Past Entrance Papers

Hint

(a) Both \(A\) and \(R\) are true and \(R\) is the correct explanation of \(A\).
A person goes to a high altitude and experiences altitude sickness like heavy breathing and heart palpitation. It is due to low atmospheric pressure at high altitudes, the body does not get sufficient oxygen.

Hint

(a) The favourable conditions for the formation of oxyhaemoglobin is high \(\mathrm{pO}_{2}\), lesser \(\mathrm{H}^{+}\)concentration and lower temperature found in alveoli, whereas low \(\mathrm{pO}_{2}\), high \(\mathrm{H}^{+}\)concentration and high temperature are favourable for dissociation of oxygen from the oxyhaemoglobin found in tissues.

Hint

(a) Partial pressures of oxygen( in \(\mathrm{mm} \mathrm{Hg}\) ) and carbon dioxide at alveoli are \(\mathrm{pO}_{2}\) \(=104\) and \(\mathrm{pCO}_{2}=40\).

\(
\begin{array}{cccccc}
\hline \begin{array}{c}
\text { Respir } \\
\text { atory } \\
\text { gas }
\end{array} & \begin{array}{c}
\text { phos- } \\
\text { air }
\end{array} & \begin{array}{c}
\text { Blood } \\
\text { (deoxy } \\
\text { genated) }
\end{array} & \begin{array}{c}
\text { Blood } \\
\text { (oxyge } \\
\text { nated) }
\end{array} & \text { Tissue } \\
\hline 0_{2} & 159 & 104 & 40 & 95 & 40 \\
\hline \mathrm{CO}_{2} & 0.3 & 40 & 45 & 40 & 45 \\
\hline
\end{array}
\)

Hint

(d) Inspiration is initiated by the contraction of the diaphragm, which increases the volume of the thoracic chamber in the anterio-posterior axis. The contraction of external inter-coastal muscles lifts up the ribs and the sternum causing an increase in the volume of the thoracic chamber in the dorso-ventral axis.
Inspiratory Reserve Volume (IRV): Additional volume of air, a person can inspire by a forcible inspiration. This averages \(2500 \mathrm{~mL}\) to \(3000 \mathrm{~mL}\).

Hint

(d) The Total Lung Capacity (TLC) is the total volume of air accommodated in the lungs at the end of a forced inspiration. This includes Residual Volume(RV), Expiratory Reserve Volume (ERV), Tidal Volume (TV), and Inspiratory Reserve Volume(IRV). TLC is also equal to the vital capacity of residual volume. 

Hint

(b) Pneumotaxic centre is present in the pons region of the brain.
\(\mathrm{O}_{2}\) dissociation curve is useful in studying the effect of factors like \(\mathrm{pCO}_{2}\), \(\mathrm{H}^{+}\)concentration, etc., on binding of \(\mathrm{O}_{2}\) with haemoglobin. Carbonic anhydrase is an enzyme present on the surface of RBC. The primary site of the exchange of gases is the alveoli of the lungs.

Hint

(b) Statement in option (b) is incorrect with reference to transport of oxygen. It can be corrected as

In alveoli high \(\mathrm{pO}_{2}\), low \(\mathrm{pCO}_{2}\), low \(\mathrm{H}^{+}\) concentration and lower temperature are the factors that favour the formation of oxyhaemoglobin.

Hint

(b) According to Central Pollution Control Board (CPCB), particulate size \(2.5\) micrometers or less in diameter (PM 2.5) are responsible for causing the greatest harm to human health. These fine particulates can be inhaled deep
into the lungs and can cause breathing and respiratory symptoms, irritation,
inflammation and damage to the lungs and premature deaths.

Hint

(b) Statement (b) is correct as intrapulmonary pressure is lower than the atmospheric pressure during inspiration. Other statements can be corrected as
(a) Inspiration occurs due to external intercoastal muscles.
(c) Inspiration occurs when atmospheric pressure is more than intrapulmonary pressure.
(d) Inspiration is initiated due to the contraction of the diaphragm.

Hint

(b) Vital Capacity (VC) is the maximum volume of air a person can breathe in after a forced expiration. This includes ERV, TV, and IRV, or the maximum volume of air a person can breathe out after a forced inspiration.

