Directions: In the following questions, a statement of assertion is followed by a statement of reason. Mark the correct choice as :
(a) If both assertion and reason are true and reason is the correct explanation of assertion.
(b) If both assertion and reason are true but reason is not the correct explanation of assertion.
(c) If the assertion is true but the reason is false.
(d) If the assertion is false but the reason is true.
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Assertion: RBCs are devoid of nucleus in most of the mammals.
Reason: Entire cytoplasm of RBCs is filled with red coloured, iron containing complex protein called haemoglobin.
(b) Red blood cells carry a red pigment called haemoglobin. Each haemoglobin molecule can bind to oxygen. This is how RBCs help in the transport of oxygen.
Assertion: Type ‘ O ‘ blood group individuals are called ‘universal donors’.
Reason: RBCs of ‘ O ‘ blood group have both ‘ A ‘ and ‘B’ surface antigens.
If a blood transfusion is made between an incompatible donor and recipient, the reaction of antigens on the cells and antibodies in the plasma produces clots that clog the capillaries. Type \(\mathrm{O}\) blood group individuals are without \(A\) and \(B\) antigens on their RBCs, but have antibodies for both the antigens in their plasma. Hence, these can donate blood to anyone.
Assertion: Fibrins are formed by the conversion of inactive fibrinogens in the plasma by the enzyme thrombin.
Reason: Plasma without fibrinogen and blood corpuscles is called serum.
(b) A clot or coagulum is mainly formed of a network of threads called fibrins in which dead and damaged formed elements of blood are trapped. Fibrins are formed by the conversion of inactive fibrinogens in the plasma by the enzyme thrombin. Thrombins, in turn are formed from another inactive substance present in the plasma called prothrombin. An enzyme complex, prothrombinase, is required for the above reaction. The clot seals the wound and stops bleeding. After its formation, clot starts contracting and a pale yellow fluid, the serum, oozes out. This serum is blood plasma minus fibrinogen and blood corpuscles.
Assertion: The process of clotting can occur in absence of all cellular elements except platelets.
Reason: Vitamin \(\mathrm{K}\) is essential for blood clotting.
(b) Activated platelets are essential for blood clotting because several of the cascade reactions take place on the surface of platelets. Vitamin K is essential for production of prothrombin. Prothrombin and many plasma clotting factors are produced in the liver. The liver requires vitamin K to produce prothrombin and several other clotting factors.
Assertion: Lymph in lymphatic system is known as tissue fluid.
Reason : It consists of plasma proteins, RBCs and WBCs.
(c) Lymphatic system consists of lymph, lymphatic capillaries, lymphatic vessels, lymphatic nodes and lymphatic ducts. Lymph is a transparent fluid derived from blood and other tissues, which accumulates in the interstitial spaces as the interstitial fluid. Lymph contains leucocytes, mainly the lymphocytes. This fluid has a composition similar to that of plasma, except that it is low in proteins.
Assertion: Closed circulatory system is less efficient than open circulatory system.
Reason: The blood flow is fast in closed circulatory system than in open circulatory system.
(d) The closed circulatory system is more efficient than the open circulatory system because in open circulatory system, the blood flows through open spaces and channels whereas in closed circulatory system, the blood flows through proper blood vessels. Also, in closed circulatory system arterial musculature and precapillary sphincters regulate the flow of blood, so the volume of blood flowing through a tissue or an organ may be regulated according to needs. Moreover, in closed circulatory system, the blood flows far more rapidly in closed blood vessels than in wide open channels and body cavities. Thus, blood takes much shorter time to circulate through the closed system than in open circulatory system.
Assertion: Double circulation is incomplete in amphibians and reptiles.
Reason: Unlike in birds and mammals, in amphibian and reptiles, the left atrium receives oxygenated blood and right atrium receives deoxygenated blood.
(c) Amphibians and the reptiles (except crocodiles) have a 3-chambered heart with two atria and a single ventricle, whereas crocodiles, birds and mammals possess a 4-chambered heart with two atria and two ventricles. In amphibians and reptiles, the left atrium receives oxygenated blood from the gills/lungs/skin and the right atrium gets the deoxygenated blood from other body parts. However, they get mixed up in the single ventricle which pumps out mixed blood (incomplete double circulation). In birds and mammals, oxygenated and deoxygenated blood received by the left and right atria respectively passes on to the ventricles of the same sides. The ventricles pump it out without any mixing up, i.e., two separate circulatory pathways are present in these organisms, hence, these animals have double circulation.
