Blood: Composition, Functions, Circulation, Formation of Blood Cells & Disorders

Table of Contents

• Introduction to Blood
• Components of Blood
• Formation of Blood Cells
• Functions of Blood
• What is Blood Circulation?
• Types of Blood Circulation
• Blood Disorders and Diseases of Blood
• References

Introduction to Blood

• Blood is a liquid connective tissue composed of blood cells and plasma, which continuously circulates through blood vessels under the pumping action of the heart.
• In humans, blood is a red-colored body fluid that flows throughout the circulatory system to perform essential transport functions necessary for life.
• The primary role of blood is to transport gases, including oxygen from the lungs to body tissues and carbon dioxide from tissues back to the lungs for removal.
• Blood also carries nutrients, hormones, and other essential chemical substances to every cell of the body, ensuring proper growth, repair, and maintenance of body functions.
• In addition to delivering useful substances, blood is responsible for transporting metabolic waste products away from body cells to excretory organs, where these wastes are removed from the body.
• In simple terms, blood can be described as the body’s transportation system, connecting all organs and tissues by delivering necessary substances and removing unwanted materials.
• In medical terminology, blood is commonly referred to by the prefixes haemo (hemo) or haemato (hemato), which are frequently used in scientific and clinical contexts.
• Blood accounts for approximately 7% to 10% of the total body weight of an average human.
• For a healthy adult weighing around 70 kilograms, the average total blood volume is approximately 5.5 liters.
• The amount of blood present in the body varies with age, with children having a higher blood volume relative to their body weight compared to adults.
• Blood volume also differs by sex, as females generally have a lower total blood volume than males of the same age, body weight, and health condition.
• Human blood is slightly basic (alkaline) in nature, with a normal pH range of approximately 7.35 to 7.45, which is essential for maintaining physiological balance and proper cellular activities.
• The temperature of human blood is usually slightly higher than the normal body temperature of 37°C, although under normal physiological conditions it rarely exceeds 38°C.

Components of Blood

• Blood is composed of blood cells (hematocytes or hematopoietic cells) and plasma.
• These components work together to perform essential functions such as transportation of substances, immunity, blood clotting, and maintaining homeostasis.

Blood Cells (Hematocytes)

• Blood cells are the cellular components of blood, making up approximately 45–46% of total blood volume.
• In humans, blood cells are classified into three major types:
• Red Blood Cells (RBCs)
• White Blood Cells (WBCs)
• Platelets

Red Blood Cells (RBCs)

• Red blood cells, also called erythrocytes, are red-colored, biconcave, anucleated cells.
• They are the most abundant blood cells, making up about 99% of total blood cells.
• They have a disc or biconcave shape with an average diameter of 7–7.5 μm and thickness of about 2.5 μm at the edges.
• Their average lifespan is 120 days.
• RBCs are more numerous in males than females.
• Males: about 42–54% of total blood volume or 4.7–6.1 million RBCs per cubic microliter
• Females: about 37–47% of total blood volume or 4.2–5.4 million RBCs per cubic microliter
• The most important components of RBCs are:
• Hemoglobin, an iron-rich protein responsible for carrying oxygen and carbon dioxide, making up about 95% of RBC dry weight
• Antigenic glycoproteins, which determine blood groups
• Major blood group antigens include:
• A antigen
• B antigen
• O antigen
• Rhesus (Rh) antigen
• The primary function of RBCs is the transportation of oxygen and carbon dioxide.
• White Blood Cells (WBCs)
• White blood cells, also called leukocytes or polymorphonuclear cells, are colorless, nucleated defensive blood cells.
• They are the largest blood cells but the least numerous, with approximately 4,000–11,000 WBCs per cubic microliter of blood.
• WBCs are polymorphic, meaning different types have different structures.
• Their major role is to provide immunity and defense against infections.
• WBCs are broadly divided into two groups:

Granulocytes

• Characterized by multi-lobed nuclei and cytoplasmic granules.
• Types include:
• Eosinophils
• Have bilobed nuclei of roughly equal size
• Stain red or pink with acidic dye
• Constitute 1–6% of total WBCs
• Function in defense against parasitic infections and inflammation
• Basophils
• Have bilobed nuclei of unequal size
• Stain blue or bluish-purple
• Make up less than 1% of total WBCs
• Function in allergic reactions, inflammation, and allergen destruction
• Neutrophils
• Smallest and most abundant WBCs
• Have multilobed nuclei (up to 6 lobes)
• Stain purple
• Account for 40–75% of total WBCs
• Provide defense mainly against bacterial infections

