Blood

• Definition of Blood
  Blood is defined as specialized connective tissue containing homogenous fluid composed of liquid substance known as plasma and the formed elements RBCs , WBCs and the platelets.
• It is slightly alkaline in nature pH=7.4 .
• Saltish in taste , heavier than water and viscous sticky fluid.
• Adult man has 5-6 litres and adult woman has 4-5 litres of blood.
• The science that deals with the study of blood is called Hematology .

Composition of Blood

Blood consists of two major parts -:

[A] PLASMA

• • It is the fluid part of the blood.
  • Slightly yellow coloured and alkaline.

      Functions of Plasma

1. Transportation of digested food – Digested food, such as glucose , amino acids, fatty acids, e.t.c. , are supplied from the alimentary canal to all body cells by the way of blood.
2. Transportation of Oxygen – About 3% of oxygen supplied to body cells is transported in dissolved state in the plasma.
3. Transportation of CO₂ – About 7% of carbon dioxide produced during oxidation of food dissolves in water of plasma.
4. Transportation of metabolic wastes – Various nitrogenous waste products are transported to kidneys by way of plasma of blood for elimination.
5. Transportation of Hormones – Hormones are the secretions of endocrine glands. They are transported to the target organs by the way of blood.
6. Transportation of heat – Blood transports heat from the organs such as liver and muscles to the organs from where it is being continuously lost.
7. Homeostasis – Maintenance of the internal environment constant is called Homeostasis. Plasma of blood helps to maintain the internal environment steady by maintaining different salts and organic substances in constant amount.

[B] FORMED ELEMENTS OR CELLULAR COMPONENTS

• • These include microscopic cells or cell – like structures which float freely in the plasma.

These are of three types -:

1. Red Blood Corpuscles (RBCs) or Erythrocytes

• • These are very small , 7.7 micrometer in diameter , biconcave, disc-like , de-nucleated cells in the mammalian blood.
  • RBCs are red in color because of the presence of a respiratory pigment called hemoglobin.
  • Hemoglobin is a conjugated protein which is composed of hematin – an iron containing pigment and globin – a protein.
  • Hb has a great affinity for oxygen.
  • Number of RBCs in-
    1. Adult man – 5 to 5.4 million/mm³
    2. Adult Woman – 4 to 4.5 million/mm³
  • Polycythemia – Increase in RBC count.
  • Erythropenia – Decrease in RB count.
  • The average life of a RBC is about 120 days.
  • In adults RBCs are produced in Red Bone Marrow of long bones – Ribs, Ilium of hip bones, vertebrae.
  • In an embryo, these are produced in liver and spleen.
  • RBCs are destroyed in the liver and spleen. Their iron part is stored in the liver cells whereas the rest is excreted as bile pigment.

    

      Functions of RBCs

• Transportation of O₂ – Hb has a great affinity for oxygen and it forms an unstable compound oxyhemoglobin (oxyHb) with O₂ . This oxyhemoglobin dissociates into Hb and O₂ in the body tissues and the oxygen is supplied to cells.
• Transportation of CO₂ – About 23% of carbon dioxide produced in the cells is carried from the tissue to respiratory organs by hemoglobin in the form of carbaminohemoglobin (HbCO₂) .

2. White Blood Corpuscles (WBCs) or Leucocytes

• • These are amoeboid (i.e. , without any definite shape) nucleated corpuscles.
  • These are larger than RBCs.
  • Number of WBCs – 5000-9000 /mm³
  • Increase in number of WBC count is called Leukemia .
  • Decrease in number of WBC count is called Leukopenia .
  • They are produced in bone marrow or in the lymph nodes, spleen.
  • They are known as phagocytes – as they engulf the foreign bodies.
  • They are known as scavengers – as they engulf the worn out tissues of the body.
  • They are the soldiers of the body – as they accumulate at the site of injury and fight with the invading microbes that may cause infection.
  • Average life-span of leucocytes is about 1 week.

Types of WBCs

(a) Agranulocytes

Cytoplasm lacks granules. They have single large nucleus.

These are further of two types :

1. Monocytes

• They are largest sized leucocytes.
• Size – 12 to 20 micrometer .
• They have large kidney shaped nucleus.
• They help in engulfing foreign particles as well as worn out tissues of the body (phagocytosis).

     2. Lymphocytes

• Round in shape and are of the size of RBCs.
• Nucleus large and rounded.
• They are produced in lymph nodes and spleen.
• These are not phagocytic.
• They produce antibodies that provide immunity against specific diseases.

(b) Granulocytes

• They have granular cytoplasm and are produced in Red Bone Marrow.

Depending upon the chemical nature of their cytoplasm they are of three types :

1. Acidophils (Eosinophils)

• Cytoplasm is alkaline in nature.
• They have bilobed nucleus.
• Their function is to engulf bacteria, secrete anti-toxin and is associated with allergy.

     2. Basophils

• Their cytoplasm is acidic in nature.
• Nucleus is three lobed.
• They release chemicals called histamine for inflammation that dilates blood vessels.

     3. Neutrophils

• Their cytoplasm is neutral.
• Nucleus is three to four lobed.
• They are phagocytic.

Functions of WBCs

1. Phagocytosis – The engulfing of particles like solid substances such as bacteria, worn out tissues e.t.c. is called Phagocytosis.
   • Neutrophils and Monocytes engulss pathogens which may enter the body and protect us against diseases.
   • WBCs have the property to squeeze through the walls of capillaries known as diapedesis at the site of injury to fight with the pathogens

     

2. Formation of antibodies and anti-toxins
   • Lymphocytes produce antibodies which fight with the antigens or pathogens (disease causing microbes) and protect the body against specific disease.
   • Acidophils produce antitoxins that neutralize the toxins produces by germs and protect the body.
3. Immunity
   • Resistance of the bodies against diseases is called immunity.
   • Lymphocytes are responsible to promote immunity.
4. Inflammation
   • Increased local heat , redness, swelling and pain due to reaction of tissues to injury and to localized invasion of germs is called inflammation.

[C] Platelets or Thrombocytes

• • These are smallest sized blood cells.
  • Number varies from 2 to 3 lakh /mm³ of blood.
  • These are denucleated.
  • Produced in Red Bone Marrow and destroyed in spleen.

     Functions of Platelets

• They are responsible for clothing of the blood at injuries.
• It produces an enzyme called thrombokinase which initiates a chain of reaction involved in blood clotting.

*Hematopoiesis – Production of blood cells is called hematopoiesis.