The Blood

Introduction:

Blood, often called the fluid connective tissue, plays a fundamental role in maintaining the body’s equilibrium. Comprising both plasma and blood corpuscles, this remarkable substance constitutes approximately 30-35 per cent of the extracellular fluid in our bodies. An average adult weighing 70 kilograms harbours around 5.5 litters of blood, a testament to its paramount significance. Characterised by a slightly alkaline nature with a pH of 7.4, blood showcases intriguing variations in pH levels across arteries and veins. Arteries have a higher pH than veins.

Plasma: The Liquid Gold

At the heart of our circulatory system lies plasma, a translucent, pale-yellow fluid constituting 60% of the total blood volume. Plasma comprises water and essential mineral salts and is crucial in maintaining the body’s osmotic balance. Sodium bicarbonate is a vital buffer, preserving the blood’s pH stability. This fluid treasure trove carries various nutrients, including glucose, fatty acids, phospholipids, cholesterol, fats, amino acids, and nucleosides, ensuring cells receive sustenance for optimal functioning.

Plasma proteins, constituting 7 to 8% of plasma, are instrumental in various physiological processes. Albumin, globulin, immunoglobulin, prothrombin, fibrinogen, and other defence compounds safeguard the body against potential threats. Moreover, plasma houses excretory substances like ammonia, urea, and uric acid, aiding in waste elimination. Notably, albumins, synthesised in the liver, are the most abundant plasma proteins. The serum is the remaining portion of plasma post-clotting, retaining essential components such as dissolved gases (oxygen, carbon dioxide, and nitrogen), anticoagulants, hormones, vitamins, and enzymes.

It’s essential to maintain specific parameters within the blood, like glucose levels ranging from 80 to 100 mg per 100 mL of blood 12 hours after a regular meal and cholesterol levels within 80-180 mg in 100 mL of blood plasma. Elevated cholesterol levels can lead to arterial disposition, raising the risk of high blood pressure and cardiovascular problems.

The RBC: red blood cell

Blood corpuscles, comprising erythrocytes, leucocytes, and thrombocytes, are the dynamic agents that empower blood’s functions. Among them, erythrocytes, commonly known as red blood cells (RBCs), take centre stage. These cells, numbering 5 million per cubic millimetre in men and 4.5 million in women, are crucial for oxygen transport. An increased count, known as polycythemia, can strain the circulatory system, while low counts (erythrocytopenia) lead to oxygen shortage, affecting tissue health.

RBCs stand out due to their biconcave circular shape and their role in the body’s vibrant hue (camel and llama RBCs are oval). The red colour is attributed to haemoglobin, a complex protein consisting of globin and heme, which harbours iron-porphyrin complexes. Comprising four heme and globin molecules each, haemoglobin transports oxygen and carbon dioxide, ensuring efficient gas exchange. Approximately 15g of haemoglobin per 100ml of blood in men and 10g in women underscores its critical role.

The concave surface of mammalian red blood cells (RBCs) serves a remarkable purpose by increasing their surface area, enabling efficient oxygen exchange. These unique cells, lacking all cell organelles, are filled with haemoglobin molecules consisting of two alpha chains with 141 amino acids each and two beta chains with 146 amino acids each. Interestingly, individuals residing in high-altitude areas, such as hills, tend to have elevated RBCs, a physiological adaptation to their oxygen-thin environments. The life span of an RBC is around 120 days, after which they are destroyed in the spleen and liver. This destruction process produces bilirubin, a yellowish pigment excreted through bile.

Erythropoiesis, the process responsible for producing red blood cells (RBCs), predominantly occurs within the confines of the red bone marrow. During this process, young RBCs, known as reticulocytes, retain a web-like appearance due to the presence of a few ribosomes. The seamless production of RBCs hinges on crucial elements such as iron, folic acid, and vitamin B12. Iron, stored within the liver as ferritin, is indispensable in synthesising haemoglobin. Folic acid and vitamin B12 collaborate harmoniously, ensuring optimal DNA synthesis and cellular growth, significantly impacting RBC production.