Agranulocytes

• Characterized by a large single nucleus and lack of granules.
• Types include:
• Monocytes
• Largest type of WBC
• Have a single large round nucleus
• Stain purple or bluish
• Constitute 2–10% of total WBCs
• Exist as:
• Phagocytes in circulation
• Macrophages in tissues
• Important in microbial defense, inflammation regulation, and T-cell activation
• Lymphocytes
• Second most abundant WBCs
• Have a large kidney-shaped nucleus
• Make up 20–50% of total WBCs
• Include:
• T-lymphocytes
• B-lymphocytes
• Responsible for immune memory and antibody production
• Major functions of WBCs include:
• Providing immunity against infections
• Regulating the inflammatory response

Platelets

• Platelets, also called thrombocytes, are tiny anucleated blood cells.
• They are disc-shaped and measure approximately 2–4 μm in diameter.
• Their normal count ranges from 200,000–500,000 per cubic microliter of blood.
• Their average lifespan is 8–11 days.
• Their primary function is hemostasis (blood clotting).

Blood Plasma

• Plasma is the fluid component of blood, making up approximately 54–55% of total blood volume.
• It is a straw-yellow colored fluid composed mainly of water and dissolved substances.
• Plasma without clotting proteins is called serum.
• Major components of plasma include:
• Water (90–92%)
• Maintains blood fluidity
• Dissolves substances for transport
• Regulates body temperature
• Plasma proteins (7–8%)
• Include albumins, globulins, clotting proteins (fibrinogen and prothrombin), lipoproteins, and other conjugated proteins
• Electrolytes / ions
• Such as Na⁺, K⁺, Ca²⁺, PO₄³⁻, Mg²⁺, and Fe⁺
• Nutrients
• Including glucose, amino acids, fatty acids, and vitamins
• Hormones
• Secreted by endocrine glands and transported to target tissues
• Dissolved gases
• Mainly oxygen and carbon dioxide
• Waste products
• Including urea, creatinine, and other metabolic wastes transported for excretion

Formation of Blood Cells

• The formation of blood cells is called hematopoiesis (hemopoiesis/hematogenesis).
• In a normal adult body, approximately 200 billion red blood cells, 10 billion white blood cells, and 400 billion platelets are produced every day.
• All blood cells originate from a single type of cell called a multipotent hematopoietic stem cell (HSC).
• HSCs are self-replicating cells with the ability to differentiate and mature into various blood cells and lymphocytes.
• During development, hematopoiesis occurs in different organs depending on the stage:
• In the fetal stage, it occurs in the yolk sac and aorta-gonad-mesonephros (AGM) region.
• In the developing embryo (2–5 months), it occurs in the thymus, spleen, and liver.
• In the developed embryo (5+ months), infants, and adults, it mainly occurs in the bone marrow and lymph nodes.
• Blood cell formation inside the bone marrow is called medullary hematopoiesis, while blood cell formation outside the bone marrow is called extramedullary hematopoiesis.
• In the bone marrow, HSCs are present in hematopoietic tissue islands.
• HSCs first develop into precursor cells (progenitor cells) under the influence of fate-determining factors.
• These HSCs then differentiate into two main lineages:
• Common myeloid progenitor
• Common lymphoid progenitor
• The common myeloid progenitor can differentiate into RBCs, WBCs, or platelets, while the common lymphoid progenitor differentiates into lymphocytes.
• The time required for blood cell formation is closely related to the lifespan of each cell type:
• RBCs: about 120 days
• WBCs: a few hours to days
• Platelets: about 5–10 days
• This ensures continuous replacement and maintenance of blood cells in the body.
• Erythropoiesis (formation of RBCs):
• The common myeloid progenitor develops into a proerythroblast.
• It then undergoes sequential stages of development:
• Basophilic erythroblast
• Polychromatic erythroblast
• Orthochromatic erythroblast
• Polychromatic erythrocyte
• Finally matures into an erythrocyte (RBC)
• Leukopoiesis (formation of WBCs):
• The common myeloid progenitor differentiates into a myeloblast.
• The myeloblast further develops into different leukocyte lineages:
• Granulocyte formation:
• Myeloblast → basophil promyelocyte → basophil myelocyte → basophil metamyelocyte → basophil band → basophil
• Myeloblast → neutrophil promyelocyte → neutrophil (through similar intermediate stages)
• Myeloblast → eosinophil promyelocyte → eosinophil (through similar stages)
• Agranulocyte formation:
• Myeloblast → monoblast → monocyte / macrophage / myeloid dendritic cell
• Thrombopoiesis (formation of platelets):
• The common myeloid progenitor differentiates into a megakaryoblast.
• It then develops sequentially into:
• Promegakaryocyte
• Megakaryocyte
• Finally forms thrombocytes (platelets)

