Comprehensive Study Notes on Blood Composition and Formed Elements

Composition and Properties of Blood Plasma

Blood is a specialized connective tissue consisting of a fluid matrix, plasma, and formed elements. Plasma is the liquid component of blood that serves as the medium for the suspension of formed elements. It contains several critical proteins, including fibrinogens, globulins, and albumins. Fibrinogens are essential for the process of blood clotting or coagulation. Globulins are primarily involved in the defense mechanisms of the body, acting as antibodies to fight infections. Albumins play a vital role in maintaining the osmotic balance of the blood, ensuring proper fluid distribution between the blood vessels and tissues.

In addition to proteins, plasma contains a variety of minerals and nutrients that are always in transit throughout the body. These minerals include sodium ions ($Na^{+}$), calcium ions ($Ca^{2+}$), magnesium ions ($Mg^{2+}$), bicarbonate ions ($HCO_{3}^{-}$), and chloride ions ($Cl^{-}$). Other substances found in plasma include glucose, amino acids, and lipids. Furthermore, factors required for the coagulation or clotting of blood are present in the plasma in an inactive form to prevent spontaneous clotting within the vessels. A specialized term, serum, refers to plasma that is devoid of these clotting factors. Mathematically, this can be expressed as $\text{Serum} = \text{Plasma} - \text{Clotting Factors}$.

Erythrocytes (Red Blood Cells)

Erythrocytes, commonly known as red blood cells (RBCs), are the most abundant cells found in human blood. In a healthy adult male, the average RBC count ranges from $5$ million to $5.5$ million per $mm^{3}$ of blood. These cells are produced in the red bone marrow of adults. A defining characteristic of mammalian erythrocytes is that they are devoid of a nucleus (enucleated) and possess a biconcave shape, which increases the surface area for gas exchange.

The red color of these cells is due to the presence of a complex, iron-containing protein called haemoglobin ($Hb$). A healthy individual typically has between $12-16\,gm$ of haemoglobin in every $100\,ml$ of blood. These molecules are critical for the transport of respiratory gases, specifically oxygen and carbon dioxide. RBCs have a definitive lifespan of approximately $120\,days$. Once they reach the end of their functional life, they are destroyed in the spleen, which is famously referred to as the "graveyard of RBCs."

Leucocytes (White Blood Cells)

Leucocytes, or white blood cells (WBCs), are nucleated cells that lack haemoglobin, making them colorless. Compared to erythrocytes, leucocytes are relatively fewer in number, averaging between $6000-8000\,mm^{3}$ of blood. They are generally short-lived cells. Leucocytes are classified into two main categories based on the presence or absence of granules in their cytoplasm: granulocytes and agranulocytes.

Granulocytes include neutrophils, eosinophils, and basophils. Agranulocytes consist of lymphocytes and monocytes. Neutrophils are the most numerous type of WBC, constituting approximately $60-65\%$ of the total leucocyte count. In contrast, basophils are the least abundant, making up only $0.5-1\%$ of the total WBCs. Monocytes account for about $6-8\%$ of the leucocytes, while eosinophils represent $2-3\%$.

Functional Roles of Leucocyte Subtypes

Each type of white blood cell has a specialized role in the body's immune and inflammatory responses. Neutrophils and monocytes are phagocytic cells, meaning they are capable of engulfing and destroying foreign organisms, such as bacteria, that enter the body. Basophils are involved in inflammatory reactions and secrete vital chemical mediators, including histamine, serotonin, and heparin.

Eosinophils are primarily responsible for resisting infections and are frequently associated with allergic reactions in the body. Lymphocytes, the second major category of agranulocytes, are divided into two main types: B lymphocytes and T lymphocytes. Both B and T lymphocytes are essential for the body's immune responses, enabling the system to recognize and remember specific pathogens.

Platelets and Blood Clotting

Platelets, also known as thrombocytes, are cell fragments produced from specialized cells in the bone marrow called megakaryocytes. A normal blood concentration of platelets ranges from $150,000$ to $350,000$ per $mm^{3}$ of blood. Platelets are essential for the coagulation of blood. They release a variety of substances that initiate and promote the clotting process.

A significant reduction in the platelet count can lead to clotting disorders, characterized by the inability of the blood to coagulate properly. This condition results in excessive blood loss from the body even after minor injuries, which can be life-threatening if not managed.