Comprehensive Biology Notes on Blood Composition, Formed Elements, and Lymphatic Systems

Blood is a complex fluid comprising two primary components: Plasma and Formed Elements. The total volume is divided as follows:

Plasma: Constitutes approximately $55\%$ of the blood volume. It serves as the liquid matrix.
Formed Elements: Constitutes approximately $45\%$ of the blood volume. This section includes Red Blood Cells (RBCs), White Blood Cells (WBCs), and Platelets. For example, in a $100\,ml$ sample, one might find $55\,ml$ of plasma and $45\,ml$ of formed elements.
Buffy Coat: A thin layer consisting of White Blood Cells and Platelets that forms between the plasma and RBC layers when blood is centrifuged.

Plasma is a straw-coloured (pale yellow) liquid that contains the following components:

Water: Comprises $90-92\%$ of the plasma.
Proteins: Comprise $6-8\%$ of the plasma. These are primarily synthesized in the liver.
- Albumin: The smallest and most abundant plasma protein. Its primary role is maintaining the Osmotic Balance of the blood.
- Globulins: For example, $\gamma$ -Globulins (Gamma Globulins) function as antibodies that help the body fight infections.
- Fibrinogen: The largest plasma protein. It is produced by the liver and is essential for the clotting of blood. It exists in an inactive form within the plasma until needed.
Clotting Factors: There are many factors involved in the coagulation cascade.
Other Substances:
- Minerals/Electrolytes: $\text{Na}^+$ , $\text{Ca}^{2+}$ , $\text{Mg}^{2+}$ , $\text{K}^+$ , $\text{Cl}^-$ , and $\text{HCO}_{3}^-$ .
- Nutrients and Regulatory Substances: Hormones, Glucose, Amino Acids, and Lipids.

Definition of Serum: $Serum = Plasma - \text{Clotting factors (e.g., Fibrinogen)}$

Count: There are about $5$ to $5.5$ million RBCs per $mm^3$ (cubic millimeter) of blood.
Colour: They are red in colour due to the presence of an iron-containing pigment called Haemoglobin ( $\text{Hb}$ ). The iron is in the $Fe^{2+}$ state.
Haemoglobin Content: Every $100\,ml$ of blood contains approximately $12-16\,g$ of haemoglobin.
Shape:
- In mammals, they are generally Circular and Biconcave.
- Exceptions: In Camels and Llamas, the RBCs are oval in shape.
Nucleus: RBCs in mammals are enucleated (devoid of a nucleus) to provide more space for haemoglobin.
Membrane: The membrane of RBCs is highly flexible and is referred to as the Donnan's Membrane.
Lifespan: The average lifespan of an RBC is about $120$ days.
Recycling: After $120$ days, RBCs lose their structural integrity and are sent to the Spleen, which is known as the "graveyard of RBCs," for recycling.
Clinical Notes:
- A deficiency in hemoglobin leads to a disorder called Anaemia.
- Iron and Vitamin B12 (along with Folic Acid) are critical for the maturation of RBCs.

Carbon Monoxide ( $CO$ ) Binding: When Carbon Monoxide binds with Haemoglobin, it forms Carboxy-haemoglobin.
Affinity: The binding affinity of Haemoglobin for Carbon Monoxide is $200-250$ times greater compared to its affinity for Oxygen ( $O_2$ ).
Binding Site: The binding site for $CO$ on the Haemoglobin molecule is the same as the binding site for $O_2$ .

Leucocytes are colourless because they lack haemoglobin. They are nucleated cells and serve as the primary defense against infectious agents.

Total Leucocyte Count (TLC): The normal range is $6000-8000$ WBCs per $mm^3$ of blood.
Infection Response: Their number increases during infection to help fight the disease.

WBCs are divided into Granulocytes and Agranulocytes.

Eosinophils (Acidophils):
- Constitute $2-3\%$ of TLC.
- Have a Bilobed nucleus.
- Associated with allergic reactions and defense against parasites (such as Ascariasis and Taeniasis caused by Taenia Solium or Tape Worm).
Basophils:
- Constitute $0.5-1\%$ of TLC.
- Have an S-shaped nucleus.
- Involved in inflammatory reactions.
- Non-phagocytic.
Neutrophils:
- The most numerous WBC, constituting $60-65\%$ of TLC.
- Have a Multilobed nucleus; they are also called Poly-morpho-nuclear leucocytes (PMNL).
- The number of lobes in the nucleus increases as the cell ages.
- Phagocytic in nature.

Monocytes:
- Constitute $6-8\%$ of TLC.
- The largest type of WBC.
- Have a Bean/Kidney shaped nucleus.
- Phagocytic; they transform into macrophages when they enter tissues.
Lymphocytes:
- Constitute $20-25\%$ of TLC.
- The smallest type of WBC.
- Have a large, rounded nucleus and are non-phagocytic.
- Types of Lymphocytes:
  1. B-lymphocyte: Formed and matured in the Bone Marrow.
  2. T-lymphocyte: Formed in the Bone Marrow but matured in the Thymus.
  3. Natural Killer (NK) Cells.

Origin: Platelets are cell fragments produced from larger cells called Megakaryocytes in the bone marrow.
Nature: They are non-nucleated.
Count: Normal concentration is $1.5$ to $3.5$ lakhs per $mm^3$ of blood.
Function: Essential for blood clotting.
Clinical Note: A decrease in platelet count below normal levels leads to a group of disorders known as Purpurea disease.

Lymph is the fluid that resides in the lymphatic system.

Composition: It has the same mineral composition as plasma but lacks RBCs and larger proteins. It does, however, contain WBCs.
Clotting: Like blood, lymph has the ability to clot.
Channels: It flows through special channels called Lymphatic Vessels.
Circulation: Lymph is recycled back into the circulatory system via the sub-clavian vein after passing through lymph nodes.
Function in Digestion: Fats from the villi of the small intestine are absorbed through small lymph capillaries called Lacteals.

While there are more than 30 blood grouping systems, the most significant are the ABO and Rh systems.

Discovery: Discovered by Karl Land Steiner (Karl Landsteiner) in 1901.
Antigen (Agglutinogen): Found on the surface of RBCs. "Antigen" refers to antibody-generating cells.
Antibody (Agglutinin): Found in the plasma.

Additional Note: Antigen + Antibody reactions can lead to agglutination.