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NCERT Solutions for Class 11 Biology Chapter 18 - Body Fluids and Circulation

In the 'Body Fluids and Circulation' chapter, the significance of fluids for the body, and how they are circulated have been explained. All living cells' basic requirements to function properly are oxygen, nutrients, and a few other essential elements. Along with this, the harmful substances or the waste inside the body need to be eliminated regularly for the body tissues' healthy functioning. In higher organisms that include humans, the body fluid that is most commonly used is the blood. Next, highly used body fluid is lymph that helps in transporting essential creatine substances.

This chapter will explore the areas of the properties and composition of blood and lymph (tissue fluid), and the method of blood circulation. For a deeper understanding of the topics explained herein, read the short explanation given below.

  • Blood
  • Lymph - The Tissue Fluid
  • Circulatory Pathways
  • Double Circulation
  • Managing Cardiac Activity
  • The disarray of the Circulatory System

In chapter 18, students will be taken through the concepts of body fluids and circulation, and a detailed discussion on the connective tissue made up of plasma, formed elements, like blood, and fluid matrix. Other than blood, the chapter also explains about lymph, which is a tissue fluid present in the body. The sub-topic blood is defined as a special connective tissue of living entities that comprises plasma, fluid matrix, and formed elements, such as leukocytes, thrombocytes, and erythrocytes.

Sub-parts of blood, i.e. blood coagulation, different blood groups, different circulatory channels of blood, and a few more. Explanations of the human circulatory system have also been made in detail where the Heart occupies the central position. Here, a few other topics that have been elaborated on are electrocardiography, cardiac cycle, double circulation, and more.

Frequently Asked Questions - FAQ

Q1. Name the components of the formed elements in the blood and mention one major function of each of them.
Answer: The formed elements found in the blood are as follows:
1. Erythrocytes- These are the most abundant cells (red blood cells) and they contain the red pigment called hemoglobin that imparts a red color to these cells. Erythrocytes carry oxygen to all parts of the body. These are produced continuously in some parts of the body such as the marrow of long bones, ribs, etc. There are about 4 to 6 million RBCs per cubic millimeter of blood.

2. Leukocytes- These are colorless cells that do not contain hemoglobin. They are divided into two main categories.
a.) Granulocytes- These leukocytes have granules in their cytoplasm and include neutrophils, eosinophils, and basophils. Neutrophils are phagocytic cells that protect the body against various infecting agents. Eosinophils are associated with allergic reactions, while basophils are involved in inflammatory responses.
b.) Agranulocytes- Lymphocytes and monocytes are agranulocytes. Lymphocytes generate immune responses against infecting agents, while monocytes are phagocytic.

3. Platelets- These are small irregular bodies present in the blood. They contain essential chemicals that help in clotting. The main function of platelets is to promote clotting.

Q2. What is the importance of plasma proteins?
Answer: Plasma is the colorless fluid of blood that constitutes about 55% of the blood. It helps in the transport of food, CO2, waste products, and salts, etc. About 6.8% of the plasma is constituted by proteins such as fibrinogens, globulins, and albumins. Fibrinogen is a plasma glycoprotein synthesized by the liver. It plays a role in the clotting of blood.
Globulin is a major protein of the plasma and it protects the body against infecting agents. Albumin is a major protein of the plasma that maintains the fluid volume within the vascular space.

Q3. Match Column I with Column II :

Column I Column II
(a) Eosinophils (i) Coagulation
(b) RBC (ii) Universal Recipient
(c) AB Group (iii) Resist Infections
(d) Platelets (iv) Contraction of Heart
(e) Systole (v) Gas transport

Answer: The correct matching is a- iii, b- v, c- ii, d-i, e- iv

Q4. Why do we consider blood as a connective tissue?
Answer: We consider blood as connective tissue because of these reasons:

  • Connective tissues bind, link, or support the other organs of the body.
  • They are the most abundant and widely distributed tissue in the body.
  • Blood transports gases, nutrients, and hormones from one body organ to the other.
  • It flows throughout the body.

Hence, blood is considered connective tissue.

Q5. What is the difference between blood and lymph?

Blood Lymph
(i) Blood is red liquid connective tissue. (i) Blood is red liquid connective tissue.
(ii) It flows through blood vessels—arteries, veins, and capillaries (ii) It flows in the lymph vessels.
(iii)It also contains hemoglobin, red blood cells, white blood cells, and platelets. (iii) It also contains white blood cells called lymphocytes.
(iv)It transports gases and other nutrients in the body. (iv) Exchange of gases and nutrients between the blood and the cells occurs through the lymph.

Q6. What is meant by double circulation? What is its significance?
Answer: Double circulation is a process during which blood passes twice through the heart during one complete cycle. This type of circulation is found in amphibians, reptiles, birds, and mammals. However, it is more prominent in birds and mammals as in them the heart is completely divided into four chambers:
1. the right atrium,
2. the right ventricle,
3. the left atrium, and
4. the left ventricle.

The movement of blood in an organism is divided into two parts:
1. Systemic circulation
2. Pulmonary circulation

Systemic circulation involves the movement of oxygenated blood from the left ventricle of the heart to the aorta. It is then carried by the blood through a network of arteries, arterioles, and capillaries to the tissues. From the tissues, the deoxygenated blood is collected by the venules, veins, and vena cava, and is emptied into the left auricle.

