In human blood, carbon dioxide is transported mainly in which form?

The correct answer is A, inhalation. Inhalation, also called inspiration, is the phase of breathing in which the diaphragm flattens and the intercostal muscles pull the ribs up and out, so the chest cavity enlarges, the pressure inside the lungs falls below the pressure of the atmosphere and oxygen rich air rushes in through the nostrils and the windpipe. Option B is wrong because exhalation is the opposite phase, in which the muscles relax, the chest cavity shrinks and carbon dioxide rich air is pushed out; it is normally a passive process. Option C is wrong because flotation is a physical property of bodies in a fluid, described by the law of Archimedes, and has nothing to do with breathing. Option D is wrong because exploration simply means searching or investigating and is not a term of biology at all. Remember the pair as inhalation for taking air in and exhalation for sending it out.

The correct answer is C, the liver. It is the largest gland as well as the heaviest internal organ of the human body, it lies in the upper right part of the abdomen, and its chief contribution to digestion is bile, which is stored in the gall bladder and emulsifies fat. Option A is wrong because the pancreas, although important as the only gland that is both exocrine and endocrine, is much smaller and weighs less than a hundred grams. Option B is wrong because the thyroid, the largest endocrine gland of the body, is still far smaller than the liver; the question asks for the largest gland of any kind. Option D is wrong because the parotid is only the largest of the three pairs of salivary glands, which is a much narrower title. A common trap in papers is to offer the thyroid, so read whether the question says gland or endocrine gland.

The correct answer is C, salivary amylase, also called ptyalin. It is secreted by the three pairs of salivary glands and breaks starch into the smaller sugar maltose, which is why starch is the only class of food whose digestion actually begins in the buccal cavity. Option A is wrong because pepsin is secreted by the gastric glands of the stomach and works only on protein, and only in a strongly acidic medium. Option B is wrong because trypsin comes from the pancreas, acts in the duodenum and also digests protein, not starch. Option D is wrong because lipase, again from the pancreas, acts on fat that bile has already emulsified, and breaks it into fatty acids and glycerol. Note that a long chew makes bread taste sweet, which is the everyday proof that salivary amylase is turning starch into sugar.

The correct answer is B. Bile is made in the liver, stored and concentrated in the gall bladder and poured into the duodenum, and it carries bile salts and bile pigments but not a single digestive enzyme. Its bile salts break the large globules of fat into very fine droplets, a process called emulsification, which multiplies the surface on which pancreatic lipase can act, and it also neutralises the acid arriving from the stomach so that the alkaline pancreatic and intestinal enzymes can work. Option A is wrong because that chemical breakdown is the work of lipase, not of bile. Option C is wrong because absorption through the villi is a separate step that follows digestion and is not performed by bile. Option D is wrong because no secretion of the body converts fat into starch; the two are different classes of nutrient.

The correct answer is C, the small intestine. At roughly six to seven metres it is by far the longest part of the nine-metre alimentary canal, it is where digestion is completed by pancreatic and intestinal juices, and its inner wall carries millions of finger-like villi that give a vast surface for absorbing glucose, amino acids, fatty acids and glycerol into the blood. Option A is wrong because the stomach is only a muscular bag about twenty-five centimetres long, and it absorbs almost nothing besides water, alcohol and some medicines. Option B is wrong because the oesophagus is a plain conducting tube about twenty-five centimetres long that only pushes food down by peristalsis. Option D is wrong because the large intestine, though wider, is much shorter and mainly absorbs water and salts from the undigested residue.