Hint

(d) The Expiratory Capacity of athlete will be \(1500 \mathrm{~mL}\).
It can be calculated as Given, Tidal Volume (TV) \(=500 \mathrm{~mL}\)
Expiratory Reserve Volume \((E R V)=1000 \mathrm{~mL}\)
Expiratory Capacity \(=\) TV+ERV
\(=500+1000=1500 \mathrm{~mL}\)

Hint

(b) Tidal Volume (TV) is the volume of air inspired or expired during the normal breath. It is about 500-550 mL.

Inspiratory Reserve Volume (IRV) is the extra amount of air that can be inspired directly after a normal inspiration. It is about 2500-3000 mL.

Expiratory Reserve Volume (ERV) is the extra amount of air that can be expired forcibly after a normal expiration. It is about 1000-1100 mL. Residual Volume (RV) is the volume of air that remains still in the lung after the most forceful expiration. It is about 1100-1200 mL.

Hint

(b) Silicosis is an occupational respiratory disorder that is caused due to excessive inhalation of silica dust. It usually affects the workers of grinding or stone-breaking industries. The long-term exposure can cause lung fibrosis (or stiffening), leading to breathing difficulties. Anthracis or Anthrax is a bacterial infection caused by Bacillus anthracis. Botulism is a food poisoning infection caused by Clostridium botulinum. Its
symptoms include diarrhoea, vomiting, abdominal distention, etc.

Hint

(c) Asthma is inflammation of bronchioles. Its symptoms include wheezing, coughing, and difficulty in breathing mainly during expiration. Emphysema is an inflation or abnormal distension of the bronchioles or alveolar sacs of the lungs. Many of the septa between the alveoli are destroyed and much of the elastic tissue of the lungs is replaced by connective tissue. As a result, alveolar septa collapse, and the surface area gets greatly reduced.

Hint

(b) ‘Smack’ also called as brown sugar/ Heroin is formed by acetylation of morphine. It is obtained from the latex of unripe capsule of Poppy plant (Papaver somniferum).

Hint

(b) Asthma (an allergic condition) is a difficulty in breathing causing wheezing due to inflammation of the bronchi and bronchioles. It can be due to increasing air-born allergens and pollutants. Many people in urban areas are suffering from this respiratory disorder.

Hint

(d) Residual volume is the volume of air that remains in the lungs after the most forceful expiration. This residual air enables the lungs to continue the exchange of gases even after maximum exhalation. Due to this, lungs do not collapse even after forceful expiration.

Hint

(b) The partial pressure of oxygen in alveolar air is \(104 \mathrm{mmHg}\) whereas it is \(40 \mathrm{mmHg}\) in deoxygenated blood and \(95 \mathrm{mmHg}\) in oxygenated blood.

Hint

(b) Intrapleural pressure is the pressure of air within the pleural cavity. Intrapleural pressure is always negative, which acts like a suction to keep the lungs inflated and prevent them from collapsing. The negative intrapleural pressure is due to three main factors: surface tension of the alveolar fluid; elasticity of lungs; elasticity of thoracic wall. Normally, there is a difference between intrapleural and intrapulmonary pressure, which is called transpulmonary pressure. This transpulmonary pressure creates the suction to keep the lungs inflated. If there is no pressure difference, there is no suction and the lungs will collapse.

Hint

(a) Reduction in \(\mathrm{pH}\) of blood causes oxygen haemoglobin dissociation curve to shift to the right which indicates dissociation of oxygen from haemoglobin. This decreases the affinity of haemoglobin for oxygen.

Hint

(c) Emphysema is a chronic obstructive pulmonary disease (COPD) caused due to cigarette smoking. It is an inflation or abnormal distention of the bronchioles or alveolar sacs of the lungs which causes irreversible distension and loss of elasticity of alveoli of the lungs.

Hint

(d) Asthma is an allergic condition in which the tissue surrounding the bronchioles of the lungs swell up and compress the bronchioles thus causing difficulty in breathing. This allergy mainly involves \(\mathrm{IgE}\) antibodies and chemicals like histamine and serotonin from the mast cells.

Hint

(d) Emphysema is an inflation or abnormal distension of the bronchioles or alveolar sacs of the lungs. Many of the septa between the alveoli are destroyed and much of the elastic tissue of the lungs is replaced by connective tissue. As the alveolar septa collapse, the surface area for gas exchange is greatly reduced. There is loss of elasticity in the walls of bronchioles or alveolar sacs. As a result, the alveolar sacs remain filled with air even after expiration. The exhalation becomes more difficult. The lungs remain inflated. Major causes of emphysema are cigarette smoking and the inhalation of smoke or other toxic substances over a period of time.

