NCERT Solutions for Class 11 Biology Chapter 18

Q.1 Name the components of the formed elements in the blood and mention one major function of each of them.

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Erythrocytes, leucocytes and platelets are the formed elements of blood. They constitute approximately 45% of the blood.

Erythrocytes: Erythrocytes or red blood cells (RBCs) are essential for the transport of respiratory gases (oxygen and carbon dioxide) across the body.

Leucocytes: Leucocytes are essential for providing immunity against pathogens and other foreign substances.

• Granulocytes: Neutrophils, eosinophils and basophils phagocytose infecting pathogens.

• Agranulocytes: B lymphocytes and T lymphocytes are essential for both innate and acquired immunity.

Platelets: Platelets release the factors which are required for blood clotting.

Q.2 What is the importance of plasma proteins?

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The proteins constitute 6-8% of plasma. Major proteins of plasma are fibrinogen, globulins and albumins. The function of these plasma proteins is as follows:

• Fibrinogen is essential for blood clotting.
• Globulins (immunoglobulins or antibodies) form the defence system of the body and prevent infections.
• Albumins maintain the osmotic balance of the blood.
• Plasma proteins also help in the transportation of lipids and hormones.

Q.3 Match Column I with Column II:

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Q.4 Why do we consider blood as a connective tissue?

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Connective tissue supports and connects different types of tissues and organs of the body. Blood is considered as a connective tissue due to the following reasons:

a) It is mesodermal in origin like other connective tissues.

b) Like connective tissues, blood has plasma as the extracellular matrix in which various formed elements (cellular components) like erythrocytes, leucocytes and platelets are suspended.

c) It connects various body parts and organ systems. Nutrients, hormones, oxygen and other essential elements are transported from one part of the body to another through the blood. Blood also carries waste products from various organs to the site of excretion.

Q.5 What is the difference between lymph and blood?

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Q.6 What is meant by double circulation? What is its significance?

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Double circulation refers to the process wherein blood passes two times through the heart in one complete cycle. In this circulation, the blood circulates in two distinct and separate pathways/loops (pulmonary circulation and systemic circulation). It is observed in birds and mammals as they have a four-chambered heart.

Pulmonary circulation: The pumping of blood from the heart to the lungs forms the pulmonary circuit. The right atrium of the heart receives deoxygenated blood from all the body parts. The deoxygenated blood then moves to the right ventricle. This deoxygenated blood is carried to the lungs from the right ventricle of the heart via the pulmonary artery. The blood is oxygenated in the lungs and is transported back to the left atrium by the pulmonary vein.

Systemic circulation: Flow of oxygenated blood, from the left ventricle of the heart, via arteries, arterioles and capillaries to the tissues is known as a systemic circuit. In the lungs, deoxygenated blood acquires oxygen and goes into the left atrium of the heart. From the left atrium, the oxygenated blood goes to left ventricle. Thereafter, the blood moves into the aorta and is carried to different body tissues by a network of arterioles, arteries, and capillaries. The deoxygenated blood is then collected from body tissues by a system of venules, veins and vena cava and emptied into the right atrium. This forms the systemic circulation. Systemic circulation provides nutrients, oxygen and other necessary substances to various body parts and carries back carbon dioxide and other harmful waste products away for elimination from the body.

Significance: The importance of double circulation is that it does not allow the mixing of oxygenated and deoxygenated blood. Thus, it ensures the efficient oxygen supply to the body organs.

Q.7 Write the differences between:

(a) Blood and Lymph

(b) Open and Closed system of circulation

(c) Systole and Diastole

(d) P-wave and T-wave

Ans-

(a) Blood and Lymph

(b) Open and Closed system of circulation

(c) Systole and Diastole

(d) P-wave and T-wave

Q.8 Describe the evolutionary change in the pattern of heart among the vertebrates.