Assertion: There is no mixing of oxygenated and deoxygenated blood in the human heart.
Reason: Valves are present in the heart which allows the movement of blood in one direction only.
(b) There is no mixing of deoxygenated and oxygenated blood in human heart due to the presence of inter-atrial and inter-ventricular septum. These septa completely divide the atria and ventricles into right and left to avoid mixing of blood. The backflow of the blood is prevented by the presence of valves. The two atria are separated from the ventricles by menbranous valves. They allow the flow of blood from atria to ventricles and not in the backward direction. At the base of aorta and pulmonary artery, are present three membranous, pocket-shaped semilunar valves. These valves check the return of the blood to the ventricles.
Assertion: Left atrium has the thickest muscles.
Reason: Left atrium receives blood from the lungs.
(d) Left ventricle has the thickest muscles because it pumps the blood to the whole body. The left atrium receives blood from the lungs.
Assertion: Sino-atrial node (SAN) is called the pacemaker.
Reason: SAN generates the maximum number of action potentials and is responsible for initiating and maintaining the rhythmic contractions of the heart.
(a) In the myogenic heart, contraction is initiated by specilised muscle. The SAN can generated the maximum number of action potentials, i.e., \(70-75 \mathrm{~min}^{-1}\) and is responsible for intiating and maintaining the rhythmic contractile activity of the heart. Therefore, it is called the pacemaker. The SAN controls the beat of the heart because its rate of rhythmical discharge is faster than that of any other part of the heart. Therefore, the SAN is virtually always the pacemaker of the normal heart.
Assertion: A contraction of heart is called systole and its relaxation is called diastole.
Reason: The contraction and relaxation of heart constitute heartbeat.
(b) The human heart works by contraction and relaxation of the cardiac muscles. The contraction phase is called systole and relaxation phase is called diastole.
Assertion: There are 72-75 heartbeats per minute on an average when a person is performing normal work.
Reason: One heartbeat is completed in 0.8 second.
(a) One heartbeat completes in \(0.8-0.83\) second. Therefore, there are \(60 / 0.8\) or \(60 / 0.83\), i.e., \(72-75\) heart beats per minute on an average when a person is performing normal work.
Assertion: The cardiac output of an ordinary man and of an athlete is the same.
Reason: Cardiac output gets altered with change in stroke volume and heart rate.
(d) The stroke volume multiplied by the heart rate (no. of beats per minute) gives the cardiac output. Therefore, the cardiac output can be defined as the volume of blood pumped out by each ventricle per minute and averages \(5000 \mathrm{~mL}\) or 5 litres in a healthy individual. The body has the ability to alter the stroke volume as well as the heart rate and thereby the cardiac output. For example, the cardiac output of an athlete will be much higher than that of an ordinary man.
Assertion: The enlarged Q and R waves indicate myocardial infarction.
Reason: Any deviation in the normal recording of ECG indicates possible abnormality or disease.
(b) A normal electrocardiogram (ECG) is composed of P wave, a QRS wave (complex) and a T wave. The ECGs obtained from different individuals have roughly the same shape and any deviation from this shape indicates a possible abnormality or disease. For example, the enlarged \(Q\) and \(R\) wave indicate a myocardial infraction (heart attack), enlargement of the \(P\) wave indicates enlargement of the atria. T wave is flat when the heart muscles receive insufficient oxygen as in atherosclerotic heart disease.
Assertion: Neural signals through sympathetic nerves can increase the strength of ventricular contraction.
Reason: Parasympathetic neural signals synergistically act with sympathetic neural signals to increase the cardiac output.
(c) Neural signals through the sympathetic nerves (part of ANS) can increase the rate of heartbeat, the strength of ventricular contraction and thereby the cardiac output. On the other hand, parasympathetic neural signals (another component of ANS) decrease the rate of heartbeat, speed of conduction of action potential and thereby the cardiac output.
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