Blood is a specialized fluid connective tissue that circulates throughout the body, performing transport, regulation, and protection. This fluid is divided into two main components: a liquid matrix called plasma and the components suspended within it, known as the formed elements. Plasma constitutes over half of the total blood volume and is primarily water, carrying proteins, nutrients, and waste products. The formed elements make up the remaining volume and include true cells and cell fragments.
Red Blood Cells (Erythrocytes)
Erythrocytes, or red blood cells, are the most abundant formed elements, making up approximately 45% of the total blood volume. Their primary role is the transport of oxygen from the lungs to the body’s tissues and the return of carbon dioxide. This function is supported by their unique structure: mature red blood cells lack a nucleus and most other organelles, a condition known as being anucleated.
The flexibility and shape of an erythrocyte stem from its biconcave disc structure, which resembles a flattened doughnut with a depressed center. This shape maximizes the surface area-to-volume ratio, facilitating the rapid exchange of gases across the cell membrane. The cell is packed with the protein hemoglobin, which gives blood its red color.
Hemoglobin is an iron-containing molecule that reversibly binds to oxygen in areas of high concentration, such as the lungs. Each hemoglobin molecule can carry up to four molecules of oxygen, enabling high oxygen-carrying capacity. Red blood cells typically circulate for about 120 days before being removed by specialized cells in organs like the spleen and liver.
Platelets (Thrombocytes)
Platelets, also known as thrombocytes, are small, irregularly shaped fragments that are not complete cells. Their primary function is hemostasis, the process of stopping blood loss following an injury to a blood vessel. They are much smaller than red or white blood cells.
When a blood vessel is damaged, platelets quickly adhere to the exposed inner lining, a process mediated by proteins like von Willebrand factor. This adhesion triggers platelet activation, causing them to change shape and release chemical messengers like ADP and Thromboxane A2. These messengers recruit neighboring platelets, causing them to aggregate and form a temporary plug that seals the breach.
The platelet plug provides the initial barrier to blood loss, which is reinforced by the coagulation cascade. Activated platelets enhance the subsequent steps of the clotting process, leading to the generation of thrombin. Thrombin converts the soluble protein fibrinogen into insoluble fibrin threads, which weave across the plug to form a stable blood clot.
White Blood Cells (Leukocytes)
Leukocytes, or white blood cells, are dedicated to immune defense, protecting the body against pathogens, foreign material, and damaged cells. Unlike red blood cells, leukocytes are the only formed elements that are complete cells, possessing a nucleus and standard organelles. They use the bloodstream as a transport system to reach sites of injury or infection in the tissues.
Leukocytes are broadly categorized based on the appearance of granules in their cytoplasm after staining. Granulocytes include neutrophils, eosinophils, and basophils, and contain distinct cytoplasmic granules. Agranulocytes, comprising lymphocytes and monocytes, either lack these specific granules or have granules that are much less visible.
The five main types of leukocytes perform specialized immune functions:
- Neutrophils are the most common type, acting as first responders to bacterial and fungal infections by engulfing and destroying microorganisms (phagocytosis).
- Monocytes circulate briefly before maturing into macrophages in tissues, which are large, long-lived cells that clean up cellular waste and present foreign antigens.
- Lymphocytes are responsible for specific and adaptive immunity, including B-cells (antibody production) and T-cells (direct cell attack and immune regulation).
- Eosinophils modulate allergic reactions and defend against parasitic worm infections.
- Basophils are the least numerous type and release histamine and other chemicals that promote inflammation.
The Origin of Formed Elements
The continuous production of all formed elements is a regulated process called hematopoiesis. This manufacturing occurs primarily within the red bone marrow in adults, a soft tissue found in the spongy bone of the skull, pelvis, vertebrae, and the ends of long bones. This continuous renewal is necessary because the lifespans of blood components vary significantly.
All blood cells and cell fragments originate from a single type of cell, the hematopoietic stem cell (HSC), which is multipotent. These stem cells differentiate into two main lines of progenitor cells: the myeloid and lymphoid lineages. Myeloid stem cells give rise to red blood cells, platelets, and most types of white blood cells, including neutrophils, eosinophils, basophils, and monocytes.
Lymphoid stem cells are responsible for generating the various types of lymphocytes. The entire process is controlled by a network of chemical signals, such as hormones and colony-stimulating factors, which ensure the body produces the correct number and type of formed elements. For example, the hormone erythropoietin, mainly produced by the kidneys, stimulates the production of red blood cells in response to low oxygen levels.