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What are components of blood

What are components of blood

The system, which is primarily meant for the circulation of nutritive products, disposal and excretion of waste material in the body is know as circulatory system.

The two basic types of circulatory system have evolved in animals such as

Open Circulatory System

It is the type in which blood is pumped by heart into an aorta, which branches off into a number of arteries that ultimately open into a series of spaces collectively called as haemocoel,

e.g., most arthropods and some cephalopods.

Closed Circulatory System

It is the type in which blood stays in blood vessels. It does not come into direct contact with the body tissues. The only entry and exit to the system is through the walls of blood vessels. e.g., in annelids and chordates. This is considered to be the most advantageous method as it supplies blood to the deepest tissues of the body.

Human circulatory system is of closed type and consists of blood vascular system and lymphatic system.

Blood Vascular System

Human blood vascular system comprises of blood, blood vesseles and heart.


It is a fluid connective tissue which forms 30-32% of total extracellular fluid. Volume of blood in an adult person is about 5-5.5 L. Blood is a red coloured liquid, which circulates in our body. It is red because it contains a red pigment called haemoglobin in its red blood cells.

Components of Blood

Blood is a fluid connective tissue comprising of fluid part called plasma and the cells known as blood corpuscles. 


It is a straw coloured, viscous fluid, slightly alkaline and aqueous solution. It forms about 55% of the blood.

Composition of Plasma It is composed of many organic any organic and inorganic substances, which includes 90-92% water and 6-8% solutes in it.

The solutes found in plasma are various ions (Na+, Mg2+, CaC+, HCO3, etc.), giucose, traces of other sugars, plasma proteins, amino acids, hormones, cholesterol, hipias, urea, other wastes and organic acids.


Factors for clotting or coagulation of blood are also present actively in the plasma. Plasma without the blood clotting factors is called serum.

plasma Proteins

Proteins found in plasma are the important components. These are responsible for providing viscosity to the plasma. Over 70 different plasma proteins are found out of which the major proteins found in plasma are fibrinogen, globulins and albumins. 

 Minerals and Inorganic Salts

They occur in plasma in the form of ions i.e., potassium, magnesium, calcium, iron and manganese. Sodium and chloride are principal cation and anion of the plasma. Bicarbonate and phosphate also occur in smaller amounts.

The inorganic salts are sometimes referred to as blood electrolytes. The kidneys maintain plasma electrolytes at precise concentrations-an example of homeostasis.


A natural strong  anticoagulant present in the plasma  is a heteropolysaccharide named  antiprothrombin, or heparin, which  checks clotting of blood in uninjured blood vessels by preventing conversion  of prothrombin into thrombin. It is produced in the liver.

 Functions of Plasma

  •   Helps in transport and uniform distribution of heat all over the body. 
  • Provides body immunity. 
  • Maintenance of blood pH.
  • Provides prevention of blood loss.
  • Fibrinogen helps in blood clotting, globulin help in defense mechanism, albumin maintains osmotic balance.

Formed Elements

The formed elements or blood corpuscles include erythrocytes, leucocytes and platelets. These constitute about 45% of the blood.


Blood Cells


  1. Red Blood Cells or Erythrocytes ::-  Hemoglobin containing cells that carry oxygen in the blood. Anucleate in humans.
  2. Blood Platelets or Thromobocytes ::-  Anucleate, disc-shaped fragments of bone marrow cells, involved in blood clotting 

  3. Blood Cells or Leucocytes ::Colourless, motile, nucleated cells, involved in body defense  mechanism

  • Agranulocytes :: – Granules are not found in cytoplasm
  • Granulocytes ::- Contains granules in their cytoplasm


Lymphocytes ( 20-25% ) ::- 

  • Large rounded  nucleus 
  • Non-phagocytic
  • They produce antibodies 
  • They are further  of 2 types namely B-cells and T-cells

Monocytes ( 2-10% ) ::- 

  • Bean-shaped nucleus 
  • Phagocytic
  • They engulf  bacteria and cellular debris

Eosinophils ( 2-3%) ::- 

  • Bilobed nucleus Non-phagocytics
  • Play role in allergy and hypersensitivity  reactions (correspond to lysosomes)

Basophils  ( 0-1%  )::- 

  • Three-lobed nucleus
  •  Non-phagocytic
  • Contain heparin, histamine and serotonin (correspond to mast cells)

Neutrophils ( 60-65%  )::- 

  • Multi-lobed nucleus Phagocytic 
  •  Multi-lobed nucleus Phagocytic

Functions of Blood

  •  Maintains body pH, water and ionic balance.
  •  Helps in healing of wounds. 
  • Also helps in transportation of hormones from endocrine glands to target organs.
  • Coagulation.
  •  Helps in transportation of body wastes from different body parts to kidneys. 
  • Maintains normal body temperature. 
  • Fight against infections by forming body immunity.
  • Helps in transportation of respiratory gases (i.e., O2, CO2, etc).

