Blood and Its Composition 🩸

Shivi Forensics
Blood and Its Composition 🩸

Written by,
Suramya Suresh
Volunteer, Shivi Forensics

Blood
Blood is a fluid connective tissue that consists of plasma, blood cells and platelets. It circulates throughout our body delivering oxygen and nutrients to various cells and tissue.

Properties Of Human Blood
• Blood makes up about 8% of body weight.
• Adult females normally have 4–5 litres of blood, while adult males have 5–6 litres, mainly due to differences in body size.
• Blood is composed of plasma and blood cells. Plasma is a straw-coloured liquid forming more than half of the blood, while the cells include RBCs, WBCs, and platelets.
• Blood appears red and opaque because of haemoglobin, which becomes brighter with oxygen and darker when oxygen is low.
• It is about five times thicker (more viscous) than water.
• Blood has a slightly alkaline pH of 7.35–7.45.
• Its temperature is around 38°C (100.4°F), about one degree higher than normal body temperature.

Function
• Transport: Delivers oxygen, nutrients, hormones, and essential substances to all body tissues.
• Waste Removal: Carries carbon dioxide and metabolic wastes to the lungs, kidneys, and digestive organs for elimination.
• Temperature Regulation: Helps maintain and balance body temperature.
• Infection Protection: Transports white blood cells and antibodies that detect and destroy harmful pathogens.
• Clotting Mechanism: Platelets form clots to prevent excessive bleeding and protect injured tissues.
• Immune Response: White blood cells actively identify, attack, and remove invading organisms and abnormal cells.

Composition Of Blood
When a sample of blood is spun in a centrifuge machine, they separate into the following constituents:
• Plasma
• buffy coat
• Erythrocytes
• Plasma forms the uppermost layer in a centrifuged blood sample and makes up slightly less than 55% of the total blood volume. It appears as a pale-yellow fluid and is composed of approximately 90% water and 10% dissolved substances. These dissolved components include antibodies, enzymes, hormones, various blood proteins, and metabolic waste products such as carbon dioxide. Plasma also carries essential nutrients including amino acids, glucose, lipids, and electrolytes. In addition, foreign substances—such as medications, toxins, and alcohol—may also be present in the plasma. Because of this, plasma is often tested in forensic science to detect drugs or other chemicals during criminal investigations.
• Red Blood Cells (RBCs) are much denser than plasma, causing them to settle at the bottom of the tube during centrifugation. They account for about 45% of the total blood volume. This proportion is measured as the haematocrit, an important clinical indicator used to assess oxygen-carrying capacity, hydration status, and various medical conditions.
• White Blood Cells (WBCs) and platelets are less dense than RBCs but denser than plasma. As a result, they form a thin, cream-coloured intermediate layer between the plasma and RBCs. This layer is called the buffy coat. Although it makes up only a very small percentage of the total blood volume, the buffy coat is significant because it contains the immune cells and clotting components essential for fighting infection and preventing blood loss.
Plasma
The liquid portion of blood is due to the presence of plasma, which makes up approximately 55% of the total blood volume. Plasma is pale yellow in colour when separated from the other components. It is composed mainly of water, salts, nutrients, and enzymes, along with important proteins such as albumin, fibrinogen, and antibodies. These components play essential roles in maintaining overall health by supporting nutrient transport, clotting, immune function, and the regulation of fluid balance within the body.

Blood plasma has several protein components. Proteins in blood plasma are:
• Serum globulin
• Serum albumin
• Fibrinogen

Erythrocytes
Erythrocytes are disc-shaped cells with a thick outer edge and a thin, depressed centre. In humans, mature erythrocytes do not contain a nucleus, which allows additional space for haemoglobin. They contain the iron-rich protein haemoglobin, responsible for the red colour of blood. Haemoglobin binds with oxygen in the lungs and transports it to tissues throughout the body. RBCs are the most abundant type of blood cell, produced continuously in the bone marrow. Their primary role is to deliver oxygen to organs and tissues and transport carbon dioxide back to the lungs for exhalation. The plasma membrane of RBCs carries distinctive glycoproteins and glycolipids, which are used to determine an individual’s blood group. During early development, erythrocytes contain typical cellular organelles such as mitochondria, but these are lost as the cell matures. This adaptation increases haemoglobin content and enhances oxygen-carrying capacity. Haemoglobin itself is an iron-containing protein that forms a temporary bond with oxygen, producing oxyhaemoglobin, which allows efficient oxygen transport to all parts of the body.

