Blood Components and Functions

• Whole blood is separated via centrifuge, resulting in three components

  • Erythrocytes: Also known as red blood cells, they make up roughly 44% of a blood sample
  • Buffy coat: The middle layer, containing leukocytes (white blood cells) and platelets, comprises about 1% of a blood sample
  • Plasma: A straw-colored liquid, which constitutes approximately 55% of blood

• Erythrocytes, leukocytes, and platelets are collectively termed "formed elements"

• Blood transports oxygen, carbon dioxide, hormones, nutrients, and wastes

• Blood helps maintain temperature, pH, and fluid levels within the cardiovascular system

• Regulation is achieved through:

  • pH levels: pH is a measure of acidity or alkalinity in a fluid
  • Acidic pH: A pH between 0 and 7
  • Alkaline pH: A pH between 7 and 14
  • pH imbalances: Conditions like acidosis and alkalosis

• Leukocytes guard against infection by triggering immune responses to antigens (foreign substances); blood carries antibodies

• Plasma is a mix of water, proteins, and other solutes

  • Serum: Defined as plasma without clotting proteins.

• Water accounts for 92% of plasma, while proteins make up 7%

  • Electrolytes (sodium, potassium, chloride, etc.)
  • Nutrients
  • Respiratory gases (oxygen and carbon dioxide)
  • Hormones
  • Waste products

Plasma Proteins

• Albumins: Smallest and most abundant, regulating water movement between blood and interstitial fluid and transporting ions, hormones, and lipids
• Globulins:
  • alpha- and beta-globulins are involved in substance transport
  • gamma-globulins are antibodies for immune defense
• Fibrinogen: Responsible for clot formation, converted to fibrin during the clotting process
• Regulatory proteins: Enzymes and proenzymes

Plasma versus Interstitial Fluid

• Plasma is a type of extracellular fluid (ECF), found outside cells
• Plasma and interstitial fluid (fluid surrounding cells) share similar concentrations of most solutes, except plasma proteins
• Oxygen concentration is higher in plasma than in interstitial fluid
• Carbon dioxide concentration is higher in interstitial fluid than in plasma

Formed Elements

• Formed elements include:

  • Erythrocytes: Transport respiratory gases and comprise more than 99% of formed elements
  • Leukocytes: Aid in mounting an immune response against pathogens, accounting for less than 0.01% of formed elements
  • Platelets: Assist in blood clotting and represent less than 1% of formed elements

• Observation: Formed elements can be observed via a blood smear

• Hematocrit: Represents the percentage of the volume of all formed elements in the blood

  • Clinically: Percentage volume of erythrocytes only, but true and clinical hematocrit are virtually the same

• Hematocrit: Varies with age (more variable in children) and sex (higher testosterone levels result in higher hematocrit)

Characteristics of The Formed Elements

• Erythrocytes
  • Size: 7.5 µm
  • Function: Transport of oxygen and carbon dioxide, with a lifespan of ~120 days
  • Density: ~4.8 million for Genetic females and ~5.4 million for Genetic males
• Leukocytes
  • Size: 1.5 to 3 times larger than an erythrocyte (11.25 to 22.5 m)
  • Function: Initiate immune response
  • Varies from 12 hours (neutrophil) to years (lymphocyte)
• Platelets
  • Size: Less than one-fourth the size of an erythrocyte; ~2 m with a lifespan of 8 to 10 days
  • Function: Participate in blood clotting
  • Density: 150,000 to 400,000

Erythrocytes

• Erythrocytes, commonly called red blood cells (RBCs), lack a nucleus and organelles
  • They measure about 7.5 micrometers (µm) in diameter.
  • Have a biconcave disc shape
  • The count of the blood ranges between 4.2 and 6.2 million per cubic millimeter (or microliter).
• Every erythrocyte contains about 280 million molecules of hemoglobin
  • Hemoglobin transports oxygen and carbon dioxide
  • Oxygenated when maximally loaded with oxygen
  • Deoxygenated: When some oxygen is lost and carbon dioxide gained
  • Each hemoglobin consists of four globin polypeptides
• Each of the four globins possesses a heme group containing an iron (Fe2+)ion for oxygen binding

Erythrocytes life cycle

• Erythrocytes have short life spans, with ~120 days

• Old erythrocytes are phagocytized in liver and spleen by cells called macrophages

  • The heme group is converted into biliverdin and then to bilirubin, becoming part of bile
  • Iron is transported by transferrin to the liver and stored by ferritin
  • Erythrocyte proteins are broken down into free amino acids

• Erythrocytes contain surface antigens for the ABO blood group

  • Presence or absence of A and B surface antigens determine the ABO blood type
  • Example: Type B has antigen B only, while Type O has neither surface antigen A nor B
  • Specific surface antigens present are accompanied in the plasma by antibodies to antigens that would be foreign to that individual

• Rh blood type is determined by presence or absence of surface antigen D (Rh factor)

  • When present, individual is Rh positive, when not, they are Rh negative
  • Anti-D antibodies only form when an Rh negative individual is exposed to Rh+ blood
  • ABO and Rh blood types are typically reported together, for example: AB+ blood

Leukocytes

• Leukocytes have a nucleus and organelles

  • They are 1.5 to 3 times larger in diameter than erythrocytes
  • They help initiate immune responses
  • Pathological conditions can result in abnormal numbers of leukocytes - Leukopenia means reduced number of cells
    • Leukocytosis means elevated number of cells

• Leukocyte flexibility and motility assist them in reaching pathogens

  • Diapedesis: is the process of squeezing through the blood vessel to exit the bloodstream
  • Chemotaxis: is the process of moving toward chemicals at the site of infection
  • There are five types of leukocytes are divided into two classes - Granulocytes: Include neutrophils, eosinophils, and basophils - Agranulocytes: Include lymphocytes and monocytes

Thrombopoiesis

• Thrombopoiesis: Production of platelets
  • Megakaryoblast forms from myeloid stem cell
  • It matures into a megakaryocyte
  • Each megakaryocyte produces thousands of platelets

Leukopoiesis

• Leukopoiesis: Production of leukocytes
  • All granulocytes derive along the myeloid line
• Monocyte maturation:
  • Monocytes derive from myeloid stem cells.
  • It then differentiates into a monoblast
  • Monoblast becomes a promonocyte
• Lymphocyte maturation
  • Lymphocytes derive from lymphoid stem cells
  • These stem cells differentiate into B-lymphoblasts and T-lymphoblasts
  • Some stem cells differentiate directly into NK cells