Hint

(d) Excess \(\mathrm{CO}_{2}\) mainly stimulates the respiratory centre of the brain and increases the inspiratory and expiratory signals to the respiratory muscles. \(\mathrm{O}_{2}\) does not have a significant direct effect on the respiratory centre of the brain in controlling respiration.

Hint

(a) About \(70 \%\) of \(\mathrm{CO}_{2}\) (about \(2.5 \mathrm{ml}\) per 100 \(\mathrm{ml}\). of blood), received by blood from the tissues, enters the RBCs where it reacts with water to form carbonic acid \(\left(\mathrm{H}_{2} \mathrm{CO}_{3}\right)\).

Carbonic anhydrase, exclusively found in \(\mathrm{RBCs}\), speeds up the formation of \(\mathrm{H}_{2} \mathrm{CO}_{3}\) and rapidly converts it back to carbon dioxide and water when blood reaches th lungs. Almost as rapidly as formed, all carbonic acid of \(\mathrm{RBCs}\) dissociates into hydrogen \(\left(\mathrm{H}^{+}\right)\)and bicarbonate ions \(\left(\mathrm{HCO}_{3}^{-}\right)\).

Hint

(a) In the given figure \(A\) is trachea. It is supported by incomplete cartilaginous rings which prevent its collapse during inspiration. \(\mathrm{B}\) is pleural membrane. It encloses the lungs. \(\mathrm{C}\) are alveoli. They are thin-walled sacs having an extensive network of capillaries for gaseous exchange. \(\mathrm{D}\) is diaphragm.

Hint

(b) A is pulmonary vein which takes pure blood from the lungs to the heart. B is the dorsal aorta which takes pure blood from the heart to various body parts. C is vena cava which takes impure blood from various body parts to the right auricle of the heart. D is the pulmonary artery which takes impure blood from the heart to the lungs.

Hint

(c) In certain industries, especially those involving grinding or stone breaking so much dust is produced that the defense mechanism of the body cannot fully cope with the situation. Long exposure can give rise to inflammation leading to fibrosis (proliferation of fibrous tissues) and thus causing serious lung damage. Workers in such industries should wear protective masks.

Hint

(a) As a person moves up a hill the \(\mathrm{pO}_{2}\) and total atmospheric pressure decreases. Decrease in \(\mathrm{pO}_{2}\) due to the increasing altitude, stimulates the Juxtaglomerular cells of the kidney to secrete erythropoietin hormone which increases the number of RBCs (polycythemia) to compensate the supply of \(\mathrm{O}_{2}\). At higher altitude, haemoglobin has a lower binding affinity to \(\mathrm{O}_{2}\) because the primary factor responsible for binding is \(\mathrm{pO}_{2}\) which decreases at higher altitude.

Hint

(b) In the given figure the exchange of respiratory gases occurs in alveolar cavity.

The thickness of the alveolar-capillary barrier varies from \(0.2\) to \(2.5 \mu \mathrm{m}\). The wall of the capillary endothelial cell is fused to that of the alveolar cell with only a very thin basement membrane between these two cells. This produces a very narrow gap across which oxygen and carbon dioxide can rapidly diffuse.

Hint

(a) A large portion of oxygen is left unused in the human blood even after its uptake by the body tissues. This \(\mathrm{O}_{2}\) acts as a reserve during muscular exercise.

Hint

(c) The process of breathing involves the pathway of the human respiratory system and includes the nasal cavities, oral cavities, pharynx, trachea, primary bronchi, secondary bronchi, and lungs with bronchioles and alveoli present. Breathing is a 2-fold process which includes breathing in and breathing out. During this process, the diaphragm which is the dome-shaped sheet of muscle located below the lungs contracts and expands along with the intercostal muscles to force air in and out of the lungs. Since intercostal muscles are also involved in the process of respiration, deeper respiration results in the movement of ribs too. Hence most of us can consciously breathe in and breathe out by moving the diaphragm alone.

