### Theory:

What are the main characteristics of connective tissue?
• It arises from the mesoderm of the embryo.
• It has elements such as matrix, fibres and cells.
There is a connective tissue that is in the form of fluid and has the above-mentioned characteristics.
• It also arises from the mesoderm of the embryo.
• It has a non-living fluid matrix called plasma.
• It has soluble protein molecules as fibres.
• It also has different types of cells.
Hence this tissue is called fluid connective tissue. We call it "blood".
Blood - a liquid connective tissue:
It is a fluid connective tissue that connects various parts of the body. It helps to maintain the continuity of the body. It contains a liquid matrix called plasma, blood cells like RBCs (Red blood corpuscles), WBCs (White blood corpuscles) and platelets suspended in it. Plasma is a pale yellow colour fluid. It contains proteins, water, salt and enzymes.
• It transports nutritive materials, gases, vitamins and hormones to tissues present all around the body.
• It carries waste products from tissues to excretory organs like the liver and kidney.
• It also conducts heat and maintains the body temperature.
Blood has mainly two components,
1. Blood plasma
2. Blood corpuscles
Blood and its components
Blood plasma:
• It is a transparent, clear fluid with pale yellow colour.
• It contributes $$55-60%$$ of blood and has $$90-92%$$ of water.
• Apart from water, it has inorganic salts such as sodium, potassium, magnesium and calcium.
Blood corpuscles:
Blood cells or corpuscles are one of the elements of the blood. It contributes $$40-45%$$ of blood and is heavier than plasma.
It is of three types.
1. Red blood corpuscles or erythrocytes
2. White blood corpuscles or leukocytes
3. Blood platelets or thrombocytes
Types of blood cells
1. Red blood corpuscles:
The red blood cell contains a pigment called haemoglobin, and due to that, blood is red.
• The life span of RBC is $$120$$ days.
• Mature RBC do not contain a nucleus.
• Most of the cell organelles like mitochondria and lysosomes are absent in these cells.
• The shape of the RBC is known to be a biconcave disc.
• Red blood cells, also termed erythrocytes, play a critical role in transporting gases such as oxygen from the lungs. They also aid in carbon dioxide transport from tissues to the lungs.
Important!
Note:
• In humans, RBCs are smaller than WBCs. RBCs are $$7.2 µm$$ in diameter.
• Per cubic mm of blood contains about $$5.5$$ and $$4.5 million$$ RBCs for a normal adult man and woman, respectively.
• A decrease in RBC count results in a condition called anaemia.
• An increase in RBC count results in a condition called polycythemia.
2. White blood corpuscles:
• It is colourless.
• It is irregular in shape.
• The life span of WBC is $$13$$ to $$20$$ days.
• WBCs are also called leucocytes.
• WBC contain nucleus and cell organelles like ER, golgi apparatus and mitochondria.
• WBCs are larger than RBCs but smaller in number.
• Per cubic mm of blood contains $$5000$$ to $$9000$$ WBCs are present in human.
• There are five different types of cells (WBC) are there. i.e. monocytes, lymphocytes, neutrophils, basophils and eosinophils.
Different types of white blood cells

Leucocytes are of two types:

A. Granulocytes (with granules in the cytoplasm)
B. Agranulocytes (without granules in the cytoplasm)
A. Granulocytes:
They have irregularly shaped nuclei and small cytoplasmic granules. The specific types of granulocytes are basophils, neutrophils and eosinophils.

a. Eosinophils:
• It makes $$1-2%$$ of total leucocytes count, i.e., $$70-300 cells per cubic mm$$ of blood.
• It has a bi-lobed nucleus.
• Granules of these cells are the largest among all components of leucocytes.
Function: It plays an essential role in immunity, allergy and hypersensitive reactions.
b. Basophils:
• It makes $$0.5-2.7%$$ of leucocytes count. i.e., $$35-150 cells per cubic mm$$ of blood.
• It has an S-shaped nucleus with two or three lobes.
• It constitutes the mast cells which consists of heparin and histamine.
• It is the least component present in leucocytes based on count.
c. Neutrophils:
• It makes $$60 - 70%$$ of total leucocytes. i.e., $$4000-5000 cells per cubic mm$$ of blood.
• It has two to five-lobed nucleus.
• Granules of these cells are the smallest among all components of leucocytes.
• It is the highest component present in leucocytes based on count.
B. Agranulocytes:
They do not have distinct cytoplasmic granules. The specific types of agranulocytes are lymphocytes and monocytes.

a. Lymphocytes: It has a spherical cell and less motile.

Function: Responsible for the production of antibodies.
b. Monocytes: It is the largest white blood cell and has a large nucleus indented on one side (horse-shoe shaped). Cells are actively motile and phagocytic.
Function: White blood cells are a major part of the immune system, helping the body fight against disease-causing pathogens by producing antibodies.
Important!
Do you know!
• A decrease in WBC count in the blood creates a condition called leukopenia.
• An increase in WBC count in the blood creates a condition called leukaemia (cancer of the blood).
• Monocytes are the largest cell of white blood cell, while lymphocytes are the smallest.