BLOOD AND IT'S CONSTITUENTS

UPDATED ON Tue, 21 Jan, 2025 BY

Blood is a fluid connective tissue that continuously circulates throughout the body, enabling ongoing communication between distant tissues. It plays a vital role in transporting oxygen, nutrients, hormones, heat, protective substances, and clotting factors.

Blood is made up of a clear, straw-colored, watery fluid known as plasma, in which various types of blood cells are suspended. Plasma typically makes up 55% of the blood's volume, while the cell fraction accounts for 45%.

Blood cells and plasma can be separated through centrifugation (spinning) or by allowing the blood to stand, which uses gravity to achieve separation.

The cells in blood are denser than plasma and therefore settle at the bottom of any sample. Blood comprises approximately 7% of body weight, which is around 5.6 liters in a 70 kg man.

This percentage is lower in women and significantly higher in children, gradually decreasing until reaching adult levels. Blood within the blood vessels is constantly in motion due to the heart's pumping action.

This continuous flow ensures a stable environment for body cells. Blood volume and the concentration of its various components are tightly regulated by homeostatic mechanisms.

Heat generated by metabolically active organs, such as working skeletal muscles and the liver, is circulated throughout the body by the bloodstream, helping to maintain core body temperature.

Plasma is chiefly composed of water (90–92%) and includes a variety of dissolved and suspended substances such as plasma proteins, inorganic salts, nutrients from digested foods, waste materials, hormones, and gases.

Plasma Proteins

Plasma proteins, making up about 7% of plasma, are essential for maintaining blood's osmotic pressure, which keeps plasma fluid within circulation. A reduction in plasma protein levels can lead to fluid moving into tissues (oedema). Plasma viscosity is mainly due to albumin and fibrinogen. Except for immunoglobulins, most plasma proteins are produced in the liver. Example of Plasma include; Albumins, Globulins etc.

Albumins:

Albumins are the most abundant plasma proteins, constituting about 60% of the total. Their primary function is to maintain normal plasma osmotic pressure. They also serve as carrier molecules for free fatty acids, some drugs, and steroid hormones.

Globulins:

Globulins have several functions, including acting as antibodies (immunoglobulins) crucial for immunity, transporting hormones and mineral salts, and inhibiting proteolytic enzymes. They also include clotting factors responsible for blood coagulation, with fibrinogen being the most abundant.

Electrolytes:

Electrolytes in plasma are involved in various functions, such as muscle contraction, nerve impulse transmission, and maintaining acid-base balance. The pH of blood is kept slightly alkaline, between 7.35 and 7.45, by a buffering system.

Nutrients:

Nutrients from digestion, including glucose, amino acids, fatty acids, and glycerol, are absorbed from the alimentary tract and used by body cells for energy, repair, and synthesis of other blood components and secretions.

Waste Products:

Waste products like urea, creatinine, and uric acid, resulting from protein metabolism, are formed in the liver and carried by blood to the kidneys for excretion. Carbon dioxide from tissue metabolism is transported to the lungs for excretion.

Hormones:

Hormones are chemical messengers produced by endocrine glands. They are released directly into the blood, which transports them to target tissues and organs, where they influence cellular activities.

Gases:

Oxygen, carbon dioxide, and nitrogen are dissolved in plasma and transported throughout the body. Oxygen and carbon dioxide are also carried by hemoglobin in red blood cells, with most oxygen bound to hemoglobin and most carbon dioxide as bicarbonate ions in plasma. Atmospheric nitrogen is present in plasma but has no physiological role. Atmospheric nitrogen enters the body in the same way as other gases and is present in plasma but it has no physiological function.

Erythrocytes (Red Blood Cells

Erythrocytes, or red blood cells, are the most prevalent type of blood cell, accounting for 99% of all blood cells. These cells are biconcave discs, lacking a nucleus, and measure about 7 µm in diameter. Their primary role is to transport gases, predominantly oxygen, though they also carry some carbon dioxide. Their unique shape enhances their function: the biconcave form increases the surface area for gas exchange, and the thin central region allows for the rapid movement of gases. Additionally, erythrocytes are flexible, enabling them to pass through narrow capillaries, and they lack intracellular organelles, maximizing space for hemoglobin, the large pigmented protein essential for gas transport. Assessments of red cell count, volume, and hemoglobin content are routine and valuable in clinical practice, with commonly used laboratory abbreviations provided in reports.

Life span and function of erythrocytes

Erythrocytes, or red blood cells, have a lifespan of about 120 days, during which they cannot divide due to the absence of a nucleus. They are continuously replenished by new cells produced in the red bone marrow found in the ends of long bones and in flat and irregular bones. Erythropoiesis, the process of red blood cell development from stem cells, takes about 7 days. Immature cells, called reticulocytes, are released into the bloodstream and mature into erythrocytes within a day or two.

Vitamin B12 and folic acid are essential for red blood cell synthesis, and both are absorbed in the intestines. Vitamin B12 absorption requires intrinsic factor, and both vitamins are found in dairy products, meat, and green vegetables. The liver stores a significant amount of vitamin B12, but folic acid deficiency can manifest within a few months if intake is insufficient.

