NCERT Class 11-Biology: Chapter – 18 Body Fluids and Circulation Part 6 (For CBSE, ICSE, IAS, NET, NRA 2023)

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Question 9:

Answer the following

a. Name the major site where RBCs are formed.

b. Which part of heart is responsible for initating and maintaining its rhythmic activity?

c. What is specific in the heart of crocodiles among reptilians?


a. In early embryonic stage RBC՚s are produced in: Yolk Sac

In later embryonic stage RBC՚s are produced in: Liver and Spleen

In mature adults RBC՚s are produced in: Bone Marrow

b. Part of heart which is responsible for initiating and maintaining its rhythmic activity is SA node or Sino Atrial Node because of which it is also known as Pacemaker of the heart.

c. Reptilians have a three chambered heart (2 atria and 1 ventricle) whereas crocodiles have four chambered heart (2 atria and 2 ventricles) in which the mixing of oxygenated and deoxygenated blood does not take place.

Long Answer Type Questions

Question 1:

Explain Rh-incompatibility in humans.


1. There are four types of blood group A, B, O and AB.

2. There is one other type of blood group known as Rh factor or Rhesus Monkey factor which is an inherited factor.

3. Most of the people are Rh positive which means that the person have Rh factor in them.

4. People who do not have Rh factor are called as Rh negative people (Rh-ve) .

5. Rh-incompatibility in humans may lead to a disorder called as Erythroblastosis foetal is.

6. In this disorder, at the time of pregnancy if the foetus is and the mother is and if the blood from foetus enters the mother՚s bloodstream, antibodies against the foetus blood is created.

7. At the time of first pregnancy it does not have any affect.

8. But in subsequent pregnancies if the baby is , the antibodies formed against blood during first pregnancy may enter the foetus through placenta.

9. This may lead to destruction of foetus՚s RBC՚s which may lead to death of the foetus or extreme Anaemia.

Question 2:

Describe the events in cardiac cycle. Explain “double circulation” .


Cardiac cycle is the cycle of initiation of a beat till the beginning of another heartbeat.

Events of a Cardiac Cycle:

1. Atrial Systole (0.7 sec) : In this event atria contracts due to the wave of contraction initiated by the SA node.

2. Now, the blood is forced into the ventricles as the bicuspid and tricuspid valves are open.

3. Ventricle Systole (0.5 sec) : After this, contraction of ventricles start taking place due to the wave of contraction initiated by the AV node.

4. This leads to the closing of bicuspid and tricuspid valve and the first heart sound is produced ‘lub’ .

5. Complete Ventricular Systole: After ventricular contraction the blood flows into the pulmonary trunk and aorta as the semilunar valves are opened.

6. Ventricular Diastole: In this event the ventricles relax and the semilunar valves close and the second heart sound is made ‘dub’ .

7. Complete Ventricular Diastole: The opening of bicuspid and tricuspid valves take place as the pressure of ventricles lowers down and blood flows from atria to the ventricles.

8. The duration of this cycle is about 0.8 seconds.


Cardiac Cycle

Double Circulation:

1. Birds and mammals have four chambered heart which are two atria and two ventricles.

2. The two atria are separated by inter-atrial septum and the two ventricles are separated by inter-ventricular septum.

3. Oxygenated blood received by the left atria passes to the left ventricle and Deoxygenated blood received by the right atria passes to the right ventricle without getting mixed

4. The blood is circulated to the body organs through the systemic circulation and to the lungs through pulmonary circulation.

5. This shows that birds and mammals have two separate circulatory pathways which are Systemic and Pulmonary Pathway

Advantage of Double Circulation:

1. Deoxygenated and oxygenated blood remains separated and do not mix.