Functions of Blood

• Transportation of respiratory gases: Blood transports oxygen and carbon dioxide throughout the body, mainly through red blood cells (RBCs) and hemoglobin, as well as in dissolved form or combined with other substances.
• Transportation of essential substances: Blood carries nutrients, hormones, ions, and other chemical constituents to body cells, where they are required for various metabolic and physiological processes.
• Removal of metabolic wastes: Blood collects metabolic waste products from cells and tissues and transports them to excretory organs, such as the kidneys and lungs, for filtration and elimination.
• Immune defense: Circulating white blood cells (WBCs), antibodies, and other immune components protect the body against infections and provide immunological defense mechanisms.
• Blood clotting and hemostasis: Platelets and blood clotting factors help initiate and maintain hemostasis (blood clotting) to prevent excessive blood loss after injury and also participate in the breakdown of clots when healing is complete.
• Regulation of body temperature: Continuous blood circulation helps distribute heat throughout the body, thereby assisting in maintaining normal body temperature.
• Maintenance of homeostasis: Blood helps maintain the body’s internal balance (homeostasis) by regulating the chemical environment of cells and tissues.
• Maintenance of osmolarity: Blood maintains the osmotic balance of body cells and tissues, ensuring proper movement of fluids and preventing cellular dehydration or swelling.

What is Blood Circulation?

• Blood circulation is the continuous movement of blood through blood vessels due to the mechanical pumping action of the heart.
• The heart pumps oxygenated blood to all parts of the body through the systemic arteries, delivering oxygen and essential nutrients to tissues and organs.
• At the same time, the heart collects deoxygenated blood containing carbon dioxide and metabolic wastes from different body parts through the systemic veins.
• This deoxygenated blood is then transported to the lungs, where it undergoes reoxygenation through gas exchange.
• The complete process of pumping oxygenated blood to body tissues, collecting deoxygenated blood, and sending it back to the lungs for oxygenation is called the blood circulation process.
• The group of organs responsible for this process is known as the blood circulation system (circulatory system).
• The blood circulation system consists of:
• The cardiovascular system (heart and blood vessels)
• Blood
• Human blood circulation is classified as a closed type double circulation system.
• Closed circulation means that blood always flows within closed blood vessels and is returned to the heart for continuous recirculation.
• Double circulation means blood passes through the heart twice during one complete cycle of circulation.
• Human blood circulation has two separate circulatory pathways:
• Pulmonary circulation:
• The circulation of blood between the heart and the lungs
• Responsible for oxygenating blood and removing carbon dioxide
• Systemic circulation:
• The circulation of blood between the heart and the rest of the body organs and tissues
• Responsible for delivering oxygen and nutrients and collecting wastes from body cells

Types of Blood Circulation

Blood circulation in humans is broadly divided into two main types:

1. Pulmonary circulation
2. Systemic circulation
3. Coronary Circulation

Pulmonary Circulation

• Pulmonary circulation is the process of blood circulation between the heart and lungs for the purpose of blood purification and reoxygenation.
• In this circulation:
• The heart collects deoxygenated (impure) blood in the right atrium.
• The blood then passes from the right atrium to the right ventricle.
• The right ventricle pumps this blood to the lungs through the pulmonary artery.
• The pulmonary artery divides into smaller branches that further form capillaries surrounding the alveoli of the lungs.
• In these alveolar capillaries, gaseous exchange takes place, where:
• Blood releases carbon dioxide
• Blood absorbs oxygen from the air present in the alveoli
• After oxygenation, the capillaries unite to form pulmonary veins.
• The pulmonary veins carry the oxygenated blood back to the heart, delivering it to the left atrium for further circulation through the systemic circulation pathway.
• The primary function of pulmonary circulation is to oxygenate blood and remove carbon dioxide.

Systemic Circulation

• Systemic circulation is the process of blood circulation between the heart and all body tissues through systemic arteries and veins.
• In this circulation:
• The heart receives oxygenated blood from pulmonary circulation in the left atrium via the pulmonary veins.
• The left atrium transfers blood to the left ventricle.
• The left ventricle pumps blood into the aorta, from where it is distributed throughout the body via systemic arteries.
• The arteries gradually branch into smaller arterioles and finally into capillaries, which deliver blood to every tissue and cell of the body.
• In systemic capillaries, several exchanges occur:
• Oxygen is delivered to body cells
• Carbon dioxide is collected from cells
• Nutrients are delivered to tissues
• Metabolic wastes and other materials are collected
• After exchange, the capillaries collect carbon dioxide and waste products and merge to form venules.
• Venules combine to form veins, which eventually drain into the vena cava.
• The vena cava returns all deoxygenated blood to the right atrium of the heart, from where it is sent to the lungs again through pulmonary circulation for purification.
• The primary function of systemic circulation is to supply oxygen and nutrients to body tissues and remove metabolic wastes.

Coronary Circulation

• Coronary circulation is a specialized part of systemic circulation.
• In this circulation:
• Blood is supplied to the heart muscles (myocardium) through the coronary arteries.
• Deoxygenated blood from the heart muscles is collected by the coronary veins.
• Its main function is to provide the heart muscle with oxygen and nutrients necessary for continuous pumping activity.