Haemoglobin:

Haemoglobin is a large, complex molecule made up of a globular protein (globin) and an iron-containing pigment called haem. Each haemoglobin molecule has four globin chains and four haem units, each with one iron atom, allowing it to carry up to four oxygen molecules. An average red blood cell contains about 280 million haemoglobin molecules, giving it a vast oxygen-carrying capacity. Iron, essential for haemoglobin production, is transported in the bloodstream by transferrin and stored in the liver. Iron absorption from the alimentary canal is slow, making deficiency common if losses exceed intake.

Oxygen Transport:

Haemoglobin binds reversibly to oxygen to form oxyhaemoglobin. When fully saturated, haemoglobin carries oxygen efficiently, giving oxygen-rich blood (usually arterial) a bright red color. In contrast, oxygen-poor blood (usually venous) appears dark bluish. Oxyhaemoglobin readily releases oxygen, especially in low pH environments, low oxygen levels (hypoxia), and higher temperatures, such as in metabolically active tissues. In the lungs, cooler temperatures favor oxyhaemoglobin formation, ensuring oxygen is transported from the lungs to the tissues effectively.

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Control of Erythropoiesis:

Erythropoiesis, the production of red blood cells, is regulated by a homeostatic negative feedback mechanism, primarily controlled by the hormone erythropoietin, produced mainly by the kidneys. Hypoxia, or low oxygen levels, stimulates erythropoietin production, which increases the production and maturation of red blood cells, boosting the blood's oxygen-carrying capacity. When hypoxia is resolved, erythropoietin levels decrease. If erythropoietin production is low, red cell formation is impaired, leading to anemia, where the blood cannot carry enough oxygen to meet the body's needs.

Destruction of erythrocytes (Haemolysis)

Erythrocytes have a lifespan of about 120 days, after which they are broken down in a process called haemolysis by phagocytic reticuloendothelial cells, primarily in the spleen, bone marrow, and liver. As erythrocytes age, their membranes become fragile, making them more prone to haemolysis. The iron released during this process is retained and reused in the bone marrow to produce new hemoglobin. The haem component of hemoglobin is converted into biliverdin, which is then reduced to bilirubin. Bilirubin is transported to the liver, where it is converted from a fat-soluble to a water-soluble form and excreted in bile.

Blood groups

Early blood transfusions often failed because the recipient's immune system would react to incompatible blood. This reaction is due to specific proteins, or antigens, on the surface of red blood cells, which vary among individuals and determine their blood group. When blood from a donor with different antigens is transfused, the recipient's immune system produces antibodies against the foreign antigens, leading to a potentially fatal transfusion reaction. Successful transfusions occur when the donor's blood group matches the recipient's, preventing the immune system from rejecting the transfused blood. The most important blood group systems are ABO and Rhesus.

The ABO system

The ABO blood group system classifies people based on the presence of A-type or B-type antigens on their red blood cells. About 55% of people have either A (blood group A), B (blood group B), or both antigens (blood group AB), while 45% have neither (blood group O). Blood group A individuals produce anti-B antibodies, group B produces anti-A, group AB produces neither, and group O produces both. Group AB individuals are known as universal recipients, as they can receive blood from any ABO group, while group O individuals are considered universal donors since their blood can be safely transfused to any ABO type. However, the terms "universal donor" and "universal recipient" are somewhat misleading, as other antigen systems besides ABO must be considered to prevent transfusion reactions. Therefore, cross-matching blood is essential before transfusion to ensure compatibility.

The Rhesus system

The Rhesus (Rh) system classifies people based on the presence of the Rh antigen on their red blood cells. About 85% of people are Rh positive (Rh+), meaning they have this antigen and do not produce anti-Rhesus antibodies. The remaining 15% are Rh negative (Rh−) and can produce anti-Rhesus antibodies, but only if exposed to Rh+ blood, such as during pregnancy or after an incompatible blood transfusion.

Leukocytes (White Blood Cells)

responsible for detecting and destroying foreign or abnormal substances in the body. Although they account for only about 1% of the blood volume, they are essential for maintaining health and defending against infections. Leukocytes are the largest cells in the blood and contain nuclei. They are categorized into two main types: granulocytes and agranulocytes.

Granulocytes

Granulocytes, also known as polymorphonuclear leukocytes, have multi-lobed nuclei and granules in their cytoplasm. They are further divided into three types: neutrophils, eosinophils, and basophils.

Neutrophils

Neutrophils are the most abundant type of white blood cell. They are small, fast, and highly active, acting as the body's first line of defense against bacterial infections. Neutrophils are attracted to sites of infection by chemical signals known as chemotaxins, released by damaged cells. They move quickly to the affected area, squeezing through capillary walls in a process called diapedesis. Once at the site of infection, neutrophils engulf and destroy bacteria through phagocytosis. They have complex nuclei with multiple lobes and granules containing lysosomes, which are enzymes that digest engulfed material. Neutrophils have a short lifespan of about 6 to 9 hours in the bloodstream. Pus that forms in an infected area is composed of dead tissue cells, microbes, and neutrophils killed by microbes.