Hint

(a) At the tissue site where partial pressure of \(\mathrm{CO}_{2}\) is high due to catabolism, \(\mathrm{CO}_{2}\) diffuses into blood
(RBCs and plasma) and forms \(\mathrm{HCO}_{2}{ }^{-}\)and \(\mathrm{H}^{+}\). At the alveolar site where \(\mathrm{pCO}_{2}\) is low, the reaction proceeds in the opposite direction leading to the formation of \(\mathrm{CO}_{2}\) and \(\mathrm{H}_{2} \mathrm{O}\). Thus, \(\mathrm{CO}_{2}\) trapped as bicarbonate at the tissue level and transported to the alveoli is released out as \(\mathrm{CO}_{2}\).

Hint

\(
\begin{aligned}
&\text { (c) }\\
&\begin{array}{lc}
\text { Respiratory capacities } & \text { Respiratory volumes } \\
\text { Residual volume } & 1200 \mathrm{~mL} \\
\text { Vital capacity } & 4500 \mathrm{~mL} \\
\text { Inspiratory reserve volume } & 2500 \mathrm{~mL} \\
\text { Inspiratory capacity } & 3500 \mathrm{~mL}
\end{array}
\end{aligned}
\)

Hint

(a) Blood is the medium of transport for \(\mathrm{O}_{2}\) and \(\mathrm{CO}_{2}\). About 97 percent of \(\mathrm{O}_{2}\) is transported by \(\mathrm{RBCs}\) in the blood. The remaining 3 percent of \(\mathrm{O}_{2}\) is carried in a dissolved state through the plasma. Nearly 20-25 percent of \(\mathrm{CO}_{2}\) is transported by RBCs whereas 70 percent of it is carried as bicarbonate. About 7 percent of \(\mathrm{CO}_{2}\) is carried in a dissolved state through plasma.

Hint

(a) When a person moves to higher altitudes, the \(\mathrm{pO}_{2}\) and total atmospheric pressure decrease. Hypoxia stimulates the Juxtaglomerular cells of the kidney to release erythropoietin hormone which stimulates erythropoesis in the bone marrow causing polycythemia (increase in \(\mathrm{RBC}\) production). Hypoxia will also increase the breathing rate. Initially, the size of RBCs will also increase but with an increase in the number of RBCs, the size of RBC s becomes normal.

Hint

(b) Vital capacity is the amount of air which one can inhale or exhale with maximum effort. It is the sum of tidal volume, inspiratory reserve volume, and expiratory reserve volume, while total lung capacity is the total amount of air present in the lungs and the respiratory passage after a maximum inspiration. It is the sum of the vital capacity and the residual volume. \(\mathrm{TLC}=\mathrm{VC}+\mathrm{RV}\). So, vital capacity is also total lung capacity (TLC) – residual volume (RV).

Hint

(b) Oxygen is needed for aerobic respiration and diffuses from a region of high to low concentration from the mother’s blood to the blood of the foetus. The haemoglobin of the foetus has a higher affinity for oxygen than that of adult haemoglobin and so the efficiency of exchange is increased. Carbon dioxide, a waste product of aerobic respiration diffuses in the opposite direction.

Hint

(d) Vital capacity is the sum of inspiratory reserve volume, tidal volume, and expiratory reserve volume. It is about \(4800 \mathrm{~mL}\).

Hint

(c) When systemic arterial blood flows through capillaries, carbon dioxide diffuses from the tissues into the blood. Some carbon dioxide is dissolved in the blood. Some carbon dioxide reacts with haemoglobin to form carbaminohaemoglobin. The remaining carbon dioxide is converted to bicarbonate and hydrogen ions. Most carbon dioxide is transported through the blood in the form of bicarbonate ions.

Hint

(d) The circulatory system of insects is open, whereby blood (haemolymph), flows freely through the body cavity (haemocoel). There is a dorsal vessel which is closed at the posterior end of the abdomen, and runs forward along the dorsal midline, and opens in the head at the anterior portion (aorta). There are several chambers and openings (ostia), along the dorsal vessel where blood enters it through valves. The blood is then pumped forward to the aorta and into the body cavity.Blood contains: water – about \(90 \%\); inorganic ions – dissolved salts of \(\mathrm{Na}, \mathrm{K}, \mathrm{Ca}\), \(\mathrm{Mg}\); organic molecules – amino acids, sugars for muscle use; blood cells. Haemolymph does not contain an oxygen-carrying pigment like \(\mathrm{Hb}\), hence, does not assist in respiration.