Blood Disorders and Diseases of Blood

• Blood and blood circulation can be affected by several diseases and disorders.
• These disorders are broadly categorized into infectious blood disorders and non-infectious blood disorders.

Infectious Blood Disorders

• These are blood disorders caused by infectious microorganisms and/or their products.
• They are commonly referred to as blood infections, and the pathogens involved are known as blood pathogens.
• Sepsis: It is a condition in which bacteria, fungi, viruses, or parasites are present in the blood, leading to the manifestation of clinical symptoms and triggering the body’s immune response against such pathogens.
• Bacteremia: It refers to the presence of bacteria in the blood or bloodstream circulation. Common bacteria causing bacteremia include Staphylococcus spp., Streptococcus spp., Acinetobacter baumannii, Escherichia coli (E. coli), Klebsiella spp., Salmonella spp., and Pseudomonas aeruginosa.
• Viremia: It refers to the presence of viruses in the blood. Common viruses detected in blood include HIV, Hepatitis B virus, Hepatitis C virus, Dengue virus, Measles virus, Chickenpox virus, Ebola virus, and Lassa virus.
• Fungemia: It is defined as the presence of fungi in the blood, including spores and vegetative forms such as yeasts. Common fungi causing fungemia include Histoplasma spp., Candida spp., Aspergillus spp., Cryptococcus spp., and Saccharomyces spp.
• Parasitemia: It refers to the presence of parasites in the blood. Common blood parasites include Plasmodium spp., Trypanosoma spp., Babesia spp., and Haemoproteus spp.

Non-Infectious Blood Disorders

• These are blood disorders caused by non-microbial factors.
• They may result from defects in blood circulation, abnormalities in blood components, or disturbances in body physiology.
• Anemia: It is a blood disorder characterized by a reduction in the oxygen-carrying capacity of blood (RBCs) and/or reduced quantity or function of hemoglobin. About one-third of the world’s population is affected by anemia. It may be caused by disturbances in RBC and hemoglobin formation, destruction of RBCs, excessive bleeding, over-dilution of blood (hypervolemia), or intestinal inflammation.
• There are over 400 types of anemia.
• Clinically, anemia is classified as:
• Normocytic anemia (normal-sized RBCs)
• Megaloblastic anemia (enlarged RBCs)
• Microcytic anemia (smaller RBCs)
• Scientifically, it is classified as:
• Aplastic anemia (bone marrow damage)
• Megaloblastic anemia (vitamin B12 and/or folic acid deficiency)
• Hereditary anemia, including sickle cell anemia
• Hypochromic anemia (hemoglobin deficiency)
• Hemorrhagic anemia (excessive bleeding)
• Anemia due to reduced RBC count
• Erythrocytosis (Polycythemia): It is a condition characterized by an increase in RBC count above normal levels. It may be primary (caused by bone marrow abnormalities) or secondary (caused by disorders outside the bone marrow).
• Leukopenia: It is a condition in which WBC count decreases below normal levels. It is diagnosed when WBC count is less than 4.0 × 10⁹/L of peripheral blood. A decrease in neutrophils is called neutropenia, while a decrease in lymphocytes is called lymphocytopenia.
• Leukocytosis: It is a condition in which WBC count increases above normal levels. It is diagnosed when WBC count exceeds 11.0 × 10⁹/L of peripheral blood. Common forms include neutrophilia, lymphocytosis, and eosinophilia.
• Thrombocytopenia: It is a condition in which platelet count decreases below normal levels, specifically when platelet count is less than 150,000 per μL of blood.
• Thrombocytosis: It is a condition characterized by an increase in platelet count above normal levels.
• Blood Cancer: It is a condition involving uncontrolled growth of blood cells and their precursors, affecting bone marrow and normal blood cell production. The three major types are:
• Leukemia: Excessive production of abnormal WBCs that interfere with bone marrow function and produce large numbers of immature blast cells.
• Myeloma (Multiple Myeloma): Excessive growth of plasma cells that crowd the bone marrow and affect production of RBCs, platelets, and other WBCs.
• Lymphoma: A blood cancer that begins in the lymphoid system and forms tumors from lymphocytes.

• Hemorrhage: It refers to the leakage of blood outside blood vessels.
• Hematoma: It refers to the collection of blood within tissues or organs.
• Hemochromatosis: It is a condition characterized by an excessive amount of iron in the blood.
• Disseminated Intravascular Coagulation (DIC): It is a condition in which continuous hemorrhage and clotting occur simultaneously in small blood vessels.
• Hemophilia: It is a genetic blood disorder caused by deficiency of blood clotting proteins, leading to uncontrolled bleeding and delayed hemostasis.

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