Human Circulatory System

The System, which is primarily meant for the circulation of the nutritive products, disposal and excretion of waste material in the body is known 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.

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 tissue. The only entry and exit to the system is through the walls of blood vessels e.g.. in annelids and chordates. This considered to be the most advantageous method as it supplies blood to the deepest tissues of the body.

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

Blood vascular system

Human blood vascular system comprises blood, blood vesseles and heart


It is fluid connective tissue which forms 30-32% of total extracellular fluid. Volume of blood in an adult person is about 5-5.5L. 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. These are described below


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

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

The solutes found in plasma are various ions, glucose, tracces of other sugars, plasma proteins, amino acids, hormones, cholesterol, lipids, urea, other wastes and organic acids.

  • Serum: 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 principle cation and anion of the plasma. Bicarbonate and phosphate also occur in smaller amounts.
  • Anticoagulant: A natural strong anticoagulant present in the plasma is heteropolysaccharide named anti prothrombin, 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

It performs various functions in the blood as follows

  1. Helps in transport and uniform distribution of heat all over the body
  2. Provides body immunity.
  3. Maintananc of blod pH.
  4. Provides prevention of blood loss.
  5. Fibrinogen helpsin blood clotting, globulin help in defense mechanism, albumin maintains osmotic balance

Formed Elements

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

Functions of Blood
Blood performs the following important functions

  1. Maintains body pH, water and ionic balance
  2. helps in healing of wounds
  3. Also helps in transportation of hormones from endocrine glands to target organs.
  4. Coagulation
  5. Helps in transportation of body wastes from different body parts to kidneys.
  6. Maintains normal body temperature
  7. Fight against infections by forming body immunity
  8. Helps in transportation of respiratory gases (i.e. Oxygen, carbon dioxide)

A woman has approximately 4.5 litres of blood in her body while man has 5.6 litres

Blood Groups

Although blood of every human being appears to be similar in apperance 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.

Two important common types of blood grouping found in human beings are

1. ABO Blood Grouping

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).

2. 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 Rh+ are called universal receivers.
  • Rh+ blod can never be given to some with Rh- blood, but the other way around works. e.g. O Rh+ blood can not be given to someone with the blood type AB Rh.

Rh Incompatibility During Pregnancy

This is 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 RH+ man).

In such a case, mother becomes sensitive, while carrying a RH+ baby in her uterus. THis reason is that some of 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 remain separated by Placenta).

But is case of her subsequent pregnancies (i.e., 2nd and 3rd) Rh+ foet uses 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 erythroblastrosis 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 flouid 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 capiliaries of the arterial system into the tissues, some water along with many water soluble substances comes 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

Lymph performs the following important functions

  1. It acts as an important carrier of nutrients, hormones, etc.
  2. Also, helps in the renewal of ECF.
  3. Absorption of fat also occurs through lymph in the lacteal present in the intestinal villi.
  4. Helps in keeping tissue cells moist.
  5. 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 posses a muscular chambered heart

Sepending upon the different types of circulation in theom heart is of following 3 types

Four Chambered: Posses 2 atria and 2 ventricles; oxygenated and deoxygenated blood do not get mixed and pumped separately; double circulation e.g. birds and Mammanls

Three Chambered: Posses w atria and 1 ventricle; left atrium receive oxygenated and right atrium receive de-oxygenated blood. which gets mixed in venticle, incomplete doouble circulation, e.g., amphibians and reptiles (except corocodile)

Two Chambered: Posses atrium and 1 ventricle, heart always receive deoxygentaed blood which passes through it for once (single circulation) e.g., fishes

Human Heart

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

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

  1. 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 Nod) is situated in the upper wall of right atrium.
  2. Ventricles: 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 teh 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 typeso f valves i.e. two atrioventricular (AV) valve and two semilunar (SL) valve
  • 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
  • Semilunar valves are present in the arteries levving the hear
  • Aorta It caries oxygenated blood
  • Septum It prevents mixing of oxygenated and deoxygenated blood
  • Pulmonary Artery It caries deoxygenated blood to the lung
  • Pulmonary Vein It caries oxygenated blood from olung 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.

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