Hint

(c) At high altitudes composition of air remains almost the same as at sea level, but density (barometric pressure) of air gradually decreases due to which arterial \(\mathrm{pO}_{2}\) is also decreased (hypoxemia). High altitudes present with complex conditions to which the human body has to acclimatize. The number of red blood cells per unit volume of blood is likely to be higher in a person living at high altitudes. This is in response to the air being less dense at high altitudes. More number of red blood cells are needed to trap \(\mathrm{O}_{2}\) from rarefied air having low \(\mathrm{pO}_{2}\) (partial pressure of oxygen).

Hint

(d) The effect of rising \(\mathrm{CO}_{2}\) tension is to decrease the affinity of \(\mathrm{Hb}\) for \(\mathrm{O}_{2}\). Thus, when \(\mathrm{CO}_{2}\) concentration in blood increases, breathing becomes faster and deeper.

Hint

(d) Carboxyhaemoglobin, a stable compound, is formed when haemoglobin readily combines with carbon monoxide. Carbon monoxide converts iron (II) to iron (III) in its reaction with haemoglobin. In this form haemoglobin does not carry oxygen resulting in its (oxygen) starvation and leads to asphyxiation and in extreme cases to death. The affinity of haemoglobin for \(\mathrm{CO}\) is 250 times its affinity for \(\mathrm{O}_{2}\) and \(\mathrm{COHb}\) liberates \(\mathrm{CO}\) very slowly and also due to that compound the dissociation curve of the remaining \(\mathrm{HbO}_{2}\) shifts to the left, decreasing the amount of \(\mathrm{O}_{2}\) released.

Hint

(d) An average person, not doing hard labour i.e., leading a rather sedentary life, needs about 2800 kcal of energy per day. This is called routine metabolic rate (RMR).

Hint

(b) Haemoglobin \((\mathrm{Hb})\) is a conjugated protein. It consists of a basic protein globin joined to a nonprotein group heme. Heme is an iron-porphyrin ring. A mammalian \(\mathrm{Hb}\) molecule is a complex of 4 heme molecules joined with 4 globin molecules. It is present in \(\mathrm{RBC}\), and carries \(\mathrm{O}_{2}\) from the lungs to the tissues and transports \(\mathrm{CO}_{2}\) from the tissues to the lungs

Hint

(b) The respiratory centre is the medulla oblongata, which regulates the rate and depth of breathing. The dorsal group of neurons located in the dorsal portion of the medulla oblongata regulates inspiration and the ventral group of neurons located in the ventrolateral part of the medulla oblongata regulates both inspiration and expiration.

Hint

(d) The brain cells are highly specialised. They cannot regenerate and respire without \(\mathrm{O}_{2}\). Therefore, the shortage of \(\mathrm{O}_{2}\) leads to the death of brain cells.

Hint

(a) To maintain electrostatic neutrality of plasma, many chloride ions diffuse from plasma into RBCs and bicarbonate ions pass out. The chloride content of RBC s increases when oxygenated blood becomes deoxygenated. This is termed as chloride shift or Hamburger shift.

Hint

(d) Diffusion is the net flow of a substance from a region of higher concentration to a region of lower concentration. The exchange of gases between the alveoli and blood in the lung is the result of the difference in partial pressure of respiratory gases. The partial pressure of oxygen \(\left(\mathrm{pO}_{2}\right)\) of the alveolar air is higher than the \(\mathrm{pO}_{2}\) of blood in alveolar capillaries, thus \(\mathrm{O}_{2}\) diffuses rapidly from the alveolar air into the blood of alveolar capillaries. The \(\mathrm{pCO}_{2}\) of blood reaching the alveolar capillaries is higher than the \(\mathrm{pCO}_{2}\) of alveolar air. Therefore, \(\mathrm{CO}_{2}\) into the alveolar air.

Hint

(c) The atmosphere (air) is a mixture of several gases. Near the earth’s surface it consists of \(78 \%\) nitrogen, 21% oxygen, \(0.93 \%\) argon, \(0.03 \%\) carbon dioxide, and small quantities of hydrogen, helium, neon, krypton and traces of many other gases.

Hint

(b) In the lungs, the air is separated from the venous blood through the squamous epithelium and endothelium of blood vessels. As a result, the barriers between the air in an alveolus and the blood in its capillaries is only about \(0.5 \mathrm{~mm}\).

Hint

(b) Alveoli are the site of the respiratory exchange of gases. Oxygen from the alveolar air diffuses through the alveolar epithelium and the capillary endothelium into the capillary blood, and carbon dioxide diffuses in the opposite direction.

