### Theory:

All organisms require nutrients, water, oxygen, etc., for their survival. Therefore, living things must transport these substances to all parts of the body to function correctly. Unicellular organisms use diffusion and osmosis for the transportation of substances.

In multicellular organisms, a more sophisticated and structured system for the transportation of various substances is required. Similarly, it must eliminate wastes from the body.

In multicellular organisms, the transport of nutrients, oxygen, minerals, salts, and other waste products around the body are performed by the circulatory system. The circulatory and excretory system together constitute the transportation system. In this chapter, the circulatory system of animals is dealt in detail.
Circulatory system:
The circulatory system consists of the circulating body fluids - the blood and lymph, the heart and the blood vessels that form the collecting and transporting system. The circulatory system of animals is discussed in depth in this chapter.
Blood:
Blood is an important circulatory medium present in complex multicellular organisms, including the human body. It is a red coloured fluid connective tissue. Blood has a slightly alkaline pH of $$7.4$$.

Components of Blood:

The blood mainly consists of $$two$$ components:
1. Plasma - Liquid portion of the blood
2. Formed elements or blood cells - Solid portion of the blood
Plasma is the liquid or the fluid portion of the blood. The formed elements or the blood cells are suspended in the plasma.

1. Plasma - The liquid portion of the blood

Plasma is a slightly alkaline, pale yellow, transparent fluid that comprises $$90 - 92 \%$$ water. It contains non-cellular substances and makes up to $$55 \%$$ of the total volume of the blood.

Organic substances like vitamins, minerals, glucose, protein, urea, enzymes, clotting factors, hormones, amino acids etc., are present in the plasma. In addition, it transports food, $$CO_2$$ and the nitrogenous wastes in the dissolved state.

2. Formed elements or blood cells:

Blood corpuscles constitute about $$40 - 45 \%$$ of the blood. They are of $$three$$ types:

1Red blood corpuscles (RBCs) or Erythrocytes

2White blood corpuscles (WBCs) or Leucocytes

3Blood platelets or Thrombocytes

The components of blood