Human Blood and Composition

Blood Overview

• Blood is a continuously regenerated connective tissue
• It carries gases, nutrients, wastes, and hormones
• Blood is transported through the cardiovascular system
• The heart pumps blood
• Arteries carry blood away from the heart
• Veins carry blood towards the heart
• Capillaries facilitate exchange between blood and body tissues

Blood Functions

• Transportation:
  • Carries oxygen from the lungs to the body tissues
  • Carries nutrients from the gastrointestinal tract (GIT) to the tissues
  • Transports hormones from endocrine glands to target tissues
  • Removes metabolic waste products
• Regulation:
  • Regulates blood pH (acidity or alkalinity)
  • Regulates body temperature
  • Regulates water content of cells
• Protection:
  • White blood cells (WBCs) fight infection by phagocytosis
  • Blood contains substances to help with clotting

Physical Characteristics of Blood

• Color: High oxygen = bright red; Low oxygen = dark red
• Volume: Approximately 5 liters in a normal adult
• Viscosity: Thick; high erythrocyte concentration = high viscosity
• Solutes Concentration: Determines osmosis direction
• Temperature: Slightly higher than body temperature (one degree Celsius)
• pH: 7.35-7.45 slightly alkaline (crucial for plasma protein integrity)

Whole Blood Separation

• Blood is separated into Plasma (55%), Buffy Coat (<1%) and Erythrocytes (44%) during centrifugation
• Plasma, mostly water, contains proteins, nutrients, wastes, regulatory substances, etc
• Buffy Coat consists primarily of leukocytes (white blood cells) and platelets
• Erythrocytes (red blood cells) are the most abundant

Hematocrit

• Percentage of blood volume comprised of erythrocytes
• Normal values:
  • Males: 42-56%
  • Females: 38-46%

Formed Elements: Erythrocytes

• Small, flexible, biconcave discs
• Lack a nucleus and organelles; packed with hemoglobin
• Transport oxygen and carbon dioxide between tissues and lungs

Hemoglobin

• Red-pigmented protein
• Transport oxygen and carbon dioxide
• Each hemoglobin molecule contains 4 globin protein chains (2 alpha and 2 beta) each with a heme group containing a central iron atom

Erythropoiesis

• Process of red blood cell production in red bone marrow
• Stimulated by erythropoietin (EPO) released by the kidneys in response to low oxygen levels

Erythropoietin

• Hormone that stimulates erythrocyte production
• Testosterone increases EPO levels, thus increasing erythrocyte count in males
• Environmental factors like altitude affect EPO levels

Fate and Destruction of Erythrocytes

• Aged/damaged erythrocytes are eliminated by macrophages
• Hemoglobin is broken down into heme and globin
• Iron is recycled, and bilirubin is eliminated in bile

Blood Types (ABO)

• Determined by the presence or absence of antigens (A, B) on the surface of erythrocytes and matching antibodies (anti-A, anti-B) in the plasma

Blood Types (Rh)

• Rh factor (antigen D) is another factor determining blood type (positive or negative)
• Individuals with Rh+ blood, lacking anti-Rh antibodies will not have a transfusion reaction following the initial exposure to Rh+ blood, However subsequent exposures will induce antibody that may cause a reaction.

Blood Transfusions and Agglutination

• Transfusion reactions occur when incompatible blood types are mixed
• Recipient antibodies bind to donor erythrocytes, causing clumping (agglutination)
• Agglutinated cells can block blood vessels and lead to serious complications

Rh Incompatibility in Pregnancy

• If an Rh-negative mother carries an Rh-positive fetus, anti-Rh antibodies can form in the mother
• These antibodies can attack fetal red blood cells, potentially causing serious complications
• RhoGAM is an immunoglobulin that can prevent this problem by suppressing antibody formation