Note ::- A woman has approximately 4.5 L of blood in her body while man has 5.6 litres.

Blood Groups

Although blood of every human being appears to be similar in appearance but, it differ in certain aspects. The plasma membrane of RBCs contain certain glycoproteinaceous molecules known as antigen, which differ in different persons. Thus, providing them different blood groups.

 ABO Blood Grouping

We have already studied in the chapter Animal tissue that ABO blood grouping is based on the presence or absence of antigen A or antigen B on the surface of RBCs (chemicals that can induce immune response). Similarly, the plasma of different individuals contain two natural antibodies (which are proteins produced in response to antigens).

 Rh Grouping

Another antigen, known as Rh antigen (similar to the one present in Rhesus monkey) is also found on the RBCs surface in majority of humans (nearly about 80%). Individuals having Rh antigen are called Rh positive (Rh”) and those without the Rh antigen are called Rh negative (RH-).

  • People with blood group O RH are called universal donors and people with blood group AB Rht are called universal receivers. 
  • * Rh* blood can never be given to someone with Rh- blood, but the other way around works. e.g., O Rht blood can not be given to someone with the blood type AB Rh.

Rh Incompatibility During Pregnancy

This is a special case of mismatching of Rh group or Rh incompatibility which has been observed between the RH blood of a pregnant mother with RH+ blood of the foetus (born out of a marriage between RH woman and a RHman). In such a case, mother becomes sensitive, while carrying a RH+ baby in her uterus. The reason is that some of the RBCs from the developing foetus enters into the blood stream of the mother during development.

This causes the development of anti-Rh antibodies. This does not happen in her first pregnancy (because two blood remain separated by placenta). But in case of her subsequent pregnancies, (i.e., 2nd and 3rd) RH+ foetuses get exposed to the anti-Rh antibodies, which will leak into the blood of the foetus (RH+) and destroy foetal RBCs.

This could be fatal to the foetus or could cause severe anaemia and jaundice in the body. This is known as erythroblastosis foetalis. The condition can be avoided by administering anti-Rh antibodies to the mother immediately after the delivery of the first child.


It is a colourless fluid and its composition is same as that of tissue fluid, which in turn is same as that of blood plasma. It contains very small amount of nutrients and oxygen but contains abundant carbon dioxide and other metabolic wastes. Amoeboid shaped white blood corpuscles are also present in lymph.

 Formation of Lymph

As the blood passes through the capillaries of the arterial system into the tissues, some water along with many water soluble substances come out in the spaces between the cells of tissues. But a very small amount of proteins come out from the capillary with the plasma (leaving the larger proteins and most of the formed elements in the blood vessel).

The fluid thus, released out is called interstitial fluid (tissue fluid) or Extra Cellular Fluid (ECF). After entering the lymph vessel, the ECF becomes lymph.

Functions of Lymph

  •  It acts as an important carrier of nutrients, hormones, etc.
  •  Also helps in the renewal of ECF.
  •  Absorption of fat also occurs through lymph in the lacteals present in the intestinal villi.
  •  Helps in keeping tissue cells moist.
  •  Maturation of lymphocytes, i.e., B-cells and T-cells occur with the help of lymph nodes, releasing them into the lymph.

Heart (The Pumping Organ)

All vertebrates possess a muscular chambered heart.

Types of Heart

Two Chambered

  • Possess atrium and 1 ventricle, heart always receive deoxygenated blood which passes through it for once (single circulation)
  • Example ::-  fishes.

Three Chambered

  • Posses 2 atria and 1 ventricle; left atrium receive oxygenated and right atrium receive de-oxygenated blood. which gets mixed in ventricle, incomplete double circulation,
  • Example ::-  amphibians and reptiles (except crocodile).

Four Chambered

  • Posses 2 atria and 2 ventricles; oxygenated and de-oxygenated blood do not get mixed and pumped separately; double circulation,
  • Example ::-, birds and mammals.

Human Heart

Human heart is a muscular organ of mesodermal origin, situated between the lungs of thoracic cavity. An average adult human heart is about 12cm. Its weight ranges from 280-340 g   (average 300 g) in males and 230-280 g (average 250 g) in females. The heart is surrounded by a protective covering called pericardiurn (two-layered) which is filled with pericardial fluid.

Human Heart
Human Heart

The heart is divided into four chambers in human beings; these chambers are

Auricles or Atrium

There are two auricles or atrium, left and right separated by inter-auricular septum and are superior in position. SA node (Sinoatrial Node) is situated in the upper wall of right atrium.


The two interior chambers of the heart are the right and left ventricle separated by inter-ventricular septum.

The walls of ventricle are thicker than that of atria because ventricles have to pump blood into various organs, while atria receives blood.

