15.1. Blood

Blood is a special connective tissue consisting of a fluid matrix, plasma, and formed elements.

Blood is classified as a connective tissue due to its composition of cells suspended in a liquid matrix. The fluid matrix, known as plasma, makes up a significant portion of blood, while the formed elements include cells and cell fragments that serve various functions within the bloodstream.

Think of blood like a soup where the broth is the plasma (the liquid) and the vegetables and grains are the formed elements. Just as the soup nourishes and provides sustenance, blood helps transport nutrients and waste in the body.

Plasma is a straw coloured, viscous fluid constituting nearly 55 per cent of the blood. 90-92 per cent of plasma is water and proteins contribute 6-8 per cent of it. Fibrinogen, globulins and albumins are the major proteins.

Plasma serves multiple roles in the body, primarily made up of water and proteins. Water in plasma helps maintain blood volume and pressure, while proteins like fibrinogen are crucial for blood clotting, globulins play roles in immune response, and albumins help regulate osmotic pressure in the blood.

Imagine plasma as a river that carries different types of cargo. The water (solvent) represents volume, while different cargo types (proteins) are specific to tasks: some are there to help with healing (fibrinogen), others to fight infections (globulins), and some to keep things balanced (albumins).

Erythrocytes, leucocytes and platelets are collectively called formed elements and they constitute nearly 45 per cent of the blood. Erythrocytes or red blood cells (RBC) are the most abundant of all the cells in blood. A healthy adult man has, on average, 5 millions to 5.5 millions of RBCs mm–3 of blood. RBCs are formed in the red bone marrow in the adults.

The formed elements in blood include three main types of cells: red blood cells (RBCs), white blood cells (WBCs), and platelets. RBCs are responsible for transporting oxygen throughout the body due to the presence of hemoglobin. WBCs are involved in the immune response, fighting infections, while platelets are crucial for blood clotting.

Think of RBCs as delivery trucks carrying oxygen to different parts of the city (body) while WBCs are like security guards monitoring for intruders (infections) and platelets are first responders ready to handle any accidents (cuts or injuries).

Leucocytes are also known as white blood cells (WBC) as they are colourless due to the lack of haemoglobin. They are nucleated and are relatively lesser in number which averages 6000-8000 mm–3 of blood. Leucocytes are generally short lived.

WBCs are critical for the immune system, and they are categorized into two main types: granulocytes (like neutrophils, eosinophils, and basophils) and agranulocytes (such as lymphocytes and monocytes). Each type has a specific role in protecting the body from infections and foreign substances.

You can think of granulocytes as the frontline soldiers launching direct attacks against intruders (infectious agents), while lymphocytes act more like strategists, deciding how to best respond to threats based on their previous experiences.

Platelets also called thrombocytes, are cell fragments produced from megakaryocytes (special cells in the bone marrow). Blood normally contains 1,500,000-3,500,000 platelets mm–3.

Platelets are crucial for blood clotting, preventing excessive blood loss when injuries occur. They help to form a clot by clumping together and releasing growth factors that aid in tissue repair, making them essential in healing processes.

Imagine platelets as workers at a construction site. When there's a break in the structure (an injury), they quickly come to the scene to patch it up and ensure that no further damage occurs to the building (body).

As you know, blood of human beings differ in certain aspects though it appears to be similar. Various types of grouping of blood has been done. Two such groupings – the ABO and Rh – are widely used all over the world.

Blood groups are determined by the presence of specific antigens on the surface of RBCs. The ABO system classifies blood into four groups (A, B, AB, O) based on these antigens. The Rh system further categorizes blood as Rh-positive or Rh-negative based on the presence of the Rh factor, which is important in blood transfusions.

You can think of blood types as security badges. Just like you need to show a valid badge to enter a building, a patient needs to receive the correct blood type during a transfusion to avoid complications or rejection.

You know that when you cut your finger or hurt yourself, your wound does not continue to bleed for a long time; usually, the blood stops flowing after a while. Blood exhibits coagulation or clotting in response to an injury or trauma.

The coagulation process involves a complex cascade of reactions that lead to the formation of a clot, primarily involving platelets and clotting factors. When a blood vessel is injured, platelets adhere to the injury site and attract more platelets, forming a plug. Fibrin threads weave through this plug to solidify it.

This process can be compared to a dam being built to stop an overflow of water. After a breach in the dam (injury), workers (platelets) rush to fill in the gap, and then concrete (fibrin) is poured to make it firm and secure.

--