Overview of Blood

Learning Objectives

• By the end of this section, students will be able to:

  • Identify the primary functions of blood in:

  • Transportation

  • Defense

  • Maintenance of homeostasis

  • Name the fluid component of blood and the three major types of formed elements, and identify their relative proportions in a blood sample

  • Discuss the unique physical characteristics of blood

  • Identify the composition of blood plasma, including its most important solutes and plasma proteins

  • Recall that blood is a connective tissue

Composition of Blood

• Blood is classified as a connective tissue, consisting of two main components:

  • Cellular elements (formed elements)

  • Red blood cells (RBCs)

  • White blood cells (WBCs)

  • Platelets (cell fragments)

  • Extracellular matrix (plasma)

• Plasma makes blood unique among connective tissues because it is fluid.

  • Plasma is mostly water and perpetually suspends the formed elements, enabling circulation within the cardiovascular system.

Functions of Blood

• The primary function of blood encompasses:

  • Transportation

  • Blood delivers oxygen and nutrients to body cells and removes wastes from them.

  • Nutrients are absorbed in the digestive tract and transported to the liver for processing before being sent to cells.

  • Oxygen diffuses from the lungs into the blood, transported by the heart to the body.

  • Hormones released by endocrine glands travel through the bloodstream to target cells.

  • Blood carries cellular wastes to organs like the lungs (for carbon dioxide exhalation) and kidneys/liver (for excretion via urine or bile).

  • Defense

  • White blood cells (WBCs) play a protective role:

    • They combat external threats (like bacteria from wounds).

    • They eliminate internal threats, such as mutated cells that may become cancerous or cells infected by viruses.

    • Platelets and certain plasma proteins work to block ruptured blood vessels during bleeding.

  • Maintenance of Homeostasis

  • Blood regulates body temperature through a negative feedback loop:

    • Increased blood flow to the periphery cools the body during exercise.

    • Blood is diverted away from the skin in cold conditions to maintain body warmth, which can lead to conditions like frostbite.

  • Blood maintains the chemical balance of the body, with certain proteins acting as buffers to regulate tissue pH.

Composition of Blood Samples

• Blood tests are performed to analyze blood composition, including types and quantities of formed elements.

• Hematocrit Test

  • Measures the percentage of RBCs (erythrocytes) within a blood sample.

  • Conducted using a centrifuge to separate blood components.

  • Heaviest elements (erythrocytes) settle at the bottom.

  • A buffy coat (less than 1% of a sample) is between erythrocytes and plasma, composed of WBCs (leukocytes) and platelets (thrombocytes).

  • Normal Ranges for Hematocrit

  • Erythrocytes constitute about 45% of a blood sample.

  • Normal hematocrit values vary:

    • Females: 37-47%, mean 41%

    • Males: 42-52%, mean 47%

  • Plasma percentage calculates as 100% - hematocrit values:

    • Females: approximately 59%

    • Males: approximately 53%

Characteristics of Blood

• Blood color changes depending on oxygen saturation:

  • Oxygenated blood from the lungs appears bright red.

  • Deoxygenated blood from the tissues appears dusky red.

• Viscosity of Blood

  • Blood is about five times more viscous than water, affecting blood pressure and flow.

  • Viscosity is influenced by plasma proteins and formed elements.

• Temperature

  • Normal blood temperature is approximately 38°C (100.4°F), slightly higher than normal body temperature (37°C or 98.6°F).

• pH Range

  • Blood pH averages about 7.4, within the range of 7.35 to 7.45.

• Total Volume

  • Blood constitutes approximately 8 percent of adult body weight.

  • Males: 5-6 liters; Females: 4-5 liters.

Blood Plasma

• Plasma comprises about 7 percent of total plasma volume consisting primarily of proteins, with many solutes dissolved within.

• Major Plasma Proteins:

  • Albumin

  • Most abundant plasma protein, produced by the liver.

  • Serves as a binding protein, transporting fatty acids and steroid hormones.

  • Important for osmotic pressure of blood, helping to maintain blood volume and pressure.

  • Constitutes approximately 54% of total plasma protein (3.5–5.0 g/dL).

  • Globulins

  • A heterogeneous group of proteins, divided into:

    • Alpha and Beta Globulins: Transport substances like iron and fat-soluble vitamins (A, D, E, K).

    • Gamma Globulins: Known as antibodies or immunoglobulins, produced by plasma cells, involved in immunity.

  • Globulins make up approximately 38% of total plasma protein (1.0–1.5 g/dL).

  • Fibrinogen

  • Essential for blood clotting, produced by the liver.

  • Accounts for about 7% of plasma protein volume (0.2–0.45 g/dL).

• Other Plasma Solutes:

  • Comprise various electrolytes (sodium, potassium, calcium), dissolved gases (oxygen, carbon dioxide, nitrogen), organic nutrients (vitamins, lipids, glucose, amino acids), and metabolic wastes.

  • Contribute approximately 1% to total volume of plasma.

Career Connection

• Medical Technologists (MTs) and Clinical Laboratory Technologists (CLTs)

  • Typically hold a bachelor's degree and certification.

  • Perform various tests on body fluids like blood, aiding in diagnosis and treatment monitoring.

• Phlebotomists

  • Trained to draw blood (perform venipuncture, capillary sticks, arterial sticks).

  • May be certified through the American Society of Phlebotomy Technicians after passing a national examination.

• Medical Laboratory Technicians (MLTs)

  • Typically have an associate's degree and can perform duties similar to MTs.

• Medical Laboratory Assistants (MLAs)

  • Focus on processing samples and routine lab tasks, clinical training often required.