All Free Biology MCQs with Answers

271. The rate and depth of breathing are regulated mainly by the level of

• A. oxygen in the alveoli
• B. glucose in the blood
• C. nitrogen in the blood
• D. carbon dioxide in the blood, monitored by the medulla oblongata

Explanation: Chemoreceptors in the medulla and in the great arteries respond to the fall in pH that a rise in carbon dioxide produces, and the respiratory centre then increases the rate and depth of breathing until the excess is blown off. Oxygen level acts only as a weak backup stimulus and matters mainly at high altitude. This is why holding the breath becomes unbearable from the build up of carbon dioxide, not from the lack of oxygen.

272. Carbon monoxide is dangerous because it

• A. binds haemoglobin far more strongly than oxygen does and does not readily let go
• B. dissolves the walls of the alveoli
• C. destroys the red blood cells immediately
• D. prevents the diaphragm from contracting

Explanation: Carbon monoxide has roughly two hundred times the affinity of oxygen for haemoglobin and forms stable carboxyhaemoglobin, so those binding sites are taken out of use and the blood carries far less oxygen even though it looks bright red. The victim feels no shortage of breath, because breathing is driven by carbon dioxide, which is why poisoning from a gas heater can be fatal during sleep.

273. The chloride shift refers to the movement of chloride ions

• A. out of the red cell as oxygen enters
• B. into the red cell to balance the hydrogen carbonate ions leaving it
• C. from the plasma into the alveolar air
• D. into the plasma from the kidney tubules

Explanation: As hydrogen carbonate ions formed inside the red cell diffuse out into the plasma, the cell would be left with a net positive charge, so chloride ions move in to keep it electrically neutral. This exchange lets the red cell keep converting carbon dioxide rapidly all the way through the tissue capillaries. In the lungs the whole sequence runs in reverse and chloride moves back out.

274. The air that remains in the lungs even after the most forceful expiration is the

• A. tidal volume
• B. inspiratory reserve volume
• C. residual volume
• D. vital capacity

Explanation: About 1,200 cm3 cannot be expelled, and this residual volume keeps the alveoli partly inflated so they do not collapse and stick shut, and it also lets gas exchange continue between breaths. Because it can never be breathed out, it cannot be measured with a simple spirometer. Its presence is also why lungs float in water, a fact used in forensic medicine to show that a baby breathed after birth.

275. The component of cigarette smoke responsible for addiction and for raising heart rate and blood pressure is

• A. tar
• B. carbon monoxide
• C. benzene
• D. nicotine

Explanation: Nicotine reaches the brain within seconds and acts on receptors there to give the reward that makes smoking so hard to give up, while also constricting arterioles and stimulating adrenaline release. Tar is the sticky carcinogenic residue that damages the airways, and carbon monoxide reduces the oxygen carrying power of the blood. All three are harmful, but only nicotine causes the dependence.

276. Lung cancer in smokers is caused mainly by

• A. the carcinogenic chemicals in tar
• B. the heat of the smoke
• C. nicotine acting on the brain
• D. the loss of surfactant from the alveoli

Explanation: Tar contains carcinogens such as benzopyrene that mutate the genes controlling cell division in the bronchial epithelium, and years of exposure eventually produce an uncontrolled tumour. Nicotine is highly addictive but is not itself the main carcinogen, which is why a nicotine patch is far safer than a cigarette. The risk rises with the number of cigarettes and the number of years smoked.

277. In emphysema the patient becomes breathless because

• A. the trachea is blocked by mucus
• B. the walls between alveoli break down, reducing the surface area for gas exchange
• C. the diaphragm is paralysed
• D. the blood contains too few white cells

Explanation: Smoke attracts phagocytes that release elastase, which digests the elastic tissue, so alveoli merge into large air spaces with much less total surface and the lungs lose the recoil needed to breathe out. Gas exchange falls and expiration becomes an effort, so even mild exertion leaves the patient gasping. The damage is permanent, since alveolar walls cannot regenerate.

278. The persistent cough of chronic bronchitis in a smoker develops because

• A. the alveoli fill with blood
• B. the ribs lose their cartilage
• C. smoke paralyses the cilia and stimulates extra mucus, which then collects in the airways
• D. the vocal cords are permanently inflamed

Explanation: Cilia that normally sweep the mucus escalator upwards are stopped by the smoke, while the irritation makes goblet cells secrete more mucus, so it pools in the bronchi and can only be cleared by coughing. The stagnant mucus also becomes a breeding ground for bacteria, which is why chest infections are frequent. Much of this damage is reversible when a person stops smoking.

279. Passive smoking is a health hazard because

• A. the smoke changes chemically once it leaves the smoker's lungs
• B. non smokers absorb nicotine through the skin
• C. it affects only people who are already ill
• D. sidestream smoke contains the same tar, nicotine and carbon monoxide, which bystanders inhale

Explanation: Smoke drifting from the burning tip is unfiltered and carries the same carcinogens, so people sharing the room inhale them and their risk of lung cancer and heart disease rises measurably. Children exposed at home suffer more asthma attacks and chest infections, and exposure during pregnancy lowers birth weight. This evidence is what public smoking bans are based on.

280. The number of bones in the skeleton of a normal adult human is

• A. 200
• B. 206
• C. 213
• D. 306

Explanation: An adult has 206 bones, 80 in the axial skeleton and 126 in the appendicular skeleton. A newborn has roughly 300 separate bones, and the number falls as several of them fuse during growth, for example the ilium, ischium and pubis into one hip bone and the five sacral vertebrae into the sacrum. That fusion is why the infant figure is the standard distractor.