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Blood: Structure, Function, and Components

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BLOOD

Overview of Blood in the Circulatory System

Blood is a specialized fluid of connective tissue that plays a vital role in the circulatory system. It consists of cells suspended in a fluid matrix and interacts closely with the heart (the pump) and blood vessels (the conducting system).

  • Pump: The heart circulates blood throughout the body.

  • Conducting system: Blood vessels transport blood to and from tissues.

  • Fluid medium: Blood itself, containing cells and plasma.

THE OBJECTIVE

Functions of Blood

The primary objective of blood is to transport materials to and from cells, supporting homeostasis and cellular function.

  • Oxygen and carbon dioxide: Transported between lungs and tissues.

  • Nutrients: Delivered from the digestive tract to cells.

  • Hormones: Carried from endocrine glands to target organs.

  • Immune system components: Circulated to defend against pathogens.

  • Waste products: Removed from cells for excretion.

BLOOD HAS A JOB

Major Functions of Blood

  • Transport of dissolved substances: Including gases, nutrients, hormones, and wastes.

  • Regulation of pH and ions: Maintains acid-base balance and electrolyte composition.

  • Restriction of fluid losses at injury sites: Through clotting mechanisms.

  • Defense against toxins and pathogens: Via white blood cells and antibodies.

  • Stabilization of body temperature: Distributes heat throughout the body.

THE FORMED ELEMENTS

Types of Formed Elements in Blood

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

  • Red blood cells (RBCs) or erythrocytes: Transport oxygen.

  • White blood cells (WBCs) or leukocytes: Part of the immune system.

  • Platelets: Cell fragments involved in blood clotting.

CHARACTERISTICS OF BLOOD

Composition of Whole Blood

  • Plasma: Fluid component consisting of water, dissolved plasma proteins, and other solutes.

  • Formed elements: All cells and cell fragments (RBCs, WBCs, platelets).

CHARACTERISTICS

General Physical and Chemical Properties

  • Temperature: 38°C (100.4°F) is normal.

  • Viscosity: Blood is highly viscous (thicker than water).

  • pH: Slightly alkaline, ranging from 7.35 to 7.45.

  • Blood volume: Approximately 7% of body weight.

    • Adult male: 5 to 6 liters

    • Adult female: 4 to 5 liters

PLASMA

Composition and Function of Plasma

Plasma makes up 50–60% of blood volume, with more than 90% of plasma being water. It contains several important proteins and solutes.

  • Albumins (60%): Transport substances such as fatty acids, thyroid hormones, and steroid hormones.

  • Globulins (35%): Include antibodies (immunoglobulins) and transport globulins for small molecules (e.g., hormone-binding proteins, metalloproteins, apolipoproteins, and steroid-binding proteins).

  • Fibrinogen (4%): Forms clots and produces long, insoluble strands of fibrin.

  • Serum: The liquid part of a blood sample in which dissolved fibrinogen has converted to solid fibrin (after clotting).

RED BLOOD CELLS

Structure and Function of RBCs

Red blood cells (RBCs) make up 99.9% of the formed elements in blood. They contain hemoglobin, a protein responsible for the red color of blood and for transporting oxygen and carbon dioxide.

  • Hemoglobin: Binds and transports oxygen from the lungs to tissues and carbon dioxide from tissues to the lungs.

  • Shape: Biconcave discs, thin in the middle and thicker at the edge, optimizing gas exchange.

RBC DETAILS

RBC Count, Hematocrit, and Lifespan

  • RBC count: Number of RBCs in 1 microliter of whole blood.

    • Male: 4.5–6.3 million

    • Female: 4.2–5.5 million

  • Hematocrit (packed cell volume, PCV): Percentage of RBCs in centrifuged whole blood.

    • Male: 40–54%

    • Female: 37–47%

  • Lifespan: RBCs lack nuclei, mitochondria, and ribosomes, so they cannot repair themselves and rely on anaerobic metabolism. They live about 120 days.

MORE RBC DETAILS

Importance of RBC Shape and Size

  • High surface-to-volume ratio: Facilitates rapid gas exchange.

  • Rouleaux formation: RBCs form stacks to smooth flow through narrow blood vessels.

  • Flexibility: RBCs can bend and flex to pass through capillaries as small as 4 μm in diameter (RBC diameter ~7.8 μm).

Example: The biconcave shape allows a red blood cell to squeeze through capillaries smaller than its own diameter, maximizing oxygen delivery to tissues.

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