Hint

(a) Spleen receives only oxygenated blood from the heart through the splenic artery. The liver receives a blood supply from two sources. The first is the hepatic artery which delivers oxygenated blood from the general circulation. The second is the hepatic portal vein delivering deoxygenated blood from the small intestine containing nutrients. The blood flows through the liver tissue to the hepatic cells where many metabolic functions take place. The blood drains out of the liver via the hepatic vein. Gill and Lung receive deoxygenated blood as these are the organs where oxygenation of blood takes place.

Hint

(c) The transport of \(\mathrm{O}_{2}\) and \(\mathrm{CO}_{2}\) occurs with the help of RBCs and blood plasma. \(97 \%\) of \(\mathrm{O}_{2}\) is transported by \(\mathrm{RBCs}\) and \(3 \%\) of \(\mathrm{O}_{2}\) is carried by plasma. About \(7 \%\) of \(\mathrm{CO}_{2}\) is transported in plasma and rest by RBCs \((23 \%)\) by binding with \(\mathrm{Hb}\) and \(70 \%\) reacts with water to form carbonic acid in RBCs.

Hint

(a) Residual volume is the amount of air that remains in the lungs after forcible expiration. It is about \(1500 \mathrm{ml}\). It enables the lungs to continue exchange of gases even after maximum exhalation or holding the breath. Inspiratory reserve volume is the extra amount of air that can be inhaled forcibly after a normal inspiration. It is about 2000 to \(2500 \mathrm{ml}\). Vital capacity is the amount of air which one can inhale and also exhale with maximum effort. It is about \(3.5-4.5\) litres. Tidal volume \((500 \mathrm{ml})\) is the volume of air normally inspired or expired in one breath without any effort.

Hint

(d) Each lung is enclosed in two membranes, the pleura. The inner membrane is called the visceral pleuron and the outer membrane is called the parietal pleuron. A very narrow space exists between the two pleura. It is called the pleural cavity and contains a watery fluid called the pleural fluid that lubricates the pleura. The periosteum is the outer membrane of the bone. The perichondrium is a layer that surrounds the cartilage and the pericardium is the membrane that encloses the pericardial cavity, containing the vertebrate heart.

Hint

(d) RBCs and plasma both transport \(\mathrm{O}_{2}\) and \(\mathrm{CO}_{2}\) either in chemically bonded state or in dissolved state.

Hint

(b) Residual air is the volume of air that remains in the lungs after the most forceful expiration. It equals to \(1500 \mathrm{~mL}\). Residual air mostly occurs in alveoli. Maximum air that can be breathed in and breathed out is the vital capacity (3500-4500 mL) while the air that can be expired over and above the tidal air by most forceful expiration is the expiratory reserve volume (1200 \(\mathrm{mL})\).

Hint

(a) At high altitudes, arterial \(\mathrm{pO}_{2}\) decreases as density of air decreases. The number of RBCs per unit volume of blood is likely to be higher in a person living at high altitudes. More number of \(\mathrm{RBCs}\) are needed to trap \(\mathrm{O}_{2}\) from air having less \(\mathrm{O}_{2}\).

Hint

(b) Buffer is a solution that resists change in \(\mathrm{pH}\) when an acid or alkali is added or when the solution is diluted. Acidic buffers consists of a weak acid with a salt of the acid. The salt provides the negative ion \(\mathrm{A}\), which is the conjugate base of the acid HA. An example is carbonic acid and sodium hydrogen carbonate in which molecules \(\mathrm{H}_{2} \mathrm{CO}_{3}\) and ions \(\mathrm{HCO}_{3}{ }^{-}\)are present. About \(70 \%\) of \(\mathrm{CO}_{2}\) released combines with water in the RBCs to form carbonic acid. Carbonic acid dissociates into bicarbonate and hydrogen ions. Addition of \(\mathrm{H}^{+}\)ions would make the blood acidic. However, most of the hydrogen ions are neutralized by combination with \(\mathrm{Hb}\), forming acid haemoglobin. This reduces the acidity of the blood and also releases additional \(\mathrm{O}_{2}\).
\(
\mathrm{HbO}_{2}^{-}+\mathrm{H}^{+} \rightleftharpoons \mathrm{HHb}+\mathrm{O}_{2}
\)

Hint

(c) About \(70 \%\) of \(\mathrm{CO}_{2}\) released diffuses into the plasma and then into the \(\mathrm{RBCs}\). Here, it combines with water to form carbonic acid. Carbonic acid dissociates into bicarbonate and hydrogen ions. Hydrogen ions are picked up by proteins and a small amount of bicarbonate ions is transported in the RBCs, whereas most of them diffuse into the plasma to be carried by it. About \(7 \%\) of \(\mathrm{CO}_{2}\) is transported as dissolved in plasma and \(23 \%\) of \(\mathrm{CO}_{2}\) combines with \(\mathrm{Hb}\) to form carbaminohaemoglobin. 

