Blood Composition and Functions

• Blood is a specialized bodily fluid in animals that delivers necessary substances such as nutrients and oxygen to the cells and transports metabolic waste products away from those same cells.
• In vertebrates, blood is composed of blood cells suspended in blood plasma.
• Plasma, which constitutes 55% of blood fluid, is mostly water (92% by volume) and contains dissolved proteins, glucose, mineral ions, hormones, carbon dioxide, and blood cells.
• The blood cells are mainly red blood cells (also called erythrocytes), white blood cells (also called leukocytes), and platelets (also called thrombocytes).
• Blood performs many important functions, including:
  • Supplying oxygen to tissues
  • Supplying nutrients such as glucose, amino acids, and fatty acids
  • Removing waste products such as carbon dioxide, urea, and lactic acid
  • Immunological functions, including circulation of white blood cells, and detection of foreign material by antibodies
  • Coagulation, which is one part of the body's self-repair mechanisms
  • Messenger functions, including transport of hormones and signaling of tissue damage
  • Regulation of body pH

Blood Components

• Blood consists of plasma and several kinds of cells.
• Plasma is the liquid component of blood, in which the blood cells are suspended and constitutes about 55% of blood volume.
• Plasma is about 92% water, 7% proteins, and 1% electrolytes, nutrients, and waste.
• Key plasma proteins include:
  • Albumin: Regulates osmotic pressure and transports lipids, hormones, and calcium.
  • Immunoglobulins: Antibodies released by plasma cells during immune response.
  • Fibrinogen: Essential for blood clotting.
• The cellular components of blood are:
  • Red blood cells (erythrocytes)
  • White blood cells (leukocytes)
  • Platelets (thrombocytes)

Red Blood Cells (Erythrocytes)

• Red blood cells are the most abundant cells in the blood, responsible for transporting oxygen from the lungs to the body tissues and carbon dioxide from the tissues back to the lungs.
• Erythrocytes contain hemoglobin, an iron-containing protein that binds to oxygen.
• Mature red blood cells are anucleate (lack a nucleus) in mammals, maximizing space for hemoglobin.
• Erythrocytes have a biconcave disc shape, which increases their surface area for gas exchange and allows them to squeeze through narrow capillaries.
• The lifespan of a red blood cell is approximately 120 days.

White Blood Cells (Leukocytes)

• White blood cells are involved in the immune response, protecting the body against infection and foreign invaders.
• Leukocytes are nucleated and can move independently, allowing them to migrate to sites of infection or inflammation.
• There are several types of leukocytes, each with a specific function:
  • Neutrophils: Phagocytic cells that engulf and destroy bacteria and fungi and are the most abundant type of leukocyte.
  • Lymphocytes: Include B cells (produce antibodies), T cells (kill infected cells and regulate immune responses), and natural killer (NK) cells (attack and kill abnormal cells).
  • Monocytes: Differentiate into macrophages, which are phagocytic cells that engulf pathogens, cellular debris, and foreign substances.
  • Eosinophils: Involved in allergic reactions and defense against parasitic infections.
  • Basophils: Release histamine and other mediators of inflammation.

Platelets (Thrombocytes)

• Platelets are small, anucleate cell fragments derived from megakaryocytes in the bone marrow.
• Thrombocytes play a critical role in blood clotting (hemostasis), preventing excessive bleeding after injury.
• When blood vessel damage occurs, platelets adhere to the damaged site, aggregate, and form a platelet plug and also release factors that activate the coagulation cascade, leading to the formation of a fibrin clot.

Blood Groups and Transfusions

• Human blood is classified into different blood groups based on the presence or absence of specific antigens on the surface of red blood cells.
• The ABO blood group system is based on the presence or absence of A and B antigens, where individuals can be type A, type B, type AB, or type O.
• The Rh blood group system is based on the presence or absence of the Rh D antigen, where Individuals are either Rh-positive (Rh+) or Rh-negative (Rh-).
• Blood transfusions must be compatible between the donor and recipient to avoid a transfusion reaction, where the recipient's immune system attacks the donor's red blood cells.
• Type O-negative blood is considered the "universal donor" because it lacks A, B, and Rh antigens and can be transfused to individuals of any ABO and Rh blood type in emergencies.
• Type AB-positive blood is considered the "universal recipient" because individuals with this blood type can receive red blood cells from any ABO and Rh blood type.

Hematopoiesis

• Hematopoiesis is the process of blood cell formation, which occurs in the bone marrow.
• All blood cells originate from hematopoietic stem cells, which are pluripotent cells capable of differentiating into any type of blood cell.
• Hematopoiesis is regulated by various growth factors and cytokines, which stimulate the proliferation and differentiation of blood cell precursors.

Blood Disorders

• Anemia: A condition characterized by a deficiency of red blood cells or hemoglobin, resulting in reduced oxygen-carrying capacity of the blood and can be caused by iron deficiency, blood loss, genetic disorders, or chronic diseases.
• Leukemia: A type of cancer of the blood or bone marrow characterized by the abnormal proliferation of white blood cells.
• Thrombocytopenia: A condition characterized by a deficiency of platelets, leading to an increased risk of bleeding.
• Hemophilia: A genetic disorder characterized by a deficiency of certain clotting factors, resulting in impaired blood clotting and excessive bleeding.
• Blood clots: Can form in veins (deep vein thrombosis, DVT) or arteries (arterial thrombosis), obstructing blood flow and potentially leading to serious complications such as pulmonary embolism or stroke.