Histology: Blood Composition and Blood Cells

• Histology is the study of the microscopic structure of tissues.

Blood Composition

• Blood is a specialized bodily fluid in animals, delivering nutrients and oxygen to cells.
• Blood transports metabolic waste products away from cells.
• Blood consists of blood cells suspended in blood plasma.
• Blood cells are mainly red blood cells (erythrocytes), white blood cells (leukocytes), and platelets (thrombocytes).
• Plasma, the liquid fraction of blood, is composed of water, ions, proteins, and dissolved gases.
• Blood's primary function includes transporting oxygen and carbon dioxide.
• Blood also facilitates immune responses.
• Blood maintains homeostasis.

Blood Cells

• Blood cells are produced in the bone marrow through hematopoiesis.
• Erythrocytes transport oxygen, leukocytes provide immune defense, and thrombocytes facilitate blood clotting.

Erythrocytes (Red Blood Cells)

• Erythrocytes are the most abundant cell type in blood.
• Erythrocytes are specialized for oxygen transport and contain hemoglobin.
• Mature mammalian erythrocytes lack a nucleus (anucleate), maximizing space for hemoglobin and enhancing oxygen-carrying ability.
• Erythrocytes are flexible, biconcave discs, optimizing their surface area-to-volume ratio for gas exchange.
• Erythrocytes have a lifespan of about 120 days in humans, after which the spleen and liver remove them.
• Erythropoiesis, or red blood cell production, is stimulated by erythropoietin, a hormone produced by the kidneys in response to low oxygen levels.

Leukocytes (White Blood Cells)

• Leukocytes are involved in immune defense, protecting against infections and foreign invaders.
• Leukocytes are nucleated and divided into granulocytes and agranulocytes.
• This division is based on the presence or absence of specific granules in their cytoplasm.
• Granulocytes include neutrophils, eosinophils, and basophils.
• Agranulocytes include lymphocytes and monocytes.

Neutrophils

• Neutrophils are the most abundant type of leukocyte.
• Neutrophils have a multi-lobed nucleus and granules containing enzymes and antimicrobial substances.
• Neutrophils are phagocytic, engulfing and destroying bacteria and other pathogens.
• Neutrophils are typically the first responders to sites of infection or injury.

Eosinophils

• Eosinophils are characterized by a bilobed nucleus.
• Eosinophils contain large, eosin-stained (red) granules containing enzymes effective against parasites.
• Eosinophils are also involved in allergic reactions.

Basophils

• Basophils are the least common type of granulocyte.
• Basophils have a bilobed nucleus and large, basophilic (blue) granules containing histamine and heparin.
• Basophils are involved in allergic reactions and inflammation, releasing histamine and other mediators.

Lymphocytes

• Lymphocytes include T cells, B cells, and natural killer (NK) cells.
• Lymphocytes are crucial for the adaptive immune response, recognizing and targeting specific antigens.
• T cells are involved in cell-mediated immunity, directly killing infected cells or activating other immune cells.
• B cells produce antibodies, neutralizing pathogens and marking them for destruction by other immune cells.
• NK cells are involved in innate immunity, killing infected or cancerous cells without prior sensitization.

Monocytes

• Monocytes are the largest type of leukocyte.
• Monocytes differentiate into macrophages or dendritic cells in tissues.
• Macrophages are phagocytic cells that engulf and digest pathogens, cellular debris, and foreign substances.
• Dendritic cells are antigen-presenting cells that capture antigens and present them to T cells, initiating an adaptive immune response.

Thrombocytes (Platelets)

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

Plasma

• Plasma is the liquid component of blood, making up about 55% of its volume.
• Plasma consists of water, ions, proteins, and dissolved gases.
• Plasma proteins include albumin, globulins, and fibrinogen.
• Albumin helps maintain osmotic pressure.
• Globulins include antibodies and transport proteins.
• Fibrinogen is essential for blood clotting, converting into fibrin, the main component of blood clots.
• Plasma transports nutrients, hormones, and waste products throughout the body.

Overview of blood composition, including plasma, red blood cells (erythrocytes), white blood cells (leukocytes), and platelets (thrombocytes). Blood's primary functions include transporting oxygen, facilitating immune responses and maintaining homeostasis. Blood cells are produced in the bone marrow through hematopoiesis.