Transportation in Humans

• Blood cells constitute 45% of the total blood volume.
• Three kinds of blood cells are found in the blood.
  
  
  
  
  
  

Blood Grouping and Blood Transfusion

• The ABO system is based on the absence or presence of two surface antigens A and B on RBCs.
• Similarly, the plasma of other individuals contains antibodies produced in response to these antigens.
  
  
  
  
• According to the ABO blood group system, there are four blood groups—A, B, AB and O.
• Compatibility and Incompatibility in the ABO System:
  
  
  
  
• O type blood can be given to persons of all types of blood, i.e. O, A, B and AB. Hence, a person with O type of blood is called a universal donor.
• A person with AB type of blood can receive blood from all types, i.e. AB, A, B and O. Hence, such a person is called a universal recipient.
  (Note – The concept of universal donor and universal acceptor is now only theoretical. It is not in practice anymore.)

Rh System

• Nearly 80% of human beings have the Rh antigen on the surface of their RBCs.
• When the blood of an Rh-positive (Rh⁺) individual is transfused into a person lacking the Rh factor, the blood of the recipient develops antibodies against the Rh factor which may even lead to death.
• Hence, it is very important to match the Rh factor before blood transfusion.
• An Rh-negative (Rh⁻) woman may become sensitive if she carried an Rh⁺ child.
• Rh antigens of the foetus do not get exposed to the Rh⁻ blood of the mother in the first pregnancy because the blood of the two is well separated by the placenta.
• However, during the delivery of the first child, there is a possibility of exposure of maternal blood to the Rh⁺ blood of the first child. 
• It sensitises the mother, and the mother’s body starts producing antibodies against the Rh⁺ antigens.
• During the second pregnancy, if the child is Rh⁺, then there may be blood trouble.
• The mother’s Rh antibodies can leak into the blood of the Rh⁺ foetus which may destroy RBCs in the foetus.
• This may cause severe anaemia or jaundice in the baby, or it may sometimes even lead to the death of the foetus and abortion. This condition is called erythroblastosis foetalis.

Coagulation of Blood

• Blood coagulation occurs in series of steps:
• The platelets disintegrate at the site of injury and release thrombokinase.
• Thrombokinase with the help of calcium ions converts prothrombin into thrombin.
• Thrombin converts inactive fibrinogen into fibrin.
• Fibrin forms threads and a meshwork at the site of the wound.
• Blood cells are trapped in the network of the fibrin. The blood shrinks and squeezes out the rest of the plasma in the form of a clear liquid. The solid mass which is left behind is called a clot

Lymphatic System

• As blood flows in the capillaries of tissues, the plasma of leucocytes leak out through their walls and bathe the cells. Large protein molecules remain behind in blood vessels. This fluid released out is called tissue fluid or intercellular fluid.
• Most of the tissue fluid enters another set of vessels called lymphatic vessels, and this fluid is called lymph.

Functions of Lymph

• It supplies oxygen and nutrients to the parts where the blood cannot reach.
• Lymphocytes and monocytes of the lymph play role in defence mechanism.
• The lymph nodes localise the location and prevent them from spreading.

Circulatory Pathways

• There are two circulatory pathways — open circulatory system and closed circulatory system.
  
  

Human

Circulatory System

Conducting System

• SA (sino-atrial) node: It is located in the wall of the upper part of the right atrium, near the opening of the superior vena cava. It initiates and maintains the rhythmic contractile activity of the heart. Hence, it is also called a pacemaker.
• AV (Atrio-ventricular) node: It lies in the wall of the lower left corner or the base of the right atrium near the interatrial septum.
• Bundle of His: It is a bundle of atrioventricular fibres continues from the AV node. It gives rise to minute fibres called Purkinje fibres. Purkinje fibres spread out and connect with the ventricular muscle fibres.

Mechanism of Conduction of Impulse

• The SA node which acts as a pacemaker generates an action potential and produces a wave of contraction called cardiac impulse.
• The cardiac impulse first spreads to the muscle of atria causing their contraction.
• Later, it spreads on through the AV node and the bundle of His.
• From the bundle of His, it is further transmitted to the muscle of ventricles resulting in the ventricular contraction from the tip of the base or apex.