Leukocytes (White Blood Cells – WBCs)
White blood cells form a crucial part of the body’s immune system, offering protection against disease-causing microorganisms. They circulate continually through the bloodstream and body tissues, detecting and responding to signs of infection, injury, or inflammation. When foreign organisms enter the body, WBCs act quickly to identify, attack, and destroy them, helping to maintain overall health and immunity.
WBCs are classified as:
• Granulocytes - Eosinophils, Basophil, Neutrophils
• Agranulocytes - Lymphocytes, Monocytes
1. Granulocytes
Granulocytes are a category of leukocytes (white blood cells) characterized by the presence of granules in their cytoplasm. They include eosinophils, basophils, and neutrophils. These granules contain enzymes and chemicals that assist in immune defens.
1. Eosinophils
• Eosinophils are a type of leukocyte involved in immune system function.
• They play a significant role in defending the body against parasitic infections, particularly in vertebrates, and also regulate allergic reactions and asthma-related responses.
• They are small granulocytes produced in the bone marrow, making up 2–3% of total WBCs.
• Eosinophils are especially abundant in the digestive tract, where they help control inflammation and respond to allergens.

2. Basophils
• Basophils are the least common granulocytes, forming only 0.5–1% of circulating WBCs.
• They possess large cytoplasmic granules that play key roles in non-specific immune responses, allergic reactions, and inflammatory processes.
• Basophils release chemicals such as histamine, which dilates blood vessels, and serotonin and heparin, which assist in inflammation and anticoagulation.
• They are called basophils because they stain readily with basic dyes.
• Basophils are strongly associated with asthma and contribute to inflammation and bronchoconstriction within the airways.

3. Neutrophils
• Neutrophils circulate primarily in the bloodstream and are the most abundant WBCs.
• They make up 60–65% of total WBCs and measure 10–12 μm in diameter.
• They have a multi-lobed nucleus (2–5 lobes) and contain fine cytoplasmic granules.
• Neutrophils are the body’s first line of defense, rapidly responding to bacterial or viral invaders.
• They destroy pathogens using lysosomes and strong oxidizing agents.
• Neutrophils are stained using neutral dyes, which is why they are named so.
• They are the predominant cells found in pus at infection sites.
• Their lifespan is around 8 hours, and they are continuously produced in the bone marrow.

2. Agranulocytes
They are leukocytes, with the absence of granules in their cytoplasm. Agranulocytes are further classified into monocytes and lymphocytes.
1. Monocytes
• Monocytes have a large, folded nucleus, measuring about 12–20 μm in diameter.
• They make up 6–8% of WBCs and typically have a half-moon or kidney-shaped nucleus.
• Their key functions include:
(i) Migrating into tissues to become macrophages
(ii) Removing dead or damaged cells
(iii)  Defending against blood-borne pathogens
(iv)  Responding quickly to infection sites
(v)  They have a single bean-shaped nucleus, giving them the name monocytes.

2. Lymphocytes
• Lymphocytes play a crucial role in antibody production and overall immune defense.
• They measure 8–10 μm in diameter.
• They are often referred to as natural killer cells when functioning in innate immunity.
• Lymphocytes are colourless and produced in lymphoid tissues, such as the spleen, lymph nodes, and thymus.
• There are two major types:
(i) B lymphocytes, which produce antibodies (humoral immunity)
(ii) T lymphocytes, which destroy infected or abnormal cells (cell-mediated immunity)
• They are vital for both humoral and cellular immune responses and act as one of the body’s main defense mechanisms.

Platelets (Thrombocytes)
Platelets are specialized cell fragments produced in the bone marrow from large cells called megakaryocytes. They are essential for controlling bleeding and blood clotting. Since platelets are fragments of larger cells, they are not considered true cells.
Functions of Platelets:
• They secrete vasoconstrictors, which constrict blood vessels and reduce blood loss by causing vascular spasms.
• They form temporary platelet plugs to seal small breaks in blood vessels.
• They release procoagulants (clotting factors) to promote formation of a stable blood clot.
• They help dissolve clots once healing is complete.
• They assist in the destruction of bacteria.
• They secrete chemicals that attract neutrophils and monocytes to sites of inflammation, aiding in the immune response.

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