Parts of the Human Heart and their Functions

 Left Atrium ::-  It receives oxygenated blood from the lungs via pulmonary vein.
 Left Ventricle ::–  It pumps blood to all parts of the body except the lungs via aorta. 
Right Atrium  ::- It receives deoxygenated blood coming from the body via vena cava. 
Right Ventricle ::-  It pumps blood to the lungs for oxygenation. 
Valves ::-  It prevents backflow of blood. There are four types of valves je two atrioventricular (AV) valve and two semilunar (SL) valve.
  1.  Atrioventricular (AV) valves  ::– are present between the atria and the ventricles. It mainly prevent the back flow from the ventricles into the atrium during systole. It includes mitral valve and tricuspid valve.  Mitral valve or bicuspid valve allows the blood to flow from left atrium into the left ventricle.   Tricuspid valve present between the right atrium and right ventricle and stops the back flow of blood between them.
  2.  Semilunar valves are present in the arteries leaving the heart. 

Aorta It carries oxygenated blood.
Septum It prevents mixing of oxygenated and deoxygenated blood. 
Pulmonary Artery It carries deoxygenated blood to the lung. 
Pulmonary Vein It carries oxygenated blood from lung to the heart.
Superior Vena Cava It returns deoxygenated blood from head and arms to the heart.
Inferior Vena Cava It returns deoxygenated blood from lower limbs and organs to the heart.

 Working of Human Heart

The working mechanism of heart includes contraction (systole) and relaxation (diastole) of the atria and ventricles. The atria and ventricles contract alternately and constitutes a heart beat. SA node spreads waves of contraction across the walls of the atria which further reaches to the AV node (pacesetter).

AV node further stimulates the ventricles to contract simultaneously by transmitting the impulse via bundle of his and purkinje’s fibres. The ventricles force blood through arteries and hence exert great pressure on the blood.

Pumping Action of the Heart

The human heart functions to pump oxygenated (left portion) and deoxygenated blood (right portion) to various body organs.

Double Circulation
Double Circulation

It is the passage of blood occurring twice (two times) in the heart through separate pathways for completing one cycle. It consists of pulmonary and systemic circulation.


Double circulation is generally found in fishes, amphibians, birds, reptiles, and man where arterivenous heart (means when it receives both venous or deoxygenated and arterial or oxygenated blood) is present. The advantage of double circulation is that the blood can be sent to the lungs to pick up oxygen and then be returned to the heart to be pumped again before travelling around the body.

Heart Beat and its Regulation

Heart beat is the rhythmic contraction (systole) and relaxation (diastole) of the cardiac muscles. The heart of a healthy person beats 72 times per minute, while for a hard core physical worker it is 180 times per minute. Heart beat is regulated by the ability of the heart to contract spontaneously at a regular rate.

Heart sounds (beatings)can be heard by stethoscope. The first heart sound is ‘lub’, created by the closure of the atriventricular valves, at the beginning of the ventricular systole and opening of semilunar valves. The second heart sound is ‘dub’ created at the end of ventricular systole when the semilunar valve closes and the AV valve open.

Electrocardiograph (ECG)

ECG is the graphical record of electrical currents produced by the excitation of the cardiac muscles. Normal ECG is shown in the fig below. Any deviation from normal ECG represents abnormal heart functioning.

Electrocardiograph (ECG)
Electrocardiograph (ECG)

Blood Vessels

The intricate network of hollow tubes that transport blood throughout the body are called blood vessels. Blood vessels (except capillaries) are made up of three layers. . i.e., tunica externa, tunica media and tunica interna.

There are three types of blood vessels

 Arteries These are elastic vessels that transport blood away from the heart to various organs. Through the arteries only pure blood flows, but through the pulmonary arteries impure blood flows. 

Veins These are elastic vessels that transport blood to the heart from various organs of the body. Veins carry only impure blood (deoxygenated blood), but pulmonary veins carry only pure (oxygenated) blood. 

Capillaries These are small vessels that transport blood from the arteries to the veins and are located within the tissues of the body. Through the thin walls of the capillaries, exchange of wastes, nutrients, carbon dioxide and oxygen takes place

Blood Pressure

 Blood pressure is produced by the blood (present in blood vessels) against the walls of the blood vessels by the contraction of left ventricle. Blood pressure is recorded in millimeters of mercury, similar to barometric pressure with the help of an instrument called sphygmomanometer. The blood pressure is high in the arteries, gradually drops in the arterioles and capillaries, and become very low in the veins. The average blood pressure is expressed as 120/80 mmHg.

When the left ventricle contracts (pushing the blood into the aorta), the pressure produced is known as systolic blood pressure (120 mm Hg). When the complete diastole occurs and the heart is resting the pressure within the vessels is called diastolic blood pressure (80 mmHg).


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 What are components of blood
What are components of blood
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