Body Fluids and Circulation

15.1 Blood

Blood is a vital fluid connective tissue in humans and most higher organisms, responsible for the transport of nutrients, respiratory gases such as oxygen (O₂) and carbon dioxide (CO₂), hormones, and waste products to and from the cells. It plays a crucial role in maintaining homeostasis and protecting the body against infections. Blood consists of two main components: plasma, the fluid matrix, and formed elements, the cellular components. Plasma constitutes about 55% of the total blood volume and is a straw-coloured viscous fluid. It is mainly water (90-92%) with proteins (6-8%) such as fibrinogen, globulins, and albumins dissolved in it. Fibrinogen is essential for blood clotting, globulins are involved in immune defense, and albumins help maintain osmotic balance. Plasma also contains minerals like sodium (Na⁺), calcium (Ca²⁺), magnesium (Mg²⁺), bicarbonate (HCO₃⁻), and chloride (Cl⁻), as well as nutrients like glucose, amino acids, and lipids, which are constantly transported in the blood. The plasma also carries inactive clotting factors, which upon activation prevent excessive blood loss during injury. When plasma is devoid of clotting factors, it is called serum. The formed elements make up about 45% of the blood volume and include erythrocytes (red blood cells, RBCs), leucocytes (white blood cells, WBCs), and platelets (thrombocytes). RBCs are the most abundant cells in blood, numbering about 5 to 5.5 million per cubic millimeter (mm³) in a healthy adult male. They are biconcave, anucleate cells containing haemoglobin, an iron-containing protein that binds oxygen and imparts the red colour to blood. The average haemoglobin content is 12-16 grams per 100 ml of blood. RBCs have a lifespan of about 120 days and are destroyed in the spleen. Leucocytes are nucleated, colourless cells that defend the body against infections. Their count ranges between 6000 to 8000 per mm³. They are classified into granulocytes (neutrophils, eosinophils, basophils) and agranulocytes (lymphocytes and monocytes). Neutrophils are the most abundant (60-65%) and are phagocytic, engulfing pathogens. Basophils secrete histamine and heparin and are involved in inflammatory responses. Eosinophils (2-3%) combat parasitic infections and participate in allergic reactions. Lymphocytes (20-25%) include B and T cells responsible for immune responses. Monocytes (6-8%) are also phagocytic. Platelets are cell fragments derived from megakaryocytes in the bone marrow and number about 1,500,000 to 3,500,000 per mm³. They play a key role in blood clotting by releasing substances that activate the coagulation cascade. A decrease in platelet count can lead to bleeding disorders.

• Blood is a fluid connective tissue composed of plasma and formed elements.
• Plasma is 90-92% water and contains proteins like fibrinogen, globulins, and albumins.
• Erythrocytes are biconcave, anucleate cells containing haemoglobin for oxygen transport.
• Leucocytes defend the body and are classified into granulocytes and agranulocytes.
• Platelets are cell fragments involved in blood clotting.
• Plasma carries nutrients, minerals, hormones, and inactive clotting factors.
• 📌 Blood: Fluid connective tissue transporting substances in the body.
• 📌 Plasma: Straw-coloured fluid matrix of blood.
• 📌 Erythrocytes: Red blood cells carrying oxygen.

15.1.1 Plasma

Plasma is the liquid component of blood, constituting about 55% of the total blood volume. It is a straw-coloured, viscous fluid composed mainly of water (90-92%) which acts as a solvent and medium for transporting various substances. Plasma proteins make up 6-8% of plasma and include fibrinogen, globulins, and albumins. Fibrinogen is essential for blood clotting; it is converted into fibrin threads during coagulation to form a clot. Globulins are primarily involved in the immune defense mechanisms of the body, functioning as antibodies. Albumins maintain the osmotic balance between blood and tissues, preventing excessive loss of fluid from blood vessels. Besides proteins, plasma contains dissolved minerals such as sodium ions (Na⁺), calcium ions (Ca²⁺), magnesium ions (Mg²⁺), bicarbonate ions (HCO₃⁻), and chloride ions (Cl⁻). These electrolytes help in maintaining pH, osmotic balance, and are vital for various physiological functions. Plasma also carries nutrients like glucose, amino acids, and lipids which are transported to cells for metabolism. Hormones secreted by endocrine glands travel dissolved in plasma to reach target organs. Waste products like urea and carbon dioxide are also transported in plasma to excretory organs. Plasma contains inactive clotting factors that are activated during injury to initiate coagulation. When plasma is separated from blood after clotting, it is called serum, which lacks clotting factors. The viscous nature of plasma helps in smooth flow of blood through vessels. Thus, plasma serves as a transport medium and plays a critical role in maintaining homeostasis.

• Plasma constitutes about 55% of blood volume and is mostly water.
• Contains proteins: fibrinogen (clotting), globulins (immunity), albumins (osmotic balance).
• Carries minerals like Na⁺, Ca²⁺, Mg²⁺, HCO₃⁻, Cl⁻ essential for physiological functions.
• Transports nutrients, hormones, and waste products.
• Contains inactive clotting factors; plasma without clotting factors is serum.
• Viscous fluid facilitating smooth blood flow.
• 📌 Plasma: Fluid matrix of blood carrying substances.
• 📌 Fibrinogen: Plasma protein essential for blood clotting.
• 📌 Serum: Plasma without clotting factors.