Eosinophils

Eosinophils are less active in phagocytosis compared to neutrophils. Their specialized function is in combating parasitic infections, particularly those caused by large organisms like worms, which are too big to be engulfed by phagocytosis. Eosinophils release toxic chemicals stored in their granules when they bind to a parasite, helping to eliminate the threat. They also play a role in allergic reactions, accumulating in areas of allergic inflammation, such as in asthma or skin allergies. In addition to promoting inflammation, eosinophils can also regulate the inflammatory response by releasing chemicals like histaminase, which breaks down histamine.

Basophils

Basophils are the least common type of granulocyte and are closely associated with allergic reactions. They contain granules packed with heparin (an anticoagulant), histamine (an inflammatory agent), and other substances that promote inflammation. Basophils release these granules when stimulated by allergens, which are antigens that cause allergic reactions. They have receptors on their membranes that bind to antibodies, triggering the release of their granules. A similar type of cell, called a mast cell, is found in tissues rather than in the bloodstream. Mast cells rapidly release their granule contents upon exposure to allergens, leading to the quick onset of allergic symptoms, such as those seen in hay fever.

Agranulocytes

Agranulocytes lack granules in their cytoplasm and have large, unlobed nuclei. This category includes monocytes and lymphocytes, which together make up 25 to 50% of the total leukocyte count.

Monocytes

Monocytes are the largest type of white blood cell. Some monocytes circulate in the blood and are actively motile and phagocytic, while others migrate into tissues and develop into macrophages. Macrophages are powerful and long-lived cells that perform a wide range of protective functions. They are highly phagocytic, meaning they can engulf and digest large amounts of foreign material, including bacteria, dead cells, and other debris. Macrophages are also key players in linking the non-specific and specific immune systems by producing cytokines, such as interleukin 1, which have various roles in inflammation, immune response, and repair. Some macrophages are mobile, while others are fixed in specific tissues, providing effective defense at critical body locations. In the lungs, for example, macrophages can isolate and seal off harmful substances like tuberculosis bacteria or inhaled dust particles, preventing them from damaging surrounding tissue.

Lymphocytes

Lymphocytes are smaller than monocytes and have large, round nuclei. While some lymphocytes circulate in the blood, most are found in lymphatic tissues, such as lymph nodes and the spleen. Lymphocytes originate from pluripotent stem cells in the red bone marrow and from precursors in lymphoid tissue. Although all lymphocytes start from a common stem cell, they eventually differentiate into two distinct types: T-lymphocytes and B-lymphocytes. T-lymphocytes are primarily responsible for cell-mediated immunity, which involves the direct destruction of infected or abnormal cells. B-lymphocytes are involved in humoral immunity, which involves the production of antibodies that target and neutralize foreign invaders like bacteria and viruses. Together, T- and B-lymphocytes form the backbone of the body's specific immune response, enabling it to recognize and remember specific pathogens for more effective defense in future encounters.

Platelets (thrombocytes)

Platelets, or thrombocytes, are small, disc-shaped cell fragments about 2–4 µm in diameter, derived from megakaryocytes in the red bone marrow. They lack nuclei but are filled with granules that aid in blood clotting and stop bleeding (haemostasis). The normal platelet count ranges from 200,000 to 350,000 per mm³ of blood. Platelet production is stimulated by the hormone thrombopoietin from the liver. Platelets have a lifespan of 8 to 11 days, and those not involved in clotting are removed by macrophages, primarily in the spleen. About one-third of platelets are stored in the spleen as an emergency reserve to manage excessive bleeding.

Haemostasis

Haemostasis is the process that stops blood loss and facilitates healing after a blood vessel is damaged. It involves four main steps:

1. Vasoconstriction: Platelets adhere to the damaged vessel wall, releasing serotonin and other chemicals to narrow the vessel and reduce blood flow.

2. Platelet Plug Formation: Adherent platelets clump together and release substances like adenosine diphosphate (ADP) to attract more platelets, forming a temporary seal called a platelet plug within 6 minutes.

3. Coagulation (Blood Clotting): A complex process involving clotting factors activates prothrombin, which converts to thrombin. Thrombin then converts fibrinogen into fibrin threads, stabilizing the platelet plug and forming a stronger blood clot. This process involves two pathways: the extrinsic pathway, activated rapidly by tissue damage, and the intrinsic pathway, triggered by blood contact with damaged vessel lining.

4. Fibrinolysis: Once the clot has formed, it is eventually removed through fibrinolysis. Plasminogen in the clot is activated to plasmin, which breaks down fibrin into soluble products that are cleared by phagocytosis, completing the healing process.

Control of coagulation

Smooth Blood Vessel Lining:Healthy, undamaged blood vessels have a smooth lining that prevents unwanted platelet adhesion.

Natural Anticoagulants: Activated clotting factors are quickly deactivated by natural anticoagulants like heparin and antithrombin III, which interrupt the clotting cascade and limit its activity to the area of injury.