Cardiovascular System: Blood Cells and Hematopoiesis

Cardiovascular System: Blood

Blood Cells

The blood is a specialized bodily fluid that plays a vital role in transporting nutrients, gases, hormones, and waste products throughout the body. It consists of several types of cells, each with distinct functions essential for maintaining homeostasis and defending against disease.

Hematopoiesis

Development of Blood Cells

Hematopoiesis is the process by which the formed elements of blood are produced. This process occurs at different sites during development:

• Embryonic stage: Yolk sac

• Fetal stage: Liver, spleen, thymus, lymph nodes

• Late fetal stage (3 months prior to birth) and after birth: Red bone marrow

Hematopoiesis ensures a continuous supply of red blood cells, white blood cells, and platelets throughout life.

Pluripotent Stem Cells

Origin and Differentiation

Pluripotent stem cells in the red bone marrow are derived from mesenchymal cells. These stem cells have the ability to:

• Develop into many different types of blood cells

• Self-renew and proliferate

There are two major lineages:

• Myeloid lineage: Gives rise to red blood cells (RBCs), platelets, neutrophils, eosinophils, basophils, and monocytes

• Lymphoid lineage: Gives rise to lymphocytes (T cells, B cells, and natural killer cells)

Colony-forming units (CFUs) and precursor cells (blasts) are intermediate stages in the development of mature blood cells.

Erythropoiesis

Production of Red Blood Cells

Erythropoiesis is the process of producing red blood cells (RBCs) in the bone marrow. It is regulated by the hormone erythropoietin, which is produced by the kidneys in response to low oxygen levels (hypoxia).

• Proerythroblasts divide and produce cells that synthesize hemoglobin.

• Reticulocytes are immature RBCs that enter the bloodstream and mature within 1-2 days.

Equation:

$\text{Erythropoiesis Rate} \propto \text{Erythropoietin Concentration}$

Red Blood Cells (Erythrocytes)

Structure and Function

Red blood cells are the most abundant cells in the blood and are responsible for transporting oxygen and carbon dioxide.

• Normal count: Male: 5.4 million/μL; Female: 4.8 million/μL

• Shape: Biconcave discs

• Nucleus: Absent (anucleate)

• Reproduction: Cannot reproduce

• Lifespan: ~120 days

• Destruction: Removed from circulation and destroyed by phagocytic macrophages in the spleen and liver; breakdown products are recycled

Example: Hemoglobin within RBCs binds oxygen in the lungs and releases it in tissues.

White Blood Cells (Leukocytes)

Types and Functions

White blood cells are crucial for immune defense. They have nuclei and do not contain hemoglobin. Their lifespan ranges from a few days to several years, and their normal count is 5,000-10,000 cells/μL.

• Granular leukocytes:

  • Neutrophils (60-70%)

  • Eosinophils (2-4%)

  • Basophils (0.5-1%)

• Agranular leukocytes:

  • Lymphocytes (20-25%)

  • Monocytes (3-8%)

Leukocytes circulate in the blood and tissues, performing phagocytosis or stimulating immune responses during inflammation or infection.

Neutrophils

Characteristics and Role

Neutrophils are the most abundant type of white blood cell and are essential for fighting bacterial infections.

• Small granules evenly distributed

• 2-5 lobed nucleus

• Pale lilac color

• Phagocytosis of bacteria

• Destruction of bacteria using lysozyme, defensins, and strong oxidants

Example: Neutrophils rapidly increase in number during acute bacterial infections.

Eosinophils

Characteristics and Role

Eosinophils are involved in combating parasitic infections and modulating allergic responses.

• Large granules

• Bilobed nucleus

• Granules do not cover nucleus

• Red/orange color

• Active in allergic reactions, antigen-antibody complexes, and parasitic worm infections

Example: Eosinophil count increases in response to parasitic worm infections and allergies.

Additional info:

• Other white blood cells such as basophils, lymphocytes, and monocytes also play important roles in immunity and inflammation.

• Platelets are cell fragments involved in blood clotting and hemostasis.

• Blood cell development and function are tightly regulated to maintain health and respond to injury or infection.