Blood Composition

Overview of Blood

Blood is a specialized fluid connective tissue essential for transporting substances throughout the body. It consists of two main components: plasma and formed elements.

• Plasma: Makes up about 55% of blood volume. It is primarily water (90%), with plasma proteins (8%) and other solutes (2%).

• Formed Elements: Comprise 45% of blood and include erythrocytes (red blood cells, RBCs), leukocytes (white blood cells, WBCs), and thrombocytes (platelets).

• Buffy Coat: A thin layer between plasma and RBCs, containing WBCs and platelets (about 1% of blood volume).

• Hematocrit: The percentage of blood volume occupied by RBCs (typically 44%).

• Sex Differences: Males generally have a higher hematocrit due to testosterone.

• Anemia: Decreased RBC count, leading to lower hematocrit and higher plasma proportion.

• Polycythemia: Increased RBC count, resulting in higher hematocrit and lower plasma. Causes include genetic conditions, cancer, and lifestyle factors like smoking.

Physical Characteristics and Volume

Properties of Blood

Blood has distinct physical and chemical properties that support its functions in the body.

• Viscosity: Blood is a sticky, opaque fluid.

• Color: Ranges from scarlet (oxygen-rich) to dark red (oxygen-poor).

• pH: Slightly alkaline, between 7.35 and 7.45.

• Temperature: Approximately 38°C (100.5°F), slightly warmer than body temperature.

• Volume: 5–6 L in males, 4–5 L in females (about 8% of body weight).

Functions of Blood

Major Roles

Blood performs three primary functions: transportation, regulation, and protection.

• Transportation:

  • Delivers oxygen and nutrients to body cells.

  • Transports metabolic wastes to lungs and kidneys for elimination.

  • Carries hormones from endocrine glands to target organs.

• Regulation:

  • Maintains body temperature by distributing heat.

  • Regulates normal pH using buffers.

  • Ensures adequate fluid volume in the circulatory system.

• Protection:

  • Prevents infection via antibodies, complement proteins, and WBCs.

  • Prevents blood loss through clotting mechanisms involving plasma proteins and platelets.

Blood Plasma

Composition and Functions

Plasma is the liquid matrix of blood, serving as a medium for transporting substances.

• Water: 90% of plasma, acts as a solvent.

• Plasma Proteins (8%):

  • Albumin (60%): Maintains osmotic pressure, acts as a carrier, and buffers pH.

  • Globulins (36%): Include antibodies and transport proteins.

  • Fibrinogen (4%): Essential for blood clotting.

• Other Solutes (2%): Nutrients, electrolytes, respiratory gases, hormones, enzymes, and metabolic wastes.

Formed Elements in Blood

Types and Characteristics

The formed elements are the cellular components of blood, each with specialized functions.

• Red Blood Cells (Erythrocytes): Anucleate, lack organelles, transport oxygen and carbon dioxide.

• White Blood Cells (Leukocytes): The only complete cells, involved in immune defense.

• Platelets (Thrombocytes): Cell fragments involved in clotting.

• Most formed elements are short-lived and originate in red bone marrow.

Erythrocytes (Red Blood Cells)

Structure and Function

Erythrocytes are specialized for efficient gas transport.

• Shape: Biconcave discs, increasing surface area for gas exchange.

• Contents: Largely filled with hemoglobin (97% of cell, excluding water).

• Flexibility: Spectrin protein allows RBCs to deform as they pass through capillaries.

• Metabolism: Lack mitochondria; rely on anaerobic metabolism (do not consume transported oxygen).

• Hemoglobin (Hb): Each molecule consists of 4 globin chains, each with a heme group containing iron.

• Each iron atom binds one oxygen molecule; each Hb can carry 4 oxygen molecules.

• Each RBC contains about 250 million Hb molecules.

Hemoglobin Forms

• Oxyhemoglobin: Formed when oxygen binds in the lungs (bright red).

• Deoxyhemoglobin: Formed when oxygen is released in tissues (dark red).

• Carbaminohemoglobin: Formed when carbon dioxide binds to globin (carries ~20% of CO2 in blood).

Hematopoiesis

Blood Cell Formation

Hematopoiesis is the process of blood cell production, occurring primarily in red bone marrow.

• Occurs in axial skeleton, girdles, and proximal epiphyses of humerus and femur.

• Produces about 100 billion new blood cells daily.

Erythropoiesis

• Production of RBCs; maintains a balance between production and destruction.

• Regulated by hormonal controls (mainly erythropoietin) and adequate supplies of iron, amino acids, and B vitamins.

• Too few RBCs: tissue hypoxia; too many: increased blood viscosity.

Hormonal Control of Erythropoiesis

• Erythropoietin (EPO): Hormone released by kidneys in response to hypoxia; stimulates rapid maturation of bone marrow cells.

• Testosterone enhances EPO production, leading to higher RBC counts in males.

• Increased reticulocyte count in 1–2 days after EPO stimulation.

RBC Lifespan and Fate

• Lifespan: 100–120 days.

• Old RBCs become fragile; hemoglobin degenerates.

• Macrophages in spleen engulf dying RBCs.

• Heme and globin are separated:

  • Iron is salvaged and reused.

  • Heme is degraded to bilirubin (yellow pigment), secreted in bile by the liver.

  • Globin is broken down into amino acids.

Leukocytes (White Blood Cells)

Structure and Function

Leukocytes are the only complete cells among the formed elements and are crucial for immune defense.

• Make up less than 1% of total blood volume.

• Can leave capillaries (diapedesis) and move through tissues by ameboid motion and chemotaxis.

• Leukocytosis: Elevated WBC count (>10,000/mm3), usually in response to infection.

• Two main categories: granulocytes and agranulocytes.

Granulocytes

• Contain cytoplasmic granules that stain distinctly.

• Larger and shorter-lived than RBCs.

• Lobed nuclei; all are phagocytic.

Basophils

• Rarest type of WBC.

• Large, purplish-black granules containing histamine.

• Histamine: Inflammatory chemical acting as a vasodilator and attracting other WBCs to sites of inflammation; important in allergic reactions.

• Functionally similar to mast cells.

Table: Major Components of Blood

Key Equations

• Hematocrit Calculation:

• Oxygen Carrying Capacity: Each RBC has about 250 million Hb molecules, each binding 4 O2 molecules.

Summary Table: Plasma Proteins

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