\(
\begin{array}{r}
\mathrm{CO}_{2}+\mathrm{H}_{2} \mathrm{O} \stackrel{\text { carbonic }}{\stackrel{\text { anhydrase }}{\rightleftharpoons}} \mathrm{H}_{2} \mathrm{CO}_{3} \\
\text { carbonic acid } \\
\mathrm{H}_{2} \mathrm{CO}_{3} \rightleftharpoons \mathrm{H}^{+}+\mathrm{HCO}_{3}^{-} \\
\text {bicarbonate ion }
\end{array}
\)

Hint

(b) The medulla oblongata is the primary respiratory control center. Its main function is to send signals to the muscles that control respiration to cause breathing to occur.

Hint

(c) In mammalian respiratory system, air is breathed through nostrils, from nostrils air passes through pharynx (common passage for food and air) \(\rightarrow\) larynx(voice box) \(\rightarrow\) trachea (wind pipe) \(\rightarrow\) bronchi \((2\) for each side lungs) \(\rightarrow\) bronchioles \(\rightarrow\) alveoli (small sacs or pouches for exchange of gases).

Hint

(a) Oxygen haemoglobin dissociation curve gives the relationship between the saturation of haemoglobin and oxygen tension. The curve obtained by plotting the percent saturation of \(\mathrm{Hb}\) against time is sigmoid, at \(38^{\circ} \mathrm{C}\) and \(\mathrm{pH} 7.4\).
Dissociation of oxyhaemoglobin can be promoted by rise in the body temperature and low \(\mathrm{pH}\left(\right.\) high \(\mathrm{CO}_{2}\) ).

Hint

(a) Carbon dioxide is carried by the blood in three forms: physical solution, bicarbonate ions, and carbamino-haemoglobin. A very small amount of carbon dioxide dissolves in the plasma and is carried as a physical solution. About \(70 \%\) of carbon dioxide released by respiring tissue cells diffuses into the plasma and then into the red blood corpuscles. Here, \(\mathrm{CO}_{2}\) combines with water to form carbonic acid. Carbonic acid dissociates into bicarbonate and hydrogen ions.

Hint

(c) The erythrocyte (RBC) contains sufficient amount of carbonic anhydrase enzyme which catalyses the reaction between \(\mathrm{CO}_{2}\) and \(\mathrm{H}_{2} \mathrm{O}\) and helps in transportation of \(\mathrm{CO}_{2}\) from tissues to the lungs.

Hint

(b) In the lung, each alveolar duct opens into a blind chamber, the alveolar sac, or infundibulum. The latter consists of a central passage giving off several small pouches, the alveoli, or air sacs on all sides. The air sacs give the infundibulum the appearance of a small bunch of grapes. The alveoli have a very thin \((0.0001 \mathrm{~mm}\) thick) wall composed of simple moist, nonciliated, squamous epithelium. The number of alveoli is countless and their surface area is enormous. This further accelerates the gaseous exchange in the alveoli.

Hint

(b) In addition to the lungs, skin is also an organ of respiration in frogs. It is practically the only mode of respiration when the frog is underwater or hibernating. Skin is richly supplied with blood and is permeable to gases. That is why frogs always stay near water to keep their skin moist. It is further kept moist by secretion of mucus from its glands and does not become dry out of water.

Hint

(c) Intercostal muscles (external intercostal and internal intercostal) are attached with the ribs which help in the movement of the rib cage during breathing.

Hint

(d) Vital capacity of the lungs is the largest possible expiration after the largest possible inspiration that is greatest. The volume of air can be exchanged in single respiration or the amount of air breathed in and out with the greatest possible efforts.
\(
\begin{aligned}
\mathrm{VC} &=\mathrm{TRV}+\mathrm{TV}+\mathrm{ERV} \\
&=3000+500+1100 \\
&=4600 \mathrm{ml}
\end{aligned}
\)