Pulmonary circulation involves the movement of deoxygenated blood from the right ventricle to the pulmonary artery which then carries blood to the lungs for oxygenation.

Significance of double circulation: The separation of oxygenated and deoxygenated blood allows a more efficient supply of oxygen to the body cells. Blood is circulated to the body tissues through systemic circulation and to the lungs through the pulmonary circulation.

Q7. (a) Write the differences between Systole and Diastole

Systole Diastole
It is the contraction of the heart chambers. It is the relaxation of heart chambers.
Blood is pumped out of the chambers during systole. Blood is received by the chambers during diastole.
It increases the blood pressure inside the heart. The blood pressure in the heart falls.

Q7 (b) Write the differences between P-wave and T-wave

P-Wave T-Wave
P-Wave represents the depolarisation or electrical excitation of atria. T-Wave represents the repolarisation of ventricles.
Blood is pumped into the ventricles. Blood is received by the atria.

Q8 Describe the evolutionary change in the pattern of the heart among the vertebrates.
Answer: The evolutionary change in the pattern of heart among the vertebrates are given below:

  • The vertebrate heart has evolved from a two-chambered heart of a fish to a four-chambered heart of mammals and birds.
  • The heart of the fish is two-chambered. The heart pumps deoxygenated blood to the gills where it is oxygenated and sent to the body. The deoxygenated blood is carried to the heart.
  • In amphibians, there are three chambers
    - the left atrium
    - the right atrium, and
    - a ventricle.
  • The left atrium receives the oxygenated blood from the gills, the lungs, or the skin. The right atrium receives deoxygenated blood from the body organs. However, eventually, both types of blood are mixed in the ventricle and the body is supplied with the mixed blood.
  • In the case of reptiles, there is a half septum that divides the ventricle incompletely. Here, oxygenated and deoxygenated blood do not mix.
  • In crocodiles, birds, and mammals, the heart is completely divided into two atria and two ventricles which keep the oxygenated and deoxygenated blood separate.
  • There is a structural modification in the heart from fish to mammals, which ensures the supply of oxygenated blood to the body. At the same time, the four-chambered structure makes sure that the flow of blood is regulated.

Q9. Why do we call our heart myogenic?
Answer: The meaning of ‘mayo’ is ‘muscle’ and ‘genic’ is ‘originating from’. As the cardiac impulse originates in the heart muscles, we call it myogenic. Modified cardiac muscles or the nodal tissue of the heart the SA node can generate an impulse that spreads over the walls of the heart. This results in the beating of the heart.

Q10. The sino-atrial node is called the pacemaker of our heart. Why?
Answer: Because of the property of originating a cardiac impulse, the sino-atrial node is called the pacemaker of the heart. The sino-atrial node generates the action potential and produces cardiac impulse. This cardiac impulse then spreads over the atria and ventricles causing them to contract or relax.
The SA node can generate a maximum number of action potentials which is about 70−75 per minute. It also controls the rhythmic contractile activity of the heart.

Q11. What is the significance of the atrioventricular node and atrioventricular bundle in the functioning of the heart?
Answer: The significance of the atrioventricular node and the atrioventricular bundle in the functioning of the heart is :
1. The atrioventricular node picks up the cardiac impulse from the SA node.
2. The atrioventricular bundle which originates from the AV-node conveys the cardiac impulse further towards the walls of the ventricles.

Q12. Define a cardiac cycle and the cardiac output.

Cardiac Cycle:

  • The circulation of blood in the heart occurs because of the alternate contraction and relaxation of the heart chambers and the contraction is also known as systole, while relaxation is known as diastole.
  • Each contraction phase (systole) is followed by a relaxation or expansion phase (diastole).
  • The series of events that occur during one complete beat of the heart is called the cardiac cycle and the duration of one cardiac cycle is 0.8 seconds.

- Cardiac Output:

  • The amount of blood pumped by each ventricle per minute is called the cardiac output.
  • Cardiac output = Stroke volume * Number of beats per minute

Q13. Explain heart sounds
Answer: Heart Sound: They occur in sequence with each heartbeat.
1. Two sounds, lubb, and dubb are heard during each cardiac cycle.
2. The first sound lubb is produced by the closure of the bicuspid and tricuspid valves. It is low-pitched.
3. The second sound dub is caused by the closure of semilunar valves. It is high-pitched.
4. Both sounds are important in the clinical diagnosis of any heart-related disorder.

Q14. Draw a standard ECG and explain the different segments in it.



ECG is a graphical representation of heart activities during a cardiac cycle. ECG is Electrocardiograph. Suppose a patient is connected to the machine with three electrical leads that are one to each wrist and the left ankle and it continuously monitors the heart activity. For a detailed evaluation of the heart's function, multiple leads are attached to the chest region.

Each peak in the ECG is identified with a letter from P to T that corresponds to a specific electrical activity of the heart.

P-wave: It represents the electrical excitation (or depolarization) of the atria, which leads to the contraction of both the atria.

QRS complex: It represents the depolarization of the ventricles, which initiates the ventricular contraction. The contraction starts shortly after
T-wave: It represents the return of the ventricles from excited to the normal state (repolarisation).

The end of the T-wave marks the end of the systole. Obviously, by counting the number of QRS complexes that occur in a given period, one can determine the heartbeat rate of an individual. Since the ECGs obtained from different individuals have roughly the same shape for a given lead configuration, any deviation from this shape indicates a possible abnormality or disease. Hence, it is of great clinical significance.

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