Blood is a fundamental component of the human body, integral to sustaining life and promoting health. It plays a critical role in various bodily functions, including transportation of substances, maintenance of homeostasis, and provision of defence mechanisms. This section provides an in-depth look at the composition and functions of blood, specifically focusing on its four main components: erythrocytes, leucocytes, platelets, and plasma.
Composition of Blood
Erythrocytes (Red Blood Cells)
- Primary Function: Erythrocytes are primarily responsible for the transport of oxygen from the lungs to body tissues and facilitating the return of carbon dioxide from the tissues back to the lungs.
- Structure and Adaptations: These cells are uniquely shaped as biconcave discs, which lack a nucleus. This shape increases the surface area for gas exchange and allows for flexibility as they navigate through capillaries.
- Haemoglobin Content: Erythrocytes contain haemoglobin, a protein that can bind both oxygen and carbon dioxide. The binding of oxygen to haemoglobin in the lungs forms oxyhaemoglobin, which is released into tissues.
- Life Span and Recycling: They have a life span of approximately 120 days. After their lifespan, they are phagocytosed primarily by the spleen and liver, where their components are recycled.
Leucocytes (White Blood Cells)
- Role in Defence Mechanisms: Leucocytes are crucial in the body's defence against infectious disease and foreign invaders. They are part of the immune system and can be classified into several types, each with distinct functions.
- Variety and Functions:
- Lymphocytes: They are responsible for the production of antibodies and the cellular immune response. T-lymphocytes and B-lymphocytes are the two primary types, playing roles in cell-mediated and humoral immunity, respectively.
- Neutrophils: These are the first responders to microbial infection and are effective in engulfing and destroying pathogens, particularly bacteria and fungi.
- Monocytes: These cells differentiate into macrophages and dendritic cells, which are essential for antigen presentation and initiating an immune response.
- Eosinophils and Basophils: These cells are primarily involved in allergic reactions and combating parasitic infections. Basophils release histamine, contributing to inflammation.
Platelets (Thrombocytes)
- Function in Clotting: Platelets are critical in the blood clotting process, which prevents excessive bleeding. They adhere to the site of a blood vessel injury, aggregate, and interact with various clotting factors to form a clot.
- Formation and Activation: They are formed as cell fragments from megakaryocytes in the bone marrow. Upon vascular injury, they are activated, leading to a change in shape and the release of clotting factors.
Plasma
- Composition: Plasma is a straw-coloured liquid that makes up about 55% of total blood volume. It comprises about 90% water, with the rest being proteins, glucose, mineral ions, hormones, carbon dioxide, and various other components.
- Functional Roles:
- Transport Medium: It serves as the primary medium for transporting hormones, nutrients, gases, and waste products throughout the body.
- Regulatory Functions: Plasma helps in maintaining blood pressure and pH balance in the body.
- Immune Response: It contains antibodies and other proteins essential for immune system responses.
Functions of Blood Components
Transport of Gases, Nutrients, and Waste Products
- Oxygen Transportation: Erythrocytes carry oxygen from the lungs to tissues. Oxygen is essential for cellular respiration and energy production.
- Carbon Dioxide Removal: Carbon dioxide, a waste product of cellular respiration, is transported back to the lungs for exhalation.
- Nutrient Delivery: Essential nutrients like glucose, amino acids, and lipids are carried to cells for energy, growth, and repair.
- Waste Product Removal: Metabolic waste products such as urea are transported to excretory organs like the kidneys and liver for removal from the body.
Hormonal Transport and Regulation
- Endocrine Integration: Blood plays a vital role in the endocrine system by transporting hormones from glands to their target organs. This transport is crucial for regulating various body functions, including growth, metabolism, and reproduction.
Defence Mechanisms and Immune Response
- Pathogen Defence: Leucocytes play a key role in identifying and destroying pathogens and foreign bodies. They are essential components of the body’s immune response.
- Antibody Production: Lymphocytes produce antibodies, which are proteins that bind to and neutralise foreign substances like bacteria and viruses.
Role in Homeostasis
- Temperature Control: Blood helps distribute heat generated by metabolism throughout the body, thereby aiding in temperature regulation.
- pH and Electrolyte Balance: Blood contains buffers that help maintain pH at a constant level, essential for enzyme function and metabolic processes. It also helps regulate electrolyte balance, important for nerve and muscle function.
Nutrient Supply and Energy Provision
- Digestive System Link: After digestion, nutrients absorbed from the gastrointestinal tract are distributed throughout the body via the blood.
