO<em>2 + H</em>2O \rightarrow H<em>2CO</em>3}
      and is accelerated by carbonic anhydrase.

    • Spectrin and ankyrin: structural proteins that maintain the biconcave shape and allow deformability as RBCs pass through narrow capillaries; they help the RBC return to its shape after deformation.

  • Function and metabolism:

    • Gas transport is the primary role; oxygen diffuses across the RBC membrane to tissues; CO2 diffuses from tissues into the blood.

    • RBCs generate ATP mainly by anaerobic glycolysis (they do not use the oxygen they carry).

  • Shape and deformability:

    • The biconcave shape provides a large surface area-to-volume ratio, maximizing diffusion of O2 and CO2.

    • Defects in spectrin or ankyrin can lead to loss of the biconcave shape, producing spherocytes (rounder RBCs) that have impaired gas transport.

  • Internal RBC architecture:

    • Inside RBCs: hemoglobin, a few enzymes (glycolytic and antioxidant enzymes), and carbonic anhydrase; almost no organelles and no nucleus in mature RBCs.

  • Visual/structural notes:

    • A normal RBC shows a pale central area (biconcave hole) in smear images; spherocytes lack this clear central area.

  • Summary points:

    • Small size, high surface area, abundant hemoglobin, and anaerobic metabolism enable efficient gas transport without consuming the carried oxygen.

Practical and Clinical Connections

  • Hematology lab concepts:

    • Hematocrit: the percentage of blood volume occupied by red blood cells; typical value around 45% (roughly 42% in women, 47% in men). Routine lab tests may include hematocrit and white blood cell counts to assess health.

    • White blood cell counts provide insights into immune status and potential infections or hematologic conditions.

  • Blood components are dynamic:

    • RBCs are continually produced and degraded; they have short lifespans and are synthesized in response to hormonal signals (testosterone vs. estrogen influence).

    • The liver is a central organ for synthesizing plasma proteins (excluding gamma globulins) and amino acids; it also participates in de novo synthesis of some amino acids, contributing to systemic metabolism.

  • Inter-system relevance:

    • Blood carries substances to and from almost all organ systems (digestive, urinary, reproductive, nervous, etc.); this justifies revisiting blood in the context of each major organ system and the body as a whole.

Key Takeaways for Exam Preparation

  • Plasma proteins: albumin (osmotic pressure, liver-produced), alpha/beta globulins (transport), gamma globulins (antibodies from lymphocytes).

  • Lipoproteins and apolipoproteins: HDL is “good”; LDL is “less good”; apolipoproteins (apo A/B) are essential for lipoprotein function.

  • Complement proteins participate in innate immune defense (to be studied further).

  • Non-protein nitrogen substances reflect protein metabolism and renal function: urea, creatinine, uric acid, ammonium salts.

  • Plasma contains a wide range of nutrients and building blocks (glucose from carbohydrates; amino acids from proteins; fatty acids and glycerol from lipids; nucleotides; vitamins; minerals).

  • Electrolytes (Na^+, K^+, Ca^{2+}, Mg^{2+}, Cl^-, HCO3^-, PO4^{3-}, SO_4^{2-}) regulate acid-base balance, osmotic pressure, and nerve/muscle function.

  • RBCs: structurally optimized for gas transport (biconcave shape, high surface area); contain hemoglobin and carbonic anhydrase; rely on anaerobic glycolysis for ATP; mature RBCs are anucleate and lack most organelles.

  • CO_2 transport and carbonic anhydrase:

    • The conversion of CO2 to carbonic acid in RBCs facilitates CO2 transport; the reaction is CO2 + H2O ⇄ H2CO3, catalyzed by carbonic anhydrase.

  • Hormonal influence on RBC production and the endocrine system’s role in transport of hormones and enzymes in the blood.

  • Practical lab implications: hematocrit and WBC counts, plasma protein distribution via electrophoresis, and measurement of NPNs are common clinical tools.