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The heart is a hollow muscular organ that pumps blood to all the tissues of the body. This process provides oxygen to various body parts. Evolution of heart has helped in more efficient transport of oxygen in the body by preventing the mixing of oxygenated and deoxygenated blood as described below:

• Fish: Fish have a very simple heart structure. Its heart is a two-chambered hollow tube-like structure. It has one atrium and one ventricle. Deoxygenated blood enters the atrium of the heart from where the blood is pumped into the ventricle. It then enters into the gills for oxygenation. The blood oxygenated in the gills is circulated across the body and finally, the deoxygenated blood from different body parts is transported back into the atrium of the heart.
• Amphibians: Amphibians have a three-chambered heart: two auricles and one ventricle. Oxygenated blood from the lungs enters into the left auricle while deoxygenated blood from other body parts enters into the right auricle. Both auricles transfer blood to the ventricle. In the ventricles, the oxygenated and deoxygenated blood mixes and this mixed blood is circulated to different body parts.
• Reptiles: Reptilians (except crocodiles) have an incomplete four-chambered heart. They have two auricles and a ventricle which is partially divided into two chambers. There is partial mixing of oxygenated and deoxygenated blood due to incomplete partitioning of the ventricles.
• Mammals and birds: Mammals and birds have a four-chambered heart. This prevents the mixing of oxygenated and deoxygenated blood. Right atrium receives deoxygenated blood and transfers it into right ventricle. From the right ventricle, deoxygenated blood is pumped into lungs to get oxygenated. Oxygenated blood is returned to left auricle of the heart. From left auricle, the oxygenated blood flows into the left ventricle and is pumped to whole body.

Q.9 Why do we call our heart myogenic?

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Our heart is called myogenic heart because it receives signals for contraction from specialized cells called cardiac myocytes found in it. Cardiac myocytes form a specialized structure, known as nodal tissue, that possesses both muscular and nervous characteristics. The human heart receives the signals for contraction from the nodal tissue.

Q.10 Sino-atrial node is called the pacemaker of our heart. Why?

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Sino-atrial node is called the pacemaker of heart because it is a specialized structure made up of cardiac myocytes that have both muscular and nervous characteristics. It is located in the upper side of right atrium. This structure has the ability to generate action potentials without any external stimuli. It initiates the impulse of contraction that subsequently spreads throughout the heart. It can generate a maximum of 70-75 action potentials per minute. It initiates and maintains rhythmic contraction of heart. Our heart usually beats around 70-75 times in a minute.

Q.11 What is the significance of atrio-ventricular node and atrio-ventricular bundle in the functioning of heart?

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Atrio-ventricular (AV) node is present in right auricle at the base of inter-auricular septum, which separates right auricle from the ventricle. From this node, a bundle of nodal fibres called AV bundle arises and it passes via the AV septa. Immediately, it divides into left and right bundle. These bundles give rise to Purkinje fibres, which penetrate into the myocardium. These fibres along with the bundles form the structure called the bundle of His. This nodal system is auto-excitable, that is, it possesses the ability to generate an action potential in the absence of electric impulse. The electrical impulse is then passed on to ventricle by the bundle of His and Purkinje fibres. This impulse leads to contraction of the ventricles.

Q.12 Define a cardiac cycle and the cardiac output.

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A complete cycle refers to the complete cycle of events from the start of one heartbeat to the start of next. It includes diastole and systole that involves filling of the heart with blood and then pumping it out. During diastole, the heart relaxes and fills with the blood. During systole, the ventricle contracts and pumps out the blood.

Cardiac output is the amount of blood pumped out by each ventricle in a minute. It is calculated by multiplying stroke volume (volume of blood pumped by each ventricle in a cardiac cycle) and heart rate (beats per minute).

Q.13 Explain heart sounds.

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When the valves of heart open and close during the cardiac cycle, they produce a sound known as heart sound. These sounds are lub and dub. Lub is the first sound and is produced by the vibrations of the heart when bicuspid and tricuspid valves (valves present between atria and ventricle) close at the beginning of systole (when ventricular muscles contract). It is followed by second sound dub which is produced by vibrations caused due to closing of semilunar valves (which guard the pulmonary artery and the aorta) at the end of the systole.

Q.14 Draw a standard ECG and explain the different segments in it.

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Electrocardiogram (ECG) is the graphical representation of the cardiac cycle. It is produced by an electrocardiograph. The normal ECG indicating that the heart is functioning properly is shown below.

A typical human ECG includes the following waves:

• P-wave: It represents depolarization of the right and left atria. The impulse of contraction by the SA node is generated during this wave.

• QRS wave: It shows depolarization and contraction of the right and left ventricles. The contraction starts shortly after Q and indicates the beginning of systole.

• T-wave: It represents repolarization (return from excited to normal) and relaxation of the ventricles. Its end indicates the end of systole.

Waves P, R and T are above the base line and are called positive waves, while Q and S are below the base line and are known as negative waves.