- Energy Supply: Blood glucose and other nutrients provide energy to cells for their metabolic activities, crucial for maintaining bodily functions and supporting physical activity.
FAQ
Blood maintains pH balance in the body through a buffering system, predominantly involving bicarbonate ions. This system regulates the concentration of hydrogen ions (H+) in the blood, which determines its pH level. When the blood becomes too acidic (high H+ concentration), bicarbonate ions (HCO3-) react with the excess hydrogen ions to form carbonic acid (H2CO3). This carbonic acid then breaks down into water and carbon dioxide, which is exhaled through the lungs. Conversely, when the blood is too alkaline (low H+ concentration), carbonic acid dissociates to release hydrogen ions, thereby lowering the pH. This delicate balance is crucial for maintaining the optimal pH for enzyme activity and metabolic processes.
Plasma, constituting about 55% of blood's total volume, is a critical component due to its extensive roles in transportation, regulation, and defence. It serves as the primary medium for transporting nutrients, gases, hormones, and waste products, ensuring these substances reach their respective destinations within the body. Plasma maintains homeostasis by helping regulate blood pressure and pH balance, vital for proper bodily functions. It also plays a significant role in the immune response, containing antibodies, clotting factors, and other proteins necessary for fighting infections and healing wounds. Therefore, plasma is not just a carrier but also an active participant in many physiological processes.
The life span of erythrocytes can be affected by several factors, including diseases, nutritional deficiencies, and physical conditions. Diseases such as haemolytic anaemia or sickle cell disease can cause premature destruction of red blood cells. Nutritional deficiencies, particularly in iron, vitamin B12, or folic acid, can lead to reduced life spans or the production of less functional erythrocytes. Physical conditions like extreme physical stress or exposure to certain toxins can also shorten their life span. Once erythrocytes reach the end of their life span, typically around 120 days, they are broken down in the spleen and liver. Their components are recycled, with iron being reused for new haemoglobin synthesis and other parts forming bile pigments like bilirubin.
Leucocytes, or white blood cells, are classified into several types, each with distinct roles in the immune system. The main types include:
- Neutrophils: These are the most abundant type of leucocytes and are the body's first line of defence against infections. They rapidly respond to and ingest bacteria and fungi.
- Lymphocytes: This group includes B-lymphocytes, which produce antibodies, and T-lymphocytes, which destroy infected or cancerous cells. They are essential for specific immune responses.
- Monocytes: They are the largest type and transform into macrophages and dendritic cells that digest bacteria, dead cells, and other debris.
- Eosinophils: These cells combat parasites and are involved in allergic reactions.
- Basophils: The least common, they release histamine during allergic reactions, contributing to inflammation.
The absence of a nucleus in erythrocytes (red blood cells) provides a significant functional advantage. Without a nucleus, these cells have more space to carry haemoglobin, the protein responsible for binding and transporting oxygen and carbon dioxide. This increased capacity for haemoglobin means that erythrocytes can transport more oxygen from the lungs to the body's tissues and more carbon dioxide from the tissues back to the lungs. Additionally, the lack of a nucleus allows erythrocytes to adopt a biconcave shape, enhancing their flexibility and surface area for gas exchange. This shape enables them to efficiently navigate through the smallest blood vessels, ensuring effective gas delivery and exchange throughout the body.
Practice Questions
Erythrocytes, or red blood cells, are primarily responsible for the transport of oxygen from the lungs to the body's tissues and the removal of carbon dioxide from the tissues back to the lungs. Their biconcave shape increases the surface area for gas exchange, making them highly efficient at their job. The lack of a nucleus allows more room for haemoglobin, the protein that binds oxygen and carbon dioxide. Haemoglobin's affinity for oxygen enables erythrocytes to carry a significant amount of oxygen, releasing it to tissues where it's needed, thus playing a crucial role in cellular respiration and energy production.
Platelets, or thrombocytes, are key players in the blood clotting process, essential for preventing excessive bleeding from injuries. They are small cell fragments derived from megakaryocytes in the bone marrow. Upon vascular injury, platelets quickly adhere to the damaged site, change shape, and release chemicals that activate other platelets and the clotting cascade. This process leads to the aggregation of platelets and the formation of a clot, which is stabilized by fibrin, a protein formed from clotting factors. Platelets' rapid response and interaction with clotting factors are critical for stopping blood loss and initiating wound healing.
