RowQ
The Vault
RowQ
The Vault
CBSE Class 11 Biology · 10 questions · 24 marks
Every breath moves air along a purpose-built pipeline that ends in nearly three hundred million microscopic sacs, and this chapter maps that route from nostril to alveolus. It explains the mechanical bellows action that drives air in and out, the pressure gradients that move oxygen and carbon dioxide between alveoli, blood and tissues, and the way the brain's respiratory centres keep breathing running automatically without conscious thought.
During normal quiet inspiration, contraction of the diaphragm has the effect of:
Answer
Flattening it and increasing thoracic volume is correct — the diaphragm is dome-shaped at rest, and its contraction flattens this dome downward, enlarging the thoracic cavity and lowering pulmonary pressure so that air is drawn in.
The great majority of oxygen transported in human blood is carried:
Answer
Bound to haemoglobin as oxyhaemoglobin is correct — about 97% of the oxygen carried in blood is bound reversibly to the iron-containing haemoglobin inside red blood cells, and only a small fraction, about 3%, is simply dissolved in the plasma.
The enzyme that catalyses the conversion of CO2 and water into carbonic acid inside red blood cells, enabling most CO2 transport as bicarbonate, is:
Answer
Carbonic anhydrase is correct — this enzyme, abundant inside red blood cells, greatly speeds up the otherwise slow reaction between CO2 and water to form carbonic acid, which then dissociates to release bicarbonate ions into the plasma for transport.
The primary factor that stimulates the respiratory rhythm centre to increase the rate and depth of breathing is a rise in:
Answer
Blood carbon dioxide concentration is correct — the chemosensitive area near the medullary rhythm centre, along with peripheral chemoreceptors, responds far more strongly to a rise in CO2 (and the resulting fall in pH) than to a fall in oxygen, making CO2 the dominant signal driving increased ventilation.
Assertion (A): Actively respiring muscle tissue promotes greater release of oxygen from oxyhaemoglobin compared with resting tissue. Reason (R): The oxygen dissociation curve shifts to favour oxygen release under conditions of higher CO2, higher hydrogen ion concentration and higher temperature, all of which are elevated in actively respiring tissue.
Answer
Both A and R are true and R is the correct explanation of A — actively respiring muscle generates more CO2, acid and heat than resting tissue, and each of these factors independently lowers haemoglobin's affinity for oxygen, so oxygen is unloaded preferentially in exactly the tissue regions that need it most.
Define tidal volume and vital capacity, and explain how they are related.
Answer
Tidal volume is the volume of air breathed in or breathed out during one normal, quiet, unforced respiratory cycle, and it amounts to roughly 500 mL in an average adult human. Vital capacity is the maximum volume of air a person can expel from the lungs after taking the deepest possible breath, and it is made up of the tidal volume together with the inspiratory reserve volume (the extra air that can still be forcibly inhaled beyond a normal tidal breath) and the expiratory reserve volume (the extra air that can still be forcibly exhaled beyond a normal tidal breath); vital capacity therefore represents the full usable range of lung volume available for gas exchange during maximal effort, whereas tidal volume represents only the small portion of that range used during quiet, resting breathing.
Explain, using the concept of partial pressure gradients, how oxygen moves from alveolar air into the blood and then from the blood into body tissues.
Answer
Gases always diffuse from a region of higher partial pressure to a region of lower partial pressure, and oxygen exchange in the body follows this rule at two separate sites. In the lungs, the partial pressure of oxygen in the alveolar air is relatively high because fresh air is continually brought in by breathing, while the partial pressure of oxygen in the deoxygenated blood arriving through the pulmonary artery is comparatively low, since this blood has just returned from the tissues where oxygen was used up; because of this gradient, oxygen diffuses across the thin alveolar and capillary membranes from the air into the blood, where it binds to haemoglobin. This oxygen-rich blood is then carried by the systemic circulation to the body tissues, where active cells are continuously consuming oxygen in respiration, keeping the partial pressure of oxygen in the tissue fluid and cells low compared with that in the arriving blood; this reversed gradient now drives oxygen to diffuse out of the blood and into the tissue cells, where it is used for aerobic respiration.
Describe the mechanism of breathing, explaining the roles of the diaphragm and the intercostal muscles during inspiration and expiration.
