Human Digestive and Respiratory Systems: Organs and Enzymes

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.

The correct answer is B. The gastric glands secrete hydrochloric acid, which makes the contents of the stomach strongly acidic; pepsin can work only in such a medium, and the acid also kills most of the bacteria swallowed along with food. The same glands secrete a layer of mucus that protects the stomach wall from being digested by this acid, and damage to that layer produces a peptic ulcer. Option A is wrong because no acid digests fat; fat is emulsified by bile and then split by lipase. Option C is wrong because the absorption of vitamins happens mainly in the small intestine, not through the action of stomach acid. Option D is wrong because emulsification is the work of the bile salts made in the liver, and bile in fact neutralises this acid when the food reaches the duodenum.

The correct answer is B, three. The right lung is divided into three lobes, an upper, a middle and a lower, while the left lung has only two, because the heart is tilted towards the left side of the chest and takes up room there; the left lung even carries a notch called the cardiac notch for the heart. Option A is wrong because two is the number of lobes in the left lung, and it is offered here precisely as the trap for a student who remembers the pair but not which side is which. Option C is wrong because no human lung has four lobes. Option D is wrong because a single undivided lung is not the human arrangement at all. Remember the rule that the right lung is the bigger of the two, with three lobes, and the left the smaller, with two.

The correct answer is B, the diaphragm. It is a dome-shaped sheet of muscle that separates the chest cavity from the abdomen, and when it contracts it flattens, which increases the volume of the chest, lowers the pressure inside the lungs and draws air in; the intercostal muscles between the ribs assist by lifting the rib cage up and out. Option A is wrong because cardiac muscle is found only in the wall of the heart and its job is to pump blood, not air. Option C is wrong because the biceps is a skeletal muscle of the arm and has no role in respiration. Option D is wrong because the smooth muscle of the stomach churns food and has nothing to do with the breathing movement. A sudden involuntary spasm of the diaphragm is what we experience as hiccups.

The correct answer is B, as bicarbonate ions. Most of the carbon dioxide produced by the tissues enters the red blood cells, where the enzyme carbonic anhydrase combines it with water to form carbonic acid, which at once splits into hydrogen and bicarbonate ions; the bicarbonate then travels in the plasma to the lungs, where the reaction is reversed and the gas is breathed out. A smaller share is carried bound to haemoglobin as carbaminohaemoglobin and a little stays dissolved. Option A is wrong because oxyhaemoglobin is the form in which oxygen, not carbon dioxide, is carried. Option C is wrong because free bubbles of gas in the blood would cause an embolism and are dangerous, not the normal mode of transport. Option D is wrong because carbon monoxide is a poison from incomplete burning that blocks haemoglobin, and the body does not make it for transport.

The correct answer is A, the epiglottis. It is a leaf-shaped flap of cartilage at the root of the tongue that swings down and covers the opening of the windpipe every time food is swallowed, so that the bolus is guided into the oesophagus instead of the trachea; when it fails we choke and cough. Option B is wrong because the larynx is the sound box that houses the vocal cords, and although it lies just below the epiglottis it does not perform this closing action. Option C is wrong because the pharynx is the common chamber behind the mouth and nose through which both air and food pass, not a valve. Option D is wrong because the pleura is the double membrane wrapping each lung and its fluid only reduces friction as the lung expands and contracts.