Blood is a connective tissue in the form of fluid. It delivers necessary substances such as nutrients and oxygen the cells and transports metabolic waste products away from those same cells. Blood is pumped by the heart to all parts of the body. 

Blood is considered as-

Fluid of life, because it carries oxygen from lungs to all parts of the body and carbon dioxide from all parts of the body to the lungs. 

Fluid of growth, because it carries nutritive substances from the digestive system and hormones from endocrine gland to the all parts of the body. 

Fluid of health, because it protects our body from diseases and carries unwanted waste substances to the liver and kidney for detoxification or removal. Blood also helps to regulate body temperature.

Approximately 8% of an adult’s body weight is made up of blood. Female have around 4-5 liters, while males have around 5-6 liters of blood in their body. This difference is mainly due to the difference in body size and structure between men and women.

The normal temperature of blood is slightly higher than normal body temperature- about 38° C (or 100° F), compared to 37° C (or 98° F) for an internal body temperature.

Properties of blood

Colour:- The colour of blood is red due to the presence of hemoglobin. The blood in arteries is brighter than in the veins because of the high levels of oxygen present in the arteries.

Volume:- Blood is about 8% of body weight in a normal healthy adult’s. Accordingly, a person with 70 kilogram of the body has 5-6 liters of blood. In newborn baby, the volume is about 450 ml. Blood increases during growth and reaches 5 liters at puberty. In female it is slightly less and is about 4.5 liters.

pH of blood:- Blood is slightly alkaline and its pH in normal conditions is 7.4.

Specific gravity:-

  • Specific gravity of total blood     :     1.052 to 1.061
  • Specific gravity of blood cells     :     1.092 to 1.101
  • Specific gravity of plasma           :     1.022 to 1.026

Viscosity:- Due to the presence of red cells and plasma proteins, blood is five times viscous than water.

Composition of blood

Blood contains the blood cells which are the solid part of blood and the liquid part is plasma. There are three types of blood cells present in blood:-

  • Red blood cells or erythrocytes
  • White blood cells or leukocytes
  • Platelets or thrombocytes

Hematocrit value

Hematocrit value or packed cell volume (PCV) is the volume of red cells expressed in percentage. To measure the hematocrit value, the blood is collected in hematocrit tube along with a suitable anticoagulant and then centrifuge for 30 minutes at 3000 rpm. The red cell settles down and the supernatant is plasma. Plasma forms 55% and red cells forms 45% of the total blood. There is a thin whitish layer present between plasma and red blood cells which is called ‘buffy coat’ which contains white blood cells and platelets.

Plasma

Plasma is the liquid portion of blood which makes up to 55% of blood. It is mostly water and contains important proteins, nutrients, hormones, clotting factors, electrolytes and enzymes. Plasma's main function is the removal of waste from cellular functions that help to produce energy. Plasma accepts and transports this waste to other areas of the body, such as the kidneys or liver, for excretion. Blood plasma is separated from the blood by centrifuging the tube which contains fresh blood with anticoagulant.

Serum

Serum is liquid part of blood after coagulation. Serum does not contain white or red blood cell or clotting factor. Fibrinogen is absent in serum.  Serum is obtained by centrifuging whole blood in a tube which does not contain any anticoagulant.

Serum = Plasma - Fibrinogen

Function of blood

  • Blood carries nutritive substances like glucose, amino acids, lipids, vitamins from gastrointestinal tract to different parts of the body for growth and production of energy.
  • Blood absorbs oxygen from air in the lungs. It carries oxygen from alveoli of lungs to different tissues throughout the body and carbon dioxide from tissue to alveoli of lungs.
  • Hormones which are 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.
  • Blood transports waste products formed in the tissue during various metabolic activities to the organs (like kidney and liver) that remove and process them for elimination. Blood flows into the kidneys through the renal arteries and through renal veins. Kidney filters substances such as urea, uric acid, creatinine and many more. Liver removes toxins from blood.
  • Blood also helps to regulate water content of body cells.
  • Blood helps to maintain the chemical balance of the body. Proteins and other compounds in blood acts as buffer, which helps to regulate the pH of body tissue.
  • Blood is responsible for thermoregulatory mechanism in the body. i.e. the balance between heat loss and heat gain in the body.
  • When damage to the vessels results in bleeding, blood platelets and certain proteins dissolved in the plasma, interacts to create clots which block the rupture area and protects the body from further blood loss.
  • Blood play an important role in defense of the body. White blood cells develop immunity and remove foreign particles (microorganisms) from our body.
  • There are many substances which are constantly required by the tissue. Blood is the source of water and some important substances like proteins, glucose, sodium and potassium. These substances are taken from blood during the conditions like starvation, fluid loss, electrolyte loss, etc.