Answer
Breathing in humans is a purely mechanical event that depends on rhythmic changes in the volume of the thoracic cavity, which in turn change the air pressure inside the lungs relative to the atmosphere, following the physical principle that gas pressure and volume are inversely related. During inspiration, two sets of muscles act together: the diaphragm, a dome-shaped sheet of muscle separating the thoracic and abdominal cavities, contracts and flattens, pushing downward and increasing the vertical dimension of the thoracic cavity, while the external intercostal muscles between the ribs contract, pulling the ribs and the sternum upward and outward and increasing the cavity's front-to-back and side-to-side dimensions. Together these actions enlarge the volume of the thoracic chamber, and because the lungs are held against the thoracic wall by the pleural membranes and their negative pressure, the lungs expand along with it; this expansion lowers the air pressure inside the lungs (pulmonary pressure) below the pressure of the outside atmosphere, and air consequently flows in through the respiratory passages until the pressures equalise, filling the alveoli. Expiration is normally a passive process that follows once inspiration ends: the diaphragm relaxes and returns to its domed shape, and the external intercostal muscles relax, allowing the rib cage to fall back under gravity and the elastic recoil of the tissues; this reduces the volume of the thoracic cavity, compressing the lungs and raising pulmonary pressure above atmospheric pressure, so that air is now pushed out of the lungs to the exterior. During forceful breathing, as in heavy exercise, additional muscles are recruited: the internal intercostal muscles actively pull the ribs further down and inward to assist forced expiration, while abdominal muscles can also contract to push the diaphragm further up, and accessory muscles of the neck can assist forced inspiration by raising the ribs even more.
Describe the three ways in which carbon dioxide is transported in the blood from the tissues to the lungs, and explain what happens to each form when the blood reaches the alveoli.
Answer
Carbon dioxide produced continuously by respiring tissue cells diffuses into the blood and is carried back to the lungs in three distinct forms. The largest share, roughly 70%, is transported as bicarbonate ions. Inside red blood cells, CO2 combines with water in a reaction sped up enormously by the enzyme carbonic anhydrase to form carbonic acid, which almost immediately dissociates into a hydrogen ion and a bicarbonate ion; the bicarbonate ion then diffuses out of the red blood cell into the plasma (in exchange for a chloride ion moving in, maintaining electrical balance, an event called the chloride shift), while the hydrogen ion is largely buffered by binding to haemoglobin. A smaller share, roughly 20 to 25%, is transported bound directly to haemoglobin, forming a compound called carbaminohaemoglobin, in which CO2 attaches not to the oxygen-binding haem group but to amino groups on the globin protein chains, so this binding does not compete directly with oxygen transport. The remaining small fraction, roughly 7%, simply dissolves in the plasma as CO2 gas without undergoing any chemical reaction. When the blood reaches the alveolar capillaries in the lungs, the process reverses because the alveolar air has a much lower partial pressure of CO2 than the blood: the reactions run backward, with bicarbonate ions re-entering red blood cells and recombining with hydrogen ions to reform carbonic acid, which is then broken down by carbonic anhydrase back into CO2 and water; carbaminohaemoglobin releases its bound CO2; and the small dissolved fraction simply diffuses out directly. All three released pools of CO2 then diffuse from the blood into the alveolar air along the favourable partial pressure gradient and are expelled from the body during the next expiration.
A patient with long-standing exposure to cigarette smoke visits a doctor complaining of persistent breathlessness. Lung function tests show that his vital capacity is markedly reduced and that gas exchange is inefficient even though his airways are not obviously blocked; imaging shows that many alveolar walls have broken down and fused into larger, fewer air spaces with much less total surface area. A second patient, a lifelong industrial worker in a stone-cutting unit, shows fibrous, thickened tissue in the lungs traced to years of inhaling fine mineral dust. (a) Identify the disorder most consistent with the first patient's findings and explain, in structural terms, why gas exchange is reduced. (b) Why would this patient's vital capacity be reduced even though the airway passages themselves are not blocked? (c) Identify the type of disorder affecting the second patient and name it if you can. (d) Suggest one general workplace or lifestyle measure that could have helped prevent each patient's condition.
Answer
(a) The findings are consistent with emphysema, a chronic disorder often caused by long-term smoking. In this condition, the delicate walls between adjacent alveoli break down and adjacent air sacs fuse into fewer, larger spaces; because gas exchange depends on the total surface area of thin, moist alveolar membrane in contact with blood capillaries, this loss of alveolar walls drastically reduces the surface area available for oxygen and carbon dioxide to diffuse across, impairing gas exchange even without any airway blockage. (b) Vital capacity depends on the lungs' ability to expand and recoil effectively to move large volumes of air; the destruction of alveolar walls in emphysema also damages the elastic tissue of the lungs, reducing their elasticity and their ability to expand fully or recoil to expel air forcefully, which lowers the reserve volumes and therefore the overall vital capacity even though the bronchi and bronchioles remain open. (c) The second patient's condition is an occupational respiratory disorder caused by long-term inhalation of fine dust particles; given his occupation in stone-cutting, the specific condition is most likely silicosis, caused by inhaling silica dust, which leads to fibrosis (thickening and stiffening) of lung tissue. (d) For the first patient, avoiding tobacco smoke (not smoking, and avoiding secondhand smoke) is the single most effective preventive measure; for the second patient, using proper respiratory protective equipment such as masks and ensuring adequate dust control and ventilation in the workplace would reduce inhalation of harmful mineral dust particles.
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