Blood—The Softest Tissue | Blood Histology

Blood is a part of vascular connective tissue (tissue responsible for transport in the animal body) that helps in the transportation of oxygen, nutrients, hormones, medicines, etc.

There are two types of vascular connective tissue:

1. Blood
2. Lymph

Blood

The fluid that flows through the veins, arteries, and capillaries is known as blood. It is a specialized body fluid. It has mainly four components.

• Plasma
• Red blood cells (RBCs)
• White blood cells (WBCs)
• Platelets

‘Heam‘ means blood and ‘logy‘ means to study, in sum the study of blood is known as haematology. Blood is formed by the process of Haemopoisis. This process is done in bone marrow (in an adult human) and during the embryo stage, it is carried out in the spleen, liver, and lymph nodes.

Functions of Blood

• Transportation of oxygen, carbon dioxide, and nutrients from the lungs to the tissues.
• Carry out the immunity cells and antibodies that protect the body from infection.
• Helps in the formation of blood clots to prevent excess blood loss during injuries.
• Bring waste products from all the body parts to the kidneys and liver, which filter and clean the blood.
• Regulates the body temperature.

Characteristics of blood

• Colour: Red
• Volume: 5 – 6 litres in males and 4 – 5 litres in females
• pH: 7.4 (slightly acidic in pH)
• Weight: 7 – 8% of total body weight
• It is a false type of connective tissue. Reason:
  • Cells of blood have no power of division
  • Fibres are completely absent in the blood
  • Matrix of blood is produced and synthesized by the liver and lymphoid organs.

Components of Blood

Blood is a mixture of both liquid as well as solid parts. The liquid part contains a matrix, in which plasma is about 55% and the solid part contains blood corpuscles of about 45% (RBC, WBC & Platelets).

Plasma

The fluid matrix present in the blood is called plasma. It is pale yellow in colour due to urobilinogen (formed from the reduction of bilirubin). It is the largest part of the blood which contains water, sugar, protein, fat, and salts.

The main function of the plasma is to transport blood cells throughout the body along with nutrients, enzymes, waste products, antibodies, clotting proteins, chemical messengers such as hormones, and proteins to the parts of the body that need it, which help to maintain the body’s fluid balance.

Plasma composition

Plasma contains both organic and inorganic compounds.

• Water (90 – 92%)
• Solid parts (8 – 10%)

Solid parts

Organic parts of plasma (7 – 9%)

1. Proteins (6 – 7% viz; maximum)

• Albumin (4%) – Maximum in number and smallest in size. It maintains blood colloidal osmotic pressure (BCOP) to 28 – 30 mmHg.
• Globulin (2 – 2.5%) – Produced and secreted by the liver and lymphoid organs. Helps in the transportation of substances in the body, and helps in defence mechanisms by destroying bacteria, viruses, and other toxic substances.
  • Alpha (α) globulin – It is a copper-carrying protein produced by the liver. Example: Ceruloplasmin
  • Beta (β) globulin – Iron-carrying protein produced by the liver. Example: Transferrin
  • Gamma (γ) globulin – It is also called immunoglobulin present in the form of antibodies which destroy bacteria, viruses, and toxic substances and are produced by lymphoid organs. These are of 5 types:
    • IgG (γ Immunoglobin) – Present in blood and other body fluids, and protects against bacterial and viral infections.
    • IgA (α Immunoglobin) – Acts as an important first line of defence. Present in the linings of the respiratory tract and digestive system, as well as in saliva, tears, and breast milk. It helps in the neutralisation of bacterial toxins and viruses, both extracellularly and intracellularly.
    • IgM (μ Immunoglobin) – Found mainly in the blood and lymphatic fluid, this is the first antibody the body makes when it fights a new infection by bacteria and other germs.
    • IgD (δ Immunoglobin) – This is the least understood antibody, with only small amounts in the blood. It is primarily found on the surface of B lymphocytes where it functions as a receptor for antigens.
    • IgE (ε Immunoglobin) – Normally found in small amounts in the blood. These antibodies travel to cells that release chemicals, causing an allergic reaction. There may be higher amounts when the body overreacts to allergens or is fighting an infection from a parasite.
• Prothrombin (0.3%) – It is a blood coagulant produced by the liver.
• Fibrinogen (0.3%) – It is a blood coagulant and is the largest-sized plasma protein. Produced by the liver.

2. Lipoproteins – Plasma lipoproteins are the macromolecular assemblies of proteins and lipids found in the blood. The lipid components of lipoproteins are amphipathic lipids such as phospholipids (PLs), unesterified cholesterols (UCs), and hydrophobic lipids such as cholesteryl esters (CEs) and triglycerides (TGs).

3. Other substances

• Urea
• Amino acids
• Glucose (80 – 100mg/100mL)
• Heparin (Anticoagulant)
• Hormones
• Clotting factors

Inorganic parts of plasma (91 – 93%)

Mineral salts like chlorides (Cl^–), carbonates (CO₃^2-), phosphates (PO₄^3-), and sulphates (SO₄^2-) of –

• Sodium (Na⁺)
• Potassium (K⁺)
• Calcium (Ca⁺²)
• Magnesium (Mg⁺²)

Blood Corpuscles

Blood corpuscles are also called formed elements of blood. Formed elements are the cells and their fragments suspended in plasma. These three groups of formed elements are—

1. Erythrocytes (Red Blood Cells or RBC)
2. Leukocytes (White Blood Cells)
3. Thrombocytes (Platelets)

Components of blood corpuscles

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