Blood is a connective tissue which is a red fluid in the body. It is also denoted as
‘fluid of life’ or ‘fluid of growth’ or ‘fluid of health’ because it helps transport essential bodily substances. 

Properties of blood

1. Color: Blood is red. Arterial blood is scarlet red because it contains more oxygen and venous blood is purple red because of more carbon dioxide.

2. Volume: The average volume of blood in a normal adult is 5 L. In a newborn baby, the volume is 450 ml. It increases during growth and reaches 5 L at the time of puberty. In females, it is slightly less and is about 4.5 L. It is about 8% of the body weight of a normal young healthy adult, weighing about 70 kg.

3. Reaction and pH: Blood is slightly alkaline and its pH in normal conditions is 7.4.

4. Viscosity: Blood is five times more viscous than water. It is mainly due to red blood cells and plasma proteins.

Composition of blood

Blood contains the blood cells which are called formed elements and the liquid portion known as plasma.

Blood cells

Three types of cells are present in the blood:
1. Red blood cells or erythrocytes
2. White blood cells or leukocytes
3. Platelets or thrombocytes.

 Plasma

Plasma is a straw-colored clear liquid part of blood. It contains 91% to 92% of water and 8% to 9% of solids. The solids are the organic and the inorganic substances give the normal values of some important substances in the blood.

Serum

Serum is the clear straw-colored fluid that oozes from blood clot. When the blood is shed or collected in a container, it clots. In this process, the fibrinogen is converted into fibrin and the blood cells are trapped in this fibrin forming the blood clot. After about 45 minutes, serum oozes out of the blood clot.

Serum = Plasma – Fibrinogen

Function of Blood 

Nutritive Function

Nutritive substances like glucose, amino acids, lipids, and vitamins derived from digested food are absorbed from the gastrointestinal tract and carried by blood to different body parts for growth and energy production.

Respiratory Function

Transport of respiratory gases is done by the blood. It carries oxygen from alveoli of lungs to different tissues and carbon dioxide from tissues to the alveoli.

Excretory Function 

Waste products formed in the tissues during various metabolic activities are removed by blood and carried to the excretory organs like the kidney, skin, liver, etc. for excretion.

Transport of hormones and enzymes 

Hormones secreted by ductless (endocrine) glands are released directly into the blood. The blood transports these hormones to their target organs/tissues. Blood also transports enzymes.

Regulation of Water Balance 

The water content of the blood is freely interchangeable with interstitial fluid. This helps in the regulation of the water content of the body.

Regulation of acid-base balance 

Plasma proteins and hemoglobin act as buffers and help in the regulation of acid-base balance.

Regulation of body temperature 

Because of the high specific heat of blood, it is responsible for maintaining the thermoregulatory mechanism in the body, i.e. the balance between heat loss and heat gain in the body.

Storage Function

Water and some important substances like proteins, glucose, sodium, and potassium are constantly required by the tissues. Blood serves as a readymade source for these substances. And, these substances are taken from blood during the conditions like starvation, fluid loss, electrolyte loss, etc. Water and some important substances like proteins, glucose, sodium, and potassium are constantly required by the tissues. Blood serves as a readymade source for these substances. And, these substances are taken from blood during the conditions like starvation, fluid loss, electrolyte loss, etc.

Defensive Function

Blood plays an important role in the defense of the body. The white blood cells are responsible for this function. Neutrophils and monocytes engulf the bacteria by phagocytosis. Lymphocytes are involved in the development of immunity