Vitamin B12, or cobalamin, is required in minute quantities. Its efficient absorption necessitates the presence of an intrinsic factor, a protein secreted by the stomach. Insufficient levels of this protein indirectly affect RBC production and can lead to pernicious anaemia. Additionally, the hormone erythropoietin, synthesised by the kidneys, regulates RBC production by stimulating precursor cells. The secretion of erythropoietin is triggered by reduced oxygen delivery to the kidneys. Notably, the male sex hormone also plays a role in the release of erythropoietin, further influencing this intricate process.

White Blood Cells: The Guardians of Health

White blood cells, often called leukocytes, are vital to our immune defence. These colourless warriors patrol our bloodstream, ever-ready to combat intruders and maintain our body’s equilibrium. The total count of white blood cells typically ranges between 5,000 to 10,000 per cubic millimetre of human blood. Abnormal rises in WBC numbers lead to leucocytosis, while decreased counts result in leukopenia.

Their amoeboid movement enables the remarkable mobility of white blood cells. This unique ability allows them to squeeze out of blood capillaries through a process known as diapedesis. Notably, these cells are divided into two main categories: agranulocytes and granulocytes.

Agranulocytes and Granulocytes: Unveiling Cellular Diversity

Agranulocytes, devoid of granules in their cytoplasm, encompass two distinctive types: lymphocytes and monocytes. Lymphocytes play a pivotal role in producing antibodies to combat microbes. They come in two forms: B-lymphocytes and T-lymphocytes, each playing a specialised role in our immune defence. Monocytes, the largest white blood cells, exhibit mobility and phagocytic tendencies, efficiently engulfing bacteria and cellular debris.

Conversely, granulocytes harbour abundant granules within their cytoplasm, and their nuclei often sport multiple lobes, earning them the nickname “polymorphonuclear granulocyte.” These granulocytes are further classified based on their staining properties into eosinophils, basophils, and neutrophils.

Eosinophils (1 – 6%): Characterised by bilobed nuclei and acidic stains coarse granules, eosinophils play a role in the immune system, releasing hydrolytic enzymes and peroxidase into phagosomes.

Basophils (0 – 1%): With three-lobed nuclei and fewer coarse granules, basophils release substances like heparin, histamine, and serotonin from their basic-staining granules.

Neutrophils (40 – 75%): Displaying multilobed nuclei and abundant granules, neutrophils are the most numerous white blood cells and are highly phagocytic. They are pretty large and stain with both acidic and basic dyes.

Thrombocytes: Masters of Clotting

Within the realm of blood cells, thrombocytes, or blood platelets, play a critical role in clotting. With around 2,50,000 platelets per cubic millimetre of blood, these colourless, non-nucleated cell fragments are essential for maintaining hemostasis. Thrombocytosis and thrombocytopenia denote increased and decreased platelet counts, respectively.

Platelets, produced from portions of megakaryocytes in bone marrow, serve as the frontline responders during injuries. They release platelet factors like thromboplastin upon activation, contributing to blood clotting.

Regulating Blood Cell Production: A Symphony of Growth Factors

In the orchestra of blood cell production, bone marrow takes centre stage. While most bones produce blood cells in children, adulthood narrows this responsibility to specific bones such as the chest, skull base, spinal vertebrae, and upper limb portions. Adult bone marrow, resembling the liver in weight, orchestrates an impressive production rate of blood cells.

All blood cells trace their origins back to pluripotent hematopoietic stem cells, which can give rise to various progenitor cells. These specialised progenitors commit to different developmental pathways, yielding lymphocytes and myeloid stem cells. Hematopoietic growth factors (HGFs) play a crucial role in stimulating the proliferation and differentiation of these progenitor cells, ensuring the continuous renewal of our blood cell population.

In conclusion, the captivating world of blood cells unveils a symphony of diversity and precision. White blood cells defend us against threats, platelets ensure clotting, and the regulation of blood cell production highlights the remarkable orchestration within our bodies. Understanding these intricate mechanisms allows us to appreciate the resilience and intricacy of